Eye-head coordination in labyrinthine-defective humans

Eye-head coordination during saccadic gaze shifts normally relies on vestibular information. A vestibulo-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade to account for current head movement, and the vestibulo-ocular reflex (VOR) stabilizes postsaccadic gaze while the head movement is still going on. Acute bilateral loss of vestibular function is known to cause overshoot of gaze saccades and postsaccadic instability. We asked how patients suffering from chronic vestibular loss adapt to this situation. Eye and head movements were recorded from six patients and six normal control subjects. Subjects tracked a random sequence of horizontal target steps, with their heads (1) fixed in primary position, (2) free to move, or (3) preadjusted to different head-to-target offsets (to provoke head movements of different amplitudes). Patients made later and smaller head movements than normals and accepted correspondingly larger eye eccentricities. Targeting accuracy, in terms of the mean of the signed gaze error, was better in patients than in normals. However, unlike in normals, the errors of patients exhibited a large scatter and included many overshoots. These overshoots cannot be attributed to the loss of VSR because they also occurred when the head was not moving and were diminished when large head movements were provoked. Patients' postsaccadic stability was, on average, almost as good as that of normals, but the individual responses again showed a large scatter. Also, there were many cases of inappropriate postsaccadic slow eye movements, e.g., in the absence of concurrent head movements, and correction saccades, e.g., although gaze was already on target. Performance in patients was affected only marginally when large head movements were provoked. Except for the larger lag of the head upon the eye, the temporal coupling of eye and head movements in patients was similar to that in normals. Our findings show that patients with chronic vestibular loss regain the ability to make functionally appropriate gaze saccades. We assume, in line with previous work, three main compensatory mechanisms: a head movement efference copy, an active cervico-ocular reflex (COR), and a preprogrammed backsliding of the eyes. However, the large trial-to-trial variability of targeting accuracy and postsaccadic stability indicates that the saccadic gaze system of patients does not regain the high precision that is observed in normals and which appears to require a vestibular head-in-space signal. Moreover, this variability also permeates their gaze performance in the absence of head movements.     

Attentional capture and aging: Implications for visual search performance and oculomotor control.

Two studies examined potential age-related differences in attentional capture. Subjects were instructed to move their eyes as quickly as possible to a color singleton target and to identify a small letter located inside it. On half the trials, a new stimulus (i.e., a sudden onset) appeared simultaneously with the presentation of the color singleton target. The onset was always a task-irrelevant distractor. Response times were lengthened, for both young and old adults, whenever an onset distractor appeared, despite the fact that Ss reported being unaware of the appearance of the abrupt onset. Eye scan strategies were also disrupted by the appearance of the onset distractors. On about 40% of the trials on which an onset appeared, Ss made an eye movement to the task-irrelevant onset before moving their eyes to the target. Fixations close to the onset were brief, suggesting parallel programming of a reflexive eye movement to the onset and goal-directed eye movement to the target. Results are discussed in terms of age-related sparing of the attentional and oculomotor processes that underlie attentional capture. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Saccade latency toward auditory targets depends on the relative position of the sound source with respect to the eyes

The latency of saccadic eye movements evoked by the presentation of auditory and visual targets was studied while starting eye position was either 0 or 20 deg right, or 20 deg left. The results show that for any starting position the latency of visually elicited saccades increases with target eccentricity with respect to the eyes. For auditory elicited saccades and for any starting position the latency decreases with target eccentricity with respect to the eyes. Therefore auditory latency depends on a retinotopic motor error, as in the case of visual target presentation.     

Speed and accuracy of saccadic eye movements: Characteristics of impulse variability in the oculomotor system.

