Jordan Pola

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.

Stimuli for accommodation: blur, chromatic aberration and size.

We investigated the frequency response of the accommodative system (0.05-1 Hz) using three stimuli: defocus blur, the effects of the chromatic aberration of the eye, and changing target size. A high-speed infrared optometer monitored accommodation while the subject viewed a target in a Badal optometer. Blur was provided by moving the target sinusoidally towards and away from the subject (1-3 D) and the size of the target was varied at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white light (3300 K). Gain and phase plots changed systematically as we varied the number of stimuli presented together. This suggests that besides defocus blur both chromatic aberration and changing size are involved in accommodative control.

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.

Target position and velocity: The stimuli for smooth pursuit eye movements

Abstract Smooth pursuit eye movements are usually thought to be guided only by target velocity. We studied the effectiveness of target velocity and target position (offset from the fovea) as stimuli for pursuit movements. Under open-loop conditions, we used induced (apparent) sinusoidal motion as a “velocityonly” stimulus, and square-wave motion as a “position-only” stimulus. Over a range of frequencies, position stimuli tended to give larger responses, and response velocity increased linearly with target offset. When open-loop sinusoidal target motion was synthesized using appropriate position-only and velocity-only “components”, the response was about the same as for real sinusoidal motion, suggesting a dominant role for target position in both cases. Using non-periodic step-ramp stimuli as devised by Rashbass, but in the open-loop, we have commonly observed position-directed pursuit movements.

Active and passive smooth eye movements: Effects of stimulus size and location

We measured active smooth pursuit eye movements and passive smooth eye movements in the open-loop condition as subjects viewed moving stimuli of different sizes and at various retinal loci. Active movements have high gain and relatively large phase lag. Passive movements have lower gain and smaller phase lag, and occur with either foveal or eccentric stimuli. They appear to be similar or identical to optokinetic movements. Although different, active and passive movements show a similar increase in amplitude and phase lag as the size of the stimulus was increased. From these findings we suggest that: (1) The stimuli for the active movements are target position and velocity; (2) the stimulus for passive movements is target velocity; and (3) the active response to target velocity is related, in part, to the passive response and thus is related to optokinesis.

The role of perceived motion in smooth pursuit eye movements

Abstract We examined responses of the smooth pursuit system under open-loop conditions: (1) Subjects tracked an isolated target, oscillating sinusoidally at frequencies 0.3–1.5 Hz. When a variation of the Duncker illusion increased perceived target motion (leaving retinal target motion unaltered), pursuit responses increased. (2) Measurements of perceived target motion during open-loop pursuit support the hypothesis that pursuit eye movements per se contribute to perceived motion. (3) We interpret these results as evidence for a positive feedback loop in the pursuit system, related to perception. This loop may account for the high gain of the open-loop pursuit system.

Models of the mechanism underlying perceived location of a perisaccadic flash

A variety of experiments have shown that subjects tend to perceive a target flash as mislocalized when the flash is presented just before, during or shortly after the occurrence of a saccade. The characteristics of this mislocalization suggest that it arises from an anticipatory, slow extraretinal signal, i.e., the signal starts to change before a saccade and continues to change during and after the saccade. However, a target flash creates a visual signal that can persist for as long as 300 ms. Interaction of this visual persistence with the extraretinal signal could have a significant influence on the perceived location of the target flash, and thus on features of the extraretinal signal as inferred from the perceived location. In this study, several different types of models were used to explore how retinal signal persistence together with an extraretinal signal might affect perception. According to these models, the anticipatory, slow extraretinal signal may be an artifact of using a target flash, and the actual extraretinal signal may begin to change only after saccade onset and relatively quickly.

Dioptric and non-dioptric stimuli for accommodation: Target size alone and with blur and chromatic aberration

The frequency response of the accommodative system (0.05-1 Hz) was determined for various combinations of stimuli: changing target size was presented alone, together with defocus blur, and with both defocus blur and chromatic aberration. A high-speed infrared optometer monitored accommodation while the subject viewed the target in a Badal optometer. Target size was varied sinusoidally and blur was provided by moving the target towards and away from the subject at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white (3300 K) light. The target was presented under open-loop conditions when size was the only stimulus. We find that besides the conventional dioptric stimuli, changes in target size that result in changes in apparent distance can have substantial effects on accommodation.

Changing target size is a stimulus for accommodation

Accommodation was monitored by using a high-speed infrared optometer while subjects viewed a target that appeared to approach and recede in a sinusoidal manner. The target was presented under open-loop conditions to prevent blurring because of accommodation. The experiments suggest that changing target size can be an effective stimulus on its own. This supports the view that accommodation responds to both dioptric and nondioptric stimuli.

The perception of target motion during smooth pursuit eye movements in the open-loop condition: Characteristics of retinal and extraretinal signals

During smooth pursuit eye movement, the perception of target motion appears to come from retinal and extraretinal influences. To explore this, two open-loop conditions (experimental stimuli stabilized at the retina) were used: one to look at the combined effect of retinal and extraretinal signals on perception (using sinusoidal target motion); and the other to look at the characteristics of an extraretinal signal alone (using a complex target and square-wave motion). In both conditions subjects tracked target motion in the dark, and subsequently compared it to motion of a similar target in the light. The main findings of the study are that the magnitude of the extraretinal signal decreases with frequency, and that the retinal and extraretinal signals combine additively. This system appears to involve a transport-time, which could be in the form of a time advance. These features of perception have a variety of implications for motor control.