| The auditory cortex is known to be essential for normal sound localization behavior, yet we lack a complete understanding of how auditory space is represented by the activity of auditory cortical neurons. To investigate this representation, we presented sounds from loudspeakers at various locations while recording spike activity (action potentials) from the auditory cortex in awake and anesthetized cats. A newly developed preparation allowed simultaneous recordings at up to 16 cortical sites in awake animals. The firing patterns of neurons in awake animals depended strongly on sound-source location, although neurons often responded to sounds located throughout an entire hemifield of space. Spatial tuning was generally unaltered by changes in sound level. Spike timing carried information about source location that was not present in time-averaged firing rates. Spike patterns immediately following the onset of the sound tended to transmit the most information. In other experiments, we investigated neuronal correlates of the precedence effect by recording from cortical neurons in anesthetized cats. The precedence effect refers to the observation that, when multiple sounds arrive in quick succession, the earliest sound dominates the listener's spatial percept. Paired sounds were presented from various locations with a variable delay between the sounds. When the delay was less than 5 ms, spike patterns resembled the response to a single sound. Furthermore, the locations associated with those spike patterns were generally dominated by the earlier sound, in agreement with the reports of human listeners. When the delay was greater than 5 ms, discrete responses to the lagging sound were usually suppressed to an extent that was inconsistent with a listener's perception. Nonetheless, most neurons were sensitive to the presence of the lagging sound, and a small minority of neurons responded in a manner qualitatively consistent with the precedence effect. |
