The prefrontal cortex of a prosimian (Galago senegalensis) is reached by efferent neurons originating in the nucleus basalis of Meynert

Cortical and subcortical afferent connections of the squirrel monkey's (lateral) premotor cortex: evidence for visual cortical afferents

The afferent connections of the squirrel monkey's lateral premotor cortex were investigated with the horseradish peroxidase (HRP) technique. Seven monkeys received small, unilateral injections of HRP spaced between the most dorsal and the most lateral region of the premotor cortex. Results are based on six brains, all of which showed a considerable number of labeled neurons both cortically and subcortically. At the cortical level, labeled cells were found both ipsi- and contralateral to the injection site. Heavy labeling occurred in the fields surrounding the prefrontal, central, and precentral cortex, while the temporal regions usually innervated the premotor cortex with a moderate number of cells, and the parietal fields with only a minor number. Surprisingly, in all brains, but especially in those with the more lateral injections, we also found a considerable number of afferents originating from occipital fields, including a few neurons from the striate cortex. Subcortically, a few labeled cells were seen in basal forebrain regions, the amygdala, and the claustrum; a considerable number of thalamic nuclei and also some hypothalamic nuclei contained labeled cells; and within the brainstem, labeling was found most consistently in the locus ceruleus and in the tegmental fields. It is concluded that the (lateral) premotor cortex is involved in sensorimotor integration to a higher degree than has been recognized up to now.

Memory related role of the posterior cholinergic system

In order to establish a model for the possible neuropathology of patients with Alzheimer's disease, various behaviors of rats with different chemical lesions in cholinergic regions were studied and compared with those of sham-operated control rats. A battery of neurological tests was used as well as activity measurements and two learning tasks: a positively reinforced place-learning task with delay periods of Os, 1 min, 15 min, and 2h, and a shock-motivated two-way active avoidance task. While in general no intergroup differences were obtained in performance on the neurological test battery or the rats' activity in an open field, there were marked impairments in the three lesioned groups compared to the control group in the two learning tasks. These deficits were less severe in the two groups with lesions of the medial septal/vertical diagonal band of Broca region and the nucleus basalis of Meynert region, but rather marked in the group with lesions of neurons situated in the pontomesencephalic region, although the amount of ibotenic acid injected had been the same for all groups. We conclude from these data that changes in mesencephalic cholinergic regions might play a significant role in the pathology of Alzheimer's disease. The existence of such regions was recently established for primates as well, thus providing a basis for justifying an animal model of this human disease.

Cortical and subcortical afferent connections of the primate's temporal pole: a study of rhesus monkeys, squirrel monkeys, and marmosets

The afferent connections of the primate's temporopolar cortex were investigated with the retrograde horseradish peroxidase technique. Old World and New World monkeys received small unilateral injections of horseradish peroxidase. These labeled cells in a number of cortical, thalamic, and brainstem regions and in a few further telencephalic and diencephalic regions. Cortically, the neighboring areas of the inferior and superior temporal gyrus and the insula contained a considerable number of labeled cells. Furthermore, a substantial projection arose from the orbitofrontal and the frontopolar cortex. The cingulate gyrus contained only very few labeled cells. Interhemispherically, corticocortical connections arose mainly from temporal lobe areas. Labeled cells were seen in various regions of the basal forebrain and cells labeled only faintly in the lateral and basal accessory nuclei of the amygdala. The claustrum contained labeled neurons only in one rhesus monkey. On the diencephalic level, the caudal medial portion of the medial pulvinar was the principal thalamic source of afferents to the temporopolar cortex. Furthermore, labeled cells were found in the neighboring, caudal part of the mediodorsal nucleus, within and along the nucleus limitans, in the medial geniculate nucleus, and in several nuclei of the nonspecific system. The fields of Forel, the zona incerta, and lateral and dorsomedial hypothalamic areas contained a few labeled cells. Within the brainstem of the rhesus monkeys those regions projecting diffusely to the cortex contained a few labeled neurons. Furthermore, these brains had some labeled cells in the regions of the nuclei medialis annuli aqueductus, tractus mesencephalicus nervi trigemini, and trochlearis. Although among the three species differences in the cortical and thalamic projection patterns were observed, the regions projecting most densely to the temporal pole were similar in principle. This statement holds in particular for cortical and thalamic sites. However, the greatest number of labeled cells was found in the rhesus monkey, a fact that cannot be attributed solely to the size of the horseradish peroxidase injections and the size of the brain, but that appears rather to represent a true species difference. From our results we conclude that the temporopolar cortex constitutes a cortical area necessary for effective affectional-sensory integration.

The neuropathology of amnesia

Relations between brain damage and memory disturbance are outlined with emphasis on the so-called amnesic syndrome. Following a brief introduction into forms of memory and memory failures, the basic causes of brain damaage (with relevance to amnestic failures) are described. Thereafter, the two best-known forms of brain damage-amnesia relations are reviewed: the consequences of damage to medial temporal lobe structures and to diencephalic regions. For the cases with medial temporal lobe damage, evidence is reported in greater detail for H.M., who has been examined more than any other amnesic patient for more than 30 years now, as a considerable amount of literature has accumulated on his behavior in diverse situations. Other cases with more or less circumscribed damage to medial temporal lobe structures are reviewed so as to outline criteria for or against the hypothesis that there are regions within the medial temporal lobe whose damage might be critical for the amnesic syndrome. Two cases of diencephalic amnesia are summarized in particular (cases of Mair et al., 1979) as they have received extensive neuropsychological and neuropathological investigation. Other cases with, for example, Korsakoff's disease are reviewed, as well as cases with diencephalic, or combined mesencephalic-diencephalic damage without nutritional causes. A third group of patients with massive, but still selective amnesic disturbances are then described: cases of basal forebrain damage, followed by descriptions of Alzheimer's disease which has similarities in the underlying neuropathology. This leads over to cases with more generalized intellectual deteriorations (dementia), which may have developed on the basis of primarily cortical damage or damage principally to basal ganglia structures. After reviewing cases with mainly material-specific memory failures--usually as a consequence of restricted neocortical damage--a separate section follows on patients in whom retrograde amnesia is the prominent symptom. The contribution of animal models of human amnesia is critically reviewed and discrepancies are analyzed between human and animal memory disturbances. This section emphasizes the value of investigating inter-dependencies between brain structures by pointing out that relations between memory disturbances and brain damage may be more complicated than apparent from a simple structure-function assignment. This aspect is further followed up in the conclusions.