Technics SA-DX950 Receiver
Contents
1. Available online at www sciencedirect com science oinzcr Hearing Research 202 2005 235 247 HEARING RESEARCH www elsevier com locate heares Attribute capture in the precedence effect for long duration noise sounds Liang Li gt James G Qi Yu He Claude Alain Bruce A Schneider Department of Psychology Speech and Hearing Research Center National Key Laboratory on Machine Perception Peking University Beijing 100871 China P Centre for Research on Biological Communication Systems Department of Psychology University of Toronto at Mississauga 3359 N Mississauga Road Mississauga Ont Canada L5L 1C6 The Rotman Research Institute Baycrest Centre for Geriatric Care Toronto Ont Canada M6A 2E1 Received 7 October 2004 accepted 13 October 2004 Available online 8 December 2004 Abstract Listeners perceptually fuse the direct wave from a sound source with its reflections off nearby surfaces into a single sound image located at or near the sound source the precedence effect This study investigated how a brief gap presented in the middle of either a direct wave or simulated reflection is incorporated into the fused image For short lt 9 5 ms delays between the direct leading and reflected lagging waves no sound was perceived from the direction of the lagging wave For delays between 10 and 15 ms both sounds were perceived but the gap was heard only on the leading side When the gap was2. T C T Hartmann W M 1997 Psychophysical and physiological evidence for a precedence effect in the median sagittal plane J Neurophysiol 77 2223 2226 Litovsky R Y Colburn H S Yost W A Guzman S J 1999 The precedence effect J Acoust Soc Am 106 1633 1654 Litovsky R Y Fligor B J Traino M J 2002 Functional role of the human inferior colliculus in binaural hearing Hear Res 165 177 188 Perrott D R Strybel T Z Manligas C L 1987 Conditions under which the Haas precedence effect may or may not occur J Audit Res 27 59 72 Picton T W van Roon P Armilio M L Berg P Ille N Scherg M 2000 The correction of ocular artifacts a topographic perspective Clin Neurophysiol 111 53 65 Rakerd B Hartmann W M Hsu J 2000 Echo suppression in the horizontal and median sagittal planes J Acoust Soc Am 107 1061 1064 Shinn Cunningham B G Zurek P M Durlach N I 1993 Adjust ment and discrimination measurements of the precedence effect J Acoust Soc Am 93 2923 2932 Tollin D J Henning G B 1999 Some aspects of the lateralization of echoed sound in man II The role of the stimulus spectrum J Acoust Soc Am 105 838 849 Wallach H Newman E B Rosenzweig M R 1949 The precedence effect in sound localization J Acoust Soc Am 62 315 336 Yin T C T 1994 Physiological correlates of the precedence effect and summing localization in the infe
3. are correlated the lagging sound is treated as a reflection of the leading sound a single noise image is perceived and attributes that appear only in the lagging sound are attributed to captured by the leading sound This is not what we would expect on the basis of the physical cues to the location of the gap that are present when there is a gap only in the lagging sound Fig 4 shows that when there is a gap in the lagging right side source only there is a corresponding drop in energy especially in the high frequency region in the right ear with little evidence of any change in the left ear Hence if the loca tion of the gap were to be based on the ear with the most salient cues one would expect the gap to be heard on the side of the lagging sound Nevertheless the gap is heard as occurring on the leading side In other words it is attributed to captured by the leading stimulus When the gap is only in the correlated lagging sound the acoustic situation is ecologically anomalous because the gap in the reflection should have its origin in the source The ecological prediction is that a gap in the lag ging sound would cause a temporary breakdown in the precedence effect When the lagging loudspeaker be comes silent during the gap there is no correlated signal coming from the lagging loudspeaker to be captured Thus when the gap terminates and the lagging loud speaker is turned on again the participant should ini tially pe
4. 000 2000 3000 4000 6000 and 8000 Hz and all listeners in this and in subse quent experiments were normal for frequencies less than 6 kHz All the listeners in this and next experiments gave their written informed consent to participate in the experiments and were paid a modest stipend for their participation 2 1 2 Apparatus and materials During test sessions listeners were seated in a chair at the center of an Industrial Acoustic Company IAC sound attenuated chamber whose internal dimensions were 193 cm in length 183 cm in width and 198 5 cm in height Gaussian broadband noises 0 10 kHz whose duration was 3050 ms including 30 ms rise fall times were synthesized using a 16 bit Tucker Davis Technologies TDT System IH hardware DD1 and cus tom software at the sampling rate of 20 kHz The noise signals were converted to analog forms using TDT DD1 digital to analog converters under the control of a Dell computer with a Pentium processor The analog outputs were low passed at 10 kHz with the TDT FTS filter attenuated by two programmable attenuators TDT PA4 for the left and right channels amplified via a Technics power amplifier SA DX950 and then deliv ered from two balanced loudspeakers Electro Medical Instrument 40 W which were in the frontal azimuthal plane at the left and the right 45 positions symmetrical with respect to the median plane Fig 1 The distance between each of the two loudspeakers to the cen
5. 04 Elsevier B V All rights reserved doi 10 1016 j heares 2004 10 007 wave is sufficiently intense the reflected wave is per ceived as a distinct auditory event an echo whose per ceived location is usually different from that of the source However when the delays between the direct wavefront and its reflections are short e g 1 10 ms or more depending on the stimulus the auditory sys tem somehow gives precedence to the direct wave front over its reflections so that the listener hears only a single fused sound whose point of origin is perceived to be at or near the location of the sound source This phenomenon is called the precedence effect Clifton and Freyman 1989 Freyman et al 1991 Shinn Cunn ingham et al 1993 Wallach et al 1949 Zurek 1980 for reviews see Blauert 1997 Li and Yue 2002 Litov sky et al 1999 Zurek 1987 236 L Li et al Hearing Research 202 2005 235 247 The precedence effect reduces listeners perception of multiple images by perceptually grouping correlated acoustic waveforms from different directions thereby avoiding the perception of multiple sound images when only one source is present Furthermore because the fused image is perceived as originating at or near the location of the source localization errors are reduced in reverberant environments In experimental environ ments the direct and reflected waves are usually produced by two spatially sepa
