The Contribution of Primary Auditory Cortex to Auditory Categorization in Behaving Monkeys

The specific contribution of core auditory cortex to auditory perception –such as categorization– remains controversial. To identify a contribution of the primary auditory cortex (A1) to perception, we recorded A1 activity while monkeys reported whether a temporal sequence of tone bursts was heard as having a “small” or “large” frequency difference. We found that A1 had frequency-tuned responses that habituated, independent of frequency content, as this auditory sequence unfolded over time. We also found that A1 firing rate was modulated by the monkeys’ reports of “small” and “large” frequency differences; this modulation correlated with their behavioral performance. These findings are consistent with the hypothesis that A1 contributes to the processes underlying auditory categorization

Modulation of Cross-Frequency Coupling by Novel and Repeated Stimuli in the Primate Ventrolateral Prefrontal Cortex

Adaptive behavior depends on an animal’s ability to ignore uninformative stimuli, such as repeated presentations of the same stimulus, and, instead, detect informative, novel stimuli in its environment. The primate prefrontal cortex (PFC) is known to play a central role in this ability. However, the neural mechanisms underlying the ability to differentiate between repeated and novel stimuli are not clear. We hypothesized that the coupling between different frequency bands of the local field potential (LFP) underlies the PFC’s role in differentiating between repeated and novel stimuli. Specifically, we hypothesized that whereas the presentation of a novel-stimulus induces strong cross-frequency coupling, repeated presentations of the same stimulus attenuates this coupling. To test this hypothesis, we recorded LFPs from the ventrolateral PFC (vPFC) of rhesus monkeys while they listened to a novel vocalization and repeated presentations of the same vocalization. We found that the cross-frequency coupling between the gamma-band amplitude and theta-band phase of the LFP was modulated by repeated presentations of a stimulus. During the first (novel) presentation of a stimulus, gamma-band activity was modulated by the theta-band phase. However, with repeated presentations of the same stimulus, this cross-frequency coupling was attenuated. These results suggest that cross-frequency coupling may play a role in the neural computations that underlie the differentiation between novel and repeated stimuli in the vPFC

The neural and behavioral correlates of auditory streaming

Perceptual representations of auditory stimuli—which are called auditory streams or objects—are derived from the auditory system\u27s ability to segregate and group stimuli based upon spectral, temporal, and spatial features. However, it remains unclear how our auditory system encodes these auditory streams at the level of the single neuron. In order to address this question directly, we first validated an animal model of auditory streaming. Specifically, we trained rhesus macaques to report their streaming percept using methodologies and controls similar to those presented in previous human studies. We found that the monkeys\u27 behavioral reports were qualitatively consistent with those of human listeners. Next, we recorded from neurons in the primary auditory cortex while monkeys simultaneously reported their streaming percepts. We found that A1 neurons had frequency-tuned responses that habituated, independent of frequency content, as the auditory sequence unfolded over time; and we report for the first time that firing rate of A1 neurons was modulated by the monkeys’ choices. This modulation increased with listening time and was independent of the frequency difference between consecutive tone bursts. Overall, our results suggest that A1 activity contributes to the sensory evidence underlying the segregation and grouping of acoustic stimuli into distinct auditory streams. However, because we observe choice-related activity based upon firing rate alone, our data are at partially at odds with Micheyl et al.’s (2005) prominent hypothesis, which argued that frequency-dependent habituation may be a coding mechanism for the streaming percept