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Biomedical subjects

L Weiskrantz

Publications and source records attributed to L Weiskrantz.

At least 55 records · Page 3Linked to original sources
PubMed1987

Residual vision in a scotoma. A follow-up study of 'form' discrimination.

This study is a follow-up of a patient, D.B., who was reported (Weiskrantz et al., 1974) to be able to discriminate between simple visual forms within the scotoma caused by a lesion in calcarine cortex. Among other capacities, he was able to discriminate between lines of different orientation in the frontal plane. Given the reported deficits for form discrimination but a high sensitivity for orientation discrimination in primates without striate cortex, the question arises whether D.B.'s apparent 'form' discrimination arises from an ability to discriminate the orientations of components of the figures. It is shown in the first experiment, by using the optic disc as a control, that his ability to detect stimuli in his scotoma cannot be due to stray light falling upon the intact field. Next, his ability to discriminate orientations is confirmed, and an orientation threshold determined. A range of form discriminations is presented varying in degree of orientation cues of their components. It is confirmed that he can discriminate those forms originally studied, in which such differences are large, but not when orientation cues are small or minimal. Finally, his ability to compare two forms both projected within his scotoma is examined. Even when the components of the two forms have large orientation differences and are highly discriminable when presented successfully, D.B. appears to be unable to make a 'same-different' comparison when both are presented simultaneously. The evidence is interpreted against D.B.'s having a residual capacity for form discrimination.

Follow-Up Studies↗
PubMed1987

Residual tactile sensitivity with self-directed stimulation in hemianaesthesia.

A case of hemianaesthesia caused by right hemisphere stroke was found to have good sensitivity and cutaneous localisation when she touched her impaired hand with her good contralateral hand or with a probe held by that hand. The capacity did not depend solely upon movement or positioning of her right arm, and she could also discriminate whether her own left fingers, as opposed to someone else's inter-digitated fingers, were being touched, but only when she did the touching. Thus, the threshold of the impaired hand appeared to be markedly affected by active involvement of the contralateral limb in delivering the stimulus.

Cerebral Cortex↗
PubMed1987

Neuroanatomy of memory and amnesia: a case for multiple memory systems.

Several different dissociable memory systems are considered in terms of their known or putative sites of pathology. In man, short-term memory can be selectively impaired by left hemisphere lesions near the acoustic projection area. There may be other modality-specific deficits. Long-term memory is considered in terms of a) knowledge systems, b) stable associative memory; c) priming, and d) event memory, with special reference to the amnesic syndrome. For each of these categories, clinical neuropathological and relevant animal evidence is reviewed. It is concluded that "memory" is not holistic, and the anatomical bases must be considered independently for each of the several dissociable (but normally interacting) systems. Some are likely to involve virtually all levels of the nervous system, while others are highly specific.

Amnesia↗
PubMed1985

Introduction: categorization, cleverness and consciousness.

Various recurring themes in the history of the subject are reviewed. In the context of adaptation to a complex environment, one precondition for survival must be a capacity for object identity, which may be the most basic form of categorization. Evidence will be presented that suggests that the capacity is not learned. In considering learned associations among categorized items, a distinction is made between reflexive and reflective processes: that is between those associations in which a cue or signal provides an unambiguous route to the response, no matter how complex that route may be, in contrast to those in which learned information must be ordered and reordered 'in thought'. An example of one experimental approach to the latter is provided. Finally, the problem of conscious awareness is considered in terms of stored categorical knowledge and associations, on the one hand, and a system that monitors them, on the other. Neurological evidence of disconnections between these different levels is reviewed.

Animals↗
PubMed1984

Hemispheric asymmetry for a visuo-spatial task in monkeys.

The hypothesis that hemispheric asymmetries for visuo-spatial ability exist in rhesus monkeys was tested using a task for which the right hemisphere is specialized in man. The task required discrimination between two squares, one containing a central dot and the other a dot displaced upwards from centre. The animals' thresholds were determined before and after left-sided (LH-lesion) or right-sided (RH-lesion) unilateral occipital lobectomy combined with splenial transection, and the percentage change in threshold calculated. The LH-lesion monkeys were all worse than the RH-lesion monkeys on this measure, indicating the existence of a functional asymmetry.