Dynamic characteristics observed in the trajectories of saccadic eye movements revealed systematic variability of the force pulses used to move the eyes. This variability causes saccades to exhibit a linear speed–accuracy trade-off: As the average distance and duration of saccades toward specified target points increase, the standard deviations of saccadic-movement endpoints increase linearly with the saccades' average velocity. The linear trade-off, and other observed stochastic properties of saccades, may be attributed to noise in neuromotor processes and may be described in terms of an impulse-variability model originally designed for characterizing limb movements. According to the model, both eye and limb movements are controlled through stochastic force and time parameters that govern movement kinematics. Such an account may promote a unified conceptual framework for understanding a wide range of motor behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Visual search: eye movements and peripheral vision

The visual search task allows investigation of the way in which central processes can influence the programming of saccadic eye movements. In this paper, a simple search task is studied in which a target is presented in a ring-shaped display of eight stimuli. The subject is required to locate the target with a saccadic eye movement. Targets were colored disks and the task was to search for a target of a particular color. Sensory factors (when all nontargets are identical, the target stands out) and central factor (prespecification of the target) both contribute to search efficiency. When the display contains a double target, saccades sometimes land at an intermediate position between the two targets. This shows that the signal delivered by the search procedure is not necessarily highly localized.     

Anticipatory saccades during smooth pursuit eye movements and familial transmission of schizophrenia

BACKGROUND: Smooth pursuit eye movement (SPEM) abnormalities are a putative marker of genetic risk for schizophrenia. Accurate SPEM performance requires the subject to activate neural systems responsible for smooth pursuit tracking, while simultaneously suppressing activity of neurons responsible for saccadic movements that would move the eye ahead of the target. This study examined whether specific aspects of SPEM dysfunction cosegregate with genetic risk in parents of schizophrenic probands. METHODS: Eighteen probands and their parents had SPEM recorded. Parents with an ancestral history of schizophrenia were hypothesized to be more likely than their spouses without such a history to carry a genetic risk for schizophrenia. RESULTS: Ten families had a single parent with a positive ancestral history for schizophrenia. The frequency of anticipatory saccades, which were mostly small, and the fraction of total eye movement that they represented were the only measures that differentiated the more likely genetic carrier parents in these families from their spouses and age-matched normals. CONCLUSIONS: Failure to suppress saccadic anticipation of target motion during smooth pursuit appears an aspect of SPEM dysfunction related to presumed genetic risk for schizophrenia.     

Programming saccadic eye movements.

This article addresses questions about the preparatory processes that immediately precede saccadic eye movements. Saccade latencies were measured in a task in which subjects were provided partial advance information about the spatial location of a target fixation. In one experiment, subjects were faster in initiating saccades when they knew either the direction or amplitude of the required movement in advance compared to a condition with equal uncertainty about the number of potential saccade targets but without knowledge of the parameters required to execute the movement. These results suggest that the direction and amplitude for an upcoming saccade were calculated separately, and not in a fixed serial order. In another experiment, subjects appear to have programmed the saccades more holistically—with computations of direction and amplitude parameters occurring simultaneously. The implications of these results for models of eye movement preparation are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Saccade amplitude influences pointing movement kinematics

The amplitude of open-loop pointing movements to step displacements in target position is influenced by the amplitude of simultaneously produced saccadic eye movements. The time course over which this occurs was addressed in the present study. Analysis of the pointing kinematics showed that saccade amplitude had its effect only during the initial acceleration of the hand. Moreover, the magnitude of the initial acceleration was correlated with the difference in the onset times of the eye and hand movements: the closer in time the saccadic and pointing responses were initiated the larger the initial hand acceleration. Taken together, these results demonstrate that saccades influence the kinematics of simultaneously produced limb movements but only over a limited time frame.     

Smooth pursuit in 1- to 4-month-old human infants

The ability of human infants < or = 4 months of age to pursue objects smoothly with their eyes was assessed by presenting small target spots moving with hold-ramp-hold trajectories at ramp velocities of 4-32 deg/sec. Infants as young as 1 month old followed such target motions with a combination of smooth-pursuit and saccadic eye movements interrupted occasionally by periods when the eyes remained stationary. The slowest targets produced variable performance, but targets moving 8-32 deg/sec produced consistent pursuit behavior, even in the youngest infants. By the fourth month, eye-movement latency decreased and smooth-pursuit gain and the percentage of smooth pursuit per trial increased for all target velocities, though these measures had not yet reached adult levels.     

Eye tracking dysfunction in schizophrenia: Characterization of component eye movement abnormalities, diagnostic specificity, and the role of attention.

