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Biomedical subjects

S Bisti

Publications and source records attributed to S Bisti.

At least 37 records · Page 2Linked to original sources
PubMed1993

Visual performance in behaving cats after prenatal unilateral enucleation.

Prenatal unilateral enucleation in mammals causes an extensive anatomical reorganization of visual pathways. The remaining eye innervates the entire extent of visual subcortical and cortical areas. Electrophysiological recordings have shown that the retino-geniculate connections are retinotopically organized and geniculate neurones have normal receptive field properties. In area 17 all neurons respond to stimulation of the remaining eye and retinotopy, orientation columns, and direction selectivity are maintained. The only detectable change is a reduction in receptive field size. Are these changes reflected in the visual behavior? We studied visual performance in cats unilaterally enucleated 3 weeks before birth (gestational age at enucleation, 39-42 days). We tested behaviorally the development of visual acuity and, in the adult, the extension of the visual field and the contrast sensitivity. We found no difference between prenatal monocularly enucleated cats and controls in their ability to orient to targets in different positions of the visual field or in their visual acuity (at any age). The major difference between enucleated and control animals was in contrast sensitivity:prenatal enucleated cats present a loss in sensitivity for gratings of low spatial frequency (below 0.5 cycle per degree) as well as a slight increase in sensitivity at middle frequencies. We conclude that prenatal unilateral enucleation causes a selective change in the spatial performance of the remaining eye. We suggest that this change is the result of a reduction in the number of neurones with large receptive fields, possibly due to a severe impairment of the Y system.

Animals↗
PubMed1991

Two systems of branching axons in monkey's retina.

Several monkey retinae were stained, by using the reduced silver technique, in order to analyse long-distance intraretinal connections. Long, bifurcating processes covering very large areas were identified. Morphological investigation of these processes suggest that they are members of two different systems of branching axons. The first population of these processes originates as axon collaterals from cell in the ganglion cell layer. These cells have a relatively large, elongated soma and straight, sparsely branching dendrites, stratified in the vitreal half of the inner plexiform layer. The main axon (0.6 microns average diameter) passes along the optic fibre bundles, disappearing into the optic disk, whilst its collaterals run mainly in the inner plexiform layer. A cell showing similar morphology has also been found in the ganglion cell layer of a cat retina. The second population of processes consists of very thick fibres (2.1 microns average diameter) apparently originating from the optic disk. The main branches run in the space between the optic fibre layer and the ganglion cell layer, with short, secondary processes crossing the ganglion cel layer orthogonally. Many higher-order processes originate from the second-order branches; these run almost horizontally in the inner plexiform layer. The ganglion cells generating axon collaterals may constitute an intraretinal firing synchronization system, or they may be a residual feature of retinal development. The centrifugal fibres may be related to the sensitivity control during retinal dark adaptation.

Animals↗
PubMed1990

The visual acuity of the lynx.

Visual evoked potentials were recorded from the occipital scalp of two anaesthetized Lynx (Lynx europea) in response to alternating gratings of various spatial frequencies and contrasts. The visual acuity of the Lynx was found to be around 5-6 c/deg, i.e. very close to the visual acuity of the cat and by far inferior to human acuity.

Animals↗
PubMed1988

The organization of receptive fields in area 18 neurones of the cat varies with the spatio-temporal characteristics of the visual stimulus.

The spatial frequency tuning curves of neurones of area 18 depend upon the velocity of the visual stimulus. The higher the velocity the lower the spatial frequencies to which the cell is tuned. Since in area 17 the size of the cell receptive field is inversely related with the optimal spatial frequency to which the cell responds, we have investigated whether the shift of the optimal spatial frequency with the velocity corresponds to a "change" in the receptive field size. We recorded extracellularly from neurones in area 18; for each cell we selected two gratings, one of high spatial frequency drifting at low velocity and another of low spatial frequency drifting at high velocity to which the cell gave comparable responses. The results show that the masking of the cells receptive field which abolishes the response to the high frequency low velocity grating does not prevent the cell from responding to the low frequency high velocity grating. We conclude that the size of the receptive field of neurones in area 18 depends upon the characteristics (spatial frequency and velocity) of the visual stimulus.

