Comparison of the effects of frontal and caudate lesions on discrimination learning in monkeys

The neural basis of reversal learning: An updated perspective

Reversal learning paradigms are among the most widely used tests of cognitive flexibility and have been used as assays, across species, for altered cognitive processes in a host of neuropsychiatric conditions. Based on recent studies in humans, non-human primates, and rodents, the notion that reversal learning tasks primarily measure response inhibition, has been revised. In this review, we describe how cognitive flexibility is measured by reversal learning and discuss new definitions of the construct validity of the task that are serving as a heuristic to guide future research in this field. We also provide an update on the available evidence implicating certain cortical and subcortical brain regions in the mediation of reversal learning, and an overview of the principal neurotransmitter systems involved.

Neuroimaging studies of the striatum in cognition Part I: healthy individuals

The striatum has traditionally mainly been associated with playing a key role in the modulation of motor functions. Indeed, lesion studies in animals and studies of some neurological conditions in humans have brought further evidence to this idea. However, better methods of investigation have raised concerns about this notion, and it was proposed that the striatum could also be involved in different types of functions including cognitive ones. Although the notion was originally a matter of debate, it is now well-accepted that the caudate nucleus contributes to cognition, while the putamen could be involved in motor functions, and to some extent in cognitive functions as well. With the arrival of modern neuroimaging techniques in the early 1990, knowledge supporting the cognitive aspect of the striatum has greatly increased, and a substantial number of scientific papers were published studying the role of the striatum in healthy individuals. For the first time, it was possible to assess the contribution of specific areas of the brain during the execution of a cognitive task. Neuroanatomical studies have described functional loops involving the striatum and the prefrontal cortex suggesting a specific interaction between these two structures. This review examines the data up to date and provides strong evidence for a specific contribution of the fronto-striatal regions in different cognitive processes, such as set-shifting, self-initiated responses, rule learning, action-contingency, and planning. Finally, a new two-level functional model involving the prefrontal cortex and the dorsal striatum is proposed suggesting an essential role of the dorsal striatum in selecting between competing potential responses or actions, and in resolving a high level of ambiguity.

Some highlights of research on the effects of caudate nucleus lesions over the past 200 years

This review describes experiments on the effects of caudate nucleus lesions on behavior in monkeys, cats and rats. Early work on monkeys and cats focused on the relationship of the caudate to the cortex in motor control, leading to the idea that the caudate serves to inhibit behaviors initiated by the cortex. However, investigation of this hypothesis with systematic behavioral testing in all three species did not support this idea; rather, these studies provided evidence that caudate lesions affect memory functions. Two main types of memory tasks were affected. One type involved reinforced stimulus-response (S-R) associations, the other involved spatial information, response-reinforcer contingencies, or working memory. Recent evidence, mainly from rats, suggests that the dorsolateral part of the caudoputamen is central to the processing and consolidation of memory for reinforced S-R associations, and that the more medial and anterior parts of the same structure are part of a neural circuit that (in some cases) also includes the hippocampus, and mediates relational information and certain forms of working memory. The possibility that the spatial distribution of the patch and matrix compartments within the caudoputamen underlies these regional differences is discussed.

Role of basal ganglia–brainstem pathways in the control of motor behaviors

Here we review a role of a basal ganglia-brainstem (BG-BS) system throughout the mesopontine tegmentum in the control of various types of behavioral expression. First the basal ganglia-brainstem system may contribute to an automatic control of movements, such as rhythmic limb movements and adjustment of postural muscle tone during locomotion, which occurs in conjunction with voluntary control processes. Second, the basal ganglia-brainstem system can be involved in the regulation of awake-sleep states. We further propose the possibility that the basal ganglia-brainstem system is responsible for the integration of volitionally-guided and emotionally-triggered expression of motor behaviors. It can be proposed that dysfunction of the basal ganglia-brainstem system together with that of cortico-basal ganglia loop underlies the pathogenesis of behavioral disturbances expressed in basal ganglia dysfunction.

