Brain lesions impairing visual and spatial reversal learning in rats: Components of the “general learning system” of the rodent brain

Abstract

The "general learning system" (GLS) is conceived as an ensemble of brain structures essential for normal acquisition of a wide variety of laboratory tasks. Based upon earlier lesion studies, it was reasoned that the components of the rodent's GLS could be identified by determining which lesion placements within the rat brain would lead to defective acquisition of a spatial discrimination habit and its reversal as well as a brightness discrimination habit and its reversal. Of the 11 cortical and subcortical (frontal cortex, parietal cortex, occipital cortex, anterior cingulate cortex, posterior cingulate cortex, rostral caudoputamen, dorsal hippocampus, mammillary bodies, mediodorsal thalamic nucleus, parafascicular nucleus, and substantia nigra) sites investigated in this experiment, only two qualified as components of the GLS-the parafascicular nucleus and the substantia nigra. The possibility that the nigro-parafascicular-striatal complex constitutes a major part of the rodent's GLS is discussed.

Detour problem-solving behavior in rats with early lesions to the “general learning system”

“…The foregoing findings, which implicate the globus pallidus, substantia nigra, median raphe, and pontine reticular formation in the general learning process, are significant for a number of reasons. First, it should be recognized that a generalized learning impairment induced by specific brain lesions is not a common occurrence: Rats with discrete lesions to the cerebral cortex, limbic forebrain, caudoputamen, nucleus accumbens, thalamus, medial hypothalamus, dorsal midbrain, lateral pontomesencephalic area, or cerebellar cortex have not been found to exhibit a generalized learning impairment (Thompson, 1982a(Thompson, , 1982b; current study). Such a localized effect suggests that the neuronal elements intrinsic to (or extraneous fibers passing through) the regions of the globus pallidus, substantia nigra, median raphe, and pontine reticular formation are potentially more involved in the ability to learn diverse laboratory tasks than those associated with most other regions of the brain.…”

“…The results of the current study add to this pattern to the extent that median raphe lesions were found to induce impairments in detour problem-solving behavior as striking as those induced by hippocampal lesions (Thompson et al, 1984a). However, a visual discrimination task appears to differentiate rats with large median raphe lesions from rats with large dorsal or ventral hippocampal lesions: The former exhibit dramatic deficits in acquisition (Thompson, 1982b) and retention (Thompson, 1978) of a white-black discrimination, whereas the latter do not exhibit these deficits (Thompson, 1978(Thompson, , 1982a(Thompson, , 1982b. Conceivably, damage to certain elements within the median raphe (serotonergic cells, nonserotonergic cells, or fibers in passage) may be responsible for the pattern of deficits similar to that associated with hippocampal lesions, whereas damage to other elements may be responsible for the visual discrimination impairments.…”

“…Earlier studies have demonstrated that lesions to the globus pallidus, lateral thalamus, substantia nigra, midbrain central gray, median raphe, or pontine reticular forma-DETOUR PROBLEM-SOLVING BEHAVIOR 201 tion in weanling or adult (Thompson, 1982a(Thompson, , 1982b) rats produce learning impairments on visual as well as nonvisual discrimination habits. These results have provided the basis for the hypothesis that the foregoing structures playa nonspecific role in the learning process and that they constitute the major portion of the general learning system (GLS) of the rat brain.…”

“…This proposal is based on the findings that localized electrolytic lesions to anyone of the foregoing regions in adult rats retard acquisition and reversal of both visual and nonvisual discrimination habits (Thompson, 1982a(Thompson, , 1982b. The finding that weanling rats subjected to these lesions also show a nonspecific learning deficit (Thompson, Ramsay, & Yu, 1984; suggests the possibility that the young brain-injured rat (one bearing lesions to the general learning system) may serve as an animal model for certain classes of mental retardation, at least to the extent that both the braininjured rat and the mental retardate evidence a nonspecific (generalized) learning impairment (Denny, 1964).…”

Deficits in response inhibition and attention in rats rendered mentally retarded by early subcortical brain damage

“…Although not conclusive, evidence exists which casts doubt on such a causal relationship. For example, the hippocampus of rodents has been implicated in both inhibitory (Douglas, 1967) and attentional (Oades, 1982) processes; yet young or adult rats with hippocampal lesions do not exhibit a generalized learning impairment (Douglas, 1967;Thompson, 1982a;Thompson & Yu, 1983). Other brain regions, to mention a few, which have been implicated in one or both of these processes include the frontal cortex (Stuss Thompson, 1978b), amygdala (Nagel & Kemble, 1976;Thompson, 1978b), and lateral lemniscal midbrain area (Sprague, Levitt, Robson, Liu, Stellar, & Chambers, 1963).…”