THE LOCUS OF ORIGIN OF AUGMENTING AND REDUCING OF VISUAL-EVOKED POTENTIALS IN RAT-BRAIN

Humans who are high sensation seekers and cats who demonstrate compara ble behavioral traits show increasing amplitudes of the early componen ts of the cortical visual evoked potential (VEP) to increasing intensi ties of light flash; low sensation seekers show VEP reducing. Roman hi gh-avoidance (RHA) and Roman low-avoidance (RLA) rats have behavioral traits comparable to human and cat high and low sensation seekers, res pectively. Previously, we showed that RHA and RLA rats are cortical VE P augmenters and reducers, respectively. The goal of this study was to determine if augmenting-reducing is in fact a property of the visual cortex or if it originates at the lateral geniculate nucleus and is me rely reflected in recordings from the cortex. VEPs to five flash inten sities were recorded from the visual cortex and dorsal lateral genicul ate of RHA and RLA rats. As in the previous study, the slope of the fi rst cortical component as a function of flash intensity was greater in the RHA than in the RLA rats. The amplitude of the geniculate compone nt that has a latency shorter than the first cortical component was no different in the two lines of rats. The finding from the cortex confi rms the earlier finding of augmenting and reducing in RHA and RLA rats , respectively. The major new finding is that the augmenting-reducing difference recorded at the cortex does not occur at the thalamus, indi cating that it is truly a cortical phenomenon.