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Biomedical subjects

H J Wyatt

Publications and source records attributed to H J Wyatt.

14 recordsLinked to original sources
PubMed1992

Suppression of optokinesis by a stabilized target: effects of instruction and stimulus frequency.

Subjects viewed a foveally stabilized target presented against a background field of dots moving sinusoidally. Several different modes of viewing the target were used (subjects were instructed to gaze, look, or hold), and the frequency of sinusoidal field motion was varied from 1/32 to 2 Hz. In line with previous findings, the presence of a stabilized target resulted in substantial suppression of optokinesis. The characteristics of this suppression (gain and phase of slow residual eye movements) were dependent on both the mode of viewing the target and the frequency of field motion. When subjects used an imaginary target, little suppression occurred. These findings provide an overall profile of dynamic characteristics of mechanisms involved in the suppression of optokinesis. They support the view that this suppression is significantly determined by the presence of a target against a moving background (even without retinal slip), and by the mode of attending to the target.

Adult↗
PubMed1991

Modeling the kinetics of topically applied ophthalmic agents.

The equations for the ocular pharmacokinetics of topically applied agents are developed in a matrix form which is well suited to numerical solution on small computers. Some sample computations are presented. The technique is relatively simple to extend to additional compartments.

Administration, Topical↗
PubMed1991

Anisocoria. Variation and clinical observation with different conditions of illumination and accommodation.

Variations in anisocoria in light and dark conditions are used to help diagnose normal and pathologic conditions; however, there have been few observations of anisocoria in different lighting and accommodative conditions. The authors measured pupil size photographically in a group of normal subjects examined in six conditions that were controlled for illumination and accommodation. Greater variation and average extent of anisocoria were found in conditions that resulted in larger pupil size. A subset of subjects repeated several sessions. For this group, the average value of anisocoria and variability tended to be greater in dark conditions. These results show that the observation of anisocoria varies under different conditions, and they suggest careful consideration of conditions used clinically to assess pupil equality. Our analysis shows that for a given observation threshold, conditions that produce even modest changes in variability can cause dramatic changes in the probability of observing anisocoria.

Accommodation, Ocular↗
PubMed1989

The oculomotor "twitch"--a transient response to target motion.

The oculomotor "twitch" is a small, transient eye movement that occurs in response to onset of target motion during both passive and active regard of the target (Wyatt and Pola 1985 a, 1987). Among the properties described here, primarily tested with subjects remaining passive, the twitch (i) is elicited extrafoveally, but maximally at and near the fovea, (ii) often persists as a distinct early transient response even when target motion is onset of a simple ramp, and (iii) remains approximately constant over a wide range of target motions, varying only with direction. We suggest that the twitch reflects the working of a mechanism analyzing the direction in which a target starts to move.

Adult↗
PubMed1989

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.

Eye Movements↗
PubMed1988

Predictive behavior of optokinetic eye movements.

Optokinetic nystagmus is often thought of as a "primitive" oculomotor response, while smooth pursuit is thought of as a "higher" one. We have used conditions that are usually thought of as eliciting optokinetic responses; i.e., large-field stimuli confined to the retinal periphery, and instructions to subjects to respond passively. In spite of this, the responses showed predictive behavior similar to that described for smooth pursuit.

Eye Movements↗
PubMed1988

"Passive suppression" of optokinesis by stabilized targets.

Subjects gazed passively at a sinusoidally oscillating optokinetic stimulus. They made no attempt to look at a target which appeared, stabilized on the retina at one of several locations, yet the appearance of the target caused rapid and prolonged suppression of optokinesis. Suppression declined (optokinesis increased) as target eccentricity increased, but could be observed for eccentricities up to 15-20 deg. We propose that a target moving relative to a background is a stimulus for suppression of optokinesis, depending substantially on the visual properties of the target and not the act of attending to it.

Adult↗
PubMed1978

Effects of picrotoxin and strychnine on rabbit retinal ganglion cells: lateral interactions for cells with more complex receptive fields.

