Illusory sound perception in macaque monkeys

Abstract

In most natural listening environments, noise occludes objects of interest, and it would be beneficial for an organism to correctly identify those objects. When a sound of interest ("foreground" sound) is interrupted by a loud noise, subjects perceive the entire sound, even if the noise was intense enough to completely mask a part of it. This phenomenon can be exploited to create an illusion: when a silent gap is introduced into the foreground and high-intensity noise is superimposed into the gap, subjects report the foreground as continuing through the noise although that portion of the foreground was deleted. This phenomenon, referred to as auditory induction or amodal completion, is conceptually similar to visual induction, fill-in, illusory motion, and illusory contours. Two rhesus macaque monkeys performed a task designed to assess auditory induction. They were trained to discriminate complete stimuli from those containing a silent gap in the presence of two types of noise. Interrupting noise temporally coincided only with the gap, and in humans this causes induction. Surrounding noise temporally encompassed the entire foreground, and in humans this causes masking without auditory induction. Consistent with previous human psychophysical results, macaques showed better performance with surrounding masking noise than interrupting noise designed to elicit induction. These and other control experiments provide evidence that primates may share a general mechanism to perceptually complete missing sounds.

Figure 1.

Schematized spectrograms of induced and masked foregrounds. A single frequency tone is depicted as a horizontal bar with a narrow frequency range (A), but noise has a broader frequency spectrum (see Noise Only in D). Higher intensity components are represented by darker shading. A, A complete tone segment is reported as continuous by human listeners. B, A tone with a silent portion (gap) is reported as sounding discontinuous by listeners who readily detect the gap; however, when a higher intensity interrupting noise fills this gap (D), most listeners report that the tone was continuous throughout as if the tone were present in the noise (auditory induction). In this case they report that the stimulus in D sounds like the stimulus shown in C. E, When intense noise is changed to completely “surround” the foreground, the foreground is masked and continuity cannot be determined. Note, when lower intensity “surrounding” noise is used, no masking occurs, and the entire foreground is heard correctly. F, When a foreground with a gap is used with intense surrounding noise, the entire foreground is masked and continuity cannot be determined; however, with lower intensity noise, no masking occurs, and the discontinuity in the foreground can be readily identified. Note that there are intermediate intensities at which induction can occur without masking such that D would be reported as continuous and F would be reported as discontinuous.

Figure 2.

Behavioral task and spectrograms of some of the stimuli used. A, Schematized task. After a lever press two sound stimuli were played (hexagons). A lever release was to be made within 800 msec after a target (different from first sound) but not a standard (same as first sound) stimulus for reward. B, Spectrograms of 2 kHz tonal foregrounds with interrupting broadband noise (BBN). Macaques were trained to let go of the lever only if the second stimulus was a target (all targets were identical to standards except that they contained a silent gap in the foreground). C shows a spectrogram (left) and an amplitude plot (right) of the coo vocalization with interrupting noise.

Figure 3.

Psychometric functions and threshold determination. Plots show representative noise-intensity experiment psychometric functions from subject X for interrupting (circles) and surrounding (triangles) noise. Spectrograms of target stimuli are schematized in the legend. A shows the probability of a hit (solid lines and open symbols) or false alarm (FA) (dashed lines and black-filled symbols) for interrupting and surrounding noise. B shows the transformation to a criterion-free measure LOR. Curved lines represent sigmoidal functions (Eq. 2) fit to the points. The dashed line corresponds to the threshold level of LOR = 1. Error bars (SEM) designate the between-session variance in performance.

Figure 4.

Box plots of the thresholds of two macaques on the intensity experiment. Noise intensity thresholds for interrupting and surrounding noise are shown using 2 kHz tonal (A, B) and coo vocalization (C, D) foregrounds. Spectrograms on bottom schematize the target stimuli. Shaded circles represent macaque X and open circles represent Z; horizontal lines bisecting the rectangles represent the mean.

Figure 5.

Intensity experiment thresholds (over session) using notched-noise with 2 kHz tonal foregrounds (subject X only). White bars depict interrupting noise results; black bars depict surrounding noise results. Schematized spectrograms embedded in the bars show the type of noise used—broadband noise (BBN), BBN with a Notch at 2 or 8 kHz—relative to target tones. Error bars designate SE of the mean between sessions.

Figure 6.

Gap duration experiment thresholds. Thresholds for interrupting (A), surrounding (B), and no noise (C) with 2 kHz foregrounds. D, Gap in broadband noise thresholds. Schematized spectrograms of the stimuli are shown on bottom. Format is the same as Figure 4.

Figure 7.

Gap duration experiment thresholds (over session) with notched-noise (subject X only). Format is as in Figure 5.