Animals↗
PubMed1987

The transfer of visual information across the corpus callosum: spatial and temporal properties in the cat.

1. The spatial and temporal characteristics of the visual information transmitted across the corpus callosum have been studied in normal cats by recording directly from the corpus callosum and in split-chiasm cats by means of visual evoked potentials (v.e.p.s) and single-unit recordings at the 17/18 border. 2. The modulation transfer functions (m.t.f.s) obtained by recording from the corpus callosum are comparable to the m.t.f.s evaluated by various techniques for the whole visual system of the cat. The spatial and temporal acuities, however, do not reach the values obtained behaviourally or estimated with cortical evoked potentials. 3. In split-chiasm cats, both v.e.p.s and single-unit recordings indicate that the contrast gain of the callosal pathway is considerably lower than the gain of the direct, geniculo-cortical system. Spatial and temporal acuities are lower for the callosal than for the direct system. 4. The same differences in contrast gain between the spatial m.t.f. obtained for the callosal and the direct system have been found in alert split-chiasm cats. 5. Our data suggest that the cross-talk between the hemispheres taking place across the corpus callosum is nearly abolished at low contrasts and high spatial and temporal frequencies.

Animals↗
PubMed1986

Monocular deprivation in kittens differently affects crossed and uncrossed visual pathways.

The effects of monocular deprivation (MD) on the crossed and uncrossed visual projections were studied using both electrophysiological and behavioural criteria. Our results show that Visual Evoked Potentials (VEPs) from the deprived eye (DE) in response to contrast reversing gratings are more reduced in the ipsilateral than in the contralateral cortex. This suggests a different sensitivity of the crossed and uncrossed visual pathways to MD. In the behavioural experiments comparable findings were obtained.

Animals↗
PubMed1986

Interocular transfer of adaptation after effect in neurons of area 17 and 18 of split chiasm cats.

Responses to sinusoidal gratings for neurons in area 17 and 18 of split chiasm cats were recorded extracellularly, and the interocular transfer of the effect of adaptation to high-contrast gratings was studied. In area 17 all but one of the simple cells showed the phenomenon of adaptation and its interocular transfer; 60% of the complex cells showed the effect of adaptation, and of these cells 35% showed an interocular transfer of adaptation. The adaptation aftereffect was comparable both in strength and duration for the direct and the callosal pathway. The strength of the adaptation aftereffect through the callosal pathway was not related to the strength of the input from the contralateral eye. An interocular transfer of the adaptation aftereffect was found in several neurons with a very weak input from the contralateral eye and in five simple cells apparently responding only to the ipsilateral eye. Fifty-eight percent of the neurons in area 18 showed the effect of adaptation, and 55% of them showed interocular transfer. No interocular transfer of the adaptation aftereffect was found in those neurons where an input from the contralateral eye was undetectable. Interocular transfer of the adaptation was found in all the neurons recorded in area 17 of animals with section of the corpus callosum but intact chiasm. No interocular transfer was found in neurons recorded in area 17 of cats with both the optic chiasm and the corpus callosum sectioned. We conclude that callosal connections are sufficient for the transfer of the adaptation aftereffect, although they are not necessary.

Adaptation, Ocular↗
PubMed1985

Pattern ERG in the monkey after section of the optic nerve.

Electroretinographic responses (ERG) to homogeneous light flashes and to alternating gratings were recorded from either eye of monkeys in which one optic nerve had been previously sectioned. Three weeks after optic nerve section the ERG response of the operated eye to alternating gratings was drastically reduced in amplitude and 5 weeks after surgery it was reduced to noise level. The uniform field ERG was unaffected. Histological examination of whole-mounted retinas of monkeys sacrificed 6 and 8 weeks after optic nerve section showed a loss of ganglion cells in the operated eye. It is concluded that the integrity of ganglion cells is essential for the generation of a normal ERG response to gratings.

Animals↗
PubMed1985

Spatial-frequency characteristics of neurones of area 18 in the cat: dependence on the velocity of the visual stimulus.