Lesions of the medial and lateral striatum in the rat produce differential deficits in attentional performance

Excitotoxic lesions of the medial frontal cortex and anterior cingulate cortex in rats have been shown to produce dissociable impairments on a reaction time visual attention (5-choice) task. Because these cortical areas project to the medial striatal region, the authors predicted similar deficits after lesions of this striatal area compared with the lateral area. Compared with sham-operated controls, rats with quinolinic acid-induced medial striatal lesions showed all the behavioral changes associated with medial frontal cortex and anterior cingulate cortex lesions. In contrast, lateral striatal lesions produced profound disturbances in the performance of the task. Control tests showed little evidence of gross deficits in either group of rats in terms of motivation, locomotor function, or Pavlovian appetitive conditioning. These data suggest that the medial and lateral striatum have contrasting roles in the control of instrumental responding related to the primary sources of their cortical innervation.

The effect of dopamine depletion from the caudate nucleus of the common marmoset (Callithrix jacchus) on tests of prefrontal cognitive function

This study examined the effects of depletion of dopamine from the caudate nucleus of the common marmoset (Callithrix jacchus), on tasks sensitive to prefrontal damage (attentional set-shifting and spatial delayed response). There was a marked impairment in performance on the spatial delayed response task, but performance on the attentional set-shifting task was relatively preserved except for an impairment in re-engagement of a previously relevant perceptual dimension. This pattern of impairment is distinct from that seen after excitotoxic lesions of the prefrontal cortex and in patients with Parkinson's disease. Though it is not possible to identify specific cognitive functions that are independent of dopaminergic modulation of the caudate nucleus, due to the partial nature of the lesion, the results do provide insight into those cognitive processes that appear most dependent on caudate dopamine.

Effects of microinjection of scopolamine into the neostriatum of rats on performance of a food conditioned reflex at different levels of fixation

Chronic experiments performed on 32 Sprague-Dawley rats using a movement-feeding operant reflex (Skinner box) model showed that microinjection of scopolamine into the neostriatum had effects on this reflex which depended on the stage of learning. In animals with weakly fixed reflexes (prior to reaching the stage of memory consolidation), bilateral microinjection of 0.3 microgram of scopolamine into the caudate nucleus completely inhibited the reflex for a prolonged period of time. When the operant habit was well fixed, bilateral microinjection of the same doses of scopolamine into the neostriatum had no effect on the reflex. These results suggest that the neostriatum cholinergic system is critically involved in forming the motor engram. The cholinergic system of the caudate nucleus either takes no part in realizing the well-fixed conditioned reflex movement response and/or other forebrain structures are involved in the reflex, compensating for the disturbance in neostriatal cholinergic function.

Psychophysiological and behavioral differences as a function of age and Parkinson's disease

Differences related to age and to specific neurological (Parkinsonian) damage were studied by contrasting, respectively, old (mean age 64 years) and young (27 years) subjects (N = 15), and old-normal (N = 15) and old-Parkinsonian (N = 15) subjects. Both behavioral as well as psychophysiological dependent variables were employed. The behavioral measure was performance on a discrimination learning task, while the psychophysiological measures were based on habituation to a repeated tone stimulus and on a Pavlovian differential conditioning preparation. The behavioral task showed predominantly age-related differences, with the young learning faster. Another age-related difference was interpreted as showing a more insight-like learning process in the young. Response-bias (beta) values did not differ between groups. Age-related differences also emerged more clearly than specific-neurological-damage differences in the psychophysiological data. The older subjects manifested markedly less autonomic conditioning, which was probably due to a lowering of reactivity, as well as the emergence of habituation to the (loud-noise) unconditional stimulus. A correlational analysis of discrimination conditioning also yielded age-related differences. Most notably, reactivity played a greater role in conditioning in the old than in the young subjects. The results illustrated how psychophysiological measures can provide information that complements those provided by behavioral measures.

Use of the lesion model for examining toxicant effects on cognitive behavior

It is often beneficial to use a model to help understand unknown effects and relate those effects to an existing body of knowledge. In much of the early development of behavioral toxicology, the pharmacological model has served as a valuable theoretical guide, especially with regard to dosing and kinetic parameters. However, as with any model, it has certain limitations. The lesion model has complementary features which provide valuable insights into the behavioral effects of toxicants. This is particularly true for effects which persist long after the end of toxicant exposure. There is much literature describing effects of brain lesions on behavior. By comparing results from toxicology studies to those of lesion studies, one can take advantage of this trove of information to gain a better insight into the possible loci of toxic effects, and to identify tests which would be useful in further describing the nature of the toxic effects. In this article, we examine the theoretical and practical utility of the lesion model. Examples are given showing how it has proven useful in interpreting the cognitive effects of exposure of monkeys to lead and polychlorinated biphenyls (PCBs). These exposures produced syndromes that closely resemble the effects of lesions in the frontal cortex or limbic system.

