The gap effect and inhibition of return: interactive effects on eye movement latencies

Three experiments are reported with two types of manipulations that are known to affect the latency with which subjects can initiate saccadic eye movements. The first manipulation involves the temporal relation between the offset of a visual fixation point and the onset of a peripheral target (the gap effect). The second manipulation involves the prior allocation and removal of visual attention (inhibition of return). In two experiments, the gap effect was smaller for saccades to previously attended locations than to previously unattended locations. The results suggest an important link between the two phenomena and provide new insights into the brain mechanisms underlying visual attention and eye movements.     

Age-related differences in postural control: effects of the complexity of visual manipulation and sensorimotor contribution to postural performance

Patterns of adaptive changes to the exposure to a sinusoidal visual stimulus can be influenced by stimulus characteristics as well as the integrity of the sensory and motor systems involved in the task. Sensorimotor deficits due to aging might alter postural responses to visual manipulation, especially in more demanding tasks. The purpose of this study was to compare postural control between young and older adults at different levels of complexity and to examine whether possible sensory and/or motor changes account for postural performance differences in older adults. Older and young adults were submitted to the following tests: postural control assessments, i.e., body sway during upright stance and induced by movement of a visual scene (moving room paradigm); sensory assessments, i.e., visual (acuity and contrast sensitivity) and somatosensory (tactile foot sensitivity and detection of passive ankle motion); and motor assessments, i.e., isometric ankle torque and muscular activity latency after stance perturbation. Older adults had worse sensory and motor performance, larger body sway amplitude during stance and stronger coupling between body sway and moving room motion than younger adults. Multiple linear regression analyses indicated that the threshold for the detection of passive ankle motion contributed the most to variances in body sway and this contribution was more striking when visual information was manipulated in a more unpredictable way. The present study suggests that less accurate information about body position is more detrimental to controlling body position, mainly for older adults in more demanding tasks.     

Motor control prior to movement onset: preparatory mechanisms for pointing at visual targets

Summary The present study investigated the mechanisms involved in the preparation of pointing movements in humans. We provided visual precues on the location of the upcoming target, and registered the effect of these precues on the reaction time (RT = interval between target appearance and movement onset). Generally, precues were found to reduce RT, suggesting that some aspects of the preparatory process have been advanced in time. In Exp. 1, precues fully specified the direction required for the upcoming movement while indicating only a range of movement amplitudes; in Exp. 2, precues fully specified the amplitude and indicated a range of directions. In both experiments, RT was shorter than in control trials without precues, and gradually increased with the size of the precued amplitude or direction range. This result suggests that the preparation of either parameter is possible without knowing the precise value of the other, i.e. amplitude and direction are not prepared in a fixed order. Furthermore, our results are consistent with the view that movement preparation includes a progressive contraction of the precued range towards the final value. The speed of this process can be estimated as 0.31 cm/ms for amplitude, and 1.7 deg/ms for direction ranges. In Exp. 3 and 4, precues indicated both amplitude and direction as ranges only. The size of the amplitude range was held constant while the size of the direction range was varied (Exp. 3), or vice versa (Exp. 4). Under these conditions, RT increased with the size of the varied range. For all range sizes tested, RT when precuing both amplitude and direction as ranges corresponded to the longer of the two RTs obtained in control trials where only one parameter was precued as range (like in Exp. 1 and 2), This outcome supports the hypothesis that amplitude and direction are prepared in parallel. The contraction speeds of amplitude and direction ranges estimated from Exp. 3 and 4 were comparable to those estimated from Exp. 1 and 2, indicating that processing speed is not reduced if both parameters rather than just one have to be prepared. In Exp. 5, the precued amplitude and direction range was held constant while precue area, and thus the range of possible final arm positions, was varied. RT was independent of precue area, which argues against a major contribution of position control mechanisms. Taken together, the present data support the hypothesis that amplitude and direction are the two most predominant parameters of movement preparation.     

