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N Berardi

Publications and source records attributed to N Berardi.

At least 37 records · Page 2Linked to original sources
PubMed1990

Transplant of embryonal nervous tissue preserves the responses of rat retinal ganglion cells after section of the optic nerve.

We have investigated the effectiveness of embryonal tectal tissue transplants in preserving the physiological activity of lesioned ganglion cells by recording the visual responses from the adult rat retina after section of the optic nerve, with or without transplants of embryonal nervous tissue on the stump. We have found that transplant of embryonal nervous tissue at the level of the optic nerve section has dramatic effects in preserving visual retinal responses to patterned stimuli for times as long as five months after surgery. By this time retinal responses to patterned stimuli have almost completely disappeared in control animals with optic nerve section alone.

Action Potentials↗
PubMed1990

Pattern ERG in rats following section of the optic nerve.

The aim of this study is to investigate in the rat the properties of the pattern electroretinogram (ERG) and to assess whether it depends upon the functional integrity of ganglion cells. Flash and pattern ERG were recorded from urethane anaesthetized hooded rats. The pattern ERG was evoked by phase alternating gratings of various spatial frequencies and contrasts. In the first part of the study we determined how the amplitude of the main harmonic of the pattern ERG (2nd harmonic) varies as a function of stimulus parameters such as spatial and temporal frequency, contrast and mean luminance. In the second part of the study we investigated the effects of the retrograde degeneration of ganglion cells following optic nerve section on the amplitude of pattern ERG. We found that the section of the optic nerve leads to the progressive disappearance of the P-ERG which is almost complete 4 months after surgery. By this time only few axotomized ganglion cells are left. The flash ERG remained unaffected. Thus, the pattern electroretinogram seems to be a simple and sensitive tool to investigate the functional integrity of retinal ganglion cells in rats.

Animals↗
PubMed1989

Electrophysiological evidence for interhemispheric transmission of visual information in man.

1. Electrophysiological evidence is presented of interactions between two stimuli (sinusoidal gratings of equal spatial frequency but different contrast, phase-reversed sinusoidally at different temporal frequencies) located on opposite side of, and within a few degrees from, the vertical meridian. 2. These interactions are revealed by a depression of the cortical visual evoked potential (VEP) evoked by the grating of lower contrast in the presence of the grating of higher contrast. They are similar to, albeit weaker than, those obtained with superimposed asynchronously modulated gratings. 3. The VEP reduction occurs also if the stimuli are presented dichoptically. 4. It does not occur if the gratings are located one above the other either on the same or on opposite sides of the vertical meridian. 5. The strength of the VEP reduction depends on the relative contrast of the two gratings and vanishes for spatial frequencies beyond 4 cycles/deg and temporal frequencies of the high-contrast grating beyond 10 Hz. 6. The results are in agreement with data on visual callosal connections in animals and confirm previous psychophysical findings (Berardi & Fiorentini, 1987) indicating the particular properties of the interhemispheric cross-talk between symmetric regions of the visual field astride the vertical meridian in man.

Brain↗
PubMed1988

Functional dissociation of the hemispheres in the discrimination of complex gratings near the vertical meridian.

It was previously shown that the discrimination of complex gratings differing in the spatial phase of their harmonic components is performed more accurately when the stimuli are presented in the left than in the right visual hemifield, provided that the stimuli are separated at least 2 deg from the vertical meridian (VM) and/or their spatial frequency exceeds 2 c/deg. Here we report measurements of reaction times (RT) for the discrimination of complex gratings presented laterally at various distances from the VM. The (choice) RTs are found to be longer in the right than in the left hemifield even for stimuli of low spatial frequency at 1 deg from the VM. This rules out the possibility that a naso-temporal overlap in the retino-cortical projection in man is exploited in "bridging the gap" between the two hemispheres in the discrimination of complex gratings.

Corpus Callosum↗
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↗
PubMed1987

Interhemispheric transfer of visual information in humans: spatial characteristics.

1. The problem of the interhemispheric transfer of visual information in humans has been approached psychophysically, making use of a visual discrimination task that shows a clear left field advantage and is subject to the phenomenon of perceptual learning. 2. For this task (discrimination of complex gratings differing only by the relative spatial phase of their harmonic components) there is a left field advantage and a lack of interhemispheric transfer of learning effects at all spatial frequencies tested for stimuli removed at least 5 deg from either side of the vertical meridian. 3. For stimuli close to the vertical meridian, the left field advantage disappears and there is a complete transfer of learning effects, provided the fundamental spatial frequency is 2 cycles/deg or lower. 4. At higher spatial frequencies the left field advantage is maintained and the learning effects do not transfer from one visual hemifield to the other, even at +/- 0.5 deg from the vertical meridian, unless the contrast is very high. 5. The transfer of learning effects obtained for spatial frequencies of 2 cycles/deg or lower is peculiar to regions placed close to the vertical meridian and symmetrically located on either side of it. No transfer is obtained between non-overlapping regions on the same side of the vertical meridian. 6. These findings are consistent with an interhemispheric transfer of visual information, preferential for low spatial frequencies and high contrasts, in agreement with that found for callosal transfer in the cat (Berardi, Bisti & Maffei, 1987).

