Unilateral cortical spreading depression and conditioned eye blink responses in rabbits

Conditioned eye blinking was established in the rabbit by optimally pairing a somatosensory, auditory or visual conditioned stimulus (CS) with air puff to the left eye as the unconditioned stimulus (US). After conditioned response (CR) occurred on 80-90 percent of the presentations, single waves of CSD were triggered by micro-injections of 25 percent KCL into the frontal or occipital cortex of each hemisphere. Slow potential changes (SPC) were recorded via implanted, nonpolarizable electrodes and correlated with the presence or absence of the CR. Frontal KCL injections contralateral to the blinking eye totally abolished the CR when the peak SPC invaded the motor cortex independent of the sensory modality conditioned. Recovery took place in the order: somatosensory CS (13 min), auditory CS (18 min) and visual CS (23 min). Occipital KCL injections contralaterally caused partial supression of aurally and visually conditioned eye blinking when the SPC invaded the parieto-occipital region but no such suppression occurred when using the somatosensory CS. Total abolition of the CR ensued when the wave of CSD reached the motor (frontal) cortex and again was independent of the CS modality. Similar but less striking effects were noted during ipsilateral CSD. The conclusion is drawn that whereas the cortical projection of the CS is important but not indispensable, the motor cortex is essential for the elaboration of conditioned eye blinking. A discussion of cortical localization subserving discrete conditioned reactions, as revealed by the technique of CSD, is presented.