6. 6 p 0 001 Fig 6 Pairwise comparisons indicate that the ampli tude of N1 P2 response to the gap in Condition L U was significantly smaller than that in Condition L C p 0 022 and that in Condition R C p 0 000 but the difference between Condition L C and Condition R C was not significant p 0 125 Topographic volt age maps for the N1 component to the gap Fig 7 indi cate that in Condition L U the highest negativity was widely distributed over the midline but in both Condi tion L C and Condition R C it became more concen trated over the right hemisphere Hence when a gap is introduced the cortical response depends upon whether or not the two sounds were correlated or uncorrelated Moreover there appeared to be ERP differences in the sustained responses following the gap Fig 6 244 L Li et al Hearing Research 202 2005 235 247 200 1500 3500 Time ms Fig 6 The whole course of the averaged ERP responses recorded from the central 9 electrode sites across 12 listeners in each of the L U L C and R C conditions The N1 peak and P2 peak responses to the sound onset and gap and slow sustained potentials SPs following sound onset and the gap are indicated in the panel for FC1 electrode site The two arrows above the time base indicate the onset of the sound and onset of the gap respectively Fig 7 The ERP topographic voltage map for the N1 response to the gap across the
7. 61 scalp electrodes in each of the L U L C and R C conditions between conditions The average amplitude of the sus tained responses 550 850 ms after the gap onset was analyzed The results show that there were no significant differences in sustained responses for the two conditions L U and R C where the gap was correctly assigned to the right loudspeaker However the condition in which the gap in the right sound was perceptually captured by the left sound Condition L C differed significantly both from Condition L U across all the 9 central sites p 0 001 and from Condition R C across the 3 fronto central sites FC1 FCz FC2 p 0 016 Hence a long latency and negatively shifted sustained response in the frontal cortical region following the gap appears to be associated with gap capture 6 Discussion Most previous studies of the precedence effect have used clicks or short noise bursts as acoustic stimuli to avoid or reduce the overlap between the leading and lag ging stimuli Here long lasting sound segments were chosen for 3 major reasons First long duration sound segments e g speech or music are more prevalent in everyday environments therefore have greater ecologi cal validity than idealized brief sounds for humans Sec ond the use of longer stimuli allowed us to easily present an attribute a gap that was clearly a feature that appeared only in the lagging sound Third only when the sound duration is suf
8. HO THRESHOLD ms oOo N A A Correlated 50 ms gap CONDITION Correlated No gap Fig 2 Comparison of average attribute capture thresholds between the two conditions 1 Condition Correlated No gap the two noises from the two loudspeakers were correlated and no gaps were introduced 2 Condition Correlated 50 ms gap a 50 ms gap was introduced into the middle of each of the two correlated noise sounds from the two loudspeakers The error bars indicate the standard errors of the mean L Li et al Hearing Research 202 2005 235 247 239 lead lag time Thirteen of the 15 listeners always heard gaps in both sounds at all delays however two of the listeners occasionally reported that they did not hear a gap in the lagging sound 3 Experiment 2 In Experiment 1 when there were gaps in both lead ing and lagging correlated noises and the lead lag time was slightly longer than the echo threshold 10 15 ms so that both the leading and lagging sounds were heard listeners heard a gap in the leading but not in the lagging sound A possible explanation of this phenomenon is that some attributes of the lagging sound e g the pres ence of a gap were being suppressed even though the lagging sound was heard If that were the case then it would be expected that attributes of the lagging sound would be even more suppressed when the lead lag time was short enough that only a single fused sound was heard To check whether attr
9. a gap in the right side noise or whether the gap was only heard in the left side leading noise If they did not hear a gap in the noise coming from the right loudspeaker they were to press the other button In other words the lag time between the sounds from the two loudspeakers was reduced following responses indicating a perceived gap in the noise perceived on the right and increased following responses indicating that they did not hear a gap on the right The same 3 down 1 up procedure was employed Levitt 1971 In Condition Uncorrelated 50 ms gap a 50 ms gap was introduced into the middle of each of the two uncor related noise sounds from the two loudspeakers and the procedure was the same as that of Condition Correlated 50 ms gap There were four repetitions in each of the three conditions 2 2 Results In Condition Correlated No gap when the lead lag times were substantially longer than the individuals echo thresholds all 15 listeners perceived a distinct sound image originating from the right loudspeaker Be cause the noise sound image originating from the left loudspeaker was always perceived two spatially sepa rate noise sounds were actually heard at the longer lead lag times one on the left and one on the right When the lead lag delays were substantially below the individuals echo thresholds only one noise sound image was heard as coming from the locus of the leading loud speaker and no sound image as com
10. at would be occupied by the lis teners head The signal at the location of the eardrum in the simulated head was then recorded for both left and right ears under two conditions using the B amp K Pulse Platform In the first condition correlated noises were presented over both loudspeakers with the left loud speaker leading the right loudspeaker by 2 ms The lines with filled circles in Fig 4 depict the long term spectra of the left left panel and right right panel ear signals when both loudspeakers were playing The lines with open squares depict the long term spectra of the left left panel and right right panel ear signals when only the left loudspeaker was on i e the condition that existed when there was a gap in the right loudspeaker The dif ferences between the two spectra in the left panel iden tify the left ear spectral cues to the presence of a gap The comparable differences in the right panel identify the right ear spectral cues to the presence of a gap Clearly spectral differences in the right ear are much more pronounced than they are in the left ear especially at the high frequencies due to the head shadow effect Because gap detections thresholds did not vary with the degree of interaural correlation and because the spectral cues are much more pronounced at the right ear it is reasonable to conclude that the detection of a gap was based on the processing of intensity information in the right ear Hence on
11. ce effect J Acoust Soc Am 82 1834 1835 Clifton R K Freyman R L 1989 Effect of click rate and delay on breakdown of the precedence effect Percep Psychoph 46 139 145 Clifton R K Freyman R L Meo J 2002 What the precedence effect tells us about room acoustics Percep Psychoph 64 180 188 Clifton R K Freyman R L Litovsky R Y McCall D 1994 Listeners expectations about echoes can raise or lower echo threshold J Acoust Soc Am 95 1525 1533 Cornelisse L E Kelly J B 1987 Neuropsychologia 25 449 452 Fitzpatrick D C Kuwada S Batra R Trahiotis C 1995 Neural responses to simple simulated echoes in the auditory brainstem of the unanesthetized rabbit J Neurophysiol 74 2469 2486 Fitzpatrick D C Kuwada S Kim D O Parham R Batra R 1999 Responses of neurons to click pairs as simulated echoes auditory nerve to auditory cortex J Acoust Soc Am 106 3460 3472 Freyman R L Clifton R K Litovsky R Y 1991 Dynamic processes in the precedence effect J Acoust Soc Am 90 874 884 Freyman R L McCall D M Clifton R K 1998 Intensity discrim ination for precedence effect stimuli J Acoust Soc Am 103 2031 2041 Freyman R L Helfer K S McCall D D Clifton R K 1999 The role of perceived spatial separation in the unmasking of speech J Acoust Soc Am 106 3578 3588 Hartung K Trahiotis C 2001 Peripheral auditory processing a
