BASAL FOREBRAIN CHOLINERGIC LESIONS DISRUPT INCREMENTS BUT NOT DECREMENTS IN CONDITIONED-STIMULUS PROCESSING

Magnocellular neurons in the basal forebrain provide the major choline rgic innervation of cortex, Recent research suggests that this choline rgic system plays an important role in the regulation of attentional p rocesses, The present study examined the ability of rats with selectiv e immunotoxic lesions of these neurons (made with 192 IgG-saporin) to modulate attention within an associative learning framework, Each rat was exposed to conditioned stimuli (CS) that were either consistent or inconsistent predictors of subsequent cues, Intact control rats showe d increased CS associability when that cue was an inconsistent predict or of a subsequent cue, whereas lesioned rats were impaired in increas ing attention to the CS when its established relation to another cue w as modified, In a separate experiment designed to test latent inhibiti on, it was shown that removal of the corticopetal cholinergic neurons spared a decrement in associability that occurs when rats are extensiv ely preexposed to a CS prior to conditioning. These data indicate that the cholinergic innervation of cortex is critical for incrementing, b ut not for decrementing attentional processing, The specific behaviora l tests used to assess the role of the basal forebrain cholinergic sys tem in the present study were previously used to identify a role for t he amygdala central nucleus in attention (Holland and Gallagher, 1993b ). Those studies, together with the results in this report, indicate t hat regulation of attentional processes during associative learning ma y be mediated by projections from the amygdala to the basal forebrain cholinergic system.