Excision of visual cortex does not affect the facilitatory and inhibitory effects of a light flash prepulse on the acoustic startle reflex in the rat

We studied the inhibitory (PPI) and facilitatory (PPF) behavioural effects of brief light flashes presented as prepulses just prior to the acoustic startle reflex (ASR) in intact rats and in rats that had a bilateral excision of visual cortex (VC). The VC-lesioned and control groups (n=5, 5) were near identical in PPF for flashes presented 5-20 ms before the ASR and near identical in PPI for flashes presented 60 ms before the ASR. These findings differ from those of a case report of a human patient following occipital cortex resection, for whom light flashes failed to produce either acoustic startle PPF or PPI if the flash were presented to the portion of the visual field in which she could not see; they differ also from data obtained in functionally decorticate rats, for which a light flash produced no startle PPI, but instead a late-appearing exaggerated PPF. In the present experiment the lesion was restricted to VC, while in contrast the occipital resection in the human patient included posterior areas of the parietal and temporal lobe, and functional decortication in the rat disengaged the entire cortex. The greater extent of these two effective surgical procedures may have been responsible for their greater behavioural effects; otherwise, the complete loss of reflex control by photic stimulation in the human patient but not in the rat may reflect the greater functional importance and anatomical complexity of the VC in humans.