Biomedical subjects

A Ptito

Publications and source records attributed to A Ptito.

16 recordsLinked to original sources

PubMed1999

Residual vision in the blind field of hemidecorticated humans predicted by a diffusion scatter model and selective spectral absorption of the human eye.

The notion of blindsight was recently challenged by evidence that patients with occipital damage and contralateral field defects show residual islands of vision which may be associated with spared neural tissue. However, this possibility could not explain why patients who underwent the resection or disconnection of an entire cerebral hemisphere exhibit some forms of blindsight. We present here a model for the detection of intraocular scatter, which can account for human sensitivity values obtained in the blind field of hemidecorticated patients. The model demonstrates that, under controlled experimental conditions i.e. where the extraocular scatter is eliminated, Lambertian intraocular scatter alone can account for the visual sensitivities reported in these patients. The model also shows that it is possible to obtain a sensitivity in the blind field almost equivalent to that in the good field using the appropriate parameters. Finally, we show with in-vivo spectroreflectometry measurements made in the eyes of our hemidecorticated patients, that the relative drop in middle wavelength sensitivity generally obtained in the blind field of these patients can be explained by selective intraocular spectral absorption.

Blindness, Cortical↗

PubMed1997

Temperature during cardiopulmonary bypass for coronary artery operations does not influence postoperative cognitive function: a prospective, randomized trial.

OBJECTIVE: The objective was to examine the effect of temperature (28 degrees vs 36 degrees C) during cardiopulmonary bypass on postoperative cognitive functions in a prospective, double-blind, and randomized manner. METHODS: Sixty-two patients scheduled for coronary operations were randomized to warm or cold cardiopulmonary bypass. Preoperative and postoperative (7 days) neuropsychologic evaluations were performed by an observer unaware of cardiopulmonary bypass temperature. RESULTS: Fifty-four patients completed the study (cold bypass, n = 24; warm bypass, n = 30). Significant (p < 0.01) postoperative deterioration for tests of psychomotor coordination and verbal memory was noted in both warm and cold groups, but no differences were observed between groups. CONCLUSION: Temperature during cardiopulmonary bypass for coronary operations does not influence postoperative cognitive function.

Adult↗

PubMed1997

REM sleep dream mentation in right hemispherectomized patients.

Investigations of dream mentation in brain damaged patients have shed some light on the controversial issue of cerebral lateralization of dreaming. To examine further the relationships between brain function and dreaming, we studied REM sleep dream recall and content in four patients having undergone right functional or anatomical hemispherectomy and eight matched control subjects. Patients were found to have the capacity to report dreams to much the same extent as control subjects. Further, the patients' dream content was overall similar to that of the control subjects. The results provide strong evidence that dreaming is not a right-hemisphere function, and that the left hemisphere may be more critical for the generation of dreams. In addition, some characteristics of hemispherectomized patients' dream content (characters, smells) are consistent with the possibility that a history of epilepsy may influence REM sleep imagery over the long term.

Adolescent↗

PubMed1997

Blindsight in hemispherectomized patients as revealed by spatial summation across the vertical meridian.

The present study provides a demonstration of blindsight in two hemispherectomy patients who showed a visual spatial summation effect across the vertical meridian despite their lack of visual awareness in one hemifield. Such an effect cannot be related to light diffusion onto the sighted hemifield because it was not present when one of the stimuli fell into the retinal blind spot of control subjects. We conclude that blindsight phenomena of the simple type described in the present study can be subserved by sub-cortical mechanisms and do not necessarily require cortical processing.

Adolescent↗

PubMed1996

No blindsight following hemidecortication in human subjects?

Using a guessing paradigm we measured visual sensitivity in the blind and normal half-fields of four cerebrally hemidecorticated patients. In the blind field, sensitivity was reduced by approximately 3 long units. Stimuli which produced significant detection also evoked conscious sensations of light and colour. Control experiments showed that although sensitivity in the blind field depended in a normal fashion on background luminance it was independent of the luminance of a local platform, and showed no spatial summation. This residual vision can be explained by intraocular light diffusion and reflection.