The spatial and temporal response properties of neurones of areas 17 and 18 were studied in single units (165) of anaesthetized and paralysed cats. The visual stimuli were drifting or alternating gratings. We confirmed and extended the observation by Tolhurst & Movshon (1975) showing that the spatial-frequency characteristics of neurones of area 17 are largely independent of the temporal parameters of drifting or alternating gratings. The spatial-frequency tuning curves of neurones of area 18 shift along the spatial-frequency axis when the velocity or the temporal frequency of the drifting grating are changed. The effect of an increase either of velocity or temporal frequency is to shift the cell spatial-frequency tuning curve down the spatial-frequency scale, keeping relatively constant the strength and band width of the response. The spatial-frequency tuning curves of neurones of area 18 do not show this temporal-frequency-dependent phenomenon when the stimuli are gratings alternated in phase. In this case neurones of areas 17 and 18 show a similar behaviour. The response properties of neurones of area 18 are compared with recent psychophysical results obtained in similar experimental conditions. The hypothesis is advanced that both areas 17 and 18 are devoted to the processing of spatial information. Area 17 would be responsible for the processing of patterns in stationary or quasi-stationary situations while area 18 would be responsible for that of patterns moving at high velocities.

Animals↗
PubMed1984

Can functional reorganization of area 17 following monocular deprivation be modified by GM1 internal ester treatment?

It has been extensively reported that monocular exposure early in life leads to profound alterations in visual cortical areas, where the majority of cells become responsive only to the stimulation of the normal eye. We have investigated a possible effect of the monosialoganglioside internal ester, termed AGF2, on the neuronal cortical plasticity of the kitten's visual cortex following monocular deprivation. Results indicate that in monocularly deprived kittens treated with ganglioside the ocular dominance shift in favor of the normal eye is partially prevented.

Animals↗
PubMed1984

Electroretinographic responses and retrograde changes of retinal morphology after intracranial optic nerve section. A quantitative analysis in the cat.

Previous experiments have shown that the ERG response to alternating gratings vanishes gradually within 4 months after transection of the optic nerve, changes begin after 2-3 weeks. The response to gratings of low spatial frequencies deteriorates earlier than the response to gratings of high spatial frequencies (Maffei and Fiorentini 1981). Quantitative analysis of ganglion cell sizes in retinal wholemounts shows that ganglion cell shrinkage and ganglion cell loss begin at three weeks in the periphery of the retina, particularly in the temporal retina. The same morphological alteration subsequently becomes apparent also in the area centralis and the nasal retina, respectively. The main and early cell loss occurs among medium sized ganglion cells, supposedly the beta-cells. Among the alpha-cells only shrinkage is observed up to two months postoperatively. Light- and electron microscopic examination of cross sections through the retina show that pathological changes are restricted to the innermost layers.

Animals↗
PubMed1982

Correlation between the preferred orientation and spatial frequency of neurones in visual areas 17 and 18 of the cat.

1. In seventy-six penetrations through areas 17 and 18 of the cat, neurones were regularly sampled at intervals of 100 micrometers and preferred orientation, optimal spatial frequency and resolving power were determined for each neurone in response to drifting sinusoidal gratings. 2. As already shown for area 17, in tangential penetrations through area 18, whenever the preferred orientation rotates progressively from cell to cell, the optimal spatial frequency tends to remain constant. 3. A statistical analysis on 1574 cells in areas 17 and 18 showed that for pairs of cells separated 200-300 micrometers along a track the difference in preferred orientation delta alpha and the difference in optimal spatial frequency delta f are not randomly distributed: cell pairs with small delta alpha are most likely to have large delta f and vice versa. 4. These findings indicate that in areas 17 and 18 neurones with the same optimal frequency are aligned along a direction orthogonal to the orientation columns. 5. The optimal spatial frequency, resolving power and the velocity cut-off were averaged for cells from different penetrations located in the same cortical layer or sublayer of area 18: mean optimal spatial frequency and acuity are highest in layer IV and lowest in layers II and V, while the velocity cut-off is highest in layers II and V and lowest in layer IV. 6. Our data suggest that the layering of cells according to optimal spatial frequency is a more subtle subdivision than the six histological layers.

Action Potentials↗