Animals↗
PubMed1984

Impairments of visual object transforms in monkeys.

Monkeys were first trained on a series of standard, unchanging visual objects to obtain food reward. After reaching criterion on each subject, training with it continued but on 20 per cent of the trials the object was transformed in (1) size, (2) orientation, or in (3) its shadow configuration. Monkeys with lesions either of the inferotemporal cortex or the foveal prestriate region were impaired in identifying all three types of transforms as the rewarded object. They were also slightly impaired in continuing postcriterion high levels of performance on the nontransformed objects, but it was shown that the transform deficit was still present when this factor was partialled out. The groups with transform deficits were also impaired initially in their discrimination between solid cones of varying apical angle, although their final performance approached that of controls. The inferotemporal group, especially with the lesion placed anteriorly, had a deficit in their initial learning of the untransformed objects as well as in the retention of a preoperatively learned pattern discrimination; they were also deficient in selecting food objects from among an array of nonfood objects. No deficits were seen in animals with lesions of the parietal lobe or the fundus of the superior temporal sulcus in any task. The results and other relevant animal evidence are interpreted in terms of the view that the anterior temporal lobe is concerned with the storage of an object-centred prototype, necessary in turn for the attachment of positive or negative meaning, and the more posterior inferotemporal lesions with the addressing of view-centred visual information en route to the anterior regions. They are also consistent with the findings on the recognition of 'unusual views' made by Warrington and coworkers in patients with posterior lesions.

Animals↗
PubMed1982

Comparative aspects of studies of amnesia.

In recent years important advances have been made in reconciling some of the conflicting evidence regarding the contribution of the medial temporal lobe--hippocampal structures to long-term memory in man compared with laboratory animals. Despite the severe amnesic state that is seen clinically in patients, it has nevertheless emerged that both in animals and man damage to the structures leaves learning and retention of certain types of long-term memory tasks intact. The evidence from man suggests that in the amnesic syndrome the integrity is preserved of those forms of long-term memory that do not depend on the operation of a 'mediational' memory system. In particular, items stored in semantic memory can be facilitated by repetition, and simple associations can be formed if no mediating links are required, but impairments are seen in tasks in which memory depends upon the stored benefits of matching, reordering and comparing. A similar characterization seems possible for the results of animal studies. One interpretation of the differential sensitivity of memory tasks in the amnesic syndrome is in terms of a disconnection syndrome which a semantic memory system is detached from a mediational system. The disconnection is postulated to be caused by interruption of those temporal-frontal pathways in which pathology has been found in the brains of amnesic patients, namely the mammillary bodies and the subependymal zone of the thalamus.

Amnesia↗
PubMed1982

Amnesia: a disconnection syndrome?

An attempt is made to account for the ability of severely amnesic patients to learn and retain certain tasks but not others. The hypothesis is advanced that the amnesic subject can show learning through facilitation by repetition or of simple S-R relationships not requiring cognitive mediation. Three examinations are reported. The first demonstrates that the amnesic subjects have adequate speed of access to semantic and phonological knowledge and show the same degree of improvement on retesting as controls. The second and third experiments examine different degrees of cognitive mediation in paired-associate learning. Amnesic subjects are differentially impaired relative to controls on "distant" pairs and on those that, by hypothesis, would benefit from cognitive mediation. The results are discussed in terms of a disconnection syndrome. It is suggested that at the functional level, a "cognitive mediational memory system" and a semantic memory system are disconnected in the amnesic patient. Neuropathological evidence suggests that temporal lobe and frontal lobe structures are disconnected by pathways in the fornix-mammillary body route.

Adult↗
PubMed1980

Recency effects and lesion effects in delayed non-matching to randomly baited samples by monkeys.

