Biomedical subjects

E Kowler

Publications and source records attributed to E Kowler.

18 recordsLinked to original sources

PubMed1992

The role of saccades in the perception of texture patterns.

We studied the perception of texture patterns when observers used saccadic eye movements to scan the display and when the line of sight was maintained in the display center without saccades. Saccades improved the discrimination of the size and the shape of a central randomly-shaped polygon for display durations > 1 sec. Saccades were more important with textures that did not readily segregate into target and background regions than with those that did. Directing saccades to the curvature extrema of the central target figure was more useful than directing them elsewhere. Saccades did not enhance texture segregation, but rather improved the discriminability of individual target and background elements by overcoming lateral interference. To the extent that strong lateral interference is inevitable with poorly-segregating textures, our results show that serial inspection is best carried out by sequences of saccades, not by sequences of attention shifts.

Attention↗

PubMed1991

Saccadic localization of eccentric forms.

Saccades made to outline drawings of eccentric forms were compared with saccades made to single-point targets. Saccades could be directed to designated locations within eccentric forms nearly as accurately and precisely as they could be directed to single points. Saccades directed to the form as a whole landed at consistent locations near the center of the form. These results show that contour information is sufficient for accurate computation of a saccadic command and that this computation is constrained by the internal coding of the shape. A serial two-stage process, voluntary selection followed by a weighted-averaging process, is proposed for computation of the saccadic command based on information provided by shape.

Eye Movements↗

PubMed1990

New directions for oculomotor research.

This paper reviews major trends in the study of the oculomotor system since Westheimer published his doctoral research on this topic 35 years ago. Westheimer introduced the use of linear system analysis for the study of eye movements, an approach used a great deal by others ever since. Westheimer himself abandoned this approach within a decade, in part, because this kind of analysis becomes ambiguous when predictive properties of oculomotor system performance become prominent. We discuss the implications of ignoring the prominence of predictive eye movements and describe recent evidence for their prevelence and power. This leads us to propose that a new approach to the study of oculomotor performance is required. We also discuss the recent trend to apply the "connectionist" (or "neural network") approach in studies of the oculomotor system, and point out that the "symbolic", rather than the "adaptive", nature of predictive eye movements makes successful extension of these models to oculomotor performance unlikely. Our new approach emphasizes the use of natural stimulation in subjects free from bodily restraints. Accurate measurement of eye, head and torso movements under such conditions has become possible recently and data obtained in this manner has led to the discovery of a number of unexpected characteristics of oculomotor system performance. These developments have encouraged us to abandon the modular view of the oculomotor system, popular since Dodge launched the modern era of oculomotor research in 1903, which postulates five, or more, largely independent "subsystems". We suggest that only two subsystems (a fast saccadic and a somewhat slower smooth) are used to fixate and track a central representation of objects located in three-dimensional space. We show that this two-subsystem approach is consistent with current knowledge of oculomotor system neuroanatomy and neurophysiology.

Adult↗

PubMed1989

Cognitive expectations, not habits, control anticipatory smooth oculomotor pursuit.

Human smooth pursuit eye movements anticipate the future path of moving targets. Anticipatory pursuit is sometimes attributed to cognitive expectations about future motion and other times to the habitual repetition of previous pursuit responses. Expectations and habits were separated by having subjects smoothly pursue a target moving along a randomly-selected path that was either undisclosed to the subject before each trial or disclosed by means of auditory or visual cues. When the path was undisclosed, the direction of anticipatory smooth eye movements was determined by the direction of target motion in the previous trial. In the presence of cues-the critical condition for separating habits and expectations-effects of previous trials diminished and anticipatory smooth eye movements were primarily determined by the direction of motion the subject was told to expect. These results show a strong contribution of cognitive expectations which overrides persevering smooth oculomotor habits. Smooth pursuit eye movements are driven by a signal that combines the present target motion with the target motion expected to occur several hundred milliseconds into the future. The expected motion is based on a genuine cognitive prediction, not lower-level sensory or motor memories of past events.

Cognition↗

PubMed1989

The role of location probability in the programming of saccades: implications for "center-of-gravity" tendencies.

Short-latency saccades to targets among nontarget backgrounds are often directed to the center of the entire (target + nontarget) stimulus configuration. This "averaging" or "center-of-gravity" tendency has been attributed to an automatic, reflexive saccadic response to a poorly-resolved visual signal. We investigated the role of high-level processes by varying the probability of the target appearing in one of two locations. Subjects were asked to make a saccade to a target "+" located above-right or above-left of a central fixation point. A nontarget ("x") was in the other location (directional separation = 30 deg). The mean latencies were short (180-230 msec) in accordance with instructions. Mean saccadic direction was shifted to the right by 24-52% of the directional separation of the stimulus pair as the probability of the target appearing on the right increased from 0.2 to 0.8. The difference in saccadic directions as a function of the actual target location was small and independent of probability, showing that probability introduced a bias without affecting the discriminability of the target from the nontarget. The effect of probability was reduced when the discrimination of the target from the nontarget was easier (square vs triangle), and abolished (saccadic accuracy near perfect with the same average latencies) when the target was presented alone. The results show that the direction of short-latency saccades, initiated before the target has been distinguished from a nearby nontarget, is based on the prior history of target locations and expectations about the future location of the target. High-level plans can account for effects of nontargets on saccades. To infer that a reflexive sensorimotor averaging mechanism exists solely on the basis of observed saccadic "centering" tendencies is unwarranted.

Eye Movements↗

PubMed1987

Sensitivity of smooth eye movement to small differences in target velocity.

The precision of smooth pursuit eye movements was described by means of a new dependent measure, the "oculomotor difference threshold" (analogous to the perceptual difference threshold) which represents the smallest difference in target velocity that produces statistically distinguishable differences in eye velocity. Oculomotor difference thresholds for constant velocity motions were largest (greater than 50% of target velocity) during the initial 200 msec of target motion, despite fairly high average gains (0.7-1.4) during the same period. Oculomotor difference thresholds declined over time. By about 600-700 msec after the onset of target motion they reached values as low as the perceptual difference thresholds measured psychophysically with the same target velocities. The similarity of the difference thresholds suggests that equally precise sensory representations of target velocity influenced perception and smooth eye movements. Nonsensory influences on smooth eye movement were also found. Smooth pursuit velocity: (1) depended on the velocity of targets in preceding trials; (2) was decreased during the initial 200 msec of target motion when the duration of motion was reduced from 1 sec to 200 msec, a result which shows that high initial pursuit velocity depends on the expectation that pursuit will continue. These effects of context and expected duration allowed the eye to achieve quickly a velocity close to that of the target it was most likely to encounter. Study of the precision of pursuit may be valuable for characterizing its sensory input, but study of the effects of the context in which a stimulus appears and the effects of expectations about future target motion may be more valuable for understanding how smooth eye movements guarantee retinal image velocities optimal for vision.

Eye Movements↗

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↗

PubMed1983