Temporal resolution of gaps in noise by the rat is lost with functional decortication.

Abstract

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.

Parameters affecting gap detection in the rat

“…Consistent with previous research (lson, 1982;Ison et al, 1991), a gap in otherwise continuous noise reliably reduced the amplitude of the rat's acoustic startle reflex. Also consistent with previous research (e.g., Marsh et al, 1975;Schwartz, Hoffman, Stitt, & Marsh, 1976), an inverse relationship was found between reflex amplitude reduction and stimulus saliency-the larger the gap, the greater the degree of reflex reduction.…”

“…More recently, Ison, O'Connor, Bowen, and Bocirnea (1991) essentially replicated these findings in rats; gaps presented at short SOAs continued to increase in their ability to reduce startle amplitude as their duration increased from 2 to 30 msec. Gaps presented at long SOAs produced a maximum degree of reflex reduction at durations of 4 msec, beyond which increases in duration did not cause further reductions of startle amplitude.…”

“…In the present study, gap prestimuli produced reflex reduction that grew rapidly as the lSI moved from 25 to 50 msec, and then decreased less rapidly as the lSI moved to 75 msec. If the efficacy of very brief gaps in reducing startle amplitude is interpreted as a reflection of greater cortical processing, as suggested by Ison et al (1991), this process would appear to be maximal at ISIs of approximately 50 msec, declining thereafter. Ison (1982) found that an interaction existed between gap duration and lSI-gaps of long duration engendered the greatest degree of reflex reduction at short ISis, whereas brief gaps were most effective in reducing reflex amplitude if they were presented at long ISis.…”

“…The present series of experiments are an extension of the research conducted by Ison (1982) and Ison et al (1991). In the present research, the temporal interval between gap offset and the startle-eliciting stimulus (SES) onset, the duration of the gap itself, and the rise-fall time of the gap were each varied systematically over a large range of values.…”

“…One type was used by Ison (1982) and Ison et al (1991) and consisted of a trial in which the SES was presented after a period of unbroken background noise (a "no gap" condition). The other type consisted of the rise-fall time associated with the gap being presented before the SES, without the gap itself (a zero-duration gap condition).…”