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Biomedical subjects

S Bisti

Publications and source records attributed to S Bisti.

50 records · Page 3Linked to original sources
PubMed1977

Spatial frequency and orientation tuning curves of visual neurones in the cat: effects of mean luminance.

Experiments have been performed on unanaesthetized and paralysed cats. The tuning curves for spatial frequency of retinal, lateral geniculate and simple and complex cells of the cortex have been determined in response to sinusoidal gratings of various spatial frequencies at different levels of mean luminance. For all neurones, decreasing the mean luminance leads to a progressive loss of spatial resolution and contrast sensitivity. Retinal ganglion cells of type X show, for scotopic levels of luminance, a flattening of their spatial frequency tuning curves in the low spatial frequency range. For geniculate and cortical neurones, on the contrary, the spatial frequency characteristics at the various levels of luminance remain practically invariant in their bandwidth. On the average, complex cells still respond to mean luminances ten times lower than simple cells. The tuning curves for orientation of cortical cells maintain, to a first approximation, the same shape at the various levels of mean luminance. The results are discussed comparing the electrophysiological with psychophysical data.

Animals↗
PubMed1976

Binocular interaction in strabismic kittens deprived of Vision.

Artificial strabismus in kittens decreases the proportion of binocularly driven units in area 17 of the cortex. This change in the binocular interaction of cortical cells also takes place if the kittens are deprived of vision from the day in which the strabismus is surgically produced to the day of the electrophysiological recording. Thus, altered motility of the eyes per se is sufficient to affect binocular interaction in the neurons of area 17 of the cortex.

Action Potentials↗
PubMed1974

Behavioural contrast sensitivity of the cat in various visual meridians.

1. The contrast sensitivity of the cat as a function of the spatial frequency of the visual stimulus was determined by using behavioural techniques.2. No statistically significant difference was found for the contrast sensitivity thresholds at various orientations of the stimulus.3. A comparison of the contrast sensitivity curve of the cat with that of the human shows that the two species have rather different characteristics of visual spatial resolution.

Animals↗
PubMed1973

Neural correlate of perceptual adaptation to gratings.

Exposure of simple cells of the cat striate cortex to high-contrast drifting gratings greatly reduces the subsequent response of the cells to low-contrast gratings. This adaptation effect has an average duration of 30 seconds and shows interocular transfer and selectivity for spatial frequency and orientation. This effect is strikingly similar to the perceptual adaptation to high-contrast gratings.

Action Potentials↗
PubMedDate not supplied

Pattern-reversal electroretinogram in response to chromatic stimuli: II. Monkey.

We have recorded steady-state PERGs from five macaque monkeys in response to red-green plaid patterns reversed sinusoidally in contrast. The patterns had either a pure luminance contrast (red-black, green-black, yellow-black), pure red-green color contrast, or a variable amount of luminance and color contrast. By varying the relative luminance of the red-to-total luminance (color ratio) of red-green patterns, a value could be obtained at which the PERG amplitude was either minimum or locally maximum, and the phase was most lagged. This value was very similar to that producing equiluminance in human observers, and was considered to be equiluminance for the monkey. The phase of the PERG to chromatic stimulus was systematically lagged compared with that of luminance stimuli, by an amount corresponding to about 10-20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 80 ms for color contrast compared with 63 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. As a function of temporal frequency, the chromatic PERG function was clearly low-pass with a cutoff around 15 Hz, whereas that to luminance was double-peaked and extended to higher temporal frequencies, around 30 Hz. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the luminance and chromatic responses of appropriate contrasts, we were able to predict with accuracy the response as a function of color ratio. In two monkeys, the optic chiasm was sectioned sagittally causing total degeneration of ganglion cells in the nasal retina, without affecting the temporal retina (verified by histology). In these animals, there was a strong response to both luminance and chromatic patterns in the temporal retinae, but none to either type of pattern in the nasal retinae, suggesting that the PERG to both luminance and chromatic stimuli arises from the inner-retinal layers. Electrophysiological studies suggest that the PERG to chromatic stimuli is probably associated with the activity of P-cells. P-cells may also make a major contribution to the PERG of luminance stimuli, although M-cells may also participate. The above findings on normal monkeys all agree with those reported in the accompanying paper for humans (Morrone et al., 1994), so similar conclusions can probably be extended to human PERG.

Animals↗
PubMedDate not supplied

Eye alignment and cortical binocularity in strabismic kittens: a comparison between tenotomy and recession.

Interocular alignment was assessed by corneal light reflex photography in 15 normal and 26 strabismic kittens. Strabismus was induced at 3-4 weeks of age by severing one extraocular muscle (tenotomy), by cutting and reinserting the muscle at another position on the ocular globe (recession), or by combining recession of the medial rectus muscle with resection of the lateral rectus muscle of the same eye. Nineteen strabismic and five normal kittens were followed longitudinally from 12 days to about 6 months of age. Three out of six longitudinally followed tenotomized cats and six out of the 13 recessed cats conserved their postoperative ocular deviation throughout the testing period ("large-angle strabismics"). Three tenotomized and seven recessed cats showed a transient deviation for 1-2 weeks after surgery, after which the interocular deviation diminished to values found in normal cats ("microstrabismic" cats). Both recessed-resected cats showed a transient interocular deviation. In spite of their different developmental histories, all cats showed a clear breakdown of binocularity in area 17. Large-angle strabismics showed a dominance of the non-operated eye, while in microstrabismic cats, both eyes were equally effective in driving cortical cells. It thus appears that a transient strabismus is sufficient to produce a reduction of binocularity in area 17.

Animals↗
PubMedDate not supplied

Normal EOG values in intraretinal metastasis from cutaneous melanoma: a case report.

PURPOSE: The electro-oculogram (EOG) is a powerful test to diagnose primary and metastatic choroidal tumors. While in benign tumors light-peak to dark-trough ratio values are in the range of normal subjects, these values appear highly altered in eyes affected by malignant choroidal tumors. Here we report a clinical case of a patient with intraretinal metastasis from cutaneous melanoma; notwithstanding the malignancy of the tumor, the EOG doesn't present alterations. METHODS AND RESULTS: Standard electro-oculographic recordings were performed before and after local excision of the tumor: recordings from the normal eye were taken as control. The EOG values were always normal in both eyes. Histological sections showed no evident change in the cell population of the retinal pigment epithelium (RPE) and Bruch's membrane. CONCLUSION: Our results suggest that the presence of an intact RPE is a crucial requirement to obtain a normal EOG.

Adult↗