Lorrin A. Riggs

The Disappearance of Steadily Fixated Visual Test Objects

A system has been devised for causing an image to remain at one point on the retina regardless of eye movements. A beam of light, reflected from a plane mirror on a contact lens, is used to project onto a screen an image of a dark line against a bright background. The screen is viewed by the same eye through an optical system which compensates for the doubling of the angle of rotation of the beam projected from the mirror on the contact lens. Thus, any motion of the eye causes a deviation of the beam such that the retinal image of the projected line undergoes the same displacement as do the retinal receptor cells. By comparison with normal viewing of the same test objects it is found that (1) when first presented, the finest lines are seen with normal or slightly better than normal acuity, (2) within a few seconds the lines begin to disappear, and (3) within one minute even coarse lines are seen only intermittently. The results may be interpreted in terms of local retinal adaptation to a stationary field.

Time Course of Visual Inhibition during Voluntary Saccades

Partial inhibition of vision occurs during voluntary saccadic eye movements. This paper reports its time course, measured by detection of a target flashed at various times in relation to the saccade. One value of stimulus luminance was chosen which was nearly always detected by the fixating eye, but hardly ever by the moving eye. A dot pattern was presented to the fovea in 6-μsec flashes of this luminance under photopic viewing conditions. Photographic records of each saccade made by the subject and each stimulus flash permitted analysis of the precise time of occurrence of the flash in relation to the saccade. Under these conditions, curves from three subjects showed that detection decreases to 50% for a flash occurring about 20 msec before the onset of the saccade and reaches a minimum such that vision of the flash is almost completely absent when it occurs during the saccade. Detection then begins to improve, reaching the 50% point again for a flash occurring about 75 msec after the onset of the eye movement. Results are compared with other studies which employed different viewing conditions, and the use of the data to support a notion of partial central inhibition is evaluated.

Motions of the retinal image during fixation.

The ability of an observer to maintain steady fixation on a test object is limited by involuntary eye tremor. Such tremor is characteristic of balanced muscular systems such as those serving to rotate the eye. Rotations of the eye cause proportional displacements of the retinal image of a stationary test object. Data are given for typical excursions of the retinal image during various exposure intervals from 0.01 to 1 sec in duration. The data show that under good experimental conditions the retinal image is virtually stationary for exposures up to 0.01 sec in duration. Exposures of 0.1 sec entail an average displacement of 25 sec of arc (a visual angle corresponding to the diameter of a single foveal cone). Exposures as long as 1 sec permit an average excursion of about 3 min of arc.

Contrast sensitivity during saccadic eye movements

Abstract The experiment measured contrast sensitivity of three human observers to sinusoidal gratings presented in 10 msec exposures. The gratings were presented to the steadily fixating eye and during 6° horizontal saccades. Experimental conditions of viewing in a Ganzfeld reduced possible effects of contour masking. The use of horizontal gratings minimized retinal smear. Results showed a significant suppression of sensitivity (more than 0.6 log unit of contrast) to low spatial frequency gratings presented during saccades. The magnitude of saccadic suppression decreased as spatial frequency of the gratings increased. We conclude that optical and neural effects combine in normal viewing to produce saccadic suppression. Minimizing the optically originating effects of contour masking and retinal image smear failed to eliminate the considerable impairment of vision that occurs during a saccadic eye movement.

Eye Movements Recorded During Convergence and Divergence

The purpose of the present experiments was to record horizontal binocular eye movements during normal amounts of convergence and divergence by a method having satisfactory sensitivity and accuracy. This is a method of direct photography, based upon collimated beams of light reflected from plane mirrors mounted on tightly fitting contact lenses. The records show that the eyes exhibit relatively rapid and accurate motions of vergence when fixating alternately a near and a far test object. There is no evidence for a systematic discrepancy between the extent of vergence and the geometrical location of the test object. It is concluded that when fusion is maintained, the images of a fixation object are brought to corresponding areas of the retina to an accuracy of about two minutes of arc. This finding is consistent with the reported sizes of Panum’s area and the region of optimal visual resolution.

Curvature as a Feature of Pattern Vision

Prolonged inspection of convex lines of one color and concave lines of another color can cause the appearance of oppositely colored aftereffects in curved, achromatic test lines. These effects, strongly dependent on radius of curvature, cannot be attributed to tilt or orientation. It is concluded that these results are not encompassed by models of the visual system that are based on rectilinear arrays and that curvature is a specific feature of human visual perception.

A comparison of electrical and psychophysical determinations of the spectral sensitivity of the human eye.

Electrical responses of the human retina have been measured by the use of an electrode mounted on a contact lens. The form and magnitude of such responses have been determined for stimulation by filtered lights having various dominant wave-lengths. Two sets of spectral sensitivity data, one for the dark-adapted eye and the other for the light-adapted, have been obtained by computing the intensity of stimulation necessary at each wave-length to arouse an electrical response of a given small magnitude. Comparable data have been obtained in psychophysical experiments in which the same filter combinations and closely similar methods of computation have been employed. The electrical data, for both the light-adapted and the dark-adapted eye, agree much more closely with the psychophysically determined scotopic sensitivity curve than with the photopic. It appears, however, that lights of the shorter wave-lengths are somewhat more effective in arousing electrical responses than the scotopic sensitivity curve would predict.

Establishment and decay of orientation-contingent aftereffects of color*

We have used a null method to measure the orientation-contingent aftereffects of color first described by McCollough. After alternately inspecting, for example, a green horizontal line grating and a magenta vertical line grating, the Os report that in achromatic test gratings the horizontal lines appear pinkish and the vertical lines appear greenish. We have used a special color-mixing projector to add variable amounts of green and magenta light to the test gratings until they appear matched and nearly achromatic. The colorimetric purity needed to achieve this null setting is a quantitative measure of the strength of the colored aftereffect. Following inspections of the colored patterns ranging from 15 sec to 150 min, six Os showed aftereffects lasting from a few minutes to 7 or more days. The indices of colorimetric purity increase with inspection time and decline with time after inspection. The decay function is not quite linear either on semilog or on log-log coordinates. The rate of decay is mainly dependent on the magnitude of the effect built up during inspection. We conclude that the buildup and decay of these aftereffects show some of the time characteristics usually associated with central adaptability rather than sensory adaptation.

Human occipital and retinal potentials evoked by subjectively faded visual stimuli.

Abstract Human observers report marked fading or disappearance of visual patterns presented under conditions of image stabilisation of binocular rivalry. The present experiments show that these subjective effects are not accompanied by a corresponding diminution of such response potentials as can be recorded by cumulative methods of recording from retinal and occipital electrodes.

Binocular interactions during establishment of McCollough effects.

In four experiments we have shown that a McCollough color aftereffect (CAE) built up through one eye may or may not be influenced by simultaneous stimulation of the other eye depending on the kinds of similarity and dissimilarity between the two views. Binocular rivalry-produced suppression is not effective per se. However, there is a specifically binocular component that contributes strength to the CAE, and true interocular transfer takes place when facilitated by homochromatic stimulation of the two eyes. The establishment of CAE strength can also be impeded under certain conditions of dichoptic viewing.