Extraction of sounds’ time‐frequency features in the auditory cortex

Although ‘‘time‐frequency feature extraction’’ is a fundamental cue that is used to recognize speech sounds, its physiological mechanisms remain largely unknown. This research is aimed at understanding what time‐frequency features of sounds are extracted by auditory cortical neurons. We investigated spatiotemporal response patterns in the guinea‐pig auditory cortex, 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). Pure tones (PTs), frequency‐modulated (FM) sounds with various modulation rates, and their combinations were presented to the animals. Cortical activation was induced at the onset and offset of PTs along the isofrequency contours. The onset response was followed by strong inhibition. In contrast, FM sounds evoked an additional activation moving across the isofrequency contours in the cortex, with locations corresponding to the instantaneous frequency inpu...