Human optokinetic nystagmus in response to moving binocularly disparate stimuli

Effects of age and visual attention demands on optokinetic nystagmus suppression

INTRODUCTION

The utility of optokinetic nystagmus suppression as an index of visual attention has been demonstrated; however, a gap exists in our understanding of the effects of aging on attentional division. The purpose of this study was to explore the effect of a subject's age upon their ability to allocate visual attention among multiple salient elements which varied in location and complexity.

METHOD

Large-field optokinetic nystagmus (OKN)-inducing animations were presented along with a central flashing fixation point to 27 subjects: 15 younger adults (range 19-23, mean age 21.4); and 12 older adults (range 65-89, mean age 74). Subjects were instructed to fixate on a central point while attending to either moving features of the background or solely to the fixation target. Failure of subjects to accurately divide their attention was quantified by optokinetic gain (eye velocity/background velocity). Gain was analysed in two separate 3-way ANOVAs: one at the central location with the between-subjects variable of age and within-subjects variables of complexity and dynamism; and one using only the dynamic tasks, including a between-subjects variable of age and within-subjects variables of complexity and location.

RESULTS

A strong effect of age was found between subjects during the more attentionally demanding dynamic tasks, but there was only a marginal effect during the static tasks. All within-subjects variables were highly significant, and there were several significant 2- and 3-way interactions.

CONCLUSION

This study provides strong evidence for the compounding effects of senescence and stimulus characteristics on an adult's ability to accurately allocate visual attention. These findings show that OKN suppression may be a useful framework for quantification of attentional resources in older subjects.

Effect of visual attention on the properties of optokinetic nystagmus

It has been demonstrated that optokinetic nystagmus (OKN) gain increases through attention to peripheral motion when the central visual field is occluded. However, how the properties of OKN change when two areas containing motion in different directions are presented in the peripheral visual field is still unclear. In this study, we investigated whether OKN corresponding to the attended motion in the periphery occurred while the observer was maintaining fixation at the center. We presented two areas with different directions of motion arranged on the left and right, top and bottom, or center and surrounding (concentric) areas in the display. Observers counted targets appearing on the attended area in the stimulus to maintain their attention on it. The results indicate that attention enhances the gain and frequency of OKN corresponding to the attended motion even in the case of stimuli having several areas with different directions of motion.

Optokinetic nystagmus in harbor seals (Phoca vitulina)

Harbor seals experience motion due to self-motion and to movement in the external world. However, motion vision has not been studied yet in marine mammals moving in the underwater world. To open up this research, optokinetic nystagmus (OKN) as a basic motion sensing and retinal image stabilizing reflex was studied in four harbor seals during stimulation with moving black-and-white stripe patterns. All seals responded with optokinetic eye movements. Detailed measurements obtained with one animal revealed a moderate gain for horizontal binocular OKN. Monocularly stimulated, the seal displayed a symmetrical OKN with slightly stronger responses to leftward moving stimuli, and, surprisingly, a symmetrical OKN was found in the vertical domain.

Initial ocular following in humans depends critically on the fourier components of the motion stimulus

Visual motion is sensed by low-level (energy-based) and high-level (feature-based) mechanisms. Our interest is in the motion detectors underlying the initial ocular following responses (OFR) that are elicited at ultrashort latencies by sudden motions of large images. OFR were elicited in humans by applying horizontal motion to vertical square-wave gratings lacking the fundamental. In the frequency domain, a pure square wave is composed of the odd harmonics--first, third, fifth, seventh, etc.--such that the third, fifth, seventh, etc., have amplitudes that are one-third, one-fifth, one-seventh, etc., that of the first, and the missing fundamental stimulus lacks the first harmonic. Motion consisted of successive quarter-wavelength steps, so the features and 4n+1 harmonics (where n = integer) shifted forward, whereas the 4n-1 harmonics--including the strongest Fourier component (the third harmonic)--shifted backward (spatial aliasing). Thus, the net Fourier energy and the non-Fourier features moved in opposite directions. Initial OFR, recorded with the search coil technique, had minimum latencies of 60 to 70 ms and were always in the direction of the third harmonic, for example, leftward steps resulted in rightward OFR. Thus, the earliest OFR were strongly dependent on the motion of the major Fourier component, consistent with mediation by oriented spatiotemporal visual filters as in the well-known energy model of motion detection. Introducing interstimulus intervals of 10 to 100 ms (during which the screen was uniform gray) reversed the initial direction of tracking, consistent with extensive neurophysiological and psychophysical data suggesting that the visual input to the motion detectors has a biphasic temporal impulse response.

