Leonard Matin

Visual persistence: Effects of flash luminance, duration and energy

Abstract Dark-adapted observers reported whether the offset of a test flash (30′ to the right of fixation) occurred before or after the onset of a probe flash (2°30′ to the left of fixation) as the interstimulus interval was varied. Visual persistence (the interstimulus interval at the point of subjeptive equality for test flash offset/probe flash onset) was found to decrease with either increases in flash duration or flash luminance. These effects were shown to be independent of differential visual latencies to the onsets of flashes. For equal-energy flashes (variable luminance and duration), persistence was constant up to 100 msec, and thereafter declined linearly with log flash duration, a result attributable to changes in the shape of the function relating persistence to flash duration at lower luminances. The systematic increase for equal-energy stimuli in the duration of the total visual response with increase in flash duration suggests a basis for the recent finding that at threshold equally-detectable stimuli of different durations are discriminable.

Visual perception of direction when voluntary saccades occur. I. Relation of visual direction of a fixation target extinguished before a saccade to a flash presented during the saccade

In experiments designed to clarify the mechanisms underlying the normal stability of visual direction for stationary objects when voluntary saccades occur, Ss reported on the horizontal visual direction of a brief test [lash presented when the eye was at a specific point in the saccade (the trigger point) relative to a fixation target viewed and extinguished prior to the saccade. From these reports, PSEs (points of subjective equality) were calculated for the fixation target as measured by the test [lashes. The distance of the trigger point from the previous fixation position was systematically varied in each experiment. Different experiments required saccades of different lengths and directions. With the exception of the presentation of the test [lash the saccades were carried out in complete darkness so that the possible utilization of an extraretinal signal regarding the eye movement (change in eye position, the intention to turn the eye, or a change of attention related to the eye movement) in the determination of visual direction could be observed uncomplicated by a continuing visual context. According to classical theories, an extraretinal signal proportional to the change in eye position acts to maintain direction constancy by compensating for the Shift of the retinal image resulting from the movement of the eye. In general, direction constancy was not preserved in the present experiments, and thus the data would not be predicted by classical theories. However, the PSE varied with distance of the trigger point from the fixation target. Since this displacement of PSE from the trigger point was in the correct direction for compensation, the presence of an extraretinal signal was confirmed. However, the growth of this signal appears to be time-locked to the saccade rather than locked to eye position; it is suggested that this growth takes place over a time period which is longer than the duration of the saccade itself.

Visual Localization and Eye Movements

Displacement of the retinal image at the back of the eye may be produced either by eye movements, which the observer uses to redirect his gaze within the visual field, or by displacements of the visual field itself outside of the eye. Nevertheless while displacements of the visual field are normally seen to be displacements in the environment, stationary visual fields normally continue to appear stationary in the presence of eye movements. The main concern of this chapter is with the basis for this difference in visual localization.

Visual perception of direction when voluntary saccades occur: II. Relation of visual direction of a fixation target extinguished before a saccade to a subsequent test flash presented before the saccade

Previously reported experiments demonstrated changes in the relation of visual direction to retinal locus for stimulation during voluntary saccades as compared to this relation before saccade initiation. The quantitative features of these results led to the prediction, confirmed in the present experiments, that there are shifts in visual direction for stimulation presented before the saccade itself. In the present report, monotonically increasing shifts were mapped with stimuli presented as early as 240 msec before the saccade up to the saccade itself. Such shifts cannot be accounted for readily by “inflowing” processes, and while “outflowing” processes seem to be implicated, their quantitative characteristics would need to be considerably different from those required by classical outflow theories.

Eye movements in the dark during the attempt to maintain a prior fixation position

Abstract The horizontal eye movements of two subjects were measured during a series of 3-sec intervals of total darkness as they attempted to maintain the ocular position defined by prior viewing of a fixation target. Systematic changes in the mean and variance (over trials) of displacements of the eye occurred between successive portions of the dark interval. The pattern of correlations between eye position at a given moment in the dark interval and subsequent displacements contained statistically significant deviations from expectation based on a random walk model indicating some control of eye position by an extraretinal signal; however, these deviations were not large. The extraretinal signal is itself often in error with regard to the position toward which correction does take place. It is suggested that the extraretinal signal is the subjects perception of a discrepancy between the effort pattern exerted in “attempting to hold the same position” and his memory for what this effort pattern was during fixation of the visible target; the ocular displacements in the dark are a manifestation of the noisiness of this signal.

