Manipulations of listeners' echo perception are reflected in event-related potentials

Abstract

To gain information from complex auditory scenes, it is necessary to determine which of the many loudness, pitch, and timbre changes originate from a single source. Grouping sound into sources based on spatial information is complicated by reverberant energy bouncing off multiple surfaces and reaching the ears from directions other than the source's location. The ability to localize sounds despite these echoes has been explored with the precedence effect: Identical sounds presented from two locations with a short stimulus onset asynchrony (e.g., 1-5 ms) are perceived as a single source with a location dominated by the lead sound. Importantly, echo thresholds, the shortest onset asynchrony at which a listener reports hearing the lag sound as a separate source about half of the time, can be manipulated by presenting sound pairs in contexts. Event-related brain potentials elicited by physically identical sounds in contexts that resulted in listeners reporting either one or two sources were compared. Sound pairs perceived as two sources elicited a larger anterior negativity 100-250 ms after onset, previously termed the object-related negativity, and a larger posterior positivity 250-500 ms. These results indicate that the models of room acoustics listeners form based on recent experience with the spatiotemporal properties of sound modulate perceptual as well as later higher-level processing.

Figure 1

Depiction of the three contexts in which test click pairs were presented. All trials began with the onset of a fixation point. Test pairs were a click from the right speaker followed 1–14 ms later by an identical click from the left speaker. All trials included 1500 ms of silence before the test pair and 600 ms after the test pair before participants were prompted to indicate whether they had heard one sound or two. Click pairs in the buildup and depressed buildup trains were delivered at a rate of 4∕s.

Figure 2

Percent of trials (out of 64) on which each participant reported hearing the lag sound as a separate source for test pairs at a single SOA selected for that individual. Participants were more likely to report hearing two auditory objects (reflecting lower echo thresholds) in the single pair and depressed buildup conditions. Group averages and standard error bars are shown in the lower panel.

Figure 3

Event-related potentials time-locked to the onset of the lead sound for a single stimulus onset asynchrony for each participant when they reported hearing the lag sound on depressed buildup trials (gray) and when they reported not hearing the lag sound on buildup trials (black). Waveforms are shown for the 12 recording sites depicted on the electrode map. Topographic plots show the difference in amplitude (reported lag sound minus did not report lag sound) across all electrodes. The difference in amplitude between 100 and 250 ms over anterior regions is the ORN and between 250 and 500 ms over posterior regions is the late posterior positivity.

Figure 4

Difference waves for two pairs of conditions. Perception of the lag sound as a separate source is indexed by the difference between trials on which listeners reported hearing the lag sound following the depressed buildup context and not hearing the lag sound following the buildup context (two sound sources − one sound source, black line). Refractory effects are indexed by the difference between trials on which listeners reported hearing two sound sources on the single-pair and depressed buildup trials (single pair minus preceding context, gray line). Waveforms are shown for the 12 recording sites depicted on the electrode map.