Saccadic eye movements to visual and auditory targets

 Recent neurophysiological studies of the saccadic ocular motor system have lent support to the hypothesis that this system uses a motor error signal in retinotopic coordinates to direct saccades to both visual and auditory targets. With visual targets, the coordinates of the sensory and motor error signals will be identical unless the eyes move between the time of target presentation and the time of saccade onset. However, targets from other modalities must undergo different sensory-motor transformations to access the same motor error map. Because auditory targets are initially localized in head-centered coordinates, analyzing the metrics of saccades from different starting positions allows a determination of whether the coordinates of the motor signals are those of the sensory system. We studied six human subjects who made saccades to visual or auditory targets from a central fixation point or from one at 10° to the right or left of the midline of the head. Although the latencies of saccades to visual targets increased as stimulus eccentricity increased, the latencies of saccades to auditory targets decreased as stimulus eccentricity increased. The longest auditory latencies were for the smallest values of motor error (the difference between target position and fixation eye position) or desired saccade size, regardless of the position of the auditory target relative to the head or the amplitude of the executed saccade. Similarly, differences in initial eye position did not affect the accuracy of saccades of the same desired size. When saccadic error was plotted as a function of motor error, the curves obtained at the different fixation positions overlapped completely. Thus, saccadic programs in the central nervous system compensated for eye position regardless of the modality of the saccade target, supporting the hypothesis that the saccadic ocular motor system uses motor error signals to direct saccades to auditory targets. Received: 8 September 1995 / Accepted: 22 November 1996     

Age-related performance of human subjects on saccadic eye movement tasks

We measured saccadic eye movements in 168 normal human subjects, ranging in age from 5 to 79 years, to determine age-related changes in saccadic task performance. Subjects were instructed to look either toward (pro-saccade task) or away from (anti-saccade task) an eccentric target under different conditions of fixation. We quantified the percentage of direction errors, the time to onset of the eye movement (saccadic reaction time: SRT), and the metrics and dynamics of the movement itself (amplitude, peak velocity, duration) for subjects in different age groups. Young children (5–8 years of age) had slow SRTs, great intra-subject variance in SRT, and the most direction errors in the anti-saccade task. Young adults (20–30 years of age) typically had the fastest SRTs and lowest intra-subject variance in SRT. Elderly subjects (60–79 years of age) had slower SRTs and longer duration saccades than other subject groups. These results demonstrate very strong age-related effects in subject performance, which may reflect different stages of normal development and degeneration in the nervous system. We attribute the dramatic improvement in performance in the anti-saccade task that occurs between the ages of 5–15 years to delayed maturation of the frontal lobes. Received: 27 October 1997 / Accepted: 27 February 1998     

Structural analysis of eye movement response to visual field stimuli

The nature of the saccadic sequence made in response to a near threshold point light stimulus is considered as an index of perception. Features of the response sequence are evaluated by their ability to discriminate between searching responses to sub-threshold stimuli and acquisition responses to supra-threshold stimuli. A structural analysis of the entire sequence is constructed which achieves successful discrimination. The approach is proposed as the basic component of an alternate method of static perimetry.     

