Biomedical subjects

M Ptito

Publications and source records attributed to M Ptito.

At least 19 recordsLinked to original sources

PubMed2003

Stereoscopic processing in the human brain as a function of binocular luminance rivalry.

We investigated the neural substrates of a recent model of human stereodepth perception by obtaining measurements of regional cerebral blood flow (rCBF) using PET. Subjects experienced the perceptual properties of stereopsis by viewing rival-luminance stereograms displaying an identical random-dot pattern in their central portion while the backgrounds exhibited correspondent dots contrasting in black/white luminance. The stereoscopic vision induced by retinal luminance rivalry coincided with a significant elevation of rCBF in the dorsal visual pathway. Area V5 (MT) was activated bilaterally by the experimental condition while the remaining active loci were restricted to the right hemisphere. The neural sites that responded to this novel stereoscopic stimulus are similar to those activated by traditional stereograms containing horizontal disparities.

Adult↗

PubMed2001

Anatomical sparing in the superior colliculus of hemispherectomized monkeys.

Using the monkey as a model for human hemispherectomy, the effects of early removal of a whole cerebral hemisphere on the cytoarchitecture and cytochrome oxidase histochemistry of the superior colliculus (SC) were evaluated. Results show that the SC ipsilateral to the cortical lesion suffers a 29.9% average volume reduction and a 32.7% total loss of neurons compared to the contralateral SC. Neuronal densities and metabolic activity are similar in normal and hemispherectomized monkeys. Furthermore, the ipsi- and contralesional SC receive retinal inputs as revealed with intraocular injections of tritiated proline. These data suggest that the superior colliculus retains functional capabilities following hemispherectomy in monkey.

Animals↗

PubMed2001

Cortical representation of inward and outward radial motion in man.

We have used positron emission tomography to investigate the cortical areas of the normal human brain involved in processing inward (Expansion) and outward (Contraction) radial motion simulated with an optic flow stimulus. The optical flow display was made out of dots moving radially away from or toward the center of the display monitor. In the Control condition, the dots' motion was randomized in order to remove any sensation of radial motion. In the Expansion condition, several loci of activation were observed: visual areas V2-V3 and the superior parietal lobule (BA 7), predominantly in the right hemisphere. In the Contraction condition activation sites were found in the same visual areas (V2 and V3) in the right hemisphere but the increase in rCBF in these regions was much lower than in the Expansion condition. BA 7 was activated in both hemispheres. When the motion component of the stimulus was isolated by subtracting the static condition from the incoherent motion condition, we obtained activations of areas V2, V3, and MT (putative V5). These results indicate that the detection of radial motion derived from an optic flow stimulus is mediated by structures forming the dorsal part of the visual cortical system and confirm that area MT is not specifically involved in flow analysis.

Adult↗

PubMed2001

'Seeing' in the blind hemifield following hemispherectomy.

Destruction of the striate cortex has traditionally been thought to lead to permanent blindness in the contralateral visual field and to the dogma that this region is indispensable for vision in primates. For over 25 years now, evidence has been accumulating that hemianopic human subjects and monkeys possess wide-ranging residual visual capacities or 'blindsight' in the blind part of their visual field. For some researchers, isolated islands of the striate cortex have been associated with patches of degraded vision and made responsible for blindisight. Artefacts such as light scatter, criterion effects, macular sparing, eccentric fixation and minute eye movements have also been linked with the residual vision. For others, the fact that certain aspects of the visual information can be processed without the geniculostriate pathway suggests mediation by the visual subsystems such as extrastriate visual cortical areas which receive visual information via subcortical pathways, that escaped the cortical damage. Subjects who have had a whole cerebral hemisphere removed or disconnected (for the treatment of uncontrollable epilepsy) and who show residual vision in their blind field offer a remarkable opportunity to help clear the controversy regarding the neural substrate of blindsight. Because it is certain that no functional striate or extrastriate cortex remains on the ablated side, these subjects have contributed significantly to identifying the critical pathways involved in blindsight.

Awareness↗

PubMed2001

Visual pathways following cerebral hemispherectomy.

The anatomical consequences of unilateral cerebral hemispherectomy in some animal models are reviewed. We have shown that the retinogenigulate pathway undergoes severe degenerative changes in hemispherectomized monkeys, greater than those shown in cats and we proposed that remaining retinal terminals to the dorsal lateral geniculate nucleus have little potential for conveying visual information any further. All subdivisions of the pulvinar undergo severe degeneration following hemispherectomy showing that the ascending tectofugal pathway is also shut off. On the other hand, the retina subserving the blind field is not depleted of ganglion cells which still send normal appearing terminals to the midbrain pretectum and superior colliculus. Visual information from the blind hemifield can thus gain access to the brain and could potentially reach the contralateral cerebral cortex through the midbrain commissure and possibly through thalamic commissural cells.

