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Biomedical subjects

L A Riggs

Publications and source records attributed to L A Riggs.

At least 19 recordsLinked to original sources
PubMed1987

A retinal smear technique for varying grating contrast.

We present here a technique for modulating contrast in gratings seen in Maxwellian-view or on a projection screen. The method can be used with achromatic or chromatic grating stimuli. It is based on a high frequency oscillatory displacement of the grating image, which in turn produces a smearing of the image on the observer's retina. The waveform of the displacement and its amplitude determine the appearance and contrast of the image produced on the retina.

Form Perception↗
PubMed1987

Blink-related eye movements.

Eye movements that accompany a blink have been measured in human subjects by the use of a visual-persistence method. With straight-ahead binocular viewing, each eye typically rotates nasalward and downward 1-2 deg during the closing phase of a blink. These eye movements are more rapid than the lid movements as recorded by high-speed photography. In fact, the eyes have already completed their initial rotation and started back again before the lids are fully closed. With off-center viewing, a blink causes each eye to rotate toward its primary position of regard. Indeed, if the eye is already in that position when the blink starts, the eye moves very little. With eyelids taped open, an eye tracker can be used, and records confirming the visual persistence tracings are obtained. Sequential photography of the cornea in profile reveals that the eye moves inward and back out again during a blink. The amplitude of this retraction is typically less than 1 mm; and its time course, slower than that of the rotational eye movements, parallels the closure and opening of the lids. In normal conditions of viewing there is no evidence of conjugate saccades, or of any large, upward rotation of the eyes (Bell's phenomenon) that was once believed to take place during a blink.

Blinking↗
PubMed1986

Dependence of visual suppression on the amplitudes of saccades and blinks.

Visual suppression accompanying voluntary saccades and eyeblinks was measured for a range of amplitudes of both. Saccade amplitudes varied from 2 to 32 degrees and blink amplitudes varied from a slight movement to a full closure of the eye. In every case, thresholds for detecting full-field luminance decrements were determined with the method of constant stimuli and a two alternative forced choice procedure. Results from three subjects show a monotonic increase in the amount of suppression produced by saccades and blinks of increasing amplitude. Data are discussed with respect to theories about the origin of visual suppression.

Adult↗
PubMed1984

Vergence eye movements and visual suppression.

Visual sensitivity was measured during vergence eye movements in order to determine whether a suppression of vision similar to that associated with saccades is also present during vergence. Suppression was evaluated psychophysically by determining sensitivity to briefly presented, full-field decrements of light in a Ganzfeld. Subjects were rougly 0.5 log unit less sensitive when stimuli were presented at the beginning of a 2-3 deg convergent or divergent eye movement, than during steady fixation. Thus, the concept of saccadic suppression must be broadened to include visual suppression that also accompanies nonsaccadic eye movements. These results support the hypothesis that vision is affected by signals that accompany initiation of oculomotor activity.

Electrooculography↗
PubMed1983

Optics, the eye, and the brain.

Optics began as a visual science, and the eye was the original optical instrument. Students of physiological optics, together with their clinical colleagues, were concerned mainly with the normal and pathological functioning of the eye as a receptor organ. In recent years, however, exciting developments have changed all that. Optics has taken off in many directions that have little immediate relation to the eye, such as x-ray astronomy, lasers, and photoacoustic spectroscopy. Vision research, in turn, has gone far beyond its sole preoccupation with the optics of the eye. Most exciting are new discoveries about the visual pathways and the specialization of individual brain cells for the processing of line orientation, stereoscopic depth, spatial frequency, motion, and color. Comparative studies reveal the functional architecture of the brain together with the genetically and chemically programmed cellular development that lays the groundwork for later modification by the visual environment. Stimulated by this neurophysiological progress, and by newly available optical concepts and techniques, visual scientists have greatly expanded their research beyond the traditional topics of physiological optics and color.

Brain↗
PubMed1982

Changes in the human visually evoked cortical potential in response to chromatic modulation of a sinusoidal grating.