Eye position and target amplitude effects on human visual saccadic latencies

Gaze shifts vary in the extent of eye and head contribution; a large amplitude and/or an eccentric ocular orbital starting position alter the participation of head movement in the shift. The interval between eye onset and head onset determines compensatory counterrolling before and after the shift and the extent of vestibular ocular reflex reduction during the shift. The latency of eye saccades in the head-fixed condition was measured with respect to target amplitude and orbital position in order to establish base-line operations of these two variables as they apply to the headfree condition. Eye movements were measured during single-step saccades in nine young adult humans. The target step, hereafter called a jump, started from three possible fixation lights; e.g., rightward saccades started from the midline (0°) or from -20 or -40° left of the midline, with a maximum amplitude of 80°. The latency of saccades starting from the primary position increased with jump amplitude (amplitude-latency relation). When the eye started eccentrically, the latency was decreased (orbital position-latency relation), with the largest jump amplitudes most affected. These changes can be related to active eye-head coordination. Thus, with a leftward maximal orbital eccentricity, compensatory eye rotation would be impossible with a rightward head movement; however, incorporating the orbital position-latency relation, the forward ocular saccade is expedited by 90 ms. Conversely, with a primary starting position, the ocular component of an 80° gaze saccade could be slowed 125 ms by incorporating the amplitude-latency relation, thus facilitating a head contribution to the gaze shift. The orbital position and amplitude-latency relations were prominent in those subjects with habitually large head contributions to the gaze shift and minimal in individuals with typically small head contributions.     

Analysis of eye movement responses and dynamic visual acuity

Summary Horizontal eye movements were recorded during measurements on dynamic visual acuity to determine whether the deterioration in visual acuity during tracking is due to imperfect pursuit movements. The electrical method of eye movement recording (electro-oculography) was employed. Analysis of these recordings indicates that at low velocities (22°/sec and 43°/sec) accurate and synchronous pursuit is possible but at higher speeds (83°/sec and 167°/sec) saccadic movements persist owing to fixation failure. Above 60–70°/sec the oculomotor co-ordination system breaks down and saccadic movements replace the pursuit movements as the dominant mechanism.This work formed part of a Ph.D. thesis approved by the University of London.     

Differential effects of waking from non-rapid eye movement versus rapid eye movement sleep on cardiovascular activity

The occurrence of cardiovascular events increases in the morning, and while the mechanism responsible is yet to be determined, possible contributors include surges in sympathetic activity and concurrent rises in blood pressure (BP). This study tested the hypothesis that the increase in sympathetic dominance and the surge in BP were greater when waking spontaneously from Stage 2 sleep compared with waking from rapid eye movement (REM) sleep. Twenty healthy young adults had overnight polysomnography, including electrocardiogram measurements. Spectral analysis of heart rate variability (HRV) was conducted on 2-min blocks of stable data selected from the last 30 min of sleep and during 30 min of resting wakefulness (supine) immediately following sleep. Outputs included absolute low frequency (LF) and high frequency (HF) power, the LF/HF ratio, heart rate (HR) and BP. To investigate the effect of waking from Stage 2 or REM sleep on HRV and BP responses, two-way analyses of variance ( anova s) (Stage 2 versus REM) with repeated measures (sleep versus morning wakefulness) were performed. It was found that waking from Stage 2 sleep was associated with significant increases in HR ( P  = 0·002) and BP ( P  < 0·001), as well as a tendency towards an increase in the LF/HF ratio ( P  = 0·08), whereas measurements during REM sleep and subsequent wakefulness were similar ( P  > 0·05). The greater increase in BP and HR when waking from Stage 2 sleep compared with REM sleep suggests that in vulnerable populations, waking from Stage 2 sleep could be an adjunct risk factor of cardiovascular events during the morning period.     

Age-related decline of peripheral visual processing: the role of eye movements

Earlier work suggests that the area of space from which useful visual information can be extracted (useful field of view, UFoV) shrinks in old age. We investigated whether this shrinkage, documented previously with a visual search task, extends to a bimanual tracking task. Young and elderly subjects executed two concurrent tracking tasks with their right and left arms. The separation between tracking displays varied from 3 to 35 cm. Subjects were asked to fixate straight ahead (condition FIX) or were free to move their eyes (condition FREE). Eye position was registered. In FREE, young subjects tracked equally well at all display separations. Elderly subjects produced higher tracking errors, and the difference between age groups increased with display separation. Eye movements were comparable across age groups. In FIX, elderly and young subjects tracked less well at large display separations. Seniors again produced higher tracking errors in FIX, but the difference between age groups did not increase reliably with display separation. However, older subjects produced a substantial number of illicit saccades, and when the effect of those saccades was factored out, the difference between young and older subjects’ tracking did increase significantly with display separation in FIX. We conclude that the age-related shrinkage of UFoV, previously documented with a visual search task, is observable with a manual tracking task as well. Older subjects seem to partly compensate their deficit by illicit saccades. Since the deficit is similar in both conditions, it may be located downstream from the convergence of retinal and oculomotor signals.     

