Biomedical subjects

M A Paradiso

Publications and source records attributed to M A Paradiso.

8 recordsLinked to original sources

PubMed1991

Brightness perception and filling-in.

Three experiments were performed in which a stimulus with homogeneous color and luminance was masked by a second stimulus containing contours. In the first experiment the target was a large white disk and the mask was a white circle concentric with the disk but of smaller radius. We found that the mask had a large (up to 2 log unit) suppressive effect on the brightness of the target, but only inside the radius of the mask. With monoptic presentation of target and mask, the greatest suppression was observed with an SOA of 50-100 msec. With dichoptic presentation the strongest suppression was obtained with simultaneous stimuli. The second experiment demonstrated that the latest time at which masking was effective was correlated with the distance between the edge of the target stimulus and the contour in the mask. One possible explanation of the results from these two experiments is that the masking contour is interfering with the propagation of a brightness signal from the target's border. In the third experiment gaps were introduced into the masking circle. Surprisingly, even with rather large gaps there was significant suppression of brightness in the center of the target. We have encountered difficulties attempting to account for these findings with known physiological mechanisms such as lateral inhibition. A qualitative explanation of the results that looks promising is a two-component process involving brightness filling-in and smoothing to satisfy fixed boundary conditions at contours.

Color Perception↗

PubMed1989

Subjective contours, tilt aftereffects, and visual cortical organization.

The tilt aftereffect (TAE) was used to study interactions between real and subjective contours. Subjects adapted to either real or illusory lines and were then shown test stimuli containing real or illusory lines. In our first experiment, we found that there is a marked asymmetry in the interactions between real and subjective stimuli. Adaptation to real lines produces comparable TAEs with real and subjective test lines. With either type of test stimulus the maximum effect occurs with a 10-20 deg difference between the orientations of the adaptation and test stimuli. Also, there is a strong TAE when the adaptation and test stimuli contain only subjective lines. However, there is a significantly weaker TAE when the adaptation stimulus is subjective and the test stimulus is real. In a second experiment we find that interocular transfer of tilt aftereffects is greater when the test stimulus is subjective than when it is real. These results are consistent with physiological reports that a subset of orientation selective cells in visual cortex is responsive to subjective contours and that these cells are more binocular, on average, than those responsive only to real contours. Our findings also suggest that the perception of subjective contours is based on the activation of neurons with properties, such as orientation selectivity, which are characteristic of early visual cortical areas.

Adaptation, Ocular↗

PubMed1989

A physiological correlate of the Pulfrich effect in cortical neurons of the cat.

When a swinging pendulum is viewed with a light-attenuating filter before one eye, the pendulum bob is perceived to move in an elliptical path in depth. It is believed that the filter causes this illusion, the Pulfrich effect, by delaying processing of the image in the filtered eye relative to that of the unfiltered eye. We sought a physiological correlate of this effect by studying binocular integration in cortical neurons of cats while they viewed moving stimuli. Special attention was focused on single unit disparity tuning because it is widely believed that depth perception is related to the responses of disparity selective neurons in visual cortex. We found that placing a filter before one of the cat's eyes produced a temporal delay in the cortical response. The temporal delay was always associated with a shift in the neuron's spatial disparity tuning. The observed temporal delays and disparity shifts are comparable with the magnitude of the Pulfrich effect in humans.

Action Potentials↗

PubMed1989

Cortical processing of hyperacuity tasks.

Spatial discrimination thresholds were determined by having subjects make comparisons between stimuli presented successively at the same or at different locations in the visual field. Two tasks were employed, discrimination of line orientation and discrimination of the distance between two parallel lines (spatial interval discrimination). We find that discrimination thresholds based on the comparison of features in spatially-separated stimuli are comparable to those based on the comparison of features in two stimuli presented successively at the same location. This holds even when the stimuli are presented in nonhomologous positions in the visual field or are presented in a manner such that they activate cells in opposite cortical hemispheres in the early visual areas. This shows that discriminability is not determined solely by differences in the firing rates of striate or pre-striate neurons. Rather, it appears that the attributes of visual stimuli are precisely coded and available for comparison at higher levels of visual processing. Implications of this finding for models of hyperacuity are discussed.

Discrimination, Psychological↗

PubMed1988

Blockade of intracortical inhibition in kitten striate cortex: effects on receptive field properties and associated loss of ocular dominance plasticity.

