Thalamic mediodorsal nucleus and memory: a critical evaluation of studies in animals and man

Hypotheses on mnemonic information processing by the brain

From a neuropsychological point of view, hypotheses are offered on the possible action of the brain in the processing of mnemonic information for long-term storage (or for retrieval of long-term stored information). It is argued that strict relations between damage of circumscribed brain structures and amnesia, as they have been suggested in recent case reports, are questionable for several reasons: Firstly, the involved regions differ between cases; secondly, there is some counter-evidence from other cases in which similar neuronal damage failed to result in lasting amnesic disturbances; thirdly, it is hypothesized that even from circumscribed brain damage it is not justifiable to conclude that the lesioned structure is solely or principally responsible for the observed mnemonic changes, as the brain acts in an integrative way, that is, on the basis of a wide-spread network of neuronal information processing. On the basis of these and related arguments, hypotheses and models on mnemonic information processing in the intact and in the damaged brain are derived. With these hypotheses even the frequent observation of interindividual differences in mnemonic information processing finds a possible explanation which is in conformity to known anatomical circuits and connections and to principles of neuronal coding.

Amnesia in monkeys after lesions of the mediodorsal nucleus of the thalamus

Recent successes in developing an animal model of human amnesia in the monkey have made it feasible to try to identify with certainty the specific structures in the diencephalon and medial temporal region that cause amnesia when damaged. Monkeys with small lesions restricted largely to the posterior portion of the mediodorsal nucleus of the thalamus were given a test of memory sensitive to human amnesia and a second test that is analogous to the skill-based tasks performed normally by amnesic patients. The monkeys exhibited a marked impairment on the first test and performed normally on the second. The results show that circumscribed lesions of the mediodorsal nucleus can cause substantial amnesia.

Alcoholic Korsakoff's syndrome: some unresolved issues concerning etiology, neuropathology, and cognitive deficits

Recent neuropsychological and neuropathological investigations with long-term alcoholics suggest that the etiology and neuropathology of the Wernicke-Korsakoff syndrome are more complex than previously believed. While problem solving and visuoperceptual deficits seem to develop slowly during decades of alcoholism, the amnesic symptoms associated with Korsakoff's syndrome may appear acutely when severe malnutrition and alcoholism are combined. Furthermore, the report that alcoholic Korsakoff patients, like patients with Alzheimer's Disease, have endured a substantial neuronal loss in the nucleus basalis of Meynert has questioned the role of the medial diencephalon in the alcoholic patients' amnesic syndrome. Some initial demonstrations of similarities in the memory disorders of alcoholic Korsakoff and Alzheimer patients indicate that Korsakoff's syndrome may be more accurately characterized as a "basal forebrain" than as a "diencephalic" amnesia.

The amygdala's role in human mnemonic processing

The possible role played by the human amygdaloid complex in the processing of mnemonic information is examined. First, evidence is reviewed from case reports in which amygdaloid damage occurred due to surgical intervention or pathological or age-related changes. Then, studies are evaluated in which the amygdala was stimulated or in which electrical potentials were recorded from it. Based on this survey an hypothesis on the possible involvement of the amygdala in mnemonic information processing is proposed. In essence, it is argued that the human amygdala is responsible for activating or reactivating those mnemonic events which are of an emotional significance for the subjects' life history and that this (re-)activation is performed by charging sensory information with appropriate emotional cues. Supportive evidence for this hypothesis is given based on human case reports, on studies in animals in which information processing was determined following amygdaloid lesions, and on evidence of neuroanatomical connections of the primate amygdala.

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.

Different emotional tones significantly affect recognition performance in patients with Korsakoff psychosis

The performance in reidentifying photographs was measured in alcoholic Korsakoff patients, in non-amnesic alcoholics and in a control group. The photographs showed well-known and unknown faces and buildings and alcoholic and nonalcoholic beverages. A minimum of 15 min elapsed between the end of the original presentation of the series and the beginning of its second presentation within a large number of additional photographs. Korsakoff patients were significantly inferior in number of correctly reidentified items compared to alcoholics who were in turn significantly inferior to the control group. Korsakoff patients manifested the poorest performance in the reidentification of unknown pictures and of nonalcoholic beverages; they showed, however, comparatively good performance in the reidentification of alcoholic beverages and of known faces. Independent of the category of the stimulus material, the Korsakoff patients had low confidence in their responses. It is concluded from these data that the emotional tone of material to be remembered constitutes a major variable for delayed effective retrieval.