The assessment of position of stationary targets perceived during saccadic eye movement

Summary The experiment was performed to establish the accuracy with which visual targets perceived during saccadic eye movement are localised. Subjects were presented with the task of executing saccades of 30° plus amplitude, passing through primary gaze, about the time of peak velocity a 5 ms red flash was presented at some random position (up to 30° left or right of centre) on a horizontal visual display. Subjects were required to indicate the direction in which they thought the flash was localised by fixating in that direction. Observations were made under conditions of prolonged total darkness and in the presence of a contrasting background. Measurement was made of saccade velocity and eye displacement as an index of target positions. Eye displacement was linearly scaled with respect to true target direction. Targets were localised with an average error of 5°–6° although the variance was high. No systematic differences were found between conditions or subjects. Error was unrelated to saccade velocity. It is concluded that during saccadic eye movements the appreciation of target position is maintained with an acceptable degree of accuracy.     

The effects of visual input on open-loop and closed-loop postural control mechanisms

In an earlier posturographic investigation (Collins and De Luca 1993) it was proposed that open-loop and closed-loop control mechanisms are involved in the regulation of undisturbed, upright stance. In this study, stabilogram-diffusion analysis was used to examine how visual input affects the operational characteristics of these control mechanisms. Stabilogram-diffusion analysis leads to the extraction of repeatable center-of-pressure (COP) parameters that can be directly related to the resultant steady-state behavior and functional interaction of the neuromuscular mechanisms underlying the maintenance of erect posture. Twenty-five healthy male subjects (aged 19–30 years) were included in the study. An instrumented force platform was used to measure the time-varying displacements of the COP under each subject's feet during quiet standing. The subjects were tested under eyes-open and eyes-closed conditions. The COP trajectories were analyzed as one-dimensional and two-dimensional random walks, according to stabilogram-diffusion analysis. Using this technique, it was found that visual input affects the performance of the postural control system in one of two different ways — either it significantly modifies the steady-state behavior of the open-loop postural control mechanisms, or it significantly alters the characteristics of the other closed-loop feedback mechanisms that are involved in balance control. This result is interpreted as an indication that the visual system is integrated into the postural control system in one of two different ways. The experimental population was roughly evenly divided between these two schemes. For the first group (13 of 25 subjects), visual input principally caused a decrease in the effective stochastic activity of the open-loop control mechanisms in both the mediolateral and anteroposterior directions. For the second group (12 of 25 subjects), visual input caused an increase in the effective stochastic activity and uncorrelated behavior of the closed-loop control mechanisms in the anteroposterior direction only. On the basis of these results, it is hypothesized that visual input, in both schemes, serves to decrease the stiffness of the musculoskeletal system. In the former case, this may be accomplished by decreasing the level of muscular activity across the joints of the lower limb, whereas, in the latter case, reduced stiffness may be achieved by reducing the gain(s) of the other postural feedback mechanisms, i.e., the proprioceptive and/or vestibular systems. Using stabilogram-diffusion analysis, it was also found that the two groups of subjects behaved similarly under eyes-closed conditions. This result suggests that the open-loop postural control mechanisms and reflex-based feedback systems, respectively, of healthy, young individuals are organized in functionally equivalent ways.     