Animals↗

PubMed2001

When the auditory cortex turns visual.

We studied visually guided behavior and the visual response properties of single auditory cortex (A1) neurons in neonatally operated hamsters with surgically induced, permanent, ectopic retinal projections to auditory thalamic nuclei and to visual thalamic nuclei which normally receive little direct retinal input. The surgically induced retino-thalamo-cortical pathways can mediate visual guided behaviors whose normal substrate, the pathway from the retina to the primary visual cortex via the primary thalamic visual nucleus, is missing. The visually evoked response properties of A1 neurons resemble in many respects those of neurons in V1 of normal hamsters: many A1 neurons have well-defined visual receptive fields and preferences for orientation or direction of movement. In addition, some visually responsive cells in A1 are bimodal--they also respond to auditory stimuli. The visually responsive neurons in A1 probably account for the capacity of the auditory cortex to mediate visual behavior in 'rewired hamsters'.

Animals↗

PubMed2001

Cortical activation by tactile and painful stimuli in hemispherectomized patients.

Hemispherectomized patients are able to perceive tactile and painful stimuli on their nonparetic as well as paretic body halves. We have used functional MRI to study the cortical mechanisms underlying this preserved somatosensory capacity. Nonpainful brushing and painful heat were applied to the skin of the legs in four hemispherectomized patients and, for comparison, in four normal subjects. Cortical activation was studied with a 1.5 T scanner using a BOLD (blood oxygen level dependent) protocol. All patients rated both the brushing and the heat pain as almost equally intense on each leg and the ratings were similar to those in normals. Brushing on the nonparetic leg activated primary and secondary somatosensory cortices (S1 and S2) in all patients, similar to findings in normals. Brushing on the paretic leg activated S1 in two patients and S2 in one of these patients. Heat pain activated S2, insular cortex and anterior cingulate cortex to a similar degree for both legs, but the activation was weaker in the patients than in the normals. For the individual patient, there was generally no obvious correlation between cortical activation as studied with the BOLD technique and psychophysical performance. The findings from tactile stimulation of the nonparetic leg, that the activation was similar to the contralateral activation in normals, suggest that tactile information processing in the hemisphere contralateral to the stimulation is independent of the corpus callosum. In contrast, the pain activation for the nonparetic leg was weaker than in normals, suggesting that pain activation in the hemisphere contralateral to the stimulation is dependent on transcallosal information processing. The latter finding was corroborated by a subnormal capacity for pain localization on the nonparetic foot in two of the patients. The findings from stimulation of the paretic leg show that areas typically involved in the processing of tactile and painful stimuli can be activated by ipsilateral pathways directly from the periphery. The tactile-evoked ipsilateral S1 activation may be due to subcortical reorganization, since it was not observed in the normal subjects.

Adult↗

PubMed2001

Quantitative analysis of the retinal ganglion cell layer in the ostrich, Struthio camelus.

The total number, distribution and peak density of ganglion cells were evaluated in the Nissl-stained retina of the ostrich (Struthio camelus). The mean (n = 4) total number of retinal ganglion cells (RGC) was estimated at 2,274,128 (s.d. = 273, 152). The ostrich retina exhibited a prominent horizontal visual streak along which a central area located nasal to the pecten had a peak density of 9,500 cells/mm2. A high concentration of cells with a peak density of 2,646 cells/mm2 was also observed in the temporal retina, slightly dorsal to the visual streak. The results further showed that the ostrich eye has a 15-mm pupil entrance diameter, its mean axial length is 39.81 mm, the estimated retinal magnification factor is 0.4075 mm/deg and the maximum visual acuity along the well-defined visual streak was estimated to be 19.32 cycles/deg. The latter component of the retina might subserve vision along the horizon while the temporal region mediates binocular processing. The data also showed that the degree of retinal illumination in this bird could be comparable to that noted in some nocturnal species. The findings in this study suggest that the ostrich might not be restricted to diurnal activity.

Algorithms↗

PubMed2000

Surgically created neural pathways mediate visual pattern discrimination.

Combined lesions of retinal targets and ascending auditory pathways can induce, in developing animals, permanent retinal projections to auditory thalamic nuclei and to visual thalamic nuclei that normally receive little direct retinal input. Neurons in the auditory cortex of such animals have visual response properties that resemble those of neurons in the primary visual cortex of normal animals. Therefore, we investigated the behavioral function of the surgically induced retino-thalamo-cortical pathways. We showed that both surgically induced pathways can mediate visually guided behaviors whose normal substrate, the pathway from the retina to the primary visual cortex via the primary thalamic visual nucleus, is missing.