Brain Diseases↗

PubMed1996

Residual vision with awareness in the field contralateral to a partial or complete functional hemispherectomy.

Two patients with unilateral disconnection or removal of the entire occipital lobe were tested for residual vision in their blind field. Using image stabilization to eliminate eye motion artifacts, the central portion of each subject's visual field was tested, beginning 1 degree from fixation and extending outward to 13.5 degrees. A narrow zone of residual vision was identified along the retinal vertical meridian of each patient. The lateral edge of this zone was generally within 3.5 degrees of the vertical meridian, though extended farther outward (but not beyond 6 degrees) at one field location for each subject. In one patient, it was present in both superior and inferior quadrants; in the other, it was confined to the superior quadrant. Within their zones of residual vision, both patients could detect stimuli and perform simple shape discriminations, but could not name complex line drawings. The patients were aware of their vision within this zone. No residual vision, with or without awareness, was found in areas tested outside these zones. Given the complete absence of visual cortex contralateral to the observed residual vision, alternate structures must be mediating these abilities.

Adolescent↗

PubMed1996

Perception of motion-in-depth in patients with partial or complete cerebral hemispherectomy.

Four patients with functional hemispherectomy, one patient with a complete anatomical hemispherectomy, and one patient with unilateral removal of the temporal, parietal and occipital lobes took part in two sets of experiments designed to investigate their residual sensitivity to motion-in-depth in the hemianopic visual field. Two types of computer-generated visual displays were used; in the first set of experiments, a dot pattern and in the second, a circular checkerboard. These simulated either convergent, divergent or reversed rotational motion. Each set of experiments consisted of two parts; in the first part, electrodermal responses were monitored during stimulus presentation while the subjects performed a simple distracting task. In the second part, subjects were asked to state verbally the direction of stimulus motion. Contrary to expectations, no reliable changes in skin conductance were elicited from any of the subjects by changes in the direction of motion of the component parts of either the dot pattern display or the circular checkerboard display. Furthermore, none of the subjects were able to discriminate the direction of motion of the target patterns when presented in the hemianopic field. The most parsimonious explanation is that the subcortical visual pathways which survive hemispherectomy are unable to process visual information relating to motion in depth.

Adolescent↗

PubMed1996

Neural bases of residual vision in hemicorticectomized monkeys.

In this series of studies, we have attempted to characterize anatomically the organization of the retinofugal pathways in monkeys that underwent the surgical removal in infancy of the entire left cerebral hemisphere. Hemidecordication in baby monkeys produced a transneuronal retrograde degeneration of the retinal ganglion cells (RGCs) that affected mainly the foveal rim. Although the density of RGCs in this region was drastically diminished, the soma sizes of the surviving cells remained normal. The lateral geniculate nucleus (dLGN) ipsilateral to the removed cortex was dramatically reduced in size although it still showed normal layering. There was a marked reduction in the number of neurons in both the parvocellular and magnocellular layers and a heavy gliosis. By contrast, the superior colliculus ipsilateral to the lesion was remarkably well preserved: although slightly reduced in volume, it showed little gliosis and a metabolic activity, as revealed by cytochrome oxidase histochemistry, similar to the superior colliculus contralateral to the lesion. Behavioral perimetry indicated a partial sparing of vision up to 45 degrees in the 'blind' hemifield. We argue that the preservation of the retino-tectal pathway mediates most of the residual visual functions found in the 'blind field' of hemispherectomized human subjects.

Animals↗

PubMed1996

Extrageniculostriate vision in humans: investigations with hemispherectomy patients.

