Temporal Resolution of Gaps in Noise by the Rat is Lost with Functional Decortication

In Experiment 1 (n = 8), the rat's ability to detect brief gaps in white noise was measured by gap-produced inhibition of an acoustic startle reflex, elicited 100 ms after the gap. After bilateral application of KCl to the cortex, gaps as long as 15 ms provided no reflex inhibition; in contrast, the inhibitory threshold was between 2 and 4 ms in the saline control condition. In Experiment 2 (n = 13), noise pulses of 40, 50, or 70 dB were presented 20-500 ms before the startle stimulus, and in Experiment 3 (n = 5) noise offsets occurred so that the startle stimulus was presented at the end of a 2-30-ms gap. Noise pulses and offsets both inhibited reflex expression equally in saline- and KCl-treated animals. Differences between the normal (saline) functions of noise offsets and gaps suggest additional sensory processing with the longer lead time. The loss of gap sensitivity after KCl application indicates that gap processing, unlike pulses and offsets, depends on cortical mechanisms.