12. cy sustained components of ERP responses to gaps were measured in Conditions L U L C and R C respectively 5 1 Method 5 1 1 Participants All the 11 listeners from Experiment 3 and 1 new male young university student 21 years old with nor mal and balanced pure tone hearing participated in this physiological experiment These listeners were instructed to remain awake and keep their eyes open while they lis tened to the acoustic stimuli 5 1 2 Apparatus and materials The apparatus and materials were same as in previous experiments However this ERP recording experiment was conducted in a different IAC sound attenuated chamber that was equipped with 64 channel NeuroScan SynAmps bandpass 0 05 50 Hz 250 Hz sampling rate 5 1 3 Procedure The size of the gap in the sound from the right loud speaker was fixed at 50 ms and the delay between the sounds from the two loudspeakers was fixed at 2 ms During the recording all electrodes were referenced to the Cz site for data analysis they were re referenced to an average reference The analysis epoch included 200 ms of pre stimulus activity and 3500 ms of post stimulus activity following each of the 150 sound presen tations for each of the three conditions Conditions L U L C and R C Trials contaminated by excessive peak to peak deflection 150 uV at the electrodes not adjacent to the eyes were automatically rejected ERP waveforms were then averaged separately for eac
13. ed previously Thus there is a strong higher order cognitive component involved in the precedence effect For this reason human s cortical correlates of the precedence ef fect were investigated using the method of scalp event related potential ERP recording Since ERPs to a brief acoustic event can last a few hundred ms in the present study the sound duration was set to about 3 s so that ERP responses specific to the probe gap could be more easily separated from those to sound onset and offset 2 Experiment 1 In the first experiment echo thresholds for long dura tion noises as a function of the delay between the direct wave and its simulated reflection were measured This threshold is defined as the longest delay between the di rect and reflected wave at which no sound is perceived from the direction of the lagging stimulus A gap cap ture threshold was also determined where the gap cap ture threshold is defined as the longest delay between the direct and reflected wave at which the listener could no longer detect a gap in stimulation from the direction of the lagging sound 2 1 Materials and methods 2 1 1 Participants Fifteen young 19 25 years old six females and nine males university students with normal and balanced less than 15 dB difference between the two ears pure tone hearing confirmed by audiometry participated in this experiment The audiometric thresholds were deter mined at frequencies of 250 500 1
14. een them called the lead lag time was reduced following responses indicating a perceived noise sound from the right loudspeaker and increased following responses indicating that no noise sound was perceived from the right loudspeaker using a 3 down l up procedure Levitt 1971 All sessions were started with a 50 ms lead lag time Therefore the longest lead lag time at which no sound image from the right loudspeaker was perceived the echo inaudi ble criterion was obtained That an echo not a reflec tion is perceived or not is subjective and listeners responses cannot be categorized as either correct or incorrect Thus in this and the other two conditions of this experiment no feedback was given to listeners In Condition Correlated 50 ms gap a 50 ms gap was introduced into the middle of each of the two correlated noises from the two loudspeakers The delay between the onsets of the two gaps was equal to the delay be tween the leading and lagging sounds The left loud speaker was also the leading loudspeaker and listeners when presented with a stimulus indicated by pressing one of two buttons whether they heard a gap in the sound coming from the right lagging source Logically of course they could only hear a gap in the right side noise if they heard a noise on the right Hence the question here is whether when they heard a noise on the right lead lag delays gt echo threshold they also perceived
15. eft was leading for detecting the gap in the middle of the right sound especially when the gap occurred 1500 ms after sound onset Thus for the gap detection test Condition R U should be equivalent to Condition L U and was not included in the experimental protocol 240 L Li et al Hearing Research 202 2005 235 247 Oo GAP DETECTION THRESHOLD ms Nw kW A A we CONDITION Fig 3 Comparison of average gap detection thresholds in the following four conditions 1 right sound only RO 2 left leading uncorrelated L U 3 left leading correlated L C and 4 right leading correlated R C The error bars indicate the standard errors of the mean there was no indication that changing the left noise from lagging to leading affected the detection of a gap In deed the gap detection threshold remained unchanged even when the two sounds were uncorrelated These re sults are consistent with the notion that the detection of a gap in a stimulus depends only on the extent of the drop in acoustic energy present in the stimulus at the ears since the degree of interaural correlation and Left Ear Canal RELATIVE POWER IN dB 0 12 3 4 5 FREQUENCY IN kHz 6 7 8 9 10 the direction of the lag apparently had no effect on threshold To determine the nature of the local cues in the left and right ear that could signal the presence of a gap a B amp K head and torso simulator HATS 4128C was placed at the position th
16. experience to the gap which was introduced into the middle of either the leading or lagging sound As mentioned earlier most neurophysiological stud ies on the precedence effect have mainly focused their ef forts on determining the brainstem mechanisms involved in lag suppression in experimental animals Fitzpatrick et al 1995 1999 Litovsky 1998 Litovsky and Delgutte 2002 Litovsky and Yin 1998a b Litov sky et al 1997 Yin 1994 However there is more to precedence than simple suppression of the location information of the lagging stimulus For example sev eral studies have shown that listeners knowledge and expectations about the room acoustics can strongly af fect the precedence effect Clifton 1987 Clifton and Freyman 1989 Clifton et al 1994 Freyman et al 1991 Repeated presentations of the leading and lagging clicks which are not perceived to be fused at the begin ning can eventually cause fusion to occur suggesting that following continued exposure to a reverberant envi ronment listeners can build up a new representation of L Li et al Hearing Research 202 2005 235 247 237 the room acoustics consistent with the leading and lag ging stimulus being produced by a single source More over once fusion is established it is most readily broken when a change in the spatial relationship between the leading and lagging sounds is inconsistent with the knowledge of the room acoustics that has been acquir