Amnesic syndrome: a lesion-specific disorder?

This paper reviews various lines of evidence which suggest that organic amnesia stemming from lesions of the temporal lobe region produce an amnesia that is qualitatively different from that produced by diencephalic lesions. Differences between these two classes of amnesia were found within five dimensions of performance; (a) insight, concern and confabulation, (b) retrograde amnesia, (c) forgetting rate, (d) frontal lobe symptoms, (e) sensitivity to interference in short term memory. The range of differences found suggest that temporal lobe and diencephalic amnesics should not be considered as suffering from the same type of "amnesic syndrome". It is proposed that future experimental work on amnesia should take full account of neuropathological differences between amnesic patients.

Two neuronal systems are involved in a classical conditioning in the rat

Unit activity of the hippocampus, the centrum-medianum-parafascicularis and medialis dorsalis nuclei of the thalamus, was recorded in chronic rats during a classical conditioning; neocortical electroencephalographic and somatic responses were also recorded. Conditioned unit responses of the different groups of neurons were compared according to the precocity of their appearance, their stability, their latency and the differentiation between the positive (reinforced) and the negative (non-reinforced) conditioned stimuli. Conditioned unit responses of type I hippocampal neurons (probably pyramids) and of neurons located in the centrum-medianum-parafascicularis nucleus did not differentiate between the positive and negative conditioned stimuli; they progressed rapidly, then declined and disappeared. They were contemporary with an initial conditioning stage which was characterized by undifferentiated arousal responses (desynchrony of electroencephalographic activity) to both conditioned stimuli. Conditioned unit responses of type II hippocampal neurons (probably granule cells or interneurons of the fascia dentata) and of neurons located in the medialis dorsalis nucleus progressed slowly and differentiated between the conditioned stimuli during a late conditioning stage which was characterized by the regression of arousal responses and the differentiation of somatic responses. These data strongly suggest that two neuronal systems, each having a different role, are involved in classical conditioning in the rat.

Comparative effects of long-term ethanol consumption and forebrain lesions on maze learning and active avoidance behavior in rats

Male rats consumed a liquid diet containing 10.7% ethanol as their only source of food and fluid for 6.5 months, beginning at 2 months of age. During withdrawal, there were no differences between the alcohol group and their pair-fed or free-fed controls on EEG, body temperature, irritability and tremor measures. In behavioral tests begun 4-5 weeks after withdrawal, the rats that had consumed alcohol acquired accurate spatial behavior in a cross maze task more slowly than controls, but were unimpaired in shuttle-avoidance learning. In concurrent studies with groups of rats that had sustained lesions of the dorsal hippocampus, the mamillary bodies (MMB), or the mediodorsal thalamus, the pattern of behavioral deficits after MMB lesions was found to be qualitatively similar to that observed after the cessation of long-term alcohol consumption. These findings provide renewed hope that a useful rodent model for studying the neuropsychology of cognitive deficits associated with human alcoholism can be developed.

3H-2-deoxyglucose uptake after electrical stimulation of cardioactive sites in anterior medial cortex in rabbits

The anterior medial cortex is an important integrative area for cardiovascular adjustments occurring during learning and conditioning. The autoradiographic 3H-2-deoxyglucose (3H-2DG) method for regional cerebral metabolic activity was used to identify other forebrain regions associated with cardiovascular adjustments elicited by electrical stimulation of anterior medial cortex. Rabbits that received anterior midline stimulation that produced bradycardia and depressor responses showed increased metabolic activity in the ipsilateral mediodorsal (MD) nucleus of the thalamus and the dorsal aspect of the claustrum. Two additional animals with anterior cortical placements in the infralimbic and anterior limbic areas showed ipsilateral increased activity in perirhinal cortex, but more dorsolateral placements in the precentral agranular area did not produce increased perirhinal activity. Control animals did not show this pattern of activity. These data suggest that MD and claustrum participate in a neural circuit which mediates cardiovascular adjustments similar to those elicited by Pavlovian conditioning contingencies.

