Sensitivity of auditory cortical neurons to the locations of leading and lagging sounds

We recorded unit activity in the auditory cortex ( fields A1, A2, and PAF) of anesthetized cats while presenting paired clicks with variable locations and interstimulus delays ( ISDs). In human listeners, such sounds elicit the precedence effect, in which localization of the lagging sound is impaired at ISDs less than or similar to 10 ms. In the present study, neurons typically responded to the leading stimulus with a brief burst of spikes, followed by suppression lasting 100 - 200 ms. At an ISD of 20 ms, at which listeners report a distinct lagging sound, only 12% of units showed discrete lagging responses. Long-lasting suppression was found in all sampled cortical fields, for all leading and lagging locations, and at all sound levels. Recordings from awake cats confirmed this long- lasting suppression in the absence of anesthesia, although recovery from suppression was faster in the awake state. Despite the lack of discrete lagging responses at delays of 1 - 20 ms, the spike patterns of 40% of units varied systematically with ISD, suggesting that many neurons represent lagging sounds implicitly in their temporal firing patterns rather than explicitly in discrete responses. We estimated the amount of location- related information transmitted by spike patterns at delays of 1 - 16 ms under conditions in which we varied only the leading location or only the lagging location. Consistent with human psychophysical results, transmission of information about the leading location was high at all ISDs. Unlike listeners, however, transmission of information about the lagging location remained low, even at ISDs of 12 - 16 ms.