17. ficiently long can neuro physiological responses such as ERPs to a transient probe attribute embedded in the sound be easily distin guished from those to sound onsets and offsets and the development of sustained neurophysiological responses between transient acoustic events be segregated In the present study when the two spatially separated long lasting noise sounds were correlated only a single noise image was perceived as coming from the location of the leading loudspeaker if the lead lag time was below echo threshold These results are in agreement with previous reports that two correlated long lasting speech spectrum noise sounds which are presented by two spatially separated loudspeakers 60 separation 4 ms delay time can be perceived as a single noise image originating from the position of the leading loudspeaker Freyman et al 1999 The average echo threshold found here 9 5 ms is within the range reported in previous studies 5 10 ms for a review see Litovsky et al 1999 Interestingly when both leading and lagging sounds have comparable gaps the gap capture threshold is 15 6 ms which is sig nificantly longer than the echo threshold obtained from the same listeners Hence for delays larger than 10 ms listeners perceive two sound images one from the lead ing and one from the lagging loudspeaker But as long as the delays were less than 15 ms listeners perceived the gap in the lagging stimulus as occurring i
18. ging uncorrelated sound was also attributed to the leading sound indicating that attributes of the lagging sound may occasionally be captured by the leading sound even when the two sounds are uncorrelated Hence although listeners never reported that the two independent sounds became fused there is some indica tion of attribute capture by the leading sound The dis agreement concerning fusion between our data and those reported by Perrott et al 1987 for uncorrelated noises may be due to the differences of stimulus param eters between the two studies such as those in sound duration 50 ms vs 3050 ms onset offset duration 0 2 ms vs 30 ms and loudspeaker separation 20 vs 45 etc In a reverberant environment each sound reflection comes from a location that is usually different from that of the sound source and not all attributes of reflections are suppressed by their sound sources Clifton et al 2002 Freyman et al 1998 Perrott et al 1987 Tollin and Henning 1999 In the present study if the gap attribute in the lagging sound had been suppressed by the correlated leading sound when the precedence effect occurred the gap detection threshold in Condition L C should have been higher than those in Condition L U and Condition R C and the gap detection threshold in Condition R C should have been lower than that in Condition L U However our data show that gap detec tion thresholds were independent of whether the ga
19. h site and conditions and digitally low pass filtered to attenu ate the components with frequencies above 12 Hz Although the number of stimulus presentation trials was 150 the number of trials included in the average for each condition varied between listeners with the across listener average being 116 114 and 113 for Con dition L U Condition L C and Condition R C respec tively For each individual average ocular artifacts e g blinks and lateral movements were corrected by means of ocular source components using the Brain Electrical Source Analysis BESA software Picton et al 2000 ERP waveforms were quantified by computing mean values in selected latency regions relative to the mean amplitude of the 200 ms pre stimulus activity All ampli tude measurements were subjected to mixed ANOVA with condition and electrode as the two within subject factors Topographic voltage maps were examined using the 61 electrodes the periocular electrodes were not included 5 2 Results For the 9 central electrode sites FC1 FCz FC2 Cl Cz C2 CP1 CPz and CP2 there were no differences across these three conditions both for N1 P2 peak to peak amplitudes to sound onset F222 0 238 MSE 7 948 p 0 790 and for slow sustained poten tials following sound onset F222 1 308 MSE 1 537 p 0 290 Fig 6 However the N1 P2 responses to the gap did differ significantly across these three conditions F222 9 129 MSE 3 58
20. he right loudspeaker was presented If the listener re sponded that she he heard the gap on three consecutive trials the duration of the gap on the next trial was re duced If however the listener indicated on a trial that they could not hear a gap the duration of the gap on the next trial was increased a 3 down gap duration re duced l up gap duration increased procedure Levitt 1971 In Condition RO the right loudspeaker was turned on and the left loudspeaker was turned off In Condition L U a right side noise sound with a gap lagged 2 ms behind an uncorrelated left side noise sound without a gap In Condition L C a right side noise sound with a gap lagged 2 ms behind a correlated left side noise sound without a gap In Condition R C a right side noise sound with a gap led by 2 ms a correlated left side noise sound without a gap There were four repeti tions in each of the conditions The maximum gap at the beginning of a session was 50 ms 3 2 Results As indicated in Fig 3 the gap detection thresholds among Conditions L U L C and R C were similar and the lowest gap detection threshold was obtained when only the right loudspeaker was operative Condi tion RO A one way analysis of variance with repeated measures revealed that the differences in gap detection thresholds between these four conditions were signifi cant F342 5 146 MSE 6 030 p 0 004 Pairwise analyses indicated that Condition RO was
21. i et al Hearing Research 202 2005 235 247 243 perceptual system briefly treats the return of the lagging correlated stimulus as a new stimulus until it re estab lishes the correlation between the leading and lagging stimulus and suppresses the perception of the lagging source It is interesting to note however that this noise burst rather than being attributed to the lagging stimu lus is perceived as originating from the direction of the leading stimulus In other words it appears to be cap tured by the leading stimulus In Condition R C all the listeners perceived the gap as belonging to the right leading loudspeaker in the near threshold condition At the larger gap durations 20 and 50 ms the listeners predominately perceived the gap when it was heard as a gap as belonging to the right leading loudspeaker but they also reported hearing a noise burst image as coming from the location of the left lagging loudspeaker When there is a gap in the leading stimulus there is no leading sound present to suppress the information as to the location of the lag ging stimulus Hence one might expect to hear a brief noise burst during the gap from the location of the lag ging stimulus until the perception of the lagging stimulus is suppressed This is what appears to have happened here 5 Experiment 4 To examine how the precedence effect modulates cor tical responses to the probe gap in Experiment 4 N1 P2 and long laten
22. ibutes of the gap were sup pressed when the lagging sound was clearly captured in the second experiment gap detection thresholds the shortest duration at which a gap was perceived both when sounds were fused echo capture and when they were not were determined To see whether a listener s sensitivity to a gap de pended on whether or not fusion occurred in Experi ment 2 gap detection thresholds when fusion clearly happen correlated noises 2 ms delay were compared to a condition when it did not uncorrelated noises 2 ms delay If the gap appeared only on the lagging side and was suppressed when fusion occurred then the gap detection threshold should be higher than when there was no fusion 3 1 Materials and methods 3 1 1 Participants The fifteen people who participated in Experiment 1 also participated in this experiment 3 1 2 Apparatus and materials The apparatus and materials were same as in Experiment 1 3 1 3 Procedure Unlike Experiment 1 where there was a gap in the noises produced by both the left and right loudspeakers in Experiment 2 the gap appeared only in the noise that was delivered from the right loudspeaker The minimum size of the gap in the right loudspeaker noise that could be detected using a single interval staircase procedure was then determined for both correlated and indepen dent leading and lagging noises Specifically on each trial a stimulus with a gap in the sound emanating from t
