Harry J. Wyatt

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.

Kittens reared in a unidirectional environment: evidence for a critical period.

1. Kittens were reared in the dark from birth except for a period each day when they were put inside a stationary transparent cylinder, around which a drum with vertical black and white stripes on the inside, rotated in one direction. After the end of the period of exposure, we recorded a sample of single cells from their visual cortices, and analysed each cell for direction and orientation sensitivity and other properties. 2. Two kittens were placed inside the drum, rotating rightward, for 2 hr each seekday from 3 1/2 to 7 weeks of age. A greater proportion of the directionally sensitive cells in their cortices showed a preference for rightward movement. 3. Six other kittens were placed inside the drug for 1 hr each weekday from 2 to 12 weeks of age with the drum rotating leftward up to a particular changeover age, then rightward until 12 weeks. The changeover point occurred at 21, 26, 28, 33, 35 and 51 days for different kittens. A changeover earlier than 4 weeks of age led to a preponderance of cells preferring rightward movement. A changeover later than 5 weeks of age led to a preponderance of cells preferring leftward movement. Comparison of these results with others on monocular deprivation suggests that the peak of the critical period for directional deprivation may occur earlier than the peak of the critical period for monocular deprivation. 4. None of the samples of cells showed a preponderance of cells specific for vertical orientations. It is unclear whether this negative effect resulted from the presence of some horizontal contours during exposure, or some more fundamental cause.

The form of the human pupil

The purpose of this study was to characterize the form of the pupil in normal human subjects. Using a modified slitlamp, photographs of pupils were taken in steady illumination and 10-20 sec after darkness. Transparencies were projected and digitized, and the pupil margin was represented as a circular Fourier series. Best-fit ellipses were also determined. The placement of the pupil relative to the limbus was determined in a number of subjects. Results from 23 subjects indicated that in both darkness and light, average pupil noncircularity was 0.0166. (A value of 0.0200 is easy to detect with the unaided eye from the photographs.) On average, the best-fit ellipse accounted for about half of the noncircularity (59.6% in darkness; 47.7% in light). Most of the contribution to shape was made by the first 4 or 5 harmonics. Shapes were usually stable within a session and could remain fairly stable for at least a year; however, shapes for different subjects were not very similar, especially in the light. (Average pairwise similarity: 0.106 in darkness; 0.034 in light; similarity can have values from -1 to 1.) For a given subject, shapes in light and dark were often fairly similar (average similarity 0.260), but there were systematic differences: in eyes where the ellipse contributed > 20% of noncircularity, ellipse major axes clustered around vertical in darkness, and horizontal in light, implying greater contraction near the vertical meridian. Even pupils with little elliptical contribution turned out to contract more near the vertical meridian. There was some tendency for left and right eyes of an individual to show mirror symmetry of shape. In the dark, pupils were located 0.27 +/- 0.09 mm nasal and 0.20 +/- 0.15 mm superior to the limbus center, and usually moved slightly further nasal or superior in the light. Noncircularity increased with age (0.0015/decade). It was concluded that pupils show individuality of shape, with significant regularities within and across subjects.

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.

A ‘minimum-wear-and-tear’ meshwork for the iris

Large changes in pupil size occur frequently, yet the connective tissue components of the iris, consisting largely of relatively-inextensible collagen, last for many years. The concept of a minimum-wear-and-tear meshwork is developed in the context of the iris geometry, and an optimum form for such a meshwork is derived. A moderate improvement on the performance is obtained by allowing the iris to stretch nonlinearly. Comparison of the optimum mesh and stretch behavior with available data suggests that the iris may approximate a structure which minimizes wear and tear.

Raising rabbits in a moving visual environment: an attempt to modify directional sensitivity in the retina

1. Rabbits were raised inside drums with vertical stripes painted on the inside. The rabbits were held stationary while the drum rotated continually around them: rotation was always in the same direction for any one animal. Rabbits in one litter were put in the drum for 15 min/day from 10–15 days after birth to about 60 days after birth, with the drum rotating to the right. Rabbits in another litter were put in for 15 min/day with the drum moving left. Rabbits in three other litters were put in for 2‐3 hr/day with the drum moving right. All rabbits were kept in the dark when not in the drum.

Detecting saccades with jerk.

In studies of smooth eye movements, saccades are often detected and removed from eye movement records during analysis. A simple and effective method for saccade detection is described; the method uses jerk (the third derivative of eye position with respect to time).

The human pupil and the use of video-based eyetrackers

Video-based devices for measuring gaze direction are widespread. However, there is a built-in imprecision in such devices in the event that pupil diameter changes during the experiments. Data are presented to demonstrate this effect. The possibility of correcting eye-position records for the imprecision is discussed and preliminary examples of such correction are presented.

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.