We have investigated the importance of GABAergic inhibition for the receptive field properties and plasticity of cells in the visual cortex of kittens. Osmotic minipumps were used to continuously infuse the GABA-antagonist, bicuculline methiodide (BIC), into striate cortex. Extracellular recordings were made during BIC infusion to assess neuronal response properties during the blockade of inhibition. Recordings were also made from other kittens after concurrent monocular deprivation and BIC infusion to investigate the importance of response selectivity for ocular dominance plasticity. The minipump delivery technique was used to produce a large volume of cortex presumably free of GABA-ergic inhibition. Compared to recordings in saline-infused control hemispheres, about half of the cells in bicuculline-infused hemispheres had abnormally low orientation selectivity. The low selectivity was generally accompanied by marked anomalies in several other receptive field properties. Particularly striking was the large size of the receptive fields. At eccentricities less than 10 deg many receptive fields subtended from 10 to over 30 deg of arc. The less selective neurons also had abnormal responses to flashed stimuli, giving strong transient responses to the onset and offset of large stationary stimuli which filled their receptive fields. These results imply that intracortical inhibition normally suppresses responses to stimuli within a large excitatory zone beyond the classical receptive field. Inhibition is necessary for the normal orientation selectivity of many cells, although the selectivity may be partially established by the cell's excitatory input. Additionally, intracortical inhibition appears to be necessary for the antagonism and segregation of ON and OFF receptive field subregions. In our study of plasticity, we exploited the fact that BIC treatment greatly increases the range of stimuli that activate cortical neurons. Kittens were monocularly deprived for 7 days concurrently with cortical infusion of BIC. After cessation of the drug treatment, physiological recordings were made. Response properties had returned to normal but neurons in BIC-infused hemispheres had a significantly reduced ocular dominance shift compared to neurons in control hemispheres. This is probably related to the reduced selectivity of cells during BIC infusion. The suggestion here is that there is diminished ocular dominance plasticity in BIC-infused hemispheres because of an increased probability of correlated activity between spontaneous discharge from the closed eye and the cortical activity evoked by the open eye afferents.

Animals↗

PubMed1988

A theory for the use of visual orientation information which exploits the columnar structure of striate cortex.

A neural model is constructed based on the structure of a visual orientation hypercolumn in mammalian striate cortex. It is then assumed that the perceived orientation of visual contours is determined by the pattern of neuronal activity across orientation columns. Using statistical estimation theory, limits on the precision of orientation estimation and discrimination are calculated. These limits are functions of single unit response properties such as orientation tuning width, response amplitude and response variability, as well as the degree of organization in the neural network. It is shown that a network of modest size, consisting of broadly orientation selective units, can reliably discriminate orientation with a precision equivalent to human performance. Of the various network parameters, the discrimination threshold depends most critically on the number of cells in the hypercolumn. The form of the dependence on cell number correctly predicts the results of psychophysical studies of orientation discrimination. The model system's performance is also consistent with psychophysical data in two situations in which human performance is not optimal. First, interference with orientation discrimination occurs when multiple stimuli activate cells in the same hypercolumn. Second, systematic errors in the estimation of orientation can occur when a stimulus is composed of intersecting lines. The results demonstrate that it is possible to relate neural activity to visual performance by an examination of the pattern of activity across orientation columns. This provides support for the hypothesis that perceived orientation is determined by the distributed pattern of neural activity. The results also encourage the view that limits on visual discrimination are determined by the responses of many neurons rather than the sensitivity of individual cells.

Animals↗

PubMed1988

Orientation discrimination as a function of stimulus eccentricity and size: nasal/temporal retinal asymmetry.

Orientation discrimination threshold is a monotonically increasing function of retinal eccentricity. Increasing stimulus length extends the range of eccentricities over which fine orientation discriminations can be made. Orientation discrimination thresholds at all eccentricities are determined by the size of the cortical image of the stimulus. Thresholds obtained using either nasal or temporal hemiretina are similar up to the blind spot, beyond which the temporal retina yields increasingly higher thresholds. The results are consistent with a recent theoretical study which predicts that orientation discrimination threshold is determined by the number of cortical cells activated by the discrimination target.

Humans↗

PubMedDate not supplied

Two methods of catecholamine depletion in kitten visual cortex yield different effects on plasticity.

As first clearly demonstrated by the experiments of Wiesel and Hubel, the developing visual cortex is exquisitely sensitive to sensory deprivation. Temporary closure of one eye of a kitten during a critical period that extends from 3 weeks to 3 months of age results in a dramatic cortical reorganization such that most neurones, originally binocularly driven, are dominated exclusively by the open eye. Recently, attention has been directed to chemical factors which may influence the degree of plasticity during the critical period. The work of Kasamatsu and pettigrew suggests that cortical catecholamines, especially noradrenaline (NA), are essential for the normal plastic response to visual deprivation. In an effort to clarify the role of NA in visual cortical plasticity, we have monocularly deprived kittens whose cortex had been depleted of catecholamines by the neurotoxin 6-hydroxydopamine (6-OHDA). We used two strategies to deplete cortical NA: the first, pioneered by Kasamatsu el al., utilized osmotic minipumps to deliver 6-OHDA to visual cortex; the second involved systemic neonatal injections of 6-OHDA, a technique which has proved effective in rodents. We found, using high-pressure liquid chromatography (HPLC), that both techniques produced a substantial reduction in the level of cortical NA. However, single unit recording in area 17 revealed that the plastic response to monocular deprivation (MD) was only diminished in the kittens depleted by minipump.

Animals↗