The directional effects of passive eye movement on the directional visual responses of single units in the pigeon optic tectum

 We have investigated the visual responses of 184 single units located in the superficial layers of the optic tectum (OT) of the decerebrate, paralysed pigeon. Visual responses were similar to those reported in non-decerebrate preparations; most units responded best to moving visual stimuli, 18% were directionally selective (they had a clear preference for a particular direction of visual stimulus movement), 76% were plane-selective (they responded to movement in either direction in a particular plane). However, we also found that a high proportion of units showed some sensitivity to the orientation of visual stimuli. We examined the effects of extraocular muscle (EOM) afferent signals, induced by passive eye movement (PEM), on the directional visual responses of units. Visual responses were most modified by particular directions of eye movement, although there was no unique relationship between the direction of visual stimulus movement to which an individual unit responded best and the direction of eye movement that caused the greatest modification of that visual response. The results show that EOM afferent signals, carrying information concerning the direction of eye movement, reach the superficial layers of the OT in the pigeon and there modify the visual responses of units in a manner that suggests some role for these signals in the processing of visual information. Received: 17 June 1996 / Accepted: 29 April 1997     

The effects of increasing memory load on the directional accuracy of pointing movements to remembered targets

The directional accuracy of pointing arm movements to remembered targets in conditions of increasing memory load was investigated using a modified version of the Sternbergs context-recall memory-scanning task. Series of 2, 3 or 4 targets (chosen randomly from a set of 16 targets around a central starting point in 2D space) were presented sequentially, followed by a cue target randomly selected from the series excluding the last one. The subject had to move to the location of the next target in the series. Correct movements were those that ended closer to the instructed target than any other target in the series while all other movements were considered as serial order errors. Increasing memory load resulted in a large decrease in the directional accuracy or equivalently in the directional information transmitted by the motor system. The constant directional error varied with target direction in a systematic fashion reproducing previous results and suggesting the same systematic distortion of the representation of direction in different memory delay tasks. The constant directional error was not altered by increasing memory load, contradicting our hypothesis that it might reflect a cognitive strategy for better remembering spatial locations in conditions of increasing uncertainty. Increasing memory load resulted in a linear increase of mean response time and variable directional error and a non-linear increase in the percentage of serial order errors. Also the percentage of serial order errors for the last presented target in the series was smaller (recency effect). The difference between serial order and directional spatial accuracy is supported by neurophysiological and functional anatomical evidence of working memory subsystems in the prefrontal cortex.This work was supported by internal funding from Aeginition University Hospital     

Disruptive effects of rapid eye movement sleep deprivation on long-term memory

The fact that sleep is associated with very active endogenous neural (chemical and electrical) processes, suggests that these processes may be involved in the maintenance of long-term memory storage. The present experiments were designed to examine the hypothesis that rapid eye movement (REM) sleep deprivation will produce impairment of long-term memory. Mice deprived of REM sleep for 3, 5 or 7 continuous days, during the interval between a one-trial inhibitory avoidance training experience and a subsequent retention test, displayed a temporary retrograde amnesia when tested 30 min or three hr following termination of REM deprivation. The mice did not recover from the amnesia if electroconvulsive shock was administered immediately following the interval of REM sleep deprivation. In a further study, the generality of these findings was obtained by depriving mice of REM sleep during the interval between a discrimination training experiment in a black-white T-maze and the subsequent retention test.     

Directional selectivity in the responses of units in cat primary visual cortex to passive eye movement