To characterize oculomotor components and diagnostic specificity of eye tracking abnormalities in schizophrenia, we examined a large consecutively admitted series of psychotic patients and matched controls. The most common abnormality in schizophrenic patients was low gain (slow) pursuit eye movements (47% of cases). Pursuit and saccadic eye movement abnormalities were no more severe in schizophrenic Ss than in those with affective psychoses, except that high rates of catch-up saccades were unique to schizophrenic Ss (17% of cases). These findings indicate that impaired pursuit eye movements are a major cause of eye tracking impairments in schizophrenia, that tracking dysfunctions commonly occur in affective psychoses, and that markedly high rates of catch-up saccades during eye tracking may be specific to schizophrenia. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of bilateral eye movements on the retrieval of item, associative, and contextual information.

Two experiments are reported that investigate the effects of saccadic bilateral eye movements on the retrieval of item, associative, and contextual information. Experiment 1 compared the effects of bilateral versus vertical versus no eye movements on tests of item recognition, followed by remember-know responses and associative recognition. Supporting previous research, bilateral eye movements enhanced item recognition by increasing the hit rate and decreasing the false alarm rate. Analysis of remember-know responses indicated that eye movement effects were accompanied by increases in remember responses. The test of associative recognition found that bilateral eye movements increased correct responses to intact pairs and decreased false alarms to rearranged pairs. Experiment 2 assessed the effects of eye movements on the recall of intrinsic (color) and extrinsic (spatial location) context. Bilateral eye movements increased correct recall for both types of context. The results are discussed within the framework of dual-process models of memory and the possible neural underpinnings of these effects are considered. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Bilateral eye movements enhance the retrieval of episodic memories.

Two experiments examining effects of eye movements on episodic memory retrieval are reported. Thirty seconds of horizontal saccadic eye movements (but not smooth pursuit or vertical eye movements) preceding testing resulted in selective enhancement of episodic memory retrieval for laboratory (Experiment 1) and everyday (Experiment 2) events. Eye movements had no effects on implicit memory. Eye movements were also associated with more conservative response biases relative to a no eye movement condition. Episodic memory improvement induced by bilateral eye movements is hypothesized to reflect enhanced interhemispheric interaction, which is associated with superior episodic memory (S. D. Christman & R. E. Propper. 2001). Implications for neuropsychological mechanisms underlying eye movement desensitization and reprocessing (F. Shapiro, 1989, 2001), a therapeutic technique for posttraumatic stress disorder, are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Patterns of eye movements during parallel and serial visual search tasks.

In 2 experiments with a total of 24 undergraduates, eye movements were monitored while Ss performed parallel and serial search tasks. In Exp 1a, Ss searched for an "O" among "X"s (parallel condition) and for a "T" among "L"s (serial condition). In the parallel condition of Exp 1b, "Q" was the target and "O"s were distractors; in the serial condition, these stimuli switched roles. Displays contained 1, 12, or 24 stimuli, with both target-present and target-absent trials. RT and eye-movement measures (number of fixations, saccadic error, and latency to move) indicated that search efficiency was greatest in the parallel conditions, followed by the serial condition of Exp 1a and, finally, by the serial condition of Exp 1b. This suggests that eye movements are correlated with the attentional processes underlying visual search. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Binocular coordination in reading.

Measured the differences in time between the 2 eyes, using laboratory real-time computer methods to detect and measure the time between velocity peaks of binocular saccadic movements in reading. The hypothesis was that instead of being completely conjugate as indicated by prior methods of ocular measurement, the eyes must be coordinated in directional motion by small time differences that govern their feedback guidance and relative velocity. Results with 3 Ss indicate that the time differences between the eyes clustered around 3 values: (a) near synchrony, including no difference and left eye leading by 1 msec.; (b) left eye leading by 7-9 msec.; and (c) left eye leading by 14 msec. These time differences were not related to the difficulty of the reading material, but were changed significantly by 15– horizontal rotation of the reading display. Results change the established views that the eyes are perfectly conjugate in saccadic motion and provide initial suggestive data toward a dynamic feedback doctrine of coordinate eye motion and functional disabilities in visual perception. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spatial patterns in the control of human arm movement

Generalized Procrustes analysis was used to investigate the spatial paths of pointing movements. In Experiment 1, 3 participants produced similar spatial paths of the hand when repeating a pointing movement many times, despite variability in the position and orientation of the movements. The average spatial path indicates a fundamental spatial pattern of the motor system, or motor primitive. This pattern varied across the workspace. Anterioposterior movements were straight, but repeated movements had variable spatial patterns. Lateral movements were curved away from the body but had regular spatial patterns. Experiment 2 extended these results to movements of different amplitudes in 7 participants. The motor primitive seems to be abandoned at the end of the movement in favor of final adjustments to bring the hand to the target position. In Experiment 3, the same participants produced similar motor primitives both with and without vision.     

