Time-frequency feature extraction from frequency-modulated sounds in a population of auditory cortical neurons

This research is aimed at understanding how time-frequency features are extracted from frequency-modulated (FM) sounds in auditory cortical neurons. We investigated spatiotemporal response patterns in the primary auditory cortex in the guinea pig, using an optical recording method (MiCAM Ultima; Brain Vision) with a voltage-sensitive dye (RH795). Experiments were performed under anesthesia (ketamine, 100 mg/kg and xylazine, 25 mg/kg). A pure tone (PT) evoked a strong on-response that was followed by an inhibitory phase lasting approximately for 100 ms. An exponential FM sound evoked an additional activity moving along the frequency axis. Such an FM response became more evident when the frequency of the FM sound was modulated in a wider range. The position of the peak amplitude of the later FM response was shifted in accordance with the direction of the FM sweep, but the distance of the shift tended to be in the range of around 1 mm. Accordingly, the FM response sometimes appeared in isofrequency areas outside those covering the frequency range of the FM sound. These results indicate that interactions between excitatory and inhibitory neurons provide a robust FM detector that utilizes maximally the frequency-response area (FRA) of neurons distributed along the frequency axis in the auditory cortex.