23. ical involve ment may not be necessary to distinguish between correlated and uncorrelated noises it may be required to maintain and or re establish the percep tion of these two kinds of noise especially with respect to percepts related to precedence once there is a break in either the leading or lagging noise The use of gaps as probes may be a way of accessing the cortical mechanisms involved in the maintenance of percepts when there are sudden or unexpected changes in the sensory input Thus in order to more completely understand the neural mecha nisms involved in the precedence effect cortical neural correlates should be investigated in addition to the brainstem mechanisms Acknowledgements We thank Jane W Carey and Neda Chelehmalzadeh for their assistance during data acquisition We also thank the following people who have reviewed previous versions of the manuscript and provided helpful com ments and suggestions to improve its quality Ann Clock L Li et al Hearing Research 202 2005 235 247 247 Eddins William M Hartmann Jack B Kelly and three anonymous reviewers This work was supported by the Natural Sciences and Engineering Research Council of Canada the Canadian Institutes of Health Research the Canada Foundation for Innovation and the Ontario Innovation Trust Fund References Blauert J 1997 Spatial Hearing MIT Press Cambridge MA Clifton R K 1987 Breakdown of echo suppression in the preceden
24. ing from the right loudspeaker was perceived As shown in Fig 2 the aver age echo threshold was approximately 9 5 ms When a gap was introduced into both the leading and lagging sounds in Condition Correlated 50 ms gap the average gap capture threshold was 15 6 ms Fig 2 The gap capture threshold in Condition Correlated 50 ms gap was significantly longer than the echo threshold in the same condition Fi 14 5 769 MSE 47 617 p 0 031 At delays substantially longer than the gap capture threshold listeners perceived a gap in the sound image associated with the right loudspeaker At delays between the echo threshold and the gap capture thresh old listeners perceived sounds from both the left lead ing and right lagging loudspeakers but did not hear a gap in the lagging sound Rather the gap was heard only in the leading sound Finally at delays shorter than the echo threshold listeners only heard a sound on the left with a gap in it Hence for intermediate delays between 10 and 15 ms in Condition Correlated 50 ms gap lis teners heard two spatially separated continuous sound images a direct wave and its echo with a gap in the leading image but not in the lagging image even though both leading and lagging sounds contained a 50 ms gap In Condition Uncorrelated 50 ms gap listeners al ways perceived two spatially distinct sounds one on the left and the other on the right regardless of the 18 16 14 12 10 EC
25. ior colliculus are essential for the prece dence effect Cornelisse and Kelly 1987 reported that patients with lesions of the right temporo parietal cortex were able to localize single clicks but could not localize the fused image of two spatially separated clicks when the leading click was delivered from the left hemi field and the lagging click was delivered from the right hemifield Litovsky et al 2002 reported that a patient with lesions of the right inferior colliculus had substan tially weaker echo suppression when the leading sound was delivered in the left hemifield Hence it would be interesting to investigate attribute capture in patients with unilateral lesions of the central auditory system In summary based on the data of the present study three important features of attribute capture should be noted 1 Top down higher order processes are involved in attribute capture A probe gap introduced in the leading stimulus can temporarily break the prece dence effect whereas introducing a comparable gap in the lagging stimulus does not break the pre cedence effect in the majority of our listeners even though both situations are ecologically anomalous In addition gap capture is associated with long latency negatively shifted slow potentials in the frontal area 2 Attribute capture is not an all or none process For lead lag delays between 9 and 15 ms the location information concerning the lagging sound is not
26. lead ing loudspeaker and simultaneously a burst like image as coming from the lagging loudspeaker indicates a tran sient disappearance of the precedence effect during the gap Our electrophysiological results suggest a tight link between subjective perception of the gap and neural re sponses to the gap Surprisingly there is no difference in ERP responses between the correlated and uncorrelated sound conditions until a gap occurs even though the perceptual responses to the correlated and uncorrelated noise sounds are quite different When the two long duration sounds are correlated the N1 P2 peak to peak response to the gap is enhanced and the N1 topo graphic voltage map for the gap shifts laterally towards the right hemisphere regardless of the gap being in the lagging or leading sound Also in the frontocentral re gion a negatively shifted sustained ERP response fol lowing the gap embedded only in the lagging sound appears to be associated with the perceived capture of the gap The present neurophysiological results suggest that there is a greater need for cortical involvement to maintain fusion of leading and lagging sounds when there is a break in one or the other than to establish fu sion at sound onset This long latency neural event fol lowing the occurrence of the gap also suggests that higher order central processes are involved in attribute capture Clinical studies in humans suggest that both the cor tex and the infer
27. n the leading stimulus and heard the lagging stimulus as a continuous noise no gap Hence in the delay region between 10 and 15 ms listeners hear two spatially separated noises with the gap belonging to the leading stimulus These different capture thresholds echo versus gap imply that different processes are involved in capture for different attributes When the two long duration noise sounds are uncor related neither the lagging noise sound nor the gap in the lagging sound is captured Our results thus lay emphasis on the importance of inter sound correlation in producing perceptual fusion for long duration sounds This notion is partially in agreement with a pre vious study by Perrott et al 1987 who used 50 ms broadband free field noise bursts 0 2 ms rise fall left right 20 separation as stimuli and investigated listen ers experience of correlated or uncorrelated noise bursts at various inter stimulus onset delays Perrott et al re ported that fusion was stronger when the two short noise bursts were correlated than when the two bursts were uncorrelated However when the two noise bursts L Li et al Hearing Research 202 2005 235 247 245 were uncorrelated and the delayed was below 8 ms there were also a small proportion of trials on which fusion of the two bursts was perceived In our experiments the two uncorrelated sounds did not fuse On a few occa sions however a gap that appeared only in the right lag