From 1D to 2D via 3D: dynamics of surface motion segmentation for ocular tracking in primates

In primates, tracking eye movements help vision by stabilising onto the retinas the images of a moving object of interest. This sensorimotor transformation involves several stages of motion processing, from the local measurement of one-dimensional luminance changes up to the integration of first and higher-order local motion cues into a global two-dimensional motion immune to antagonistic motions arising from the surrounding. The dynamics of this surface motion segmentation is reflected into the various components of the tracking responses and its underlying neural mechanisms can be correlated with behaviour at both single-cell and population levels. I review a series of behavioural studies which demonstrate that the neural representation driving eye movements evolves over time from a fast vector average of the outputs of linear and non-linear spatio-temporal filtering to a progressive and slower accurate solution for global motion. Because of the sensitivity of earliest ocular following to binocular disparity, antagonistic visual motion from surfaces located at different depths are filtered out. Thus, global motion integration is restricted within the depth plane of the object to be tracked. Similar dynamics were found at the level of monkey extra-striate areas MT and MST and I suggest that several parallel pathways along the motion stream are involved albeit with different latencies to build-up this accurate surface motion representation. After 200-300 ms, most of the computational problems of early motion processing (aperture problem, motion integration, motion segmentation) are solved and the eye velocity matches the global object velocity to maintain a clear and steady retinal image.

Effects of spatial arrangement of visual stimulus on inverted self-motion perception induced by the foreground motion: examination of OKN-suppression hypothesis

Our previous study revealed that a slowly moving foreground, which is presented in front of a fast-moving orthogonal background, can induce self-motion perception in the same direction as its motion (inverted vection; Vis. Res. 40 (2000) 2915). The present study shows that inverted vection becomes stronger in the conditions where the foreground stimulus is presented in the central area of observer's visual field and the observer's eyes converge on the same depth plane. These stimulus conditions are consistent with the one where the foreground can induce observer's optokinetic nystagmus more effectively, and therefore, the results of this study support our hypothesis in that mis-registered eye-movement information caused by the suppression of optokinetic nystagmus induced by the foreground motion is a critical factor in perceiving inverted vection.

Early behavior of optokinetic responses elicited by transparent motion stimuli during depth-based attention

When two visual patterns moving in different directions are superimposed on the same depth plane (transparent motion stimulus), observers perceive transparent surfaces sliding over each other on different depth planes. Simultaneously, an optokinetic response (OKR) occurs so that one of the visual patterns is stabilized on the retina. In this study, we investigated the early behavior of the OKR elicited by transparent motion stimuli while subjects focused their attention on either the near or far surface. Two random dot patterns were superimposed and moved in orthogonal or opposite directions. Subjects were instructed to report the motion direction of the surface on which their attention was focused. The mean latency of initiation of OKR in the case of motion in opposite directions (150 ms) was significantly longer than that in the case of motion in orthogonal directions (100 ms). In the case of motion in orthogonal directions, the distribution of directions of OKR during the initial period, from 100 to 150 ms, was biased toward the mean direction of the two stimulus motions. After 160 ms, the eyes started to pursue a particular motion pattern of which the direction agreed with the far-perceived motion regardless of depth-based attention. Depth-based attention changed the direction of eye movements after 200 ms and eventually made the eyes follow a pattern on which the attention was focused. These results suggest that pursuit eye movement immediately after 160 ms may determine perceptual depth order through change of retinal image motion, because the slow-moving retinal image may be perceived in the far depth plane. Following this process of determination of perceptual depth order, depth-based attention starts to affect OKR.

