Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

/pdf/recent-models-and-findings-in-visual-backward-masking-a-4qil12hifz.pdf

Recent models and findings in visual backward masking: A comparison, review, and update

We studied the relation between anatomical structure and functional properties of cells in area V2 of the macaque. Visual function was assessed in the alert animal during fixation of gaze. Recording sites were reconstructed with respect to cortical lamination and the cytochrome oxidase pattern. We measured orientation and direction selectivity, end-stopping, sensitivity to binocular disparity and ocular dominance, and determined more complex functions like sensitivity to anomalous contours and lines defined by coherent motion. Orientation selectivity was found in all parts of area V2, with high frequencies in the pale and thick stripes of the cytochrome oxidase pattern, and with lower frequency in the thin stripes. Representations of anomalous contours were found in the pale and thick stripes with similar frequencies, but generally not in the thin stripes, which have been thought to process colour. Lines defined by coherent motion were most frequently represented in the thick stripes; they were less frequent in the pale stripes, and (as with anomalous contours) were not found in the thin stripes. Sensitivity to binocular disparity was found in all types of stripes, but more frequently in the thick stripes, where the exclusively binocular neurons were also concentrated. By contrast, no segregation was found for direction selectivity and end-stopping. All neuronal properties were distributed evenly across cortical laminae. We conclude that mechanisms for figure-ground segregation involve the pale and the thick stripes of the cytochrome oxidase pattern, perhaps with greater emphasis on 'shape from motion' and 'stereoscopic depth' in the thick stripes, while more elementary neuronal properties are distributed almost evenly across the stripe pattern.

Functional Organization of Area V2 in the Alert Macaque

/pdf/multiple-sclerosis-and-other-demyelinating-diseases-5at0dceq5f.pdf

Multiple Sclerosis and Other Demyelinating Diseases

Subjective contours, tilt aftereffects, and visual cortical organization

3. Test-retest reliability was generally lower with the Vistech grating chart (average intracJass correlation = 0.52 for normal subjects, 0.60 for patients) than with the CRT-based test (average intracJass correlation = 0.62 for normal subjects, 0.86 for patients). The difference was especially marked at low spatial frequencies (Vistech correlation = 0.33, CRT correlation = 0.87 for patients at the lowest spatial frequency tested). The Pelli-Robson letter chart had very high test-retest reliabilities (0.98 for normal subjects, 0.86 for patients). 4. The Vistech chart was twice as likely as the CRT test to produce anomalous CSFs. Twenty-four observers-IS normal, 9 patient-had a significant "notch" or low frequency specific loss on one Vistech test that did not appear on the other Vistech test. Twelve observers-5 normal, 7 patient-had anomalous CSFs with the CRT test. 5. A single measure of visual acuity was as good a predictor of CRT contrast sensitivity at each spatial frequency as was Vistech contrast sensitivity.

Reliability and sensitivity of clinical contrast sensitivity tests

The possibility that subjective contours are an artifact of brightness contrast was explored. In one experiment, inducing luminance was found to have different effects on the clarity of subjective contours and the magnitude of brightness contrast. The results of a second experiment indicated that differences of luminance in a stimulus display are necessary for subjective contours to be sustained, whereas chromatic differences are not. It is concluded that subjective contour and brightness contrast are distinct perceptual phenomena but share a dependency on the processing of edge information transmitted through the achromatic channels of the visual system.

https://www.jstor.org/stable/info/1421648

Sensory information and subjective contour.

When observers tracked moving stripes across a background either of stationary stripes, or of stripes moving in the opposite direction, they saw a clear motion aftereffect when the stripes stopped moving. The direction of this aftereffect was opposite to that of the previously tracked stripes, and was thus the same as the direction of the retinal movement of the non-tracked stripes. This aftereffect of tracking was shown not to depend upon slippage of the tracked contours on the retina during tracking, or upon the saccadic phase of optokinetic nystagmus. The effect showed storage over a period of time with the eyes shut. It appears that the effect is due to induced movement, and arises originally from stimulation of the retina by background contours in the tracking phase. This was shown by confining the view of the moving target to one eye, while permitting both eyes to be exposed to background stimulation during tracking. After such stimulation the magnitude of the aftereffect was equal in the two eyes.

The aftereffect of tracking eye movements.

Data are reported from the initial stage of development of a psychophysical technique that allows temporal resolution to be sampled in 120 retinal locations per eye in half an hour. This computer generated test consists of systematically increasing the off period of a small spot of light, temporally modulated at 5 Hz, until the light is perceived as flickering. Visual field plots based on the data from control observers reveal good temporal resolution in the central visual field with a fall-off in resolving power in peripheral regions. Data from multiple sclerosis patients are characterized by “islands” of poor temporal resolution that presumably reflect demyelination along the optic nerve.

Multi-flash campimetry in multiple sclerosis.

Multi-flash campimetry (MFC) is a computer implemented psycho-physical technique which allows sampling of temporal resolution at 120 points of a 40 degree visual field. The technique is rapid (about 20 minutes per eye), reproducible and easy to perform. We compared MFC to Octopus perimetry, colour vision and spatial contrast sensitivity in a series of thirty eyes of 30 observers. Nine were early glaucomatous eyes, 10 were glaucoma suspect eyes and 11 were normal controls.

Multi-flash campimetry and other psychophysical tests in chronic open angle glaucoma

Multi-flash campimetry is a computer-implemented technique used for the rapid assessment of temporal resolving power across the visual field. On the basis of previous reports that glaucoma patients exhibit impaired temporal sensitivity earlier than conventional perimetric field loss, the question of whether multi-flash campimetry might also be sensitive to the visual consequences of glaucoma was explored. Of the 27 patients that were tested, only one had normal temporal resolving power in both eyes, suggesting that multi-flash campimetry might be a useful aid in determining whether a patient should be diagnosed and treated as a glaucoma suspect or as a confirmed glaucoma patient.

Multi-flash campimetry as an indicator of visual field loss in glaucoma.

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