Neural Representations of Sinusoidal Amplitude and Frequency Modulations in the Primary Auditory Cortex of Awake Primates

Abstract

We investigated neural coding of sinusoidally modulated tones (sAM and sFM) in the primary auditory cortex (A1) of awake marmoset monkeys, demonstrating that there are systematic cortical representations of embedded temporal features that are based on both average discharge rate and stimulus-synchronized discharge patterns. The rate-representation appears to be coded alongside the stimulus-synchronized discharges, such that the auditory cortex has access to both rate and temporal representations of the stimulus at high and low frequencies, respectively. Furthermore, we showed that individual auditory cortical neurons, as well as populations of neurons, have common features in their responses to both sAM and sFM stimuli. These results may explain the similarities in the perception of sAM and sFM stimuli as well as the different perceptual qualities effected by different modulation frequencies. The main findings include the following. 1) Responses of cortical neurons to sAM and sFM stimuli in awake marmosets were generally much stronger than responses to unmodulated tones. Some neurons responded to sAM or sFM stimuli but not to pure tones. 2) The discharge rate-based modulation transfer function typically had a band-pass shape and was centered at a preferred modulation frequency (rBMF). Population-averaged mean firing rate peaked at 16- to 32-Hz modulation frequency, indicating that the A1 was maximally excited by this frequency range of temporal modulations. 3) Only approximately 60% of recorded units showed statistically significant discharge synchrony to the modulation waveform of sAM or sFM stimuli. The discharge synchrony-based best modulation frequency (tBMF) was typically lower than the rBMF measured from the same neuron. The distribution of rBMF over the population of neurons was approximately one octave higher than the distribution of tBMF. 4) There was a high degree of similarity between cortical responses to sAM and sFM stimuli that was reflected in both discharge rate- or synchrony-based response measures. 5) Inhibition appeared to be a contributing factor in limiting responses at modulation frequencies above the rBMF of a neuron. And 6) neurons with shorter response latencies tended to have higher tBMF and maximum discharge synchrony frequency than those with longer response latencies. rBMF was not significantly correlated with the minimum response latency.

The cellular basis for parallel neural transmission of a high-frequency stimulus and its low-frequency envelope

“…This response to low-frequency signals and envelopes of high-frequency narrowband signals can be thought of as another form of cue invariance, except in this case, it pertains to sound localization, not object perception. Observations of independence between optimal carrier and envelope frequencies in the auditory cortex also support the idea of parallel transmission of signal and envelope (29,31,32).…”

A Neural Correlate of Stochastic Echo Imaging

“…However, Lu et al (2001) showed that a distinct population of neurons exists in the auditory cortex of marmosets that encodes highmodulation frequencies as a rate code. A rate code for highmodulation frequencies in the auditory cortex was also reported in other studies (Bieser and Muller-Preuss, 1996;Liang et al, 2002;Lu and Wang, 2004). Our current results suggest that neurons in the auditory cortex of P. discolor represent IR roughness as a rate code.…”

Enduring effects of early structured noise exposure on temporal modulation in the primary auditory cortex

“…It should be noted that anesthesia has been shown to affect the cortical response to successive stimuli (16,17). However, anesthesia would not create a bias in estimations made in PN-exposed versus naïve rats because of identical anesthetic conditions during recording for both rat groups.…”