Monkeys' memory for 3-dimensional junk objects was tested in a task which required them to discriminate objects which had recently been presented in an acquisition list from new objects which had not. The accuracy of this discrimination was shown to depend on the recency of presentation of the old object. The discrimination was not significantly affected by lesions of either the foveal pre-striate cortex, the fundus of the superior temporal sulcus, or the anterior inferior temporal area. A substantial impairment was caused by transection of the fornix.

Animals↗
PubMed1979

Memory disorder in Korsakoff's psychosis: a neuropathological and neuropsychological investigation of two cases.

Neuropathological findings in the brains of two alcoholic patients with Korsakoff's psychosis are reported. Their memory defects had been studied in detail quantitatively over a period of nine years in one case and three years in the other, relevant details of which are presented. Both patients had had a relatively pure long-term memory impairment in the absence of other cognitive deficits and in the absence of a short-term memory impairment. Their retrograde amnesia for public events and famous faces had been measured and found to have extended backwards over at least twenty-five years. There was severe impairment in anterograde recognition memory for both verbal and non-verbal material. On a newly prepared memory quotient battery both patients had scored well below the bottom of the normal scale (less than 60, where 100 is the mean with a standard deviation of +/- 15). Both patients had also shown the characteristic differential improvement in retention when tested by cued recall and also the characteristic 'prior learning effect', i.e. normal retention of one list of words when tested by cued recall but impaired retention of a second list sharing the same cues as the first list. There had been a slight but significant deterioration in intelligence in one of the patients in the two years prior to his death, although his IQ still fell within the normal range. The other patient remained undeteriorated until his death, and his IQ also was close to an estimated measure of his premorbid IQ. In the brains of both patients there was marked gliosis, shrinkage and discolouration bilaterally in the medial nuclei of the mammillary bodies. In addition there was a thin band of gliosis bilaterally between the wall of the third ventricle and the medial dorsal nucleus, the rostral limit lying anterior to the medial dorsal nucleus. In the patient with no intellectual deterioration these were the only pathological changes that were seen. In neither patient was there evident local loss of nerve cells, gliosis or any other qualitative evidence of abnormality in the hippocampi, the white matter of the temporal lobes or the greater part of the medial dorsal nuclei, although it is difficult to be certain whether there was any overlap between the band of gliosis and the most medial region of the medial dorsal nueleus and other adjacent thalamic nuclei. In the other patient there was also a small zone of softening in the cerebellum and an increase in astrocytes in other regions of the cerebral hemispheres, including the basal ganglia, amygdala, and brain-stem, but without noticeable loss of cells. The question of the minimal lesion for the alcoholic Korsakoff amnesic state, and some aspects of the related anatomy, is discussed in the context of other reports in the literature which are, however, difficult to assess in the absence of details of the specificity, severity and character of the memory disorders.

Aged↗
PubMed1977

A comparison of hippocampal pathology in man and other animals.

Bilateral damage to the medial temporal lobe, including the hippocampus, in man is associated with a severe amnesic syndrome. It is still not clear whether the hippocampus (or its output pathways and related target projection sites) is the critical structure in producing this syndrome, especially as more severe learning deficits in animals are found with lesions in the anterior inferotemporal cortex than with hippocampal lesions per se. But the problem of trying to relate memory deficits in man and animals depends on the characterization of the amnesic syndrome itself. It was originally thought to be a failure of input into long-term memory store or a failure of consolidation. Medial temporal and hippocampal lesions in animals do not produce results that fit such a characterization. On re-examination of the human syndrome, however, for which some of the evidence is reviewed, it appears that the amnesic patients can learn and remember over long intervals if certain testing paradigms are used. The results are more readily matched to some of the results of hippocampal lesion studies in animals. Two main classes of current theories of the amnesic syndrome are discussed. A somewhat different approach is suggested here, based on the dissociation between human amnesic subjects' commentaries and their objective performance, which suggests a dissociation between levels of processing rather than a failure on any particular level.

Alcohol Amnestic Disorder↗