Patterns of activity coding discrimination of auditory stimuli differ between mid- and posterolateral thalamus of...

The auditory function of units in the mid- (lateralis dorsalis and centrolateral nuclei) and posterolateral (lateralis posterior-pulvinar complex) thalamus of cats was assessed during performance of conditioned eye blink responses (CRs) elicited discriminatively by a forward-paired, 70 dB click conditioned stimulus (CS) as opposed to a backward-paired, 70 dB hiss discriminative stimulus (DS). Discharges in response to the CS or DS were found in over 40% of units tested in each area, with onset latencies as short as 28-32 msec in mid-thalamus and 14 msec in posterolateral thalamus. The results provide evidence that both mid- and posterolateral thalamic regions of cats contain sufficient numbers of auditory responsive neurons to be considered part of the auditory system functionally. Patterns of activity changed after conditioning discriminative responses to the click CS. In mid-thalamus, the ratio of CS-evoked activity to baseline activity increased relative to levels found before conditioning. This increase was attributable to an increase in the magnitude of response to the CS. In posterolateral thalamus, an increase in the signal: noise ratio of activity in response to the CS was also found after conditioning, but this increase depended in large part on a decrease in the rate of baseline firing. Posterolateral thalamic units had substantially higher mean rates of baseline firing than mid-thalamic units before any conditioning sessions were begun.(ABSTRACT TRUNCATED AT 250 WORDS)