Residual vision was assessed in the blind hemifield of one hemispherectomized and one partially hemispherectomized patient, using an interval two alternative forced choice detection task. Fixation instabilities were controlled by retinal stabilization. In both patients, residual vision was found in the hemianopic field close to the vertical meridian. This residual vision is largely confined to a band not wider the 3 degrees, but there is a local region in each patient where it extends more than 3 degrees from the meridian. However, more than 6 degrees from the vertical meridian we found no indication of residual function in either patient. Within the band of spared vision, subjects are aware of stimuli and can perform simple shape discriminations. Visual acuity profiles argue against an explanation based on eccentric fixation. Explanations based on the retino-tectal pathway or on retinal naso-temporal overlap are possible.

Adolescent↗

PubMed1995

Visual-spatial localization by patients with frontal-lobe lesions invading or sparing area 46.

Monkeys with unilateral principal sulcus (PS) lesions show a contralateral deficit in localizing remembered targets, especially as the recall interval is lengthened. We tested 20 patients with unilateral frontal-lobe excisions that invaded (FI) or spared (FS) area 46 (putative homologue of PS) and 32 normal controls (NC) on a task where subjects had to indicate the location of a light dot either immediately, or after 30 s, with or without interference. The FI group was worse than the NC group following both delay conditions. NC and FS groups differed only after interference. We concluded that area 46 is involved in recalling the location of visual targets, but unlike the monkey, the deficit is not restricted to a particular part of the visual field.

Adult↗

PubMed1995

Preserved auditory spatial localization following cerebral hemispherectomy.

Auditory spatial function was assessed in six patients who had undergone unilateral cerebral hemispherectomy for the relief of intractable epilepsy. Separate localization and discrimination tests were carried out. In the first test, subjects localized the azimuthal position of a brief click presented at one of 13 positions in a free field, by pointing to its perceived position. All six patients demonstrated some preservation of localization ability, with some individual subjects performing at normal or near-normal levels. However, as a group, the patient sample localized less accurately than normal controls at extreme azimuthal positions, especially contralateral to the removal. In the second test, two clicks were presented either from the same location or from two locations separated by 30 degrees, and a same-different judgment was required. In this test the patient group performed worse than control subjects overall, but also demonstrated preserved discrimination ability in both hemifields. These findings demonstrate the expected existence of contralateral localization deficits, but the relative sparing of function contrasts with reports from animal behavioural studies, in which severe and persistent localization deficits contralateral to auditory cortex excision are described. Our findings suggest that extensive, early damage to one hemisphere may permit reorganization of function to occur, with auditory spatial ability being mediated by cortical systems in the remaining hemisphere and/or by subcortical structures.

Adult↗

PubMed1995

Retained language in dysgenic cortex: case report.

Since the advent of magnetic resonance imaging, there has been renewed interest in disorders of cortical migration in the cause of focal epilepsy. The function of dysplastic cortex is poorly understood. We report a 46-year-old woman in whom this was assessed by intraoperative stimulation. This left-handed patient had a 30-year history of complex partial seizures with secondary generalization. Prolonged electroencephalographic recordings documented an epileptic focus in the right lateral and inferomesiotemporal lobe. Computed tomographic scanning and angiography suggested a right posterotemporal hamartoma. Amytal testing showed major speech representation in the right hemisphere. At craniotomy, an ivory-colored, posterotemporal lesion originating 5 cm from the temporal tip and 4 cm in diameter occupied the posterotemporal region. Electrical stimulation of this lesion produced speech interference. A histological examination of the resected anterotemporal lobe, amygdala, and hippocampus and biopsy specimens of the lesion showed extensive multifocal cortical microdysgenesis and gliovascular and cortical hamartoma. This case shows that grossly dysplastic cortex can remain functional. Cortical mapping under local anesthesia should be performed before resection of dysplastic lesions in putatively functional areas in patients with intractable epilepsy.

Amygdala↗

PubMed1993

Localization and lateralization of stereoscopic processing in the human brain.