Hydrothermal-electrochemical deposition of hydroxyapatite

Age-related differences in the trajectories of saccadic eye movements were examined. Younger and older adult subjects produced saccades to predictable target locations. Detailed features of the movements were examined such as the time of peak acceleration and the variability in the magnitude of the peak velocity. These and other measures reveal important details of the force pulses underlying the eye movements and the mental mechanisms that control them. Although minor differences were apparent between the eye movements of younger and older adults, the general patterns were the same across age groups. These results suggest that fundamental details of the brain mechanisms involved in the control of movement are the same for younger and older adults.     

Retinal masking during pursuit eye movements: Implications for spatiotopic visual persistence.

White (1976) reported that presentation of a masking stimulus during a pursuit eye movement interfered with the perception of a target stimulus that shared the same spatial, rather than retinal, coordinates as the mask. This finding has been interpreted as evidence for the existence of spatiotopic visual persistence. We doubted White's results because they implied a high degree of position constancy during pursuit eye movements, contrary to previous research, and because White did not monitor Ss' eye position during pursuit; if White's Ss did not make continuous pursuit eye movements, it might appear that masking was spatial when in fact it was retinal. We attempted to replicate White's results and found that when eye position was monitored to ensure that subjects made continuous pursuit movements, masking was retinal rather than spatial. Ss' phenomenal impressions also indicated that retinal, rather than spatial, factors underlay performance in this task. The implications of these and other results regarding the existence of spatiotopic visual persistence are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inhibition of return: Effects of attentional cuing on eye movement latencies.

Inhibition of return refers to a bias against attending to and/or detecting visual stimuli at recently attended locations. A total of 57 Ss participated in 5 experiments, in which Ss were slower to initiate eye movements to previously attended locations. Furthermore, there was more inhibition when a peripheral (exogenous) flash signaled the target, compared with when a central (endogenous) arrow cue was used as an imperative stimulus. That pattern suggests that some of the inhibition is due to processes involved in detecting visual stimuli, and some of the inhibition is related to the movement of the eye. Subsequent experiments showed that the eye-movement component of the inhibition is not object-centered and does not move if the previously attended object moves, although the stimulus-detection component is object-centered. Results have implications for visual attention in general and for the link between overt and covert orienting. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Saccades induced electrically from the dorsomedial frontal cortex: evidence for a head-centered representation

The amplitude and direction of saccadic eye movements evoked electrically from the dorsomedial frontal cortex (DMFC) of monkeys vary with starting eye position. This observation has been used to argue that the DMFC codes saccadic eye movements in head-centered coordinates. Whether the amplitude and direction of the evoked saccades are also affected by changes in head position has never been demonstrated. Such a result would argue against a head-centered representation, and instead would suggest a representation anchored to another body part. Tests were conducted on rhesus monkeys to determine whether changing the position of the head with respect to the trunk or changing the position of the head with respect to the gravitational axis alters saccadic parameters. The amplitude and direction of saccadic eye movements remained invariant to such manipulations. These findings confirm the claim that the DMFC encodes saccadic eye movements in head-centered coordinates.     

Aging and movement: Variability of force pulses for saccadic eye movements.

Age-related differences in the trajectories of saccadic eye movements were examined. Younger and older adult subjects produced saccades to predictable target locations. Detailed features of the movements were examined such as the time of peak acceleration and the variability in the magnitude of the peak velocity. These and other measures reveal important details of the force pulses underlying the eye movements and the mental mechanisms that control them. Although minor differences were apparent between the eye movements of younger and older adults, the general patterns were the same across age groups. These results suggest that fundamental details of the brain mechanisms involved in the control of movement are the same for younger and older adults. (PsycINFO Database Record (c) 2010 APA, all rights reserved)