Collateral innervation of the medial and lateral prefrontal cortex by amygdaloid, thalamic, and brain-stem neurons

The distribution of the afferents to the rat's prefrontal cortex originating in the thalamic mediodorsal nucleus and the amygdala was investigated with two fluorescent tracers. Special emphasis was laid on detecting the loci of neurons which project via axonal collaterals into both lateral and medial portions of the prefrontal cortex. It was found that a high number of neurons of the anterior portion of the basolateral amygdaloid nucleus terminate via collaterals in both the medial and lateral subfields of the prefrontal cortex. On the other hand, only a small number of mediodorsal thalamic cells were found to project to both sides of the prefrontal hemisphere via bifurcating axonal collaterals. These cells were situated exclusively in the lateral part of the medial segment of the mediodorsal nucleus. The majority of both thalamic and amygdaloid neurons with bifurcating axons originate from subregions whose cells innervate primarily the medial prefrontal cortex. In brain-stem, neurons of the nucleus raphé dorsalis also project via collaterals to the medial and lateral prefrontal regions. Furthermore, neurons of the dorsal and ventral premamillary nuclei, the lateral mamillary nucleus, the ventral tegmental area of Tsai, and the ventral tegmental nucleus of Gudden were found to project to the medial prefrontal cortex. Our results indicate a differential collateral organization of thalamic and amygdaloid afferents to prefrontal cortical fields. The anterior basolateral amygdala (which innervates via collaterals both the medial and lateral prefrontal subfields) may add a common input to either subfield, such as information on the significance of incoming stimuli to the animal's behavior, while the mediodorsal nucleus (whose segments are principally connected to only one prefrontal subfield) may add segment-specific information, for example, of a spatial-cognitive nature for the lateral segment and of an emotional nature for the central and medial segments. The existence of a basolateral limbic circuit, composed of the amygdala, the thalamic mediodorsal nucleus, and the prefrontal cortex, is confirmed and knowledge on its interconnectivity is extended. From an anatomical point of view these data provide arguments for both unitary and diverging functions of the prefrontal cortex.

Can amnesia be caused by damage of a single brain structure?

The relation of amnesia and damage to some particular brain regions is discussed by reviewing the main findings of selected human case reports. It is argued that frequently a too straightforward and unidimensional interpretation of the relations between brain damage and a behavioral deficit is formulated in such reports. Evidence obtained by modern anatomical techniques as well as the widespread and time-dependent effects of lesions make it necessary to consider a lesion of a particular structure of the brain and correlated mnemonic disturbances as possibly due to an altered equilibrium in an extensive network of the brain. The primary lesion of one or the other specific structure may lead to severe and lasting amnesia or may fail to do so depending on its influence on other brain regions.

Projections of the amygdala to the thalamus in the cynomolgus monkey

The projections of the amygdala to the thalamus in cynomolgus monkeys (Macaca fascicularis) were studied with both anterograde and retrograde axonal tracing techniques. Horseradish peroxidase (HRP) was injected into medial and midline thalamic sites in five animals, and tritiated amino acids were injected into selected amygdaloid regions in a total of 13 hemispheres in ten animals. The findings from the two types of tracer experiments demonstrated the origins, course, and terminal pattern of amygdaloid projections to two thalamic nuclei--medialis dorsalis (MD) and reuniens. Almost all of the amygdaloid nuclei contribute projections to MD, though the greatest proportion arise from the basal group and terminate in discrete, interlocking patches within the medial, magnocellular portion of MD. In addition to this major projection, the central and medial amygdaloid nuclei send a lighter projection to the lateral portion of nucleus reuniens. The amygdalothalamic projections took a variety of routes out of the amygdala before the large majority joined the inferior thalamic peduncle and entered the rostral head of the thalamus where they turned caudally toward their targets. A small number of amygdalothalamic fibers may also run in the stria terminalis.