Corpus Callosum↗
PubMed1986

Binocular suppression in cortical neurons.

Dichoptic presentation of patterns similar in shape but of very different contrast results in the perception of only the high contrast pattern (binocular suppression). When recording from binocular neurons of the cat visual cortex, we have found an effect which is strikingly similar to this perceptual phenomenon. If a high and a low contrast grating are presented simultaneously, one to each eye, the cell's response to the low contrast stimulus is suppressed.

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↗
PubMed1984

Right-hemisphere superiority in the discrimination of spatial phase.

Visual field differences have been investigated in various detection and discrimination tasks for simple sinusoidal gratings or for complex gratings composed of two sinusoids of spatial frequencies f and 3f. Sinusoidal gratings were employed to evaluate contrast sensitivity, subthreshold summation effects, aftereffects of adaptation to a high-contrast grating, and spatial-frequency discrimination. The tasks with complex gratings were detection of the 3f component in the presence of a high-contrast f component and spatial-phase discrimination. The stimuli were presented either in the left or in the right visual hemifield. The results indicate a lack of lateralization for detection and spatial-frequency discrimination of sinusoidal gratings, and for the bandwidth of subthreshold summation effects and adaptation aftereffects, whereas the detection of the 3f component in the presence of a high-contrast f component, as well as spatial-phase discrimination of f +3f gratings, show a left-field advantage. This suggests a right-hemisphere superiority in the processing of spatial phase.

Cerebral Cortex↗
PubMed1984

The role of gamma-aminobutyric acid mediated inhibition in the response properties of cat lateral geniculate nucleus neurones.

We studied the effect of local ionophoretic application of bicuculline on the response of cat lateral geniculate nucleus (laminae A) cells to stimulation by sinusoidal gratings and spots of light. Application of bicuculline produced an increase both of spontaneous and visually driven discharge of both X and Y cells. On stimulation by drifting sinusoidal gratings, the average discharge of both X and Y cells remained constant with increasing contrast under normal conditions. Application of bicuculline caused the average discharge to increase with contrast, indicating that the constancy of the average discharge was maintained by gamma-aminobutyric acid mediated inhibition. Under normal conditions, the amplitude of response modulation of both X and Y cells to sinusoidal grating stimulation increased monotonically with stimulus contrast. During bicuculline application, the slope of the contrast-response curve for X cells but not for Y cells increased, indicating that the inhibition which dampened the modulation of X cells (but not Y cells) was contrast dependent. Application of acetylcholine also increased the average discharge and the amplitude of modulation of the cell responses, but this increase did not depend on stimulus contrast. Under normal conditions, X but not Y cells showed an attenuation of response and an increase in contrast threshold to low spatial frequencies. This attenuation vanished during bicuculline application. The shape of Y-cell response curves was unaffected by bicuculline. Bicuculline had the same effect on the non-linear component of Y-cell response as on the linear component. Although bicuculline had a different effect on the response of X and Y cells to stimulation by gratings, it reduced the antagonistic surround of both X and Y cells to a similar extent (revealed by plotting the cell receptive fields with flashed spots of light).

Acetylcholine↗
PubMed1984

Development of gamma-aminobutyric acid mediated inhibition of X cells of the cat lateral geniculate nucleus.

We studied the development of gamma-aminobutyric (GABA) mediated inhibitory processes of the lateral geniculate nucleus cells in kittens of various age groups, by measuring the effect of ionophoretic application of GABA and bicuculline on cell response to sinusoidal gratings. In young kittens (less than 30 days) we found very few Y cells only X cells and weakly responsive cells which fitted neither X nor Y classifications ('immature' cells). As with adult cells, GABA inhibits the visual response of young kitten cells. Simultaneous application of bicuculline restored responsiveness. The mean GABA current to silence cell response in young kittens was significantly higher than that obtained in adult cats. Application of bicuculline alone had little effect on young kitten X-cell responsiveness, either on the average discharge or on the amplitude of modulation to stimulation by sinusoidal gratings. For older kittens (40-45 days), bicuculline increased X-cell responsiveness, and the increase in responsiveness was dependent on stimulus contrast and spatial frequency. However, the increased responsiveness was less than that for adults. At 100 days the changes of slope of X-cell contrast-response curves during bicuculline application were similar to those observed for adult X cells. We conclude that, although GABA receptors may be present at 30 days, the GABA mediated inhibitory system does not begin to function until about 45 days and does not mature fully until about 100 days.

