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Biomedical subjects

E Kowler

Publications and source records attributed to E Kowler.

35 records · Page 2Linked to original sources
PubMed1987

Reading twisted text: implications for the role of saccades.

These experiments investigated how the quality of saccadic skill limits the acquisition of visual information during a reading task. Subjects read text in which the spatial pattern of saccades was varied by reversing the order of words in a line, the order of letters in a word, or both. Some of these transformations drastically altered the customary visual appearance of words. To distinguish visual from oculomotor influences on reading, letters, in some conditions, were rotated 180 deg to produce text in which the customary visual appearance of words was destroyed while the order of words and letters remained unchanged. We found that the directional pattern of saccades had relatively modest effects on reading speed under the instruction to read accurately. The size of saccades had large effects on reading speed. Text in which familiar patterns of letters were destroyed, either by changing letter-order or letter-orientation, was read by sequences of small (less than 30') saccades made to look at every letter, or every alternate letter. Separate tests of letter recognition showed that these small saccades were necessary because recognition of letters drops sharply as a function of eccentricity in the absence of familiar letter patterns. Frequent small saccades slowed reading in that durations of the pauses before saccades increased when saccade size fell below 30-60'. We found a similar pattern of increased saccadic latency with decreasing saccade size when subjects used saccades to track a point whose motion mimicked their own reading eye movement patterns. The long latencies before small saccades in the point-tracking task showed that saccadic programming difficulties contributed to the long pause durations before small saccades during reading. The observed difficulty in programming small saccades means that valid inferences about the duration of visual or cognitive processes cannot be drawn from the temporal pattern of saccades unless the relative difficulty and time required to program different spatial patterns of saccades is taken into account.

Eye Movements↗
PubMed1987

Shared attentional control of smooth eye movement and perception.

Subjects performed a concurrent smooth pursuit and perceptual task to determine whether smooth pursuit eye movements and perception share the same attentional mechanism. Subjects pursued a pair of eccentric rows of moving characters while simultaneously attempting to identify and locate the single numeral in these target rows and the single numeral in a pair of untracked background rows, which moved at a different velocity. Average smooth pursuit gain (eye velocity/target velocity) was 0.7 to 1. Visual search was better for target rows (approximately 65% correct) than for background rows (approximately 22% correct). Superior search performance for the target was not due to its lower retinal speed: performance on the target was 2-3 times better than on the background when retinal speeds were the same. Superior performance for the pursuit target suggests that smooth eye movements and perception share the same selective attentional mechanism. A shared attentional mechanism was further supported by findings that subjects could not: (1) maintain a stable line of sight on a central stationary point while simultaneously attending to moving rows; and (2) pursue one pair of rows and attend the other, untracked rows. Attempts to attend untracked rows did, however, produce a partial improvement in search performance which was accompanied by only a very slight change in eye velocity. This demonstrates that the effects of decisions about how to apportion attention across the visual field depend on the task. Despite the common selective attentional mechanism, smooth eye movements do not provide accurate external indicators of attention unless the consequences of attentional decisions for performance are determined separately for oculomotor and for perceptual tasks.

Attention↗
PubMed1987

Planning sequences of saccades.

Subjects used saccades to fixate a sequence of 1-5 stationary targets (separation = 90') located at the vertices of an imaginary pentagon. The latency of the first saccade in a sequence and the duration of intervals between subsequent saccades increased with sequence length at a rate of about 20 msec/target. Latency also varied with ordinal position in the sequence. These results were not due to directional differences in saccadic latency nor to latency-accuracy or to latency-precision trade-offs. Results were similar when targets were removed and saccades were directed to remembered locations. These effects may be best accounted for by models that have been proposed to account for similar effects of sequence length and ordinal position on other voluntary motor tasks, such as typing, speech and finger-tapping. In these models motor programs for a sequence of responses are planned before execution and then retrieved from memory during execution. These models are fundamentally different from the traditional saccadic models in which visual error signals evoke saccades. Instead, we propose that saccades are controlled by an organized plan for an entire sequence of saccades. Visual error signals may modify or elaborate the plans during the execution of a sequence. Our proposal is consistent with ideas developed by Lashley [Cerebral Mechanisms in Behavior: The Hixon Symposium. Wiley, New York (1951)] in his general treatment of the central organization that determines voluntary motor performance.

Eye Movements↗
PubMed1985

Smooth pursuit of small-amplitude sinusoidal motion.