Short-latency ocular following in humans is dependent on absolute (rather than relative) binocular disparity

A previous study showed that the initial ocular following responses elicited by sudden motion of a large random-dot pattern were only modestly attenuated when that whole pattern was shifted out of the plane of fixation by altering its horizontal binocular disparity, but the same disparity applied to a restricted region of the dots had a much more powerful effect [Vision Research 41 (2001) 3371]. Thus, if the dots were partitioned into horizontal bands, for example, and alternate bands were moved in opposite directions to the left or right then ocular following was very weak, but if the (conditioning) dots moving in one direction were all shifted out of the plane of fixation (by applying horizontal disparity to them) then strong ocular following was now seen in the direction of motion of the (test) dots in the plane of fixation, i.e., moving images became much less effective when they were given binocular disparity. We sought to determine if the greater impact of disparity with the partitioned images was because there were additional relative disparity cues. We used a similar partitioned display and found that the dependence of ocular following on the absolute disparity of the conditioning stimulus had a Gaussian form with an x-offset that was close to zero disparity and, importantly, this offset was almost unaffected by changing the absolute disparity of the test stimulus. We conclude from this that it is the absolute--rather than the relative--disparity that is important, and that ocular following has a strong preference for moving images whose absolute disparities are close to zero. This is consistent with the idea that ocular following selectively stabilizes the retinal images of objects in and around the plane of fixation and works in harmony with disparity vergence, which uses absolute disparity to bring objects of interest into the plane of fixation [Archives of Ophthalmology 55 (1956) 848].

Short-latency ocular following in humans: sensitivity to binocular disparity

We show that the initial ocular following responses elicited by motion of a large pattern are modestly attenuated when that pattern is shifted out of the plane of fixation by altering its binocular disparity. If the motion is applied to only restricted regions of the pattern, however, then altering the disparity of those regions severely attenuates their ability to generate ocular following. This sensitivity of the ocular tracking mechanism to local binocular disparity would help the observer who moves through a cluttered 3-D world to stabilize objects in the plane of fixation and ignore all others.

Effects of background field-of-view and depth-plane on the oculogyral illusion

This study examined the effects of background field-of-view and depth plane on the oculogyral illusion. Seven subjects viewed a stationary fixation stimulus during the postrotatory interval following a 45-sec constant-velocity chair rotation. The duration of the illusory movement of the fixation stimulus during the postrotatory interval was measured, along with the duration of the illusion of whole-body rotation (known as the somatogyral illusion) and the duration of the subject's slow-phase vestibular nystagmus. Subjects viewed the fixation stimulus by itself in a No-background condition or when surrounded by six background fields formed by the combination of two fields-of-view (35 degrees and 115 degrees) and three depth-planes (near, coplanar, and far). The different background fields inhibited the oculogyral illusion relative to the No-background condition but did not differ statistically from each other. The somatogyral durations better matched the oculogyral ones than did nystagmus decay, especially when a background field was present. These results suggest that the oculogyral illusion is more related to the experience of whole-body rotation than to oculomotor mechanisms and that the inhibitory effect of a background scene is only modestly affected by its field-of-view and depth plane.

Short-latency visual stabilization mechanisms that help to compensate for translational disturbances of gaze

Recent studies in primates have revealed short-latency visual tracking mechanisms that help to stabilize the eyes during translational disturbances of the observer, and so operate as backups to otolith-mediated vestibulo-ocular reflexes. One such mechanism generates version eye movements to help stabilize gaze when the moving observer looks off to one side, utilizing binocular disparity to help single out the images in the plane of fixation (ocular following). Two others generate vergence eye movements to help maintain binocular alignment on objects that lie ahead: one responds to the radial patterns of optic flow (radial-flow vergence) and the other to the changes in binocular parallax (disparity vergence). Accumulating evidence suggests that, despite their short latency, all are mediated by the medial superior temporal area of cortex.