28. nd investigations of the precedence effect which utilize successive transient stimuli J Acoust Soc Am 110 1505 1513 Levitt H 1971 Transformed up down methods in psychoacoustics J Acoust Soc Am 49 467 477 Li L Yue Q 2002 Auditory gating processes and binaural inhibition in the inferior colliculus Hear Res 168 113 124 Liebenthal E Pratt H 1999 Human auditory cortex electrophys iological correlates of the precedence effect binaural echo lateral ization suppression J Acoust Soc Am 106 291 303 Litovsky R Y 1998 Physiological studies on the precedence effect in the inferior colliculus of the kitten J Acoust Soc Am 103 3139 3152 Litovsky R Y Delgutte B 2002 Neural correlated of the precedence effect in the inferior colliculus effect of localization cues J Neurophysiol 87 976 994 Litovsky R Y Shinn Cunningham B G 2001 Investigation of the relationship among three common measures of precedence fusion localization and discrimination suppression J Acoust Soc Am 109 346 357 Litovsky R Y Yin T C T 1998a Physiological studies of the precedence effect in the inferior colliculus of the cat I Correlates of psychophysics J Neurophysiol 80 1285 1301 Litovsky R Y Yin T C T 1998b Physiological studies of the precedence effect in the inferior colliculus of the cat II Neural mechanisms J Neurophysiol 80 1302 1316 Litovsky R Y Rakerd B Yin
29. nstructions appropriately 4 Thus this participant s data were not used The results from the other 10 listeners appear in Fig 5 As shown in Fig 5 in Condition L U all the listeners predominately perceived the gap as coming from the right lagging loudspeaker However there were 1 4 and 3 listeners who reported that they perceived an addi tional gap image as coming from the left leading loud speaker in the near threshold 20 ms and 50 ms conditions respectively There were also 2 listeners reporting that they perceived a noise burst image as coming from the left loudspeaker in the 50 ms condi tion Hence even though the left and right noises were never fused occasionally perceptual events that were ini tiated by a gap in the right lagging sound were attrib uted to the leading sound However for all gap 3 Instructions to Listeners for Experiment 3 After you press the middle button you will hear noise presented over the loudspeakers Listen to the noise carefully because after 5 presentations of the noise you will be asked to answer the following three questions Question 1 Did you perceive in the noise 1 a gap of silence 2 a sudden burst of noise 3 both a gap and a noise burst 4 two gaps 5 two noise bursts 6 no change Question 2 For the perceived gap s in the noise please report where the gap s came from 1 the left hand loud speaker 2 the right hand loudspeaker 3 the two loudspeakers Q
30. o report their impres sions associated with gaps in Conditions L U L C and R C see Experiment 2 for the definitions of the three conditions 4 1 Materials and methods 4 1 1 Participants Eleven listeners four females and seven males with normal and balanced pure tone hearing participated in this experiment Four young male listeners also partici pated in Experiments 1 and 2 The other 7 listeners in cluded 4 young female listeners 19 31 years old and 3 male listeners 34 34 and 39 years old respectively The gap detection threshold for each of these 7 listeners who did not participated in Experiments 1 and 2 was also measured under Condition L U 4 1 2 Apparatus and materials The apparatus and materials were the same as in Experiments 1 and 2 4 1 3 Procedure Stimuli were presented in each of the three conditions L U L C and R C at the following three different gap sizes 1 2 ms above each individual s gap detection threshold as determined in Experiment 2 2 20 ms and 3 50 ms Thus there were 9 3 x3 condition gap size combinations These combinations were pre sented in a random order for each listener The lead lag time was fixed at 2 ms which was well below the echo threshold for each of the listeners After 5 stimulus presentations in each of the 9 condi tion gap size combinations the listeners were asked to report their impressions about the gap that occurred in the middle of the noise by
31. only in the correlated lagging sound at short delays it also was perceived as occurring on the leading side Moreover gap detection thresholds were the same for gaps in the leading and lagging sounds suggesting that the perception of the gap was not suppressed but rather incorporated into the leading sound Finally scalp event related potentials were not associated with the precedence effect until the gap occurred This suggests that cortical mechanisms are engaged to maintain fusion when attributes in direct or reflected waves change 2004 Elsevier B V All rights reserved Keywords Precedence effect Fusion Reverberant environment Correlation Gap Event related potential 1 Introduction In a reverberant environment each sound source pro duces both a direct wavefront and numerous filtered and time delayed reflections from the walls ceilings and other surfaces When the delay between the direct wave and a reflected wave is sufficiently long and the reflected Abbreviations B amp K Briel amp kj r ERP event related potential HATS head and torso simulator IAC Industrial Acoustic Company RO right loudspeaker was turned on only L U left leading uncor related L C left leading correlated R C right leading correlated TDT Tucker Davis technologies Corresponding author Tel 905 569 4628 fax 905 569 4326 1 416 978 4811 E mail address liang psych utoronto ca L Li 0378 5955 see front matter 20
32. p was in either the leading or lagging sound and also inde pendent of whether or not the leading and lagging sounds were correlated These results are consistent with the hypothesis that gap detection depends primarily on the detection of an energy change in the ear on the side of the loudspeaker producing the gap On the other hand when the two sounds were correlated a single compact sound image was perceived as coming from the leading side when the two sounds were not corre lated more diffused sound images were perceived as coming from the both sides Since there was no differ ence in gap detection between Conditions L U L C and R C there is no evidence in this experiment that sound image compactness diffuseness affects gap detection If information in these reflections is not being sup pressed then it has to be somehow perceptually incorpo rated into the fused image The present study shows that when the two sounds are uncorrelated the lagging sound is by and large not treated as the reflection of the leading sound by the auditory system and two dis tinct noise images coming from different directions are perceived and the gap presented in the lagging sound 3 is correctly perceived as coming from the lagging loudspeaker The only exception to this statement is that sometimes especially at the longer gap durations the gap is also attributed to captured by the leading stim ulus In contrast when the two sounds