Prefrontostriatal connections in relation to cortical architectonic organization in rhesus monkeys

Prefrontostriatal connections were investigated in rhesus monkeys using the autoradiographic technique to examine whether there are systematic relationships with regard to the architectonic organization of the prefrontal cortex. On the basis of progressive laminar elaboration, the different regions of the prefrontal cortex can be grouped into two architectonic trends. The dorsal trend, which begins in the medial proisocortical areas, can be followed through the dorsolateral prefrontal cortex, culminating in the dorsal arcuate region. The ventral trend, which originates in the orbital proisocortex, can be traced through the inferior prefrontal convexity to the ventral arcuate region. The results show that the main connections from the prefrontal cortex to the striatum are to the head and body of the caudate nucleus. These connections are topographically organized. Medial and dorsal prefrontal areas project predominantly to the dorsal and central portion of the head and body of the caudate nucleus, whereas orbital and inferior prefrontal areas are related mainly to the ventral and central portion. Moreover, prefrontostriatal connections have a medial-lateral topography. Medial and orbital prefrontal areas project medially in the head and body of the caudate nucleus, whereas the dorsal and ventral arcuate regions project laterally, adjacent to the internal capsule. The prefrontal regions above and below the principal sulcus project mainly to the intermediate sector of the head and body of the nucleus. However, there appears to be some degree of overlap of corticostriatal projections from the dorsal and ventral prefrontal regions, as well as within each trend. Relatively minor projections are directed to the putamen as well as to the tail of the caudate nucleus from certain subregions of the prefrontal cortex. Thus the distribution of prefrontostriatal connections seems to reflect the architectonic organization of the prefrontal cortex. Possible functional aspects of prefrontostriatal connectivity are considered in the light of behavioral and physiological studies.

Peduncular hallucinosis associated with isolated infarction of the substantia nigra pars reticulata

We report the clinicopathological study of an 83-year-old man who abruptly developed complex visual hallucinations, disordered sleep, and mild cognitive impairment that persisted until his death 6 months later. Postmortem neuropathological examination including serial sections through the diencephalon, midbrain, and pons disclosed isolated bilateral infarcts confined to the medial substantia nigra pars reticulata. The findings suggest that destruction of the pars reticulata may be the essential feature to the development of peduncular hallucinosis.

Cortical afferent input to the principalis region of the rhesus monkey

The sources of ipsilateral cortical afferent projections to regions along both banks of the principalis sulcus in the prefrontal cortex were studied with horseradish peroxidase in macaque monkeys. The principalis cortex receives a substantial proportion of its projections from neighboring prefrontal regions. However, differences were noted in the distribution of labeled cells projecting to the various principalis regions. These differences were most marked with respect to the relative proportion of cells originating in visual, auditory, somatosensory, premotor and limbic cortical areas. The findings indicate that the caudal ventral principalis region receives projections from both visual and visuomotor regions, whereas the anterior tip of the principalis appears to be the major target of projections from auditory association regions. The ventral bank at the middle extent of the principalis was the only case with a significant proportion of labeled cells in somatosensory association and premotor regions. There was a consistent increase in the proportion of labeled cells in limbic cortical areas projecting to more rostral principalis sites, irrespective of whether the injection was placed in the dorsal or ventral bank. These findings suggest that the caudal principalis region has a visual-visuomotor and the rostral, an auditory-limbic bias with respect to the long projections they receive.

The effect of Parkinson's disease on the ability to maintain a mental set

In two experiments with an Odd-Man-Out choice discrimination task, Parkinsonian subjects had difficulty in alternating between two rules on successive trials. The pattern of errors suggested that the difficulty arises from an instability of cognitive set rather than any loss of reasoning ability, perseveration or increased distractibility.

Activity of forebrain neurons during alternating movements in cats

The firing pattern of single units in the precruciate cortex, globus pallidus, entopeduncular nucleus and ventral thalamus of cats was studied before alternating limb movements. Seventy-eight percent of the neurons in these regions of the brain changed their activity more than 500 msec before the movement. No consistent early EMG changes occurred in the proximal or axial muscles which would correlate with the early neuronal activity changes. The data suggest that the sequence of movements required for this alternation task appears to be 'set' well in advance of the activation of the muscles involved in the necessary postural adjustments or the response itself. In addition, the units which showed early activity changes may be involved in the mnemonic processing necessary to discriminate a situational context and make an appropriate response. Over 50% of the movement related units in the globus pallidus, entopeduncular nucleus and ventral thalamus were related to movement of both the contralateral and ipsilateral paws. By contrast, only 33% of cortical units showed this relationship to the movement. Therefore, this 'set' process may be a relatively non-lateralized process. The data support the concept that the basal ganglia are involved in the enabling and sequencing of movements rather than in directly causing them to occur.

