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Biomedical subjects

J Sergent

Publications and source records attributed to J Sergent.

At least 37 records · Page 2Linked to original sources
PubMed1989

Mental rotation in a commissurotomized subject.

The commissurotomized subject L.B. showed a strong right-hemispheric advantage on a task requiring him to judge rotated letters normal or backward, but a left-hemispheric advantage in a task requiring discrimination of the same letters, implying that the right-hemispheric advantage has to do with mental rotation. However, his right hemisphere proved unable to perform a task requiring mental rotation of a simple unfamiliar pattern to a designated "upright", and was inferior to the left on a task requiring two such patterns, at varying orientations relative to one another, to be judged same or different. We suggest that the deficiency of the right hemisphere on these tasks was unrelated to mental rotation per se, but reflected a failure to comprehend the tasks properly.

Adult↗
PubMed1989

Hemispheric specialization for mental rotation.

A sample of 133 normal subjects, and one commissurotomized subject, were given a "mental-rotation" task, in which they were timed as they decided whether rotated letters, flashed in the left or right visual hemifield, were normal or backward. The normal subjects showed a significant right-hemifield advantage in reaction time, while the commissurotomized subject showed a pronounced left-hemifield advantage in both accuracy and reaction time. We argue that mental-rotation is primarily a right-hemispheric specialization, but that this was offset in the normal subjects by a stronger left-hemispheric specialization for letter identification.

Adolescent↗
PubMed1989

Image generation and processing of generated images in the cerebral hemispheres.

Recent computational models describing the contribution of the cerebral hemispheres to visual imagery have suggested an exclusive capacity of the left hemisphere to generate multipart images. A brief review of relevant findings indicates that the evidence presented in support of this suggestion is not entirely compelling; this prompted a reexamination of this issue in a lateral tachistoscopic study on normal adults. Sixteen subjects participated in two experiments in which they had to decide whether or not a lowercase letter contained a segment extending above or below the main body of the letter. This decision was made directly on lowercase letters in one experiment (perceptual task) and on their generated images in the other experiment (imagery task). The quality of the letters (clear or blurred) and the retinal eccentricity of stimulus presentation (small or large) were orthogonally manipulated. The perceptual task yielded no main effect of visual field but a significant interaction of visual field and letter quality. By contrast, the imagery task resulted in a left visual-field superiority but no interaction involving the visual fields--a departure from predictions based on current models of visual imagery. In addition, the pattern of results in the imagery task corresponded to that obtained with blurred letters in the perceptual task, suggesting limitations in spatial resolution of visual images. Implications of these results for models of cerebral lateralization and visual imagery are discussed.

Adult↗
PubMed1989

Categorization of disoriented faces in the cerebral hemispheres of normal and commissurotomized subjects.

We examined the capacity of the cerebral hemispheres to process faces that deviate from canonical perspective. In Experiment 1, normal Ss performed a gender categorization of faces presented at varying angular orientations in the left visual field (LVF) or right visual field (RVF). Orientation affected processing speed, more so in the RVF than in the LVF. The function relating reaction times to disorientation of the faces was approximately monotonic and reflected the increased difficulty in extracting relevant configurational information as the faces were rotated from canonical perspective. In Experiment 2, 3 commissurotomized Ss performed the same task. They responded above chance in the 2 visual fields, and the pattern of their results was similar to that obtained with the normal Ss, but the effect of disorientation was considerably more pronounced. It is suggested that the right hemisphere contribution becomes more critical the further the visual pattern departs from conventional view. Issues regarding the specification of processes correcting for disorientation and comparison of normal and commissurotomized Ss are discussed.

Adult↗
PubMed1989

Prosopagnosia in a right hemispherectomized patient.

The first reported case of prosopagnosia in a right hemispherectomized woman, B.M., whose intellectual and cognitive functions were otherwise normal or only slightly impaired, is presented. She was totally unable to identify, and to experience a sense of familiarity with, faces of persons she knew, but she could evoke semantic information about them and retrieve their names from visual contextual cues. She was unaware that she was lacking face-recognition skills and that faces alone could be used to access the identity of individuals. The functional nature of her deficit was investigated through sensory, perceptual, memory, and learning tasks to determine the level at which her prosopagnosic disturbance occurred. She was defective at resolving low spatial-frequency information, but this was insufficient to explain the selectivity of her impairment. She was able to carry out cognitive operations specific to faces as long as facial identity did not have to be ascertained, and she performed as well as control subjects at deriving information about the gender, age, and emotion of faces. She was impaired at matching different views of the same faces, and multidimensional scaling analysis of dissimilarity judgements between faces indicated an inability to combine the component features into a configurational facial representation that would uniquely define each face. In contrast to recently reported cases of prosopagnosia, B.M. showed no sign of covert recognition of known faces in a learning task, and there was no indication that she could, even for a few seconds, store a faithful facial representation. The occurrence of prosopagnosia in this hemispherectomized patient confirms that this deficit can emerge without damage to the left hemisphere, and her unawareness of her deficit, which had remained unnoticed for several years, raises the possibility that other hemispherectomized patients may be prosopagnosic. The pattern of cognitive impairments displayed by B.M. indicates a perceptual basis to her disturbance and is discussed in relation to other cases of prosopagnosia.