The responses of single units in the primary visual cortex (Area 17) of anaesthetized, paralysed cats, to passive movement of the ipsilateral eye were studied. Responses to passive eye movement were found in about one-third of the cortical units isolated. Appropriate control experiments excluded visual, auditory and cutaneous inputs as the source of the effective signal during passive eye movement. The magnitudes of the responses to a number (usually four) of radial directions of passive eye movement were estimated from sets of peristimulus time histograms "interleaved" in time. Units were defined as "radially selective" if the responses to movement along one radius (e.g. vertically upwards) exceeded that along at least one other orthogonal radius (e.g. horizontal-temporal). Of 60 units tested, 53 (88%) were "radially selective" according to this definition. Some of the "radially selective" units showed an additional type of specificity to passive eye movement: (a) Some units responded preferentially to movement along one of the arcs of passive eye movement which were tested (e.g. vertical movement above the equator of the orbit). These units we have called "arc selective". (b) Other units were sensitive to the direction of movement and preferred movement in a particular direction over more than one arc (e.g. horizontal movement towards the temporal side in both nasal and temporal halves of the orbit). These we have called "direction selective". Twenty-one "radially selective" units showed one of these additional properties, nine were arc selective and twelve were direction selective. The implications of these results for the understanding of the function of orbital proprioceptive signals in the cortex are discussed briefly. Responses to passive eye movement were found in all of layers II-VI in Area 17 and the implications of this for the understanding of the pathway by which orbital proprioceptive signals reach the primary visual cortex are discussed. The experiments have shown that many units in cat visual cortex respond to passive eye movement and that most of these units have some specificity for particular radial directions of movement while some have additional specific properties. We believe that these properties of radial, directional and arc sensitivity are likely to be important in understanding the function of the orbital proprioceptive signal which arises during eye movement and they are particularly interesting in relation to the findings of others that this proprioceptive signal appears to be concerned in the normal development of visual properties in the cortex and in the control of visually guided movement in adult cats.     

Delay activity and sensory-motor translation during planned eye or hand movements to visual or tactile targets

To perform eye or hand movements toward a relevant location, the brain must translate sensory input into motor output. Recent studies revealed segregation between circuits for translating visual information into saccadic or manual movements, but less is known about translation of tactile information into such movements. Using human functional magnetic resonance imaging (fMRI) in a delay paradigm, we factorially crossed sensory modality (vision or touch) and motor effector (eyes or hands) for lateralized movements (gaze shifts to left or right or pressing a left or right button with the corresponding left or right hand located there). We investigated activity in the delay-period between stimulation and response, asking whether the currently relevant side (left or right) during the delay was encoded according to sensory modality, upcoming motor response, or some interactive combination of these. Delay activity mainly reflected the motor response subsequently required. Irrespective of visual or tactile input, we found sustained activity in posterior partial cortex, frontal-eye field, and contralateral visual cortex when subjects would later make an eye movement. For delays prior to manual button-press response, activity increased in contralateral precentral regions, again regardless of stimulated modality. Posterior superior temporal sulcus showed sustained delay activity, irrespective of sensory modality, side, and response type. We conclude that the delay activations reflect translation of sensory signals into effector-specific motor circuits in parietal and frontal cortex (plus an impact on contralateral visual cortex for planned saccades), regardless of cue modality, whereas posterior STS provides a representation that generalizes across both sensory modality and motor effector.     

Neural underpinning of postural responses to visual field motion

Numerous results emerging from current research strongly implicate the effect of Visual Field Motion on the organization of postural responses. However, this is the first empirical study exploring the neural substrates underlying the subjects' response to Visual Field Motion. Two separate experiments were conducted to investigate the subject responses to Visual Field Motion. In the first experiment, the standing subjects were exposed to Visual Field Motion in the VR environment. In the second experiment, the recumbent subjects viewed the same Visual Field Motion while in a MRI scanner. A virtual reality (VR) prototype of the moving room paradigm [Lee, D.N., Aronson, E., 1974. Visual proprioceptive control of standing in human infants. Perception & Psychophysics 15, 529-532] was developed to simulate various optic flow patterns in a controlled VR environment. Postural responses (center of pressure, body kinematics, vection, egomotion) and brain activation patterns (fMRI signals) were examined. The subjects experienced egomotion and have reported vection in both experiments only when certain attributes of Visual Field Motion were introduced. This was accompanied by significant activation of specific brain structures, including prefrontal, parietal cortices and bilateral cerebellum. We propose the existence of functional interactions between modality specific areas of the brain involved in postural responses to Visual Field Motion (VFM).