Evoked potential decrements in auditory cortex. I. Discrete-trial and continual stimulation

Abstract

In Experiment 1 cats were exposed to sets of clicks (trials) with 1 min inter-trial-intervals to determine if the effects of repetitive stimulation on potentials evoked in the auditory cortex would be cumulative despite discretetrial stimulation. Evoked potentials were averaged to give one average evoked potential (AEP) for each trial for each electrode; there were four cortical electrodes per subject. To test for dishabituation pawshocks were given between trials 40 and 41, and to test for spontaneous recovery there was a break between trials 60 and 61. Subjects were paralyzed to insure stimulus constancy. The latency and peak-to-peak amplitude of each component of each AEP was measured; significant amplitude decrements were observed in all subjects and at 2 3 of the recording loci. About 1 3 of the components measured decremented, and decrements were more frequent in components with latencies > 15 msec. A few amplitude increments and latency changes were also observed. The systematic amplitude changes were specific to certain AEP components; a change could occur in one component of an AEP independent of any changes in the other components in the same AEP. The changes were also specific to certain loci; changes could occur at one cortical locus independent of any changes at other cortical loci in the same subject. However, systematic changes in amplitude were no more likely in one cortical region than another. For a significant number of the components that decreased systematically, the shocks produced further decreases; the results appeared to be the opposite of dishabituation. The break failed to have any consistent effect. Thus the decrements were similar to those observed by others under similar circumstances, but they did not demonstrate two of the most common characteristics of habituation. Apparently some aspect of the stimulus parameters or the constancy of the stimuli in Experiment 1 resulted in the opposite of dishabituation and no consistent spontaneous recovery. In Experiment 2 the cats were again paralyzed to maintain stimulus constancy, but more typical stimulus parameters were chosen: 65 dB clicks continual at 1/sec. There was a single shock as a dishabituating stimulus and a 15 min break to test for spontaneous recovery. Decrements were observed as in Experiment 1, and shock again paradoxically produced the opposite of dishabituation. In contrast to Experiment 1, spontaneous recovery was observed. Also amplitude and latency increases were observed with greater frequency than in Experiment 1. Since dishabituation and other characteristics of habituation were not observed, alternative explanations of the amplitude decreases were considered. The amplitude decrements were not artifacts of stimulus variability or deterioration of the paralyzed preparation. However, we were unable to rule out the hypothesis that the decrements were the consequence of a change in the subject's state during the experiment. This “state hypothesis” can only be tested by experiments more complex than the present ones.