Biomedical subjects

H O Karnath

Publications and source records attributed to H O Karnath.

43 records · Page 3Linked to original sources

PubMed1992

Abnormal eye movement behaviour during text reading in neglect syndrome: a case study.

The eye movement behaviour of a patient suffering from a right basal ganglia infarction with a left-sided hemineglect but without any visual field defects was investigated during reading. The eye movements were registered by means of an i.r. light technique (pupil-corneal reflection method). The main findings were abnormal return sweeps. Whereas in normal readers the end of one line of text is linked to the beginning of the new line by a long leftward saccade, the return sweeps of the hemineglect patient stereotypically ended in the middle of the next line. They were followed by sequences of short saccades indicating silent backward reading until a linguistically plausible continuation of sentences from the previous line was found, irrespective of the actual beginning of text. The shortened return sweeps could not be attributed to a general oculomotor disturbance. The spatial border for the occurrence of the patient's abnormal scanning pattern (left half of texts) clearly did not depend on a retinal coordinate frame of reference but rather has to be attributed to a different body-centred reference system.

Attention↗

PubMed1991

Mental planning and anticipatory processes with acute and chronic frontal lobe lesions: a comparison of maze performance in routine and non-routine situations.

A series of computerized mazes were masked during presentation allowing only partial view of their structure. Patients suffering from acute or chronic unilateral frontal (medial, lateral) or retrorolandic (temporal, parietal) lesions and non-brain-damaged controls were required to pass through the mazes repeatedly, until they solved them without reaching a dead end. Because the mazes could not be viewed in their entirety at any one time, the subjects were forced to mentally generate and apply a plan of the maze. The first ("non-routine" situation), the second and the last ("routine" situation) trials of the mazes were analysed separately. The prefrontal cortex is supposed to subserve important functions in the planning of behaviour in unfamiliar, non-routine situations. Shallice (Phil. Trans. R. Soc. Lond. B298, 199-209, 1982; From Neuropsychology to Mental Structure, Cambridge University Press, Cambridge, 1988) therefore postulated that the behaviour of patients with frontal lobe lesions should not be disturbed in routine situations, but rather these patients should exhibit difficulties in dealing with new and unfamiliar situations. In the present investigation, patients with acute and chronic frontomedial lesions exhibited deficits in generating mental plans. Patients with acute frontomedial lesions made a larger number of errors in their second trials and subsequently required more trials to reach task criterium. Patients with chronic frontomedial lesions showed more frequent rule-breaking behaviour in their second trials. The analysis of maze performance in the first trial and that trial in which task criterion was achieved, showed that frontal lobe patients were impaired in neither routine nor non-routine situations. The observed results suggest a modification concerning Shallice's assumptions. Patients with frontomedial lesions are not generally incapable of dealing with non-routine situations. But they seem to be slower than patients with retrorolandic lesions and controls in profiting from experiences made in an unfamiliar, non-routine situation represented by the first explorations of new maze problems. This deficit can possibly be explained by the observation that anticipatory processes or the formation of expectations about possibly correct decisions in non-routine situations were altered in acute and chronic frontomedial patients. When these patients reached a maze intersection for the first time and therefore had no information about the continuation of the correct path, they showed significantly different intuitive directional decisions when compared to controls.

Adult↗

PubMed1991

Limitations of interhemispheric extracallosal transfer of visual information in callosal agenesis.

A 57-year-old patient with callosal but not anterior commissure agenesis was investigated with a visual interfield comparison and a naming task. Stimuli were presented tachistoscopically either bilateral-simultaneously or unilaterally in the LVF and/or RVF. The stimuli presented bilaterally differed with respect to their degree of similarity. Whereas the patient was able to detect gross differences between stimuli presented in the left and right half-field, he was impaired at discriminating similar and identical stimuli across the half-fields. Identification by naming was correct on unilateral presentation of a stimulus, while with bilateral presentation of two stimuli, errors increased considerably with the stimulus named second. The data are thought to indicate a limited capacity of the extracallosal commissures, probably the anterior, for the transfer of visual information.