Positron emission tomography was used to investigate the neural substrate of stereopsis. Changes in cerebral blood flow were measured in nine volunteers while they judged the orientation of a rectangular cyclopean shape in random dot stereograms. This experimental condition was compared with two control conditions, one having a two-dimensional shape but no cyclopean stimulus, the other having neither shape nor cyclopean stimulus. Blood flow increases were observed in areas 17 and 18 in the right hemisphere when comparing experimental with control conditions; blood flow decreases occurred in the right inferotemporal cortex. Results indicate that stereopsis begins in posterior visual areas in the right cerebral hemisphere. Under ambiguous stereoscopic conditions, participation of more anterior areas in the temporal lobe may be elicited.

Cerebrovascular Circulation↗

PubMed1991

Target detection and movement discrimination in the blind field of hemispherectomized patients.

Four hemispherectomized patients were tested with a variety of tasks designed to investigate the extent of residual vision in the blind field. The first set of studies was aimed at evaluating the ability of these subjects to detect and localize at various eccentricities three types of targets which differed in their spatiotemporal properties (stationary, flashing and moving). The subjects could detect effectively the presence of any of these stimuli in their blind field and they experienced little difficulty in discerning blank from target trials. When manual pointing was used to measure localization quality, the hemianopes made more errors in their blind field but their accuracy, when detection was correct, was comparable with that observed in their intact visual field and in that of the control subjects. In the second set of experiments, the capacity of 3 of the subjects to detect in their blind field a moving grating, as well as to discriminate between relative grating velocities and directions in their blind field and in both fields simultaneously, was assessed. Two subjects could detect the movement in their blind field, although this was in part affected by stimulus velocity. When discrimination of relative velocities in the blind field and in both fields simultaneously was evaluated, individual differences were observed. However, all subjects showed some capacity to carry out the task. Finally, none of the experimental subjects was able to discriminate the relative directions of the moving gratings. The results are discussed in terms of collicular involvement to account for the residual vision observed in the blind field of hemispherectomized subjects.

Brain↗

PubMed1991

Stereopsis after unilateral anterior temporal lobectomy. Dissociation between local and global measures.

In man, an impairment in stereopsis may occur following damage outside the occipital lobes. If local and global stereopsis can be dissociated, this may imply that different mechanisms mediate these functions. We tested 44 patients with unilateral anterior temporal lobe excisions and 23 normal control subjects on two tasks. In the local stereopsis task, subjects indicated which of two pins varying in disparity between 4 and 512 s of arc was closer. Results showed no threshold impairment in any group. In the global stereopsis task, random-dot stereograms varying in binocular correlation were presented in random order, and subjects indicated if the squares perceived in depth were in front of or behind the screen. At binocular correlations between 50% and 70%, left and right temporal lobectomy resulted in a deficit, which was more marked following right-sided excisions. These results are concordant with earlier work (Ptito and Zatorre, 1988), and with other evidence of a right hemisphere superiority in treating binocular depth information. Moreover, the fact that global stereopsis is impaired in the presence of intact local stereopsis suggests that closely related but not identical mechanisms are involved, and fits the notion that there is a hierarchical organization of the visual pathways originating in the striate cortex leading into temporal cortex. Global stereopsis would thus be mediated in temporal lobe areas that receive information about local disparities from more posterior regions.

Adult↗

PubMed1988

Impaired stereoscopic detection thresholds after left or right temporal lobectomy.

The present study assessed global stereopsis as measured by random dot stereograms (RDS). Single stimuli were presented in order of increasing binocular correlation (40-100%) making it progressively easier to perceive the figures in depth, and then the same set of stimuli was presented in reverse order. The subjects tested included 50 patients with unilateral temporal-lobe lesions, 11 patients with right frontal-lobe lesions and 18 normal control subjects. Results indicate that either right or left temporal lobectomy leads to a higher than normal detection threshold for RDS. In contrast, right frontal lobectomy did not have a significant effect compared to the normal control subjects. All subjects were able to perceive correctly the figures at 100% binocular correlation. The results are interpreted in terms of a possible contribution of the inferior temporal cortex to global stereopsis.

Adult↗