Human visually evoked cortical potentials (VECPs) were recorded from 4 subjects in response to the counterphase alternation of an equal brightness chromatic grating pattern. The pattern was constructed from 2 monochromatic sinusoidal gratings registered 180 deg out of phase. Eleven wavelengths from 450 to 650 nm (at 20 nm intervals) were used. Each wavelength was paired in the grating stimulus with every other to produce a total of 55 different chromatic gratings. The chromatic modulation depth (contrast) of each grating was varied and resulting VECPs were recorded. VECP amplitude was found to vary linearly with log chromatic modulation depth. VECP threshold values were inferred by extrapolation of linear regression lines to zero VECP amplitude. Chromatic modulation sensitivity functions were derived and a multidimensional scaling analysis of the data for each subject was performed. The data were adequately described by a two-dimensional geometric configuration of the 11 wavelengths used. The configurations were similar in shape to those obtained psychophysically by Butler and Riggs (1978) Vision Res. 18, 1407-1416, who used a similar stimulus pattern. Their shapes are consistent with an opponent-color model of color vision.

Adult↗
PubMed1982

Measurements of visual suppression during opening, closing and blinking of the eyes.

We have previously shown that the sensitivity of human vision, as measured with a stimulus that bypasses the eyelids, is briefly impaired at the time of an eyeblink. We now find that the visual loss is almost equally extensive during eye closure if the eyes then remained closed. But little impairment occurs during eye opening when the eyes then remain open. We have previously concluded that, in blinking, visual suppression is associated with an inhibitory signal sent out by the brain. We now conclude that this signal arises primarily as a corollary to the efferent discharge that closes the eyelids.

Adult↗
PubMed1982

Perception of suprathreshold stimuli during saccadic eye movements.

We have investigated the extent to which a graded series of suprathreshold stimuli are perceptually weakened by saccadic suppression. Stimuli were brief decrements of variable amplitude in the illumination of a Ganzfeld. For each of several decrements presented during fixation, we determined a matching amplitude of decrement presented during saccades. The measure of saccadic suppression was the ratio of these two amplitudes. The matches made by three observers exhibit ratios as high as 6:1 (0.8 log unit of suppression) near the threshold. Suppression decreases systematically as the strength of the stimulus increases, reaching 0.1 log unit or less for stimuli 2 log units above threshold.

Eye Movements↗
PubMed1982

Saccadic suppression under conditions of whiteout.

We have measured the impairment of vision that accompanies a saccadic eye movement under whiteout conditions. Translucent plastic diffusers were fitted around the eyes to provide a luminous field without perceivable contours. Visual sensitivity of three subjects was tested by means of 10 msec luminance decrements of variable amplitude. We found that sensitivity was lower, by 0.7 to 1.1 log units, when the eyes were making 16 degree saccades than when they were at rest. Comparable amounts of saccadic suppression occurred under more usual Ganzfeld conditions. We therefore conclude that such minimal contours as are present in the Ganzfeld--fixation guides, minor surface blemishes, and the blurred outlines of the subject's nose and brow--have little effect on suppression. These results are consistent with the hypothesis that a centrally originating inhibitory process accompanies the initiation of the saccade.

Electrooculography↗
PubMed1980

Visual aftereffects derived from inspection of orthogonally moving patterns.

Alternate inspection of patterns moving in orthogonal directions induces an aftereffect in which a stationary test pattern seems to move in a new direction. This direction is the resultant of the two directions of aftereffect that would have arisen from separately inspecting each of the moving patterns. The direction in which objects appear to move, like their color and depth, can thus depend on a synthesis of unperceived components.

Afterimage↗
PubMed1980

Eyeblinks and visual suppression.

A technique for bypassing the eyelids permits equivalent visual stimulation of the retina before, during, or after a blink. Sensitivity to these stimuli decreases during voluntary blinks. This indicates that the source of the deficit is neural rather than optical. Such a visual loss may help to explain the common experience that most blinks go unnoticed.

Brain↗