Adult↗
PubMed1988

Imagery in a commissurotomized patient.

A commissurotomized patient, L.B., was tested on several imagery tasks, in which the stimuli were flashed tachistoscopically in the left or right visual half-field. On tests requiring the generation of images of lowercase letters from their uppercase versions, or the generation of the positions of the hands on a clockface from digitally presented times, there was a strong right half-field (left-hemispheric) advantage in accuracy, but a left half-field (right-hemispheric) advantage in reaction time (RT). On tests requiring the mental rotation of letters or stickfigures to the upright, however, there was a strong left half-field (right-hemispheric) advantage in accuracy, RT, and conformity of RTs to an ideal "mental-rotation" function when plotted against angular orientation. These data provide strong evidence that the right hemisphere was capable of mental rotation comparable to that of normal subjects; the left hemisphere, by contrast, seemed virtually incapable of mental rotation in the early testing sessions, and never achieved the proficiency of the right.

Adult↗
PubMed1988

An investigation into perceptual completion in blind areas of the visual field.

The perceptual completion phenomenon refers to seeing a figure as complete when part of it falls in a blind area of the visual field. This phenomenon and the conditions for its occurrence were examined in three different situations: over the retinal blind spot, across the midline of the visual field in commissurotomized patients, and in the blind visual field in hemianopic patients. Responses of complete perception were obtained in each of the three conditions, but they were not all the result of a genuine perceptual completion process. Reports of vision over the retinal blind spot qualified as genuine perceptual completion. In the other two situations, reports suggestive of completion could be entirely eliminated in all but one hemianopic patient by manipulating parameters inherently involved in the evaluation of perceptual completion. The nature of perception, the characteristics and properties of the experimental task and stimuli, the mode of response, the patient's mental status, and eccentric eye fixation, all contributed towards reports of complete perception for reasons other than a process of completing the missing information. Evidence of completion was found in one patient who could see more distinctly in his affected field with bilateral stimulation than with stimulation of the affected field alone. The results suggest that perceptual completion in the hemianopic field is not a common phenomenon and that reports of complete perception are determined by many factors, not all related to seeing more than is objectively available. The results also indicate that the hemianopic defect does not function as an extended blind spot. Possible mechanisms underlying perceptual completion over the blind spot and the hemianopic field are suggested.

Adult↗
PubMed1987

A new look at the human split brain.

The 'split-brain' syndrome results from the severance of neocortical commissures, and it is characterized by an incapacity of the cerebral hemispheres to exchange information and by the coexistence of two independent cognitive systems in the same cranium. Research on commissurotomized patients, for the most part, has focused on the disconnection and has treated the hemispheres as two independent entities. A different perspective was adopted in the present study, which considered the 'split brain' as a single organism, and the two disconnected hemispheres as integral components of this organism connected to one another through subcortical structures. Eight independent tasks were conducted on 2 commissurotomized patients who were presented tachistoscopically with simultaneous bilateral information such that neither hemisphere received sufficient information to make a final decision. Only by combining the initially segregated information could a correct response be produced. Both patients performed significantly above chance in 7 of the tasks (spatial orientation, calculation, lexical decision), suggesting that the information divided between the hemispheres could be united, related, and acted upon in a unified manner. This performance was achieved in spite of each hemisphere being unaware of the information received by the other, as typically observed in 'split-brain' patients. The results indicate the coexistence of perceptual disunity and behavioural unity, and they suggest that, even when the two disconnected hemispheres receive different information, the commissurotomized brain works as a single and unified organism.

Adult↗
PubMed1986

Role of input factors in visual-field asymmetries.

This paper examines the implications of lateral tachistoscopic presentation of information for the processing efficiency of the intact cerebral hemispheres. Considering that the understanding of the processes underlying the particular competences of each hemisphere may require, as a preliminary step, the specification of the characteristics of the input on which the brain operates, anatomical, physiological, and psychophysical consequences of briefly stimulating the retinal periphery for the representation of information in the brain are outlined, with special reference to the spatial-frequency spectral composition of the stimuli. Retinal eccentricity and brief exposure duration converge to making the representation of information qualitatively different from the information that the brain normally operates on, which constrains the interpretation of findings with respect to the normal functions of the cerebral hemispheres. A review and discussion of empirical findings relevant to these issues suggest that manipulation of procedural variables may differentially affect the processing efficiency of the cerebral hemispheres and indicate that a given pattern of visual-field asymmetry may be overdetermined by a multitude of variables interacting in complex ways.

Discrimination Learning↗
PubMed1986

Role of task factors in visual field asymmetries.

Any tachistoscopic study of cerebral hemisphere asymmetry imposes a variety of task demands on the participants, ranging from demands imposed by specific viewing conditions to demands imposed by response output requirements. The present article discusses the role of several task factors that influence processing after the initial reception of the stimulus input, suggests a theoretical rationale for some of the effects of these task factors, and considers implications for future studies of visual laterality. The task factors discussed include both those that are relevant for minimizing artifacts that have nothing to do with hemispheric asymmetry and those that are relevant for interpreting hemispheric asymmetry in terms of specific perceptual and cognitive processes.