1. The effects of picrotoxin and strychnine were tested on the receptive fields of direction sensitive cells, orientation sensitive cells, local edge detectors, uniformity detectors and large field units in the rabbit retina. 2. Picrotoxin eliminated the direction specificity and size specificity of 'on-off' and 'on' directionally sensitive cells for both black and white objects. Picrotoxin also made 'on' directionally sensitive cells responsive to faster velocities. 3. Picrotoxin eliminated the orientation specificity of orientation sensitive cells, and changed the bar-flank arrangement of the receptive field into a centre surround arrangement. Thus, the orientation specificity is due to inhibitory rather than excitatory mechanisms. 4. Picrotoxin altered the speed sensitivity of large field units so that they responded to slow speeds as well as fast ones, like centre surround Y cells. 5. Strychnine abolished the size specificity of local edge detectors and changed their speed specificity so that they responded to faster speeds. 6. Picrotoxin changed a uniformity detector into a sustained on centre cell. 7. Strychnine did not effect the direction specificity of directionally sensitive cells, the orientation specificity of orientation sensitive cells, or the speed specificity of large field units. Picrotoxin did not affect the size specificity of local edge detectors. 8. Picrotoxin and strychnine usually had opposing effects on the transient responses of these units to spots and annuli. In general picrotoxin prolonged and enhanced these responses at both on and off, and strychnine shortened them. 9. The effect of these drugs for every type of ganglion cell with complex receptive field properties was to make the receptive field more simple. The orientation selective cells, large field cells, 'on' direction selective cells and uniformity detectors seem to be centre surround cells with special properties that are abolished by these drugs. The 'on-off' direction selective cells and local edge detectors still on-off receptive fields, but in each case one of the drugs abolished the feature that was the basis for the cell's name.

Action Potentials↗
PubMed1976

Specific effects of neurotransmitter antagonists on ganglion cells in rabbit retina.

Directionally sensitive ganglion cells in rabbit retina lose their directional sensitivity when picrotoxin, an antagonist of the inhibitory neurotransmitter gamma-aminobutyric acid, is infused into the retinal blood supply. Strychnine, an antagonist of glycine, does not produce this effect. Other receptive field types are affected by strychnine but not picrotoxin. Inhibitory transmitters therefore have specific functions in information processing in the retina.

Animals↗
PubMed1976

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.

Animals↗
PubMed1975

Directionally sensitive ganglion cells in the rabbit retina: specificity for stimulus direction, size, and speed.

The receptive fields of directionally sensitive ganglion cells in the rabbit retina were analyzed. Several types of experiment showed that each point within the receptive field of the cell is inhibited by a fairly wide area of points around it, lying on each side of the preferred-null axis as well as along the preferred-null axis in the preferred direction. The excitatory or responsive receptive field of these cells has an inhibitory surround: this inhibitory surround appears to be simply an extension of the inhibition that occurs within the center of the receptive field. Points toward the edge of the responsive receptive field are inhibited from an area around them which extends into the center of the receptive field and also into the inhibitory surround. Directionally sensitive retinal ganglion cells respond to moving spots better than to moving bars. This is particularly true for objects moved perpendicularly to the preferred-null axis. In some cells a spot moved perpendicularly to the preferred-null axis will give a substantial response, whereas a bar moved in the same direction will give no response at all. This phenomenon can be explained by the inhibitory area which surrounds each point within the receptive field; since this inhibitory area is asymmetrical, it is also responsible for the cell's directional sensitivity. When two bars oriented perpendicular to the preferred null axis are flashed, one after the other, the response to the second bar is nearly always reduced by the presentation of the first bar. This is true for many temporal and spatial sequences corresponding to movement in the preferred direction, as well as those corresponding to movement in the null direction. However, there are temporal and spatial sequences, corresponding to movement in the preferred direction, for which the response to the second bar is unaffected by the presentation of the first bar. The time delay for this does not vary from cell to cell--it is always approximately 20 ms for on-off directionally sensitive cells and approximately 180 ms for on directionally sensitive cells. The spatial separation does vary from cell to cell, between 0.13 degrees and 1.2 degrees in 11 on-off directionally sensitive cells. This spatial separation, which gives linear summation of the response to two bars flashed 20 ms apart in the preferred direction, is correlated with the speed of movement which gives the best response for a bar moved through the receptive field in the preferred direction.

Animals↗