Activity related to perceptual judgment and action in primary auditory cortex.

Recent evidence is reshaping the view of primary auditory cortex (A1) from a unisensory area to one more involved in dynamically integrating multisensory- and task-related information. We found A1 single- (SU) and multiple-unit (MU) activity correlated with macaques' choices in an amplitude modulation (AM) discrimination task. Animals were trained to discriminate AM noise from unmodulated noise by releasing a lever for AM noise and holding down the lever for unmodulated noise. Activity for identical stimuli was compared between trials where the animals reported AM and trials where they did not. We found 47.4% of MUs and 22.8% of SUs significantly increased firing shortly before the animal's behavioral response to report AM when compared to the equivalent time period on trials where AM was not reported. Activity was also linked to lever release in a different task context, suggesting A1 modulation by somatosensory, or efference copy, input. When spikes were counted only during the stimulus, 19.6% of MUs and 13.8% of SUs increased firing rate when animals reported AM compared to when they did not, suggesting an attentional effect, or that A1 activity can be used by higher decision areas, or that such areas provide feedback to A1. Activity associated with AM reporting was correlated with a unit's AM sensitivity, suggesting AM sensitive neurons' involvement in task performance. A1 neurons' phase locking to AM correlated more weakly (compared to firing rate) with the animals' report of AM, suggesting a preferential role for rate-codes in A1 for this AM discrimination task.

Amplitude modulation detection as a function of modulation frequency and stimulus duration: comparisons between macaques and humans.

Previous observations show that humans outperform non-human primates on some temporally-based auditory discrimination tasks, suggesting there are species differences in the proficiency of auditory temporal processing among primates. To further resolve these differences we compared the abilities of rhesus macaques and humans to detect sine-amplitude modulation (AM) of a broad-band noise carrier as a function of both AM frequency (2.5 Hz-2 kHz) and signal duration (50-800 ms), under similar testing conditions. Using a go/no-go AM detection task, we found that macaques were less sensitive than humans at the lower frequencies and shorter durations tested but were as, or slightly more, sensitive at higher frequencies and longer durations. Humans had broader AM tuning functions, with lower frequency regions of peak sensitivity (10-60 Hz) than macaques (30-120 Hz). These results support the notion that there are species differences in temporal processing among primates, and underscore the importance of stimulus duration when making cross-species comparisons for temporally-based tasks.

Comparisons of electromagnetic and piezoelectric floating-mass transducers in human cadaveric temporal bones.

Electromagnetic floating-mass transducers for implantable middle-ear hearing devices (IMEHDs) afford the advantages of a simple surgical implantation procedure and easy attachment to the ossicles. However, their shortcomings include susceptibility to interference from environmental electromagnetic fields, relatively high current consumption, and a limited ability to output high-frequency vibrations. To address these limitations, a piezoelectric floating-mass transducer (PFMT) has recently been developed. This paper presents the results of a comparative study of these two types of vibration transducer developed for IMEHDs. The differential electromagnetic floating-mass transducer (DFMT) and the PFMT were implanted in two different sets of three cadaveric human temporal bones. The resulting stapes displacements were measured and compared on the basis of the ASTM standard for describing the output characteristics of IMEHDs. The experimental results show that the PFMT can produce significantly higher equivalent sound pressure levels above 3 kHz, due to the flat response of the PFMT, than can the DFMT. Thus, it is expected that the PFMT can be utilized to compensate for high-frequency sensorineural hearing loss.

Encoding of illusory continuity in primary auditory cortex.

When interfering objects occlude a scene, the visual system restores the occluded information. Similarly, when a sound of interest (a "foreground" sound) is interrupted (occluded) by loud noise, the auditory system restores the occluded information. This process, called auditory induction, can be exploited to create a continuity illusion. When a segment of a foreground sound is deleted and loud noise fills the missing portion, listeners incorrectly report hearing the foreground continuing through the noise. Here we reveal the neurophysiological underpinnings of illusory continuity in single-neuron responses from awake macaque monkeys' primary auditory cortex (A1). A1 neurons represented the missing segment of occluded tonal foregrounds by responding to discontinuous foregrounds interrupted by intense noise as if they were responding to the complete foregrounds. By comparison, simulated peripheral responses represented only the noise and not the occluded foreground. The results reveal that many A1 single-neuron responses closely follow the illusory percept.

Illusory sound perception in macaque monkeys.

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.