The neural processing of 3-D visual information: evidence from eye movements

Primates have several reflexes that generate eye movements to compensate for bodily movements that would otherwise disturb their gaze and undermine their ability to process visual information. Two vestibulo-ocular reflexes compensate selectively for rotational and translational disturbances of the head, and each has visual backups that operate as negative feedback tracking mechanisms to deal with any residual disturbances of gaze. Of particular interest here are three recently discovered visual tracking mechanisms that specifically address translational disturbances and operate in machine-like fashion with ultra-short latencies (< 60 ms in monkeys, < 85 ms in humans). These visual reflexes deal with motions in all three dimensions and operate as automatic servos, using preattentive parallel processing to provide signals that initiate eye movements before the observer is even aware that there has been a disturbance. This processing is accomplished by visual filters each tuned to a different feature of the binocular images located in the immediate vicinity of the plane of fixation. Two of the reflexes use binocular stereo cues and the third is tuned to particular patterns of optic flow associated with the observer's forward motion. Some stereoanomalous subjects show tracking deficits that can be attributed to a lack of just one subtype of cortical cell encoding motion in one particular direction in a narrow depth plane centred on fixation. Despite their rapid, reflex nature, all three mechanisms rely on cortical processing and evidence from monkeys supports the hypothesis that all are mediated by the medial superior temporal (MST) area of cortex. Remarkably, MST seems to represent the first stage in cortical motion processing at which the visual error signals driving each of the three reflexes are fully elaborated at the level of individual cells.

Visual stabilization of the eyes in primates

Observers moving through a textured three-dimensional world experience potentially confusing patterns of optic flow. Recent experiments on monkeys and humans have revealed the existence of rapid, yet cortically mediated, reflex eye movements that automatically single out images in the plane of fixation for stabilization and ignore images that are nearer or further.

Motion processing for saccadic eye movements during the visually induced sensation of ego-motion in humans

During ego-motion an observer is often faced with the task of controlling his heading direction while simultaneously registering the movement of objects in order to avoid possible obstacles. Psychophysical experiments have shown that the detection of moving objects is impaired by concurrent ego-motion. We investigated the interaction between ego-motion and object-motion by examining the latencies of saccades executed to moving targets under a visually induced sensation of ego-motion. Saccadic latencies increased during this sensation, with a global or non-retinotopic effect of optic flow on motion detection. Furthermore, separating stereoscopically the moving target and the optic flow into foreground and background, respectively, still resulted in increased latencies. We propose that an inhibitory influence of the perception of self-motion exists on the perception of object-motion. These results support a model of space constancy which strives to create a stable world during locomotion.