Effects of chlorpromazine on escape and avoidance responses: a closer look

While a wealth of evidence has implicated the nigrostriatal dopamine system in the initiation of movement, most or all of these movements have been in a conditioned avoidance framework, and on the order of 3-14 seconds in latency. It is proposed here that an elucidation of dopaminergic involvement in movement initiation requires a behavioral paradigm wherein experimental animals must rapidly and voluntarily respond to a stimulus to move (i.e., in less than 300 msec, paralleling human reaction time). Such a paradigm was developed and implemented in a re-analysis of earlier reports of chlorpromazine (CPZ) effects on escape from an avoidance of electric shock. Catecholaminergic or dopaminergic receptor blocking by CPZ resulted in clear impairment of the ability to initiate rapid avoidance movements, but in contrast to earlier work, some impairment of escape responses was also seen. Results are seen as further support for dopaminergic involvement in the initiation of voluntary movement.

Learning impairments following selective kainic acid-induced lesions within the neostriatum of rats

In two separate experiments, rats with kainic acid-induced lesions of the anteromedial (AMC) or ventrolateral (VLC) caudate nucleus were compared with controls on various discrete-trial maze and runway tasks: (a) the AMC lesion disrupted the direction but the degree of spontaneous bias in a T-maze, whereas the VLC disrupted the degree of bias but not its direction; (b) the AMC rats were impaired in the acquisition and reversal of a spatial position habit; (c) the VLC rats were impaired in runway extinction; (d) both lesion groups showed enhanced levels of activity when tested under the same conditions of food deprivation, but did not differ from controls when under free-feeding conditions. The histological analysis suggested that kainic acid does permit the selective lesioning of cell bodies in the neostriatum, whilst sparing fibres of passage. It was concluded that the results demonstrate functional heterogeneity within the neostriatum of rats, in direct agreement with the predictions of the 'prefrontal systems' hypothesis of Rosvold [48, 49].

Visual discrimination impairments after lesions in zona incerta or lateral terminal nucleus of accessory optic tract

Possible visual functions of zona incerta (ZI) and lateral terminal nucleus of the accessory optic tract (LTN) in rats were investigated by comparing the effects of localized lesions on a series of discrimination learning tasks. In Experiment 1 animals with ZI lesions were impaired on a simultaneous black versus white (BW) discrimination but not on a simultaneous horizontal versus vertical (HV) task. Animals with LTN lesions were impaired on HV but not on BW. Rats with ZI lesions were impaired on a successive bright versus dim but not a successive HV discrimination when subsequently tested in Experiment 2. Animals with LTN lesions were impaired on both tasks. The results are related to the possibility that visual intensity information gains access to the motor system through a pathway running from the ventral lateral geniculate nucleus to the basal ganglia via ZI. Since LTN lesions impaired acquisition of a HV discrimination, a task highly susceptible to the effects of geniculostriate ablation, it is suggested that such lesions impair the use of visual information without distroying the sensory channels mediating the discrimination.

Functional development of the dorsolateral prefrontal cortex: an analysis utlizing reversible cryogenic depression

The dorsolateral prefrontal cortex of rhesus monkeys was functionally inactivated by local hypothermia as the monkeys performed spatial delayed-response and spatial delayed-alternation tasks at different stages of postnatal development. Cryogenic depression of prefrontal cortex at a temperature sufficient to induce 21--25% decrements in delayed-response performance in 34--36-month-old-monkeys, produced deficits of only 7--8% in 19--31-month-old and no detectable loss in younger monkeys, 9--16 months of age. Delayed-alternation performance was impaired by local hypothermia as early as 8.5 months of age, but maximal cooling-induced deficits on this task were not observed before 33 months of age. Thermal gradients mapped in representative monkeys at different stages of development were remarkable similar, indicating that the age-dependent differences in behavior were not attributable to technical factors. The results obtained in the present study on normal developing monkeys confirm the interpretation of previous research on brain-damaged infants that functional maturation of the dorsolateral prefrontal cortex is protracted over several years of postnatal life, and extends the earlier studies by indicating that the lower limit for maturity of dorsolateral function is close to puberty in this species. Further, the present study revealed that delayed-response and delayed-alternation performance are dissociable dorsolateral functions which achieve maturity at different rates. The convergence of evidence from reversible neural depression and permanent lesion methods provides strong validation for neurobehavioral analysis as a general approach to the study of regional maturation of the brain.