Attention↗
PubMed1986

Subcortical coordination of hemisphere activity in commissurotomized patients.

Despite their commissurotomy, 'split-brain' patients behave as unified individuals and seldom display signs of hesitation and dissociation in their activities. Yet our current understanding of the callosal disconnection syndrome suggests that the two isolated hemispheres cannot work in parallel and that only one hemisphere at a time is responsive to bilateral stimulation. To examine this divergence, the two hemispheres of complete forebrain commissurotomy patients were simultaneously presented with colour stimuli associated with opposite responses, and the patients had to produce a single finger response whose accuracy depended on their capacity to resolve the conflicting information. Although the two patients differed in their efficiency, they were capable of accurate performance in the bilateral conflict condition, but could not correctly cross-compare the information received by each hemisphere. This performance suggests that the two disconnected hemispheres can work in parallel and process the incoming information efficiently. The outcome of these hemispheric operations may be integrated in subcortical 'centrencephalic' areas that coordinate the activities of the two sides of the brain and maintain some unity within cerebral structures.

Adult↗
PubMed1985

Influence of task and input factors on hemispheric involvement in face processing.

Two experiments examined the respective role of the cerebral hemispheres in face perception and the nature of their contribution depending on task demands and on the spatial-frequency composition of the stimuli. Sixteen faces of members of the subjects' department were presented as stimuli, with men and women, and professors and nonprofessors being equally represented. In Experiment 1, high-resolution black-and-white photographs of faces were used in three reaction-time tasks: verbal identification, manual membership categorization, and manual male/female categorization, in a within-subject design. Identification and membership categorization were significantly better performed in right-visual-field presentations, whereas the male/female categorization yielded a nonsignificant left-visual-field superiority. In Experiment 2, two versions of the same faces were used: digitized low-pass (0 to 2 cycles/degree of visual angle) and digitized broad-pass (0 to 32 cycles/degree) faces. Broad-pass faces produced the same laterality pattern as in Experiment 1, while low-pass faces were better processed in left-visual-field presentations for all three tasks. The results suggest that the two hemispheres play a role in face perception, and their contribution may vary as a function of the task demands and of the spatial-frequency components of the incoming information.

Adult↗
PubMed1985

[Positive inotropic agents. Generalities and classification].

The aim of inotropic therapy is to increase the force of myocardial fibre shortening by improving the availability of calcium to the contractile proteins. Digitalis remains the most widely used drug, but its positive inotropic effects are weak and the therapeutic index is low. Dobutamine is the most useful catecholamine because the most cardioselective and it induces the least increase in myocardial oxygen consumption. There are three groups of new inotropic agents: sympathomimetics: pirbuterol and prenalterol are effective in the short term but tolerance is usually observed within a few weeks. Salbutamol and terbutaline have only been assessed in acute studies, phosphodiesterase inhibitors (MDL 17043, MDL 19025) are powerful inotropic agents in the short and medium term. ARL 115 has mainly been studied by parenteral administration, Amrinone has a largely unknown mode of action, but is a very effective positive inotropic drug; its side-effects limit it as age. However, its derivative, milrinone, seems to be more inotropic and less toxic. The new inotropic drugs currently under assessment are active in the short term but their long-term efficacy and side-effects are still little known. The ideal inotropic agent remains to be discovered.

Adrenergic beta-Agonists↗
PubMed1984

Processing of visually presented vowels in the cerebral hemispheres.

The respective efficiency of the cerebral hemispheres in processing visually presented vowels is discussed and examined in a tachistoscopic experiment with normal subjects. It is suggested that results from dichotic listening experiments cannot be used to predict performance on the same material in the visual modality. The physical characteristics of the stimulus, in both modalities, contribute to the pattern of hemispheric advantage and cannot be ignored in the interpretation of results from dichotic listening and tachistoscopic experiments.

Adult↗
PubMed1984

Configural processing of faces in the left and the right cerebral hemispheres.

This study investigates the rules by which the component features of faces are combined when presented in the left or the right visual field, and it examines the validity of the analytic-holistic processing dichotomy, using concepts elaborated by Garner (1978, 1981) to specify stimulus properties and models of similarity relations as performance criteria. Latency measures of dissimilarity, obtained for the two visual fields, among a set of eight faces varying on three dimensions of two levels each, were fitted to the dominance metric model, the feature-matching model, the city-block distance metric model, and the Euclidean distance metric model. In addition to a right-visual-field superiority in different responses, a maximum likelihood estimation procedure showed that, for each subject and each visual field, the Euclidean model provided the best fit of the data, suggesting that the faces were compared in terms of their overall similarity. Moreover, the spatial representations of the results revealed interactions among the component facial features in the processing of faces. Taken together, these two findings indicate that faces initially projected to the right or to the left hemisphere were not processed analytically but in terms of their gestalt.

Adult↗