Role of the caudate nucleus in recovery from neonatal mediofrontal cortex lesions in the rat

Ablation of medial prefrontal cortex impairs spatial discrimination learning in adult but not in neonatally lesioned rats. Orbital prefrontal cortex and adjacent convexity neocortex need not be left intact to observe this sparing of function. This study examined the possibility that the caudate nucleus, remaining intact after early medial prefrontal cortex lesions, might be involved in the observed behavioral sparing. Neonatal rats given combined lesions of the medial prefrontal cortex and head of the caudate nucleus were compared to age/litter-matched sham-operated controls on spatial alternation and place response acquisition and "reversal" tests. The results show that the performance of these neonatally lesioned subjects was deficient on both tests. The discussion centers on possible recovery mechanisms in rats given prefrontothalamic system damage early in life.

Lesions of the mediodorsal thalamic nucleus do not change thresholds for hypothalamic defensive attack in cats

Electrolytic lesions of the mediodorsal thalamic nucleus (nMD) were made in 10 cats to evaluate their effects on thresholds for hypothalamic defensive attack and emotional reactivity to noxious stimuli. Only 2 cats showed minimal threshold changes after the lesions, one of which was accompanied by an increased emotional reactivity to noxious stimuli; however, this was attributed to damage to the fornical column. The nMD appeared not to be involved directly in the central mechanism of defensive attack.

Widespread neuroanatomical damage and learning deficits following chronic alcohol consumption or vitamin-B1 (thiamine) deficiency in rats

Consequences of long-term consumption of alcohol (20 months) and of pyrithiamine-induced blockade of vitamin-B1-uptake on the shape of individual brain structures and on the acquisition of two learning tasks have been investigated in 3 groups of rats (alcohol group, AL; thiamine-deficient group, TH; control group, CG). Groups AL and TH wee allowed an 8, or 3 week recovery period, respectively, with normal food and water available ad libitum before behavioral testing started. This consisted of training an active two-way avoidance task and a spatial reversal task. Rats of both experimental groups were, compared to rats of the control group, significantly impaired in acquiring the avoidance task and in acquiring the original discrimination of the spatial reversal task. No differences were found among the two experimental groups. Histological and microscopical examinations of the brains of the rats with a history of thiamine-deficiency or of chronic alcohol consumption revealed a variety of severely affected brain areas. In both groups hippocampal and cerebellar damage was prominent. Furthermore, the mamillary nuclei, certain brainstem regions situated around the ventricles and a few cortical areas contained loss or damage of neurons. It is concluded that the anatomical changes, especially, can be related to those seen in chronic alcoholics and that consequently animal models can be established to investigate in detail the multiple interactions of alcohol consumption, thiamine deficiency, brain damage and behavioral deterioration.

Convergence of basolateral amygdaloid and mediodorsal thalamic projections in different areas of the frontal cortex in the rat

The extent of convergence of mediodorsal thalamic and amygdalar afferents on the rat's frontal cortex was studied by tracing retrogradely labeled cells following injections of horseradish peroxidase (HRP). HRP was applied iontophoretically in extremely small injections throughout all areas of the frontal cortex. The following organization was revealed: Converging inputs from the mediodorsal nucleus and the amygdala are observed in the posterior parts of the pre- and infralimbic areas, in the posterior half of the dorsal and ventral agranular insular areas and in the lateral and dorsal precentral areas. Both mediodorsal and amygdaloid afferents reach the dorsal tip of the frontal cortex. Only the mediodorsal afferents were found to terminate in the anterior parts of the pre- and infralimbic areas and in the anterior part of the dorsal division of the anterior cingulate area and in the medial precentral area. On the lateral side of the hemisphere the anterior halves of the dorsal and ventral agranular insular areas receive mediodorsal afferents. Amygdaloid, but not mediodorsal afferents, were found following injections into the more posterior parts of the lateral precentral area. These results are discussed with respect to the extent of the prefrontal cortex in the rat and its definability as a target area of subcortical nuclei. Functional aspects of the anatomical convergence of connections within the so-called basolateral limbic circuit are outlined.