Agenesis of Corpus Callosum↗

PubMed1991

Trunk orientation as the determining factor of the 'contralateral' deficit in the neglect syndrome and as the physical anchor of the internal representation of body orientation in space.

The present study examines which egocentric coordinate system determines the border between the disturbed 'contralateral' and the normal 'ipsilateral' side in patients with hemineglect. Based on the observation of significantly longer reaction times for saccades towards stimuli presented in the left visual field (LVF) in right brain-damaged patients with hemineglect, stimuli were presented randomly to the LVF or RVF and the corresponding saccadic reaction times (SRTs) were compared. Beginning with the standard body position generally used for the investigation of neglect patients, where the midlines of head, trunk and visual field are parallel and oriented straight towards the middle of the projection screen, the spatial relation between orientation of head and trunk midlines and location of the target stimuli was systematically varied while holding the retinal projection of the stimuli constant. The deficit in SRTs towards the LVF in 4 right brain-damaged patients with left-sided hemineglect could be compensated for by turning the patients' trunk to the left, such that both LVF and RVF-stimuli were projected to the right, ipsilateral side of trunk space. The results suggest that the spatial orientation of the trunk midline divides our normal perception of space into an egocentric 'left' and an egocentric 'right' sector and seems to be the decisive factor for determining the neglected 'contralateral' part of space in patients with brain-damage. They indicate that the trunk midline constitutes the physical anchor for calculation of the internal egocentric coordinate frame for representing body position with respect to external objects. The hypothesis of Ventre et al. (1984) that deficient reactions to contralaterally located stimuli in neglect patients could be the result of a displacement of these egocentric coordinates towards the non-neglected, ipsilateral side is discussed.

Adult↗

PubMed1990

Parkinson's disease patient's behaviour in a covered maze learning task.

A computerized maze task was constructed that allowed only partial vision of the maze structure and produced measurements for separate analysis of cognitive processes described as impaired in Parkinson's disease. Eighteen patients suffering from mild Parkinson's disease and 18 individually matched normal controls were investigated. Baseline task response times were found to be identical for both groups. Differences between patients' and controls' performance could be related to (a) a response bias in Parkinson patients that favoured repetition of the previous action and slowed down shifting and (b) an impairment of multistep plan generation. It is speculated that the response bias reflects the disinhibition of cortico-thalamo-cortical reverberation loops which results from striatal dopamine depletion.

Aged↗

PubMed1988

Deficits of attention in acute and recovered visual hemi-neglect.

Three patients with right-sided lesions and, in two cases, clinically manifest neglect had to compare and to name stimuli presented tachistoscopically either bilateral-simultaneously or unilaterally in the LVF and/or RVF. Whereas there was no difference between unilateral LVF and RVF presentation, "extinction" of the LVF on bilateral stimulation occurred in both the acute and recovered stages. This deficit could be compensated by reducing information analysis in the RVF, thereby provoking covert shifting of attention to the LVF. The results suggest a common mechanism underlying visual neglect and "extinction" with three components--two affecting the directionally specific covert orienting and shifting of attention and one affecting the directionally non-specific processing of information by sequential analysis. The course of recovery is determined by improvement in the directionally specific shifting of attention.

Aged↗

PubMed1987

Residual information processing in the neglected visual half-field.

A patient with neglect of visual half-field without hemianopia performed "same/different" interfield comparisons and named stimuli presented either unilaterally or bilaterally in the visual half-fields. With unilateral presentation he could name all stimuli in both fields, whereas with simultaneous bilateral stimulation his performance decreased to about 50% correct naming of left visual field (LVF) stimuli. This performance was not improved by cueing for the presence of LVF stimuli. Interfield comparisons were 100% correct in all conditions. It is concluded that inattention for the left hemispace in patients with visual neglect occurs whenever information in the right hemi-space attracts attention or must be consciously processed. Simple "same/different" judgements are presumably performed on a pre-attentional level and are not affected by neglect.

Brain Diseases↗