Aging↗
PubMed1983

The cholinergic influence on the function of the cat dorsal lateral geniculate nucleus (dLGN).

The functional influence of the cholinergic input to cat dLGN has been examined by assessing the action of iontophoretically applied acetylcholine (ACh) on the visual responses of cells in layers A and A1. Iontophoretically applied pulses of ACh exerted a strong excitatory action on all 113 cells studied within these layers. In the presence of a sustained application of ACh, the excitatory responses to an optimal stimulus such as a spot of light located within the receptive field centre were greatly facilitated, but at the same time stimulus-specific inhibitory influences were also enhanced. The action of ACh on the stimulus-specific inhibitory influences had the consequence that the responses to non-optimal stimuli were not facilitated to the same extent as those to optimal stimuli and in some cases even diminished. The stimulus-specific inhibitory effects seen in the presence of ACh were very powerful and frequently resulted in complete suppression of the elevated background discharge. We suggest that the ACh directly excites both the relay cells and the Golgi type II inhibitory interneurones within the dLGN. The facilitation of the stimulus-specific inhibition may follow from a direct action on the presynaptic dendrites of the Golgi type II cells which arborize within the dendritic field of the relay cell. Supplementary observations on cells in the perigeniculate nucleus confirm previous findings showing that ACh has an inhibitory effect on these cells. We suggest a tripartite action for the cholinergic influence on the dLGN, involving direct facilitation of relay cells, enhancement of stimulus-specific inhibition via the Golgi type II cells, and disinhibition of the non-specific inhibitory influence form the perigeniculate nucleus.

Acetylcholine↗
PubMed1982

Role of extraocular proprioception in the orienting behaviour of cats.

We investigated the effects of unilateral deafferentation of extraocular proprioception on the orienting behaviour of cats. The ophthalmic branch of the Vth cranial nerve, in which most of the extraocular proprioceptive fibers are known to run, was cut unilaterally in adult cats. The animals had been previously trained to jump from a start box to a luminous target at various possible locations in the right or left visual hemifield. The postsurgical orienting performance of the animals, evaluated from the difference between landing position and position of the target, differed from the presurgical performance. The postsurgical errors were typically: (1) ipsiversive to the side of the section, (2) usually of a larger magnitude in the hemifield contralateral to the section, (3) present in binocular as well as in monocular viewing conditions and approximately of the same magnitude for jumps guided by either eye. These findings indicate that extraocular proprioception plays a role in the orienting behaviour of cats.

Afferent Pathways↗
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↗
PubMed1980

Perceptual learning specific for orientation and spatial frequency.

Several examples of 'perceptual learning' (improvement of some perceptual task with practice) have been reported. These studies are of great interest for neurological research because they demonstrate plasticity of the nervous system. Even for apparently basic perceptual tasks, such as visual acuity or vernier acuity, practice can facilitate a neural change which enhances performance. One question in this field is where does this learning occur? Indications about the possible neural site of a learning process may be derived from its specificity for some particular stimulus parameters. For instance, there is a hint that learning in global stereopsis may occur at a stage where visual information is processed by mechanisms selectively sensitive to different stimulus orientations. We report here an experiment on perceptual learning in the discrimination of gratings of different waveform. Our findings show that learning is specific for both the orientation and the spatial frequency of the practice stimulus.

Discrimination, Psychological↗
PubMed1980

A re-evaluation of the mechanisms underlying simple cell orientation selectivity.

Following from evidence supporting GABA as a putative inhibitory transmitter in the visual cortex, we have iontophoretically applied the GABA antagonist N-methyl bicuculline (Nmb) to simple cells in order to block the inhibitory inputs acting on them. We found that under these conditions previously sharply-tuned simple cells responded equally to all orientations. Moreover receptive field dimensions, judged by the response to stimuli at the optimal and orthogonal orientations, equated best with that expected from a single dLGN cell input. It seems thus, that asymmetries in the excitatory input are not a significant factor in the generation of simple cell orientation selectivity. The asymmetry underlying orientation selectivity rather originates from the operation of an intracortical inhibitory mechanism.

Animals↗