Positional disparity sensitivity of neurons in the cat accessory optic system

There is considerable evidence supporting the view that the accessory optic system (AOS) and the closely associated nucleus of the optic tract (NOT) provide visual signals used in the control of optokinetic nystagmus (OKN). In frontal-eyed animals such as the cat and primate, the high degree of overlap in the visual fields of each eye, along with a substantial projection from the visual cortex, gives rise to an increased incidence of binocularly responsive neurons in the AOS. In previous studies, my collaborators and I have shown that visual cortical input to the AOS mediates ipsilateral eye responses and high speed tuning, and can function independently of the contralateral eye. However, beyond fairly gross assessments such as these, the binocular interactions of AOS cells have not been subject to detailed examination. The present study set out to determine whether the responses of binocular cells in the dorsal terminal nucleus (DTN) of the AOS are sensitive to horizontal retinal disparity. Single units were recorded from the DTN of anaesthetized, paralysed cats. A large random-dot pattern was moved under computer control at a constant velocity in the preferred and non-preferred direction. Convergent and divergent disparities were generated by deviating the visual axis of the contralateral (dominant) eye using wedge prisms. The responses of DTN units fell into three categories: (1) cells showing tuned excitatory responses (29% or 7 cells) consisting of a marked facilitation for a single or a limited range of disparities; (2) cells broadly tuned for inhibition (25% or 6 cells); and (3) cells relatively insensitive to disparity (46% or 11 cells), showing a relatively flat response profile across the entire range of disparity conditions, or cells without clear tuning. In summary, this study demonstrates that some AOS cells are sensitive to positional disparity and, therefore, this system may provide signals which specify the plane of motion for ocular stabilization. Some of these results have been presented in brief form [Grasse (1991a) Society of Neuroscience Abstracts, 17, 1380].

Eye movements elicited by transparent stimuli

A transparent motion condition occurs when two different motion vectors appear at the same region of an image. Such transparency during self-motion has shown demonstrable effects on perception and on the underlying neurophysiology in the cortical and subcortical structures of primates. Presumably such stimulus conditions also influence oculomotor behavior. We investigated smooth-pursuit performance, using a transparent stimulus consisting of two oppositely-moving patterns. We found slight reduction in the mean eye velocity tracking a transparent pattern, compared with that when tracking a unidirectional pattern. Additionally, we investigated the behavior of the optokinetic system to transparency, demonstrating that it elicits antagonistic optokinetic nystagmus, with distinctly reduced gain of the slow phases. Furthermore, we observed, during optokinetic stabilization of transparent stimuli, directional dominances demonstrating that subjects preferably followed one direction. Presenting a transparent stimulus with oppositely moving patterns and different velocities we found a general velocity dominance demonstrating that patterns with a certain velocity are preferred. Performing all experiments under dichoptic conditions produced results comparable with those found under transparent stimulus conditions.

The effects of visual depth and eccentricity on manual bias, induced motion, and vection

The relationship between the effects of visual-surround roll motion on compensatory manual tracking of a central display and the perceptual phenomena of induced motion and vection were investigated. To determine if manual-control biases generated in the direction of surround rotation compensate primarily for the perceived counterrotation of the central display ('induced motion') or the perceived counterrotation of the entire body ('vection'), the depth and eccentricity of the visual surround were varied. In the first experiment, twelve subjects attempted to keep an unstable central display level while viewing rotating visual surrounds in three depth planes: near (approximately 20 cm in front of the central display), coplanar, and far (approximately 21 cm behind the central display). In the second experiment, twelve additional subjects viewed a rotating surround that was presented either in the full visual field (0-110 deg) or in central and peripheral regions of similar width. Manual-control biases and induced motion were shown to be closely related to one another and strongly influenced both by central and by peripheral surround motion at or beyond the plane of fixation. Vection, on the other hand, was shown to be much more dependent on peripheral visual inputs.

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.

Ocular compensation for self-motion. Visual mechanisms

In monkeys, there are several reflexes that generate eye movements to compensate for the observer's own movements. Two vestibuloocular reflexes compensate selectively for rotational (RVOR) and translational (TVOR) disturbances of the head, receiving their inputs from the semicircular canals and otolith organs, respectively. Two independent visual tracking systems that deal with residual disturbances of gaze are manifest in the two components of the optokinetic response: the indirect or delayed component (OKNd) and the direct or early component (OKNe). We hypothesize that OKNd--like the RVOR--is phylogenetically old, being found in all animals with mobile eyes, and that it evolved as a backup to the RVOR to compensate for rotational disturbances of gaze. Indeed, optically induced changes in the gain of the RVOR result in parallel changes in the gain of OKNd, consistent with the idea of shared pathways as well as shared functions. In contrast, OKNe--like the TVOR--seems to have evolved much more recently in frontal-eyