Biomedical subjects

U Polat

Publications and source records attributed to U Polat.

9 recordsLinked to original sources

PubMed1998

Collinear stimuli regulate visual responses depending on cell's contrast threshold.

Neurons in the primary visual cortex are selective for the size, orientation and direction of motion of patterns falling within a restricted region of visual space known as the receptive field. The response to stimuli presented within the receptive field can be facilitated or suppressed by other stimuli falling outside the receptive field which, when presented in isolation, fail to activate the cell. Whether this interaction is facilitative or suppressive depends on the relative orientation of pattern elements inside and outside the receptive field. Here we show that neuronal facilitation preferentially occurs when a near-threshold stimulus inside the receptive field is flanked by higher-contrast, collinear elements located in surrounding regions of visual space. Collinear flanks and orthogonally oriented flanks, however, both act to reduce the response to high-contrast stimuli presented within the receptive field. The observed pattern of facilitation and suppression may be the cellular basis for the observation in humans that the detectability of an oriented pattern is enhanced by collinear flanking elements. Modulation of neuronal responses by stimuli falling outside their receptive fields may thus represent an early neural mechanism for encoding objects and enhancing their perceptual saliency.

Animals↗

PubMed1998

Elongated physiological summation pools in the human visual cortex.

The visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to probability summation between the outputs of linear, independent spatial filters, although non-linear spatial summation can have similar effects [1]. In order to distinguish between probabilistic and physiological summation models, we measured contrast thresholds using the Visual Evoked Potential (VEP). Our previous work [2] suggests that spatial summation in the VEP is nonlinear and that it occurs preferentially for collinear configurations. Traditional probability summation models predict that areal summation will improve threshold independent of stimulus configuration. Contrast thresholds were derived from VEP contrast response functions for either circular or elongated Gabor patches with aspect ratios up to 6:1. The carrier orientation was either the same as the patch envelope orientation (collinear) or orthogonal to it. Response amplitudes were larger and contrast sensitivity was higher for collinear configurations. The results are consistent with nonlinear, configuration dependent summation that is more extensive along the axis of orientation.

Contrast Sensitivity↗

PubMed1997

Abnormal long-range spatial interactions in amblyopia.

Neural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia.

Amblyopia↗

PubMed1997

Improvement in Vernier acuity in adults with amblyopia. Practice makes better.

PURPOSE: To determine the nature and limits of visual improvement through repetitive practice in human adults with naturally occurring amblyopia. METHODS: A key measure the authors used was a psychophysical estimate of Vernier acuity; persons with amblyopia have marked deficits in Vernier acuity that are highly correlated with their loss of Snellen acuity. The experiment consisted of three phases: pretraining measurements of Vernier acuity and a second task (either line-detection thresholds or Snellen acuity) in each eye with the lines at two orientations; a training phase in which observers repetitively trained on the Vernier task at a specific line orientation until each had completed 4000 to 5000 trials; and posttraining measurements (identical to those in the first phase). Two groups of amblyopic observers were tested: novice observers (n = 6), who had no experience in making psychophysical judgments with their amblyopic eyes, and experienced observers (n = 5), who had previous experience in making Vernier judgments with their amblyopic eyes (with the lines at a different orientation) using the signal-detection methodology. RESULTS: The authors found that strong and significant improvement in Vernier acuity occurs in the trained orientation in all observers. Learning was generally strongest at the trained orientation but may partially have been transferred to other orientations (n = 4). Significant learning was transferred partially to the other eye (at the trained orientation) in two observers with anisometropic amblyopia. Improvement in Vernier acuity did not transfer to an untrained detection task. In two observers, the improvement in Vernier acuity was accompanied by a commensurate improvement in Snellen acuity. CONCLUSIONS: Some adults with amblyopia retain a significant degree of neural plasticity. Although several observers (primarily novices) showed evidence of generalized learning, several amblyopic patients showed evidence for improvement that was orientation and task specific. In this latter group of observers, the improvement appeared to reflect alterations that were, at least in part, in early neural processes that were orientation specific and were localized beyond the site of convergence of the two eyes.

Adult↗

PubMed1996

Neural plasticity in adults with amblyopia.

