Disruption of tactile discrimination in rats by bilateral stimulation of midbrain lemniscal pathways

Bilateral stimulation in the midbrain medial lemniscus disrupted tactile discrimination in four of nine rats. Single conditioning shocks in the midbrain of these animals attenuated by more than 80% the somatosensory cortical evoked responses elicited by test shocks to the contralateral forepaw. Midbrain stimulation did not selectively disrupt discrimination in the remaining five rats, and midbrain conditioning shocks attenuated the cortical test response by less than 80% on at least one side. The four rats that showed disrupted discrimination and bilaterally strong attenuation of cortical responses also had both midbrain electrodes in or next to the medial lemniscus. The five rats that did not show these effects had at least one midbrain electrode away from that structure.