33. rated sound sources and the shortest time delay between a direct and a re flected wave that produces a separate echo on certain percentage of experimental trials usually between 50 and 80 is called the echo threshold Blauert 1997 pp 224 225 Since a simulated reflection in an experimental envi ronment is not heard as a separate auditory event when the lead lag delay is below the echo threshold it has been assumed that some inhibition or attenuation of information in reflected sounds such as contralateral inhibition Blauert 1997 pp 230 233 must take place in the precedence effect For instance a prevalent expla nation is that the directional information associated with the reflected wave is suppressed Blauert 1997 Lie benthal and Pratt 1999 Litovsky and Shinn Cunning ham 2001 Rakerd et al 2000 Yin 1994 Zurek 1980 This suppression hypothesis has dominated the search for neural correlates of the precedence effect In most of the related physiological studies using either anesthetized or unanesthetized animals suppressed neu ral responses to the lagging sound in the presence of the leading sound were treated as the neural correlates of the precedence effect Fitzpatrick et al 1995 1999 Lie benthal and Pratt 1999 Litovsky 1998 Litovsky and Delgutte 2002 Litovsky and Yin 1998a b Litovsky et al 1997 Yin 1994 However suppression of the directional information in the reflection does not mean tha
34. rceive a new sound originating from the location of the lagging loudspeaker until the precedence of the leading sound is re established However most of our listeners did not hear any sound change as coming from the location of the lagging loudspeaker Rather they heard a gap or a burst like image as coming from the leading loudspeaker Since there is no physical gap in the sound from the leading loudspeaker the gap in the sound from the lagging loudspeaker has no leading partner to fuse with Moreover hearing a gap or a burst like image as coming from the leading loud speaker cannot be caused by a peripheral effect since there are no obvious differences in the sound spectra at the left ear the ear on the side of the leading loud speaker between the condition when there is no gap in the lagging right side stimulus versus when there is a gap in the lagging stimulus see Fig 4 Thus the shift of gap image from the lagging loudspeaker to the lead ing loudspeaker denotes the maintenance of the prece dence effect during the period of the gap and must involve a higher order attribute capturing process On the other hand when the gap is only in the lead ing sound that is correlated with the lagging sound the acoustic situation is also ecologically anomalous be cause a gap in a natural sound source will also appear 246 L Li et al Hearing Research 202 2005 235 247 in its reflections Hearing a gap as coming from the
35. rior colliculus of the cat J Neurosci 14 5170 5186 Zurek P M 1980 The precedence effect and its possible role in the avoidance of interaural ambiguities J Acoust Soc Am 67 952 964 Zurek P M 1987 The precedence effect In Yost W A Gourevitch G Eds Directional Hearing Springer Verlag New York pp 85 105
36. s important to study how the precedence effect works for long duration stimuli and determine how attributes that belong to reflections and indeed may be unique to them are incorporated into the fused image of the source In the present study a transient gap as a probe attri bute was inserted into an otherwise continuous steady state broadband noise Because this gap could be in the source the leading sound only the reflection the lag ging sound only or both source and reflection it should be easier to determine how this attribute of the direct wave and or the reflection is detected and incor porated in the overall percept of the sound Introducing a single gap into either the leading or the lagging sound but not both is also interesting from the point of view of top down control over the precedence effect For example a gap only in the lagging but not in leading stimulus is inconsistent with the lagging stim ulus being an echo a gap in a natural reflection should have its origin in the sound source and could lead to a breakdown in the precedence effect Moreover if the gap is in the lagging stimulus only and the leading and lag ging stimuli remained fused into a single percept will the listener perceive a break in the fused stimulus or will the gap in the lagging stimulus be suppressed so that the lis tener hears a continuous fused stimulus To investigate issues such as these listeners were asked to describe their
37. s perceived in the lagging sound In Condition L C the listeners predominately per ceived a change in the sound coming from the left lead ing loudspeaker even though the gap appeared only in the right lagging loudspeaker When the gap size was near threshold all the listeners reported that they per ceived only a single gap image in the sound from the left loudspeaker When the gap size was 20 or 50 ms most listeners perceived either a gap or a noise burst image as coming from the left loudspeaker Only a small num ber of listeners reported that they perceived a gap or a burst image as coming from the right loudspeaker Hence when the gap is in the lagging sound and the sounds are correlated listeners tend to incorporate any perceptual change occasioned by the gap into the fused image which is perceived to be located on the leading side In other words perceptual changes evoked by a gap in the lagging sound are captured by the leading sound It is interesting to note that at the longer gap durations listeners sometimes heard a noise burst which they attributed with one exception to the leading stimulus One possible explanation for this perception is that if the gap in the lagging stimulus is long enough there is no location information coming from the lag ging stimulus to suppress and the circuitry responsible for the suppression of location information is disen gaged Consequently when the gap is terminated the L L
38. selecting an answer from the following 6 options 1 a single gap 2 a sudden burst of noise 3 both a single gap and a noise burst 4 two gaps 5 two noise bursts or 6 no change They were then asked to report which loudspeaker s delivered the perceived gap s and or which loud speaker s delivered the perceived noise burst s for the instructions to listeners see Footnote Thus Op tions 1 and 2 were associated with perception of only one brief auditory event in the middle of the noise sound and Options 3 4 and 5 were associated with per ception of 2 brief auditory events Option 6 indicated that the participant did not perceive any event in the middle of the noise Noise burst options were incorporated into the re sponse list because there were reasons to expect that lis teners would hear a noise burst if there was any tendency for echo capture to break down during a gap For example if a gap were introduced into the lead ing stimulus only there would be no leading stimulus during the gap to suppress the information as to the location of the lagging stimulus Hence one might ex pect to hear a brief noise burst from the location of the lagging stimulus 4 2 Results All the 11 listeners reported that they perceived one or two sudden changes in the middle of the sound in all combined conditions No participant used the no change response However one male participant ap peared not to follow the i
39. significantly different from each of the other three conditions p lt 0 005 but there were no significant differences among Conditions L U L C and R C p gt 0 800 Hence We opted to use a single interval staircase procedure rather than the more standard two interval forced choice procedure for two reasons First the use of a two interval technique would have more than doubled trial length from its current 3 05 s to more than 7 s once an inter stimulus interval was added and we were concerned about tiring our volunteers Second we wanted to keep the testing situation as comparable as possible to that used in Experiment 1 where we also used a single interval staircase procedure since we were using naive listeners Although thresholds determined using single interval stair case procedures are subject to response biases such biases are not a significant problem for comparisons of thresholds as long as these biases remain constant across comparisons Because there is no reason to expect that a change from left leading to right leading or from correlated to independent noises or from the left loudspeaker on to the left loudspeaker off would affect the bias to report a gap gap detection threshold differences among these conditions should accu rately reflect relative but perhaps not absolute sensitivity to the presence of a gap Because the two uncorrelated sounds did not fuse it should not matter whether right or l
