Directional sensitivity of neurons in the primary auditory (AI) cortex of the cat to successive sounds ordered in time and space

Two transient sounds, considered as a conditioner followed by a probe, were delivered successively from the same or different direction in virtual acoustic space (VAS) while recording from single neurons in primary auditory cortex (AI) of cats under general anesthesia. Typically, the response to the probe sound was progressively suppressed as the interval between the two sounds (ISI) was systematically reduced from 400 to 50 ms, and the sound-source directions were within the cell's virtual space receptive field (VSRF). Suppression of the cell's discharge could be accompanied by an increase in response latency. In some neurons, the joint response to two sounds delivered successively was summative or facilitative at ISIs below about 20 ms. These relationships held throughout the VSRF, including those directions on or near the cell's acoustic axis where sounds often elicit the strongest response. The strength of suppression varied systematically with the direction of the probe sound when the ISI was fixed and the conditioning sound arrived from the cell's acoustic axis. Consequently a VSRF defined by the response to the lagging probe sound was progressively reduced in size when ISIs were shortened from 400 to 50 ms. Although the presence of a previous sound reduced the size of the VSRF, for many of these VSRFs a systematic gradient of response latency was maintained. The maintenance of such a gradient may provide a mechanism by which directional acuity remains intact in an acoustic environment containing competing acoustic transients.