Critical Duration, the Differential Luminance Threshold, Critical Flicker Frequency, and Visual Adaptation: A Theoretical Treatment*

A quantitative model formally analogous to an electrical filter containing a cascade of RC elements with output-controlled variation of time constant (parametric feedback) is shown to predict the relations between the differential luminance threshold, critical duration, critical flicker frequency, and adapting luminance in human psychophysical data; the model also treats some of the properties of frequency-response characteristics for sinusoidally-modulated flicker. Critical duration in human luminance discrimination and membrane resistance recorded from eccentric cells in Limulus both follow the same decreasing function of adapting luminance that is predicted by the model. Fitting the model to luminance discrimination and to CFF data requires a constant increment of peripheral neural response at threshold rather than a constant ratio to ongoing peripheral activity. A hypothesis regarding central neural adaptation provides a bridge between the latter result and signal/noise theories of visual discrimination.

Binocular summation at the absolute threshold of peripheral vision.

Binocular probability of seeing, P(B), was measured as a function of the time between onsets of 2-msec, 35-min visual angle flashes to corresponding locations 7° horizontally displaced from the foveas of both eyes of dark-adapted subjects. P(B) was greater than the value that would be predicted if the two eyes were independent detectors [P′(B)] for interstimulus intervals below 100 msec. Two peak values of P(B) were observed, one occurring close to zero and the other close to 90 msec. P(B) and P′(B) were indistinguishable for interstimulus intervals above 100 msec. It is concluded that the largest barrier to seeing at threshold lies in a pathway common to the two eyes.

Visual perception of direction in the dark: Roles of local sign, eye movements, and ocular proprioception

Abstract Subjects performing monocularly in an otherwise dark room reported the direction at which a flash (6 msec duration, 3.5 min visual angle, randomly located along the horizontal in the frontal parallel plane) appeared relative to a fixation target extinguished 3 sec earlier. Although the subjects attempted to maintain the eye in the same position as during the prior fixation period, large involuntary eye movements (monitored by a contact-lens technique) during the 3 sec dark interval caused a given flash target to strike the retina to the left of the fovea on some trials, and to the right on others. The report of flash direction depended strongly on the sign and magnitude of this varying retinal signal independently of the physical location of the flash target. The standard deviation of the function relating report of flash direction to the retinal signal was approximately half of the standard deviation of the function relating the report to physical target location. No evidence was found that proprioceptive signals regarding the eye movements systematically influenced the reports of flash direction. The accuracy of the report of the physical location of the target was thus limited by the subjects ability to maintain his eye close to the original fixation position.

Visually perceived eye level: changes induced by a pitched-from-vertical 2-line visual field

The physical elevation corresponding to visually perceived eye level (VPEL) changes linearly with the pitch of a visual field. Deviations from true eye level average more than 0.5 times the angle of pitch over a 65 degrees pitch range. A visual field consisting of 2 dim, isolated vertical lines in darkness is more than 4/5 as effective as that of a complexly structured visual field; 2 horizontal lines have a small and inconsistent effect. Differences in influence on VPEL between pitched-from-vertical and horizontal lines were predicted from an analysis that extracted differences in retinal perspective resulting from changes in pitch. The Great Circle Model (GCM), based on a spherical approximation to the erect, stationary eye, predicts the present results and results of 8 other sets of experiments. The model treats the influence of a single line on VPEL as systematically related to the elevation of the intersection between the great circle containing the image of the line and the central vertical retinal meridian; generalized GCM combines visual inputs with inputs from the body-referenced mechanism and maps onto the central nervous system.

Refractoriness in the maintained discharge of the cat's retinal ganglion cell.

When effects due to refractoriness (reduction of sensitivity following a nerve impulse) are taken into account, the Poisson process provides the basis for a model which accounts for all of the first-order statistical properties of the maintained discharge in-the retinal ganglion cell of the cat. The theoretical pulse-number distribution (PND) and pulse-interval distribution (PID) provide good fits to the experimental data reported by Barlow and Levick for on-center, off-center, and luminance units. The model correctly predicts changes in the shape of the empirical PND with adapting luminance and duration of the interval in which impulses are counted (counting interval). It also requires that a decrease in sensitivity to stimulation by light with increasing adapting luminance occur prior to the ganglion cell and is thus consistent with other data. Under the assumptions of the model, both on-center and off-center units appear to exhibit increasing refractoriness as the adapting luminance increases. Relationships are presented between the PND and PID for Poisson counting processes without refractoriness, with a fixed refractory period, and with a stochastically varying refractory period. It is assumed that events unable to produce impulses during the refractory period do not prolong the duration of the period (nonparalyzable counting). A short refractory period (e.g., 2% of the counting interval) drastically alters both the PND and PID, producing marked decreases in the mean and variance of the PND along with an increase in the ratio of mean to variance. In all cases of interest, a small amount of variability in refractory-period duration distinctly alters the PID from that obtainable with a fixed refractory period but has virtually no effect on the fixed-refractory period PND. Other two-parameter models that invoke scaling of a Poisson input and paralyzable counting yield predictions that do not match the data.