Animals↗

PubMed2000

Retinal projections to the pregeniculate nucleus in the hemispherectomized monkey.

Intraocular injections of tritiated proline were used to test the hypothesis that unilateral removal of all visual cortical areas results in increased distribution of retinal terminals in the pregeniculate nucleus (PGN) of the thalamus in monkeys. Following hemispherectomy, retinal input to the ipsilateral PGN was reduced by an average of 18.5% when compared to its contralateral homologue, which corresponded to the reduction in nuclear volume (19.3%). Our results show that removal of cortical afferents to the external layer of the PGN does not induce invasion of retinal projections into this region of the nucleus.

Animals↗

PubMed2000

Retinogeniculate projections following early cerebral hemispherectomy in the vervet monkey.

The effects of early, unilateral cerebral hemispherectomy on retinogeniculate projections were studied in the vervet monkey (Cercopithecus aethiops sabeus). Hemispherectomy eliminates all geniculocortical pathways and thus removes cortical factors involved in the survival of retinogeniculate projections. Complete removal of the left cerebral cortex was performed in two monkeys at 6 months and 8 months of age. After a post-surgical survival period of 50 months (SHG3) and 45 months (SHG4), both animals and a normal adult monkey received intraocular injections of [3H]proline (5 mCi) in the left eye and WGA-HRP (100 microliters, 5%) in the right eye. The dorsal lateral geniculate nucleuseuron (LGNd) ipsilateral to the hemispherectomy was on average 73% smaller than the contralateral LGNd. The magno- and parvocellular layers ipsilateral to the cortical ablation in both hemispherectomized subjects received a layered, eye-specific pattern of retinal input. This suggests that retinogeniculate projections could be sustained in the absence of geniculate relay cells.

Animals↗

PubMed2000

Responses of neurons in the cat posteromedial lateral suprasylvian cortex to moving texture patterns.

The posteromedial lateral suprasylvian cortex represents a point of convergence between the geniculostriate and extrageniculostriate visual pathways. Given its purported role in motion analysis and the conflicting reports regarding the texture sensitivity of this area, we have investigated the response properties of cells in PMLS to moving texture patterns ("visual noise"). In contrast to previous reports, we have found that a large majority of cells (80.1%) responds to the motion of a texture pattern with sustained discharges. In general, responses to noise were more broadly tuned for direction compared to gratings; however, direction selectivity appeared more pronounced in response to noise. The majority of cells was selective for drift velocity of the noise pattern (mean optimal velocity: 26.7 degrees /s). Velocity tuning was comparable to that of its principal thalamic input, the lateral posterior pulvinar nucleus. In general, responsiveness of cells in the posteromedial lateral suprasylvian cortex increased with increasing texture element size, although some units were tuned to smaller element sizes than the largest presented. Finally, the magnitude of these noise responses was dependent on the area of the visual field stimulated. In general, a stimulus corresponding to roughly twice the size of the receptive field was required to elicit an equivalent half-maximal response to that for gratings. The results of this study indicate that the majority of cells in the posteromedial lateral suprasylvian cortex can be driven by the motion of a fine texture field, and highlight the importance of this area in motion analysis.

Animals↗

PubMed2000

Sleep in right hemispherectomized patients: evidence of electrophysiological compensation.

OBJECTIVES: The goal of this study is to provide a better understanding of the role of the cortex in sleep's macro- and microstructure modulation. METHODS: Sleep architecture and phasic events were investigated in 4 patients having undergone right functional or anatomical hemispherectomy and 8 control subjects. Between-groups differences were assessed using the Wilcoxon-Mann-Whitney test. RESULTS: Findings provide evidence for overall similarity between patients' and control subjects' left hemispheric sleep architecture. In addition, results clearly indicate that it is possible to detect electrical activity over the operated side of a hemispherectomized patient's brain, even when resection of the hemi-cortex has been complete. Finally, findings provide evidence for similar left and right hemispheric relative spectral activities and for an increase in fast activity bands over the intact hemisphere in anatomical hemispherectomized patients. CONCLUSIONS: This study provides evidence that right hemispherectomy does not produce significant sleep architecture alterations as recorded over the intact hemisphere. In addition, residual activity detected over the operated side in anatomical hemispherectomized patients is interpreted as resulting from volume conduction originating from generators located in the intact hemisphere. Finally, there is strong evidence for electrophysiological compensation in the intact hemisphere following complete resection of the contralateral hemi-cortex.

