Processing of Complex Sounds by Prefrontal Neurons

Our research has shown that auditory afferents target several specific regions of the prefrontal cortex (Romanski et al., 1999). In one of these regions we have demonstrated robust responses to complex acoustic stimuli including species-specific and human vocalizations. In previous studies. For unimodal auditory and multimodal neurons that are predominantly responsive to auditory stimuli we are interested in the neuronal response to particular features of complex sounds. It is still unknown how prefrontal neurons encode complex communication sounds and whether these neurons encode speaker identity, behavioral significance or specific combinations of complex acoustic features.

Location of auditory responsive neurons in primate prefrontal cortex. Recordings across 3 subjects are summarized on a single schematic depicting the locations of auditory responsive cells in the ventrolateral prefrontal cortex. The corresponding coronal sections are shown below where the red lines depict electrode tracks. Locations of auditory responsive cells on these tracks are portrayed with black lines.

In the past we have used a PCA/ICA extraction method, (Averbeck and Romanski, 2004), to construct stimuli which emphasize certain features of sounds, or de-emphasize others. Further studies will employ stimulus manipulations which emphasize particular components of complex sounds including PCA and ICA components, formants, noise stimuli, time reversals, etc. Sounds filtered to emphasize these components will be tested in passive fixation and active discrimination tasks where we can record responses of VLPFC neurons to these stimuli and compare them with responses to the original sound. We hypothesize that these neurons encode a combination of complex features, some of which may be contained in our sound manipulations.

Types of responses to auditory stimuli by prefrontal neurons. The responses of two single units (column A and B) to three different exemplars of auditory stimuli are shown in raster and histogram plots. The onset of the auditory stimulus (vocalizations in the first two rows; noise and other stimuli in the last row) is at time 0 and the duration of the stimulus is depicted by the length of the gray bar. A neuron with a phasic response to the onset of auditory stimuli is shown in (A) and a neuron that produced a sustained response to auditory stimuli is depicted in (B).

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