Amblyopia is a neuronal abnormality of vision that is often considered irreversible in adults. We found strong and significant improvement of Vernier acuity in human adults with naturally occurring amblyopia following practice. Learning was strongest at the trained orientation and did not transfer to an untrained task (detection), but it did transfer partially to the untrained eye (primarily at the trained orientation). We conclude that this perceptual learning reflects alterations in early neural processes that are localized beyond the site of convergence of the two eyes. Our results suggest a significant degree of plasticity in the visual system of adults with amblyopia.

Adult↗

PubMed1996

Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex.

Long-range spatial interactions in human visual cortex were explored using a lateral masking paradigm. Visual evoked potentials (VEPs) elicited by a Gabor signal presented in isolation or in the presence of two flanking high-contrast Gabor signals (masks) were measured. Response amplitude and phase were recorded for a vertically oriented test, for horizontal and vertical masks and for combinations of vertical tests and vertical or horizontal masks. The amplitudes and phases of the test alone and mask alone responses were added coherently to predict the amplitude for collinear and orthogonal lateral masking conditions. Additivity failures were taken as evidence for neural interactions. At a target-to-mask distance of 2 deg, VEP amplitude exceeded the linear prediction for test contrasts in the range of 8-16% for the collinear, co-axial target/mask combination. Measured response phase also led predicted response phase over the same range of contrast. The VEP amplitudes were less than the linear prediction in the orthogonal target/mask combination and measured response phase lagged the predicted phase. Significant facilitation occurred with collinear test/mask combinations up to at least 3 deg of separation (nine wavelengths). Co-oriented, but non-collinear test/mask combinations (oblique test and mask, horizontal test and mask) did not produce facilitation. Contrast gain thus appears to be set over considerable distances in a configuration-specific fashion.

Contrast Sensitivity↗

PubMed1994

Spatial interactions in human vision: from near to far via experience-dependent cascades of connections.

Perceptual learning has been shown to affect early visual processes. Here, we show that learning induces an increase in the spatial range of lateral interactions. Using a lateral masking/facilitation paradigm and bandpass-localized stimuli, we measured the interaction range before and after extensive training on a threshold detection task. For naive observers, target threshold was found to be facilitated by mask presence at distances up to six times the target period. However, practice had the effect of increasing the facilitation range by at least a factor of three. We suggest that the induced longer-range facilitation is a result of internal response transmission via a cascade of local connections. The data presented also show that this chain can be broken. These results suggest a plasticity in early vision governed by Hebbian-like rules.

Humans↗

PubMed1994

The architecture of perceptual spatial interactions.

Lateral interactions between spatial filters were explored with a lateral masking paradigm. Contrast sensitivity (two-alternative forced-choice) for a Gabor signal in the presence of two flanking high contrast Gabor signals (masks) was measured. When the target to mask distance was less than 2 target wavelengths the contrast sensitivity decreased up to a factor of two relative to a no mask condition. At larger separations, up to eight wavelengths, an increase in contrast sensitivity occurred. This increase was maximal at separation distances of 2-3 wavelengths, where sensitivity increased by a factor of two. However, the enhancement magnitude and range was dependent on the offset between the Gabor signal orientation and the direction defined by the virtual line connecting the two masks (global orientation). Maximal effects occurred when this offset was zero (100% increase in sensitivity) and 90 deg (50% increase). A 45 deg offset yielded only a small enhancement (20%). The enhancement dependence on spatial arrangement was found to be invariant across different global orientations (meridian). This pattern of interactions may be involved in grouping colinear line segments into smooth curves.

Contrast Sensitivity↗

PubMed1993

Lateral interactions between spatial channels: suppression and facilitation revealed by lateral masking experiments.

We measured contrast detection thresholds for a foveal Gabor signal flanked by two high contrast Gabor signals. The spatially localized target and masks enabled investigation of space dependent lateral interactions between foveal and neighboring spatial channels. Our data show a suppressive region extending to a radius of two wavelengths, in which the presence of the masking signals have the effect of increasing target threshold. Beyond this range a much larger facilitatory region (up to a distance of ten wavelengths) is indicated, in which contrast thresholds were found to decrease by up to a factor of two. The interactions between the foveal target and the flanking Gabor signals are spatial-frequency and orientation specific in both regions, but less specific in the suppression region.

Contrast Sensitivity↗