Adolescent↗

PubMed1999

Stereological evaluation of substantia nigra cell number in normal and hemispherectomized monkeys.

The assessment of the anatomical consequences of cortical lesions on subcortical visual relays is necessary to further understand residual visual capacities. Unbiased stereological techniques were used to evaluate cell numbers in the substantia nigra (SN), a structure involved in the control of saccadic eye movements. Cell numbers were very similar in the ipsi- and contralateral SN of the hemispherectomized animal (329,926 vs. 310,248). These numbers are close to what was observed in the normal monkey (300,130 and 320, 859). In one case, part of the striatum was lesioned in addition to the cerebral hemisphere. Noticeable effects were observed in the SN ipsilateral to the cortical lesion: volume was reduced by 30.5% while the number of neurons, compared to the contralateral side, dropped by 43.2% (186,644 vs. 328,757). These results suggest that due to its anatomical sparing following hemispherectomy the SN, in addition to other subcortical structures, is in a prime position to modulate the spared saccadic behaviors seen after massive cortical injuries.

Animals↗

PubMed1999

Transneuronal degeneration of retinal ganglion cells in early hemispherectomized monkeys.

Transneuronal retrograde cell changes in the retina of the primate have been well documented after lesions to striate cortex, but little is known about the effects of hemispherectomy, a surgical procedure used in humans for the treatment of intractable epilepsy. In order to follow the time course of this degenerative process, we examined the retinae of six monkeys who underwent a total hemispherectomy at various postnatal ages with a survival period of 4 years. We demonstrate that transneuronal retrograde degeneration in the retina following hemispherectomy is inversely correlated with age at the time of the lesion. This degeneration is maximal when the lesion is induced within the first 4-6 months of life and less pronounced from 8 months to adulthood.

Animals↗

PubMed1999

Activation of human extrageniculostriate pathways after damage to area V1.

We have used positron emission tomography (PET) to specify the cortical and subcortical structures activated following visual stimulation of the scotomatous field in a patient with an asymmetric bilateral developmental anomaly of the visual cortex. Computerized perimetry indicated a left visual field defect only although MRI and 18FDG-PET scans showed abnormalities in both occipital lobes. The visual stimuli were semicircular gratings moving in opposite directions on a dynamic random-dot background. They were specifically constructed to eliminate intra- and extraocular light scatter and to optimize the activation of extra-striate cortical areas and their projecting subcortical relays. For anatomical localization PET images were coregistered to the subject's MRI in Talairach coordinates. After subtraction of the baseline conditions from the stimulation conditions, a t-statistic map was created on a voxel-by-voxel basis. Stimulation of the scotomatous hemifield yielded significant activations of Brodmann cortical areas 18-19 and 47 as well as the pulvinar thalami of the left hemisphere, in addition to a less prominent activation in the right hemisphere. Stimulation of the intact hemifield produced significant activation of Brodmann cortical areas 30 and 47 of the left hemisphere. These results suggest that in the absence of area V1, residual vision observed in the blind hemifield could be mediated by a retinofugal pathway to extrastriate cortex via the pulvinar.

Adult↗

PubMed1999

Activation of the remaining hemisphere following stimulation of the blind hemifield in hemispherectomized subjects.

We used functional magnetic resonance imaging (fMRI) to investigate the neural substrates mediating residual vision in the "blind" hemifield of hemispherectomized patients. The visual stimuli were semicircular gratings moving in opposite directions on a dynamic random-dot background. They were specifically constructed to eliminate intra- and extraocular light scatter and optimize the activation of extrastriate cortical areas and their subcortical relays. Multislice T2*-weighted gradient echo (GE) echoplanar imaging (EPI) images (TR/TE = 4 s/45 ms, flip angle 90 degrees ) were acquired during activation and baseline visual stimulation. An activation minus baseline subtraction was performed, and the acquired t statistic map transformed into the stereotaxic coordinate space of Talairach and Tournoux. In seven normal control subjects, right hemifield stimulation produced significant activation foci in contralateral V1/V2, V3/V3A, VP, and V5 (MT). Significant activation was also produced in homologous regions of the right occipital lobe with left hemifield stimulation. Stimulation of the intact hemifield in hemispherectomized patients resulted in activation of similar areas exclusively within the contralateral hemisphere. Stimulation of the anopic hemifield produced statistically significant activation in the ipsilateral occipital lobe (putative area V5 or MT) and areas V3/V3A in the only subject with blindsight. We conclude that the remaining hemisphere may contribute to residual visual functions in the blind hemifield of hemispherectomized patients, possibly through the collicular-pulvinar route since the activated areas are known to receive their afferents from these subcortical nuclei.

Adult↗

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↗