Frontal decortication in rhesus monkeys: a test of the interference hypothesis

Four monkeys with dorsolateral frontal ablations and 3 unoperated controls were run on discrimination problems which sequentially presented both relevant and irrelevant visual stimuli prior to the opportunity for a choice response. As previously reported, monkeys with undamaged brains performed significantly better on those problems presenting relevant information first, being unaffected by later occurring irrelevant stimuli. Contrary to the behavior of the normal monkeys, monkeys with lesions of the dorsolateral frontal cortex were severely impaired when irrelevant stimuli were presented after the relevant stimuli had been processed, but before the opportunity to respond had occurred. In other words, the frontal monkeys performed just the opposite of the normals in these procedures, even though the relevant and irrelevant stimuli were manipulated within the usual temporal definitions of the two-choice discrimination trial. These data demonstrate that the presentation of irrelevant stimuli before the choice response can significantly impair frontally decorticated monkeys and that this impairment does not require the use of a long temporal delay preceding the opportunity to respond. For this reason the data were interpreted as rather direct support of the interference hypothesis of frontal dysfunction. On the basis of these and other data discussed, it was concluded that one function of the dorsolated frontal cortex involves the suppression of interfering stimulus events interposed between the information necessary to guide behavior and the behavior itself.

Efferent projections of the insular and temporal neocortex of the cat

Anterograde degeneration resulting from small lesions placed in either the insular or temporal cortex were traced with the Fink-Heimer reduced silver procedure. In neocortical regions ipsilateral to the lesion axonal degeneration was present in auditory subdivisions AI, AII, Ep, I, T, in the second somatosensory area (SII), in the anterior and middle suprasylvian gyrus, in the posteromedial suprasylvian and posterior lateral gyri, in the posterior splenial gyrus, in the anterior two-thirds of the cingulate gyrus and in the orbitofrontal regions. With respect to interhemispheric connections, evidence was obtained for a dual pattern of projection. In addition to significant amounts of axonal and terminal degeneration in the corresponding insular or temporal fields, axonal degeneration was also present in posterior AII. In the thalamus degeneration was found in the medial dorsal, suprageniculate, and lateral posterior-pulvinar nuclei. In the posterior nuclear group (Po) and the principal division of the medial geniculate (GMp) evidence was obtained for a topographic pattern of projection; significantly more degeneration occurred in caudal Po following insular lesions whereas with temporal lesions more degeneration occurred in caudal GMp. Degeneration was also found in the dorsal cortex of the ipsilateral inferior colliculus, bilaterally in the deep layers of the superior colliculus and the periventricular central gray region, ipsilaterally in the ventromedial aspects of the head and body of the caudate nucleus, and in the lateral and central nuclei of the amygdala. These findings are discussed in terms of their significance for a possible role for the insular and temporal neocortex (I-T) in both multimodal sensory discrimination and sensory-visceral integrative functions.

Effects of caudate nuclei removal in cats. Comparison with effects of frontal cortex ablation

A method is described for performing one-stage ablation of the caudate nuclei through a midline approach in cats. Ten bilateral acaudate cats (BAc), with an average of 84% of caudate tissue removed, were studied for a average of over 6 months (three, for over 1 year). In addition, 11 cats with unilateral caudectomy (UAc), with an average of 95% of the nucleus removed, 10 cats with bilateral removal of the frontal-cortical areas (BFr), and four sham-operated cats (ShO) were also studied. Most cats were implanted with recording electrodes in neocortical area, the hippocampus, the potine reticular formation, neck muscles, and orbits. The results of the gross behavioral, neurological, and polygraphic observations are reported. 1. A long-lasting, behavioral change labeled "compulsory approaching syndrome" (CAs) was observed following surtery in all BAc cats. This syndrome was characterized by stereotyped, "compulsory," and prolonged approaching or following of persons, cats, or objects apparently aimed at a physical contact with the approached target and persisting under unusual conditions...