Subjects used smooth eye movements to track small-amplitude sinusoidal target motions. Target frequencies (0.05 to 5 Hz) and amplitudes (1.9 to 30 min of arc) were in the range of those found in the retinal image during fixation of a stationary target while the head is not artificially supported. Smooth pursuit was poor at high target frequencies in several ways: Large uncompensated drifts were observed for target frequencies between 1 and 4 Hz. The drifts were superimposed upon oscillations of the eye in response to the target motion. Mean retinal-image speeds were higher than retinal-image speeds during slow control (smooth eye movements with stationary targets) for target frequencies above 0.5 Hz. Mean retinal-image speeds were as high as target speed for target frequencies above 3 Hz. The ratio of eye speed to target speed decreased as target frequency and amplitude increased. The dependence on amplitude could be reduced and often eliminated by computing an adjusted ratio in which a constant (approximately equal to the mean speed of slow control) was subtracted from eye speed before dividing by target speed. Adjusted ratios declined for frequencies above 0.5 to 1 Hz and did not depend on amplitude. These results show that the response of the smooth-pursuit subsystem to target motion above 0.5 Hz is poor, even though the velocity and the acceleration of th motions are low. Models of smooth pursuit in which the response of the eye depends exclusively on the velocity, acceleration, or position of the target do not account for our results.(ABSTRACT TRUNCATED AT 250 WORDS)

Eye Movements↗
PubMed1984

Voluntary selection of the target for smooth eye movement in the presence of superimposed, full-field stationary and moving stimuli.

Prior work has shown that smooth eye movements in the presence of both stationary and moving stimuli are determined, at least in part, by the voluntary selection of either the stationary or the moving stimulus as the target for smooth eye movements. The effectiveness of voluntary selection in eliminating the influence of the stimuli not selected (i.e. backgrounds) on smooth eye movement is not known because prior studies used targets and backgrounds with different physical characteristics. Thus, effects of voluntary selection were confounded with the relative strength of target and background as stimuli for smooth eye movements. We measured eye movements (resolution 1') of two highly-experienced eye movement subjects with a target and background with the same physical characteristics: two, identical, full-field, superimposed patterns of randomly-positioned dots (1 dot/deg2 or 8 dots/deg2). One field was stationary and the other moved at 70.2 minarc/sec. The effect of the moving background on smooth eye movements when the stationary field was the target, and the effect of the stationary background on smooth eye movements when the moving field was the target was negligible (0-4% for one subject; 0-2% for the other). The influence of the background on smooth eye movements was affected by a six-fold reduction in the intensity of either the target or background, but effects of such intensity changes were small and different for each subject. Taken together, these results show that the effectiveness of voluntary selection in eliminating the influence of background stimuli on smooth eye movements can be virtually complete. Any observed influence of the background--however small--can be attributed to voluntary factors (e.g. subjects' failure to apply sufficient effort or attention) rather than to the operation of an involuntary mechanism that automatically integrates velocity information from target and background. The attention and effort required to ensure that voluntary selection is perfect may impair the accuracy of psychophysical judgments made about the background.

Attention↗
PubMed1984

The effect of expectations on slow oculomotor control--IV. Anticipatory smooth eye movements depend on prior target motions.

Prior work had shown that smooth eye movements depend both on the motion of the target on the retina and on the subject's expectations about future target motion (Kowler and Steinman, 1979a,b). Effects of expectation cannot be eliminated by making target motions unpredictable (Kowler and Steinman, 1981). The experiment reported here shows that effects of expectations on smooth eye movement depend in a lawful way on the history of prior target motions. Anticipatory smooth eye movements (involuntary drifts in the direction of future target motion) were measured while subjects fixated a stationary target that was expected to step in an unpredictable direction (right or left). Anticipatory smooth eye movement velocity depended on the sequence of steps in prior trials, e.g. velocity was faster to the right when the prior steps were to the right. The influence of prior steps diminished the further back into the past the step occurred. Sequential dependencies were also observed for the saccades used to track the target steps. Anticipatory smooth eye movement velocity was predicted by a two-state Markov model developed by Falmagne et al. (1975) for similar sequential dependencies observed in a manual reaction-time task (button-pressing). The model uses the prior sequence of target motions to predict the subject's expectation, and assumes that the expectation determines anticipatory smooth eye movement velocity. The fit of the model to the data was good which shows that taking expectations into account is both necessary and feasible. Taking expectations into account, quantitatively, allows accurate predictions about smooth eye movement velocity when target motions are unpredictable.

Eye Movements↗
PubMed1982

Eye movements of preschool children.

Accurate recordings of eye movements of children 4 and 5 years old show that their eye movements differed from those of adults. During maintained fixation, saccades were large (1 degree to 2 degrees) and smooth eye movement speeds were high (45 minutes of arc per second). Saccade latencies were highly variable during target step tracking. Smooth pursuit latencies were longer than those of adults. These hitherto unknown characteristics limit a child's ability to use eye movements to acquire visual information.

Child, Preschool↗
PubMedDate not supplied

When push comes to shove: compensation for passive perturbation of the head during natural gaze shifts.

The effects of passive displacements to the head delivered by an abrupt push to the upper body were studied in human subjects during gaze shifts to nearby targets while the head was completely unrestrained. Accurate measurements of gaze were obtained via the Maryland Revolving Field Monitor, used to measure head and eye rotations unconfounded with translations, and by an acoustic ranging system, used to measure head translations. Compensation for head perturbations was quite good, with gaze errors much the same as gaze errors in the absence of the push. Compensation along one or both meridians was achieved by means of the vestibulo-ocular response in many of the gaze shifts. The results suggest an impressive ability to coordinate head and eye movements during natural gaze shifts, carried out by one or more different kinds of compensatory systems that the subject can access at will or according to task demands.

Eye Movements↗