Adaptation reveals multiple levels of representation in auditory stream segregation

Abstract

When presented with alternating low and high tones, listeners are more likely to perceive 2 separate streams of tones ("streaming") than a single coherent stream when the frequency separation (Deltaf) between tones is greater and the number of tone presentations is greater ("buildup"). However, the same large-Deltaf sequence reduces streaming for subsequent patterns presented after a gap of up to several seconds. Buildup occurs at a level of neural representation with sharp frequency tuning. The authors used adaptation to demonstrate that the contextual effect of prior Deltaf arose from a representation with broad frequency tuning, unlike buildup. Separate adaptation did not occur in a representation of Deltaf independent of frequency range, suggesting that any frequency-shift detectors undergoing adaptation are also frequency specific. A separate effect of prior perception was observed, dissociating stimulus-related (i.e., Deltaf) and perception-related (i.e., 1 stream vs. 2 streams) adaptation. Viewing a visual analogue to auditory streaming had no effect on subsequent perception of streaming, suggesting adaptation in auditory-specific brain circuits. These results, along with previous findings on buildup, suggest that processing in at least 3 levels of auditory neural representation underlies segregation and formation of auditory streams.

Figure 1. Effect of stimulus context in Experiment 1A

Proportion of time the auditory tone sequences were heard as two streams (streaming) for the context period (left portion of panels) and test period (right portion of panels). The test sequence always had a Δf of 6 semitones. The top panels include trials in which the test sequence was 300-424-300-, the middle panels include trials in which the test sequence was 500-707-500-, and the bottom panels show the mean across these two test sequences. The context period contained silence (thick solid line), the same sequence as the test sequence (thin solid line), a sequence with a 3-semitone Δf (long dashed line), or a sequence with a 12-semitone Δf (short dashed line). (A) Trials in which the context sequences with 3 and 12 semitone Δf had the same A tone frequency as the test sequence. (B) Trials in which the context sequences with 3 and 12 semitone Δf had a different A tone frequency than the test sequence. Note that the traces for the silence and same conditions are exactly the same for the left and right panels.

Figure 2. Effect of perceptual context in Experiment 1A

Proportion of time trials heard as streaming for the context period (left portion of panels) and test period (right portion of panels) showing the effect of different perceptual contexts (“previous percept”) on perception of streaming during the test period. Data, shown only for trials in which the context and test sequences were the same, are collapsed across the two test sequences. The time course of perceptual dominance is shown both for trials in which the dominant percept at the end of the context period was one stream (solid line) and two streams (dashed line).

Figure 3. Effect of stimulus context in Experiment 1B

Similar to Figure 1 except the top panels include trials in which the test sequence was 300-424-300-, the middle panels include trials in which the test sequence was 1300-1838-1300-, and the bottom panels show the mean across these two test sequences.

Figure 4. Effect of perceptual context in Experiment 1B

See Figure 2 caption.

Figure 5. Effect of stimulus context in Experiment 2

The test sequence was 300-424-300- and the context period consisted of silence (solid line), a sequence of ABA-repetitions in which each A tone was chosen randomly from the range 600–2600 Hz and the B tone was 3 (long dashed line) or 12 (short dashed line) semitones higher.

Figure 6. Plaid stimuli presented during the context period in Experiment 3

The angle (α) between the direction of motion when perceiving one coherent plaid stimulus (vertical) and the direction of motion of the two grating components could have values of 50°, 105°, or 150°. Larger α leads to more perception of two visual gratings, analogously to the effect of increasing Δf on perception of two auditory streams.

Figure 7

A) Effect of visual stimulus context on auditory perception in Experiment 3. The test sequence was 300-424-300- and the context period consisted of a blank screen (thick solid line), a moving plaid stimulus with α=50° (thin solid line), α=105° (long dashed line), or α=150° (short dashed line). B) Effect of visual perceptual context on auditory perception in Experiment 3. Data are shown only for trials in which the context was ambiguous (α=105°). The time course of perceptual dominance is shown both for trials in which the dominant percept at the end of the context period was one object (solid line) and two objects (dashed line).