40. suppressed by the leading sound a sound is still heard as coming from the direction of the lagging sound but a gap in the lagging sound is neverthe less captured by the leading sound a gap is heard in the leading sound but not in the lagging sound This indicates that capture thresholds can differ for different attributes of the reflection e g gaps in the lagging sound are more easily captured than other aspects of the sound One may speculate that the degree to which the listener assigns spatially sepa rate and distinct images to the leading and lagging sounds will depend on the extent to which different attributes of the lagging sound are incorporated into captured by the leading sound According to this speculation all of the attributes of the reflec tion would have to be captured in order for the lis tener to perceive only a single source 3 The introduction of a distinct feature such as a gap into a direct or reflected wave may be one way of probing cortical involvement in the precedence effect In our study identical ERP responses were elicited by both correlated and uncorrelated noises even though listeners perceive correlated noises to be quite distinct from uncorrelated noises One may speculate that the differences between the two are processed primarily by brain stem mechanisms However the ERP to a gap differed substantially depending upon whether or not the noises were cor related This suggests that while cort
41. t the reflected wave is not heard because listeners are aware of the presence of reflections and even changes in them For example Freyman et al 1998 have shown that listeners are as sensitive to intensity decreases in the lagging sound as to intensity increases in the leading sound indicating that intensity information in the reflection is not sup pressed Also hearing a reflection while presumably suppressing its directional information raises some puz zles as to how the perceptual system incorporates re flected waves into the percept of a single auditory event For example it is not clear how the intensities of a source and its reflections blend to determine the loudness of the fused sound image Finally Hartung and Trahiotis 2001 have developed a model for describing how monaural peripheral processing without an inhibitory mechanism may contribute to data ob tained in binaural precedence experiments that use binaural pairs of transients as stimuli Hence it is evi dent that there is more to the precedence effect than sim ple inhibition Most studies on the precedence effect have used ideal ized brief acoustic stimuli such as clicks or transient noise bursts to avoid or reduce temporal overlap be tween the leading and lagging sounds for a review see Litovsky et al 1999 However acoustic stimuli under normal circumstances are usually complex and last for more than a few hundred milliseconds Therefore it i
42. ter of the listeners head was 1 03 m The loudspeaker height was approximately ear level for a seated listener with the average body height Fresh noise sounds were gener ated for each trail The gap a rectangular silent break in the otherwise continuous noise occurred 1500 ms after sound onset When the noises delivered to the two loudspeakers were identical except for a delay between them they were referred to as correlated When the two noises were independent they were referred to as uncor related All the single source levels were fixed at 60 dB SPL Calibration of sound level was carried out with a Br el amp Kj r B amp K sound meter Type 2209 whose microphone was placed at the location of the listeners head center when the listener was absent A weighting and a slow norm meter response were used 2 1 3 Procedure There were three stimulus conditions in this experi ment In Condition Correlated No gap the two noises from the two loudspeakers were correlated and no gaps 45 45 N ri Fig 1 Diagram showing the two loudspeaker configuration used in the present study The two loudspeakers were spatially separated in the frontal azimuthal plane at the left and the right 45 positions symmetrical with respect to the median plane 238 L Li et al Hearing Research 202 2005 235 247 were introduced The left loudspeaker led the right loud speaker and the time lag betw
43. the basis of spectral cues it would Right Ear Canal RELATIVE POWER IN dB 0 1 2 3 4 5 6 7 8 9 10 FREQUENCY IN kHz Fig 4 Long term spectra for the stimuli in this experiment when the listener was replaced by simulated head and torso Bruel amp Kjaer The lines connecting the open squares represent the spectra of sounds in the two ear canals of the simulated head when the noise stimulus was being played over the left loudspeaker only The line connecting the filled circles represents the spectra of the sounds in the two ear canals when the same sound was presented over both left and right loudspeakers with the sound on the left leading that on the right by 2 ms The left panel presents these spectra for the left ear canal the right panel for the right ear canal L Li et al Hearing Research 202 2005 235 247 241 be expected that the listener might hear the gap as occur ring on the right However for gap durations just above threshold listeners reported after the session that they perceived a gap in the sound source located to the left In other words the leading sound appeared to fully capture an attribute in the lagging sound This capture effect was explored more systematically in the next experiment 4 Experiment 3 Experiment 3 investigated how the precedence effect modified listeners perceptions of a gap that appeared either in the lagging or leading sounds but not both Specifically listeners were asked t
44. uestion 3 For the perceived noise burst s please report where the noise burst s came from 1 the left hand loudspeaker 2 the right hand loudspeaker 3 the two loudspeakers For all condition gap size combinations this listener selected the same option number which indicated that his unvarying response was noise bursts two loudspeakers 242 L Li et al Hearing Research 202 2005 235 247 Left Loudspeaker L G NUMBER OF LISTENERS REPORTING SCKFNnNwWRUDIAMWOS Threshold 2 20 50 L C NUMBER OF LISTENERS REPORTING SCEFNnNwWRUDIAMWOS Threshold 2 20 50 g A O NwWHUANDAxA COO CO NUMBER OF LISTENERS REPORTING Threshold 2 20 50 GAP SIZE ms Right Loudspeaker 10 9 8 7 6 5 4 3 2 1 0 Threshold 2 20 50 10 9 8 7 6 5 4 3 2 1 0 Threshold 2 20 50 10 9 8 7 6 5 4 3 2 1 0 Threshold 2 20 50 GAP SIZE ms Fig 5 Summary of listeners perceptions of the gap in Conditions L U left leading uncorrelated L C left leading correlated and R C right leading correlated The gap was only in the sound from the right loudspeaker The ordinates represent the numbers of listeners who attributed a gap or a noise burst to a particular left or right loudspeaker at each of the three different gap sizes Lighter bars indicate Gap responses and darker bars indicate Noise burst responses durations a gap was alway
Download Pdf Manuals
Related Search