Transient global amnesia

The syndrome of transient global amnesia (TGA) is defined and described. Characteristic features, epidemical data, variables possibly provoking TGA, its possible etiology and anatomical basis are reviewed. A transient disturbance in the formation of lasting new memories (usually of less than one day) and a retrograde amnesia (which includes the period of the attack and possibly a short time before) are considered as the main features of TGA. A further trait of TGA is the high age of most of the patients subjected to it (58 years on the average). TGA most likely is based on a transient change in the blood supply of certain regions of the brain. The mechanism by which this change happens is still hypothetical. Regions of the limbic system, in particular the area of the temporal lobe and the hippocampal formation, appear to be affected most likely. Uncertain are: the existence of factors provoking the outbreak of an amnesic attack, the likeliness of multiple episodes of TGA and the neuropsychological alterations in patients who suffered a transient global amnesic attack. The outcome of this review suggests, however, that the incidence of recurrent attacks of TGA is higher than previously assumed and that a transient global amnesic attack may be followed by lasting behavioral deteriorations.

Widespread cortical projections of the hippocampal formation in the cat

Efferent projections from the hippocampal formation to the cat's cortex were traced with the retrograde horseradish peroxidase technique. Different areas of the cortex of 31 cats were injected with small amounts of horseradish peroxidase. All subregions of the hippocampal formation were screened for labeled cells. It was found that, with the exception of the entorhinal injections, only subicular areas of the hippocampal formation contain labeled neurons. When HRP was injected into the entorhinal cortex, labeled cells are also found in the hippocampus proper. The most dense projection from the subicular cortex is directed to the medial part of the cortical hemisphere. Here, cingulate, retrosplenial and medial prefrontal fields receive a substantial number of subicular efferents. Furthermore, the entorhinal cortex is reached by a number of axons originating in the subicular area. Scarce projections from the subicular cortex terminate in the dorsal prefrontal, temporal, parietal and prepiriform cortex. It is suggested that the projection from the subicular cortex to the neocortical areas of the frontal pole (medial prefrontal cortex) is of special importance as it may constitute a link between the association areas of the neocortex and those regions of the limbic system thought to play a role in memory (subicular cortex, mamillary bodies, anterior thalamus, cingulate gyrus).

The thalamic mediodorsal nucleus receives input from thalamic and cortical regions related to vision

The mediodorsal nucleus of the thalamus (MD) so far has been regarded as being not closely connected to visual regions. Based on the method of retrograde transport of horseradish peroxidase, direct efferents to the cat's mediodorsal nucleus were demonstrated from two visual regions: from the ventral lateral geniculate nucleus and from parts of the visually responsive cortex. These projections were obtained following injections which covered most of MD, but also following injections which were restricted to medial or lateral parts and to the anterior two-thirds or merely to the center of MD. Projections from the ventral lateral geniculate nucleus arose mainly from its caudal portion; projections from the visually responsive cortex originated predominantly in area 20a, but labeled cells were also occasionally detected in areas 18 and 19.

Single and combined lesions of the cats thalamic mediodorsal nucleus and the mamillary bodies lead to severe deficits in the acquisition of an alternation task

The acquisition of a spatial alternation task was tested in five groups of adult cats. Two groups were used for control; one of them consisted of nonlesioned cats, the other of cats with chemical lesions (ibotenic acid) of the ventral tegmental area. The other three groups of cats received chemical lesions of the mediodorsal nucleus of the thalamus, the mamillary bodies, or of both structures together. Cats with lesions of the ventral tegmental area were non-significantly impaired in comparison to non-lesioned cats in the acquisition of the task. Cats with lesions of the mediodorsal nucleus, the mamillary bodies, or with combined lesions of both structures were significantly impaired, compared to the two control groups; among each other, however, their performance rates were similar. As none of the cats with single or combined lesions of the mamillary and the mediodorsal nuclei manifested observable abnormalities with respect to motor, motivational, or emotional behavior during their testing period, it is concluded that their--compared to the other two groups--inferior performance was due to a memory defect. As all three kinds of lesions resulted in a similar defect, it is suggested that the mamillary and the mediodorsal nuclei transmit information to one or more common target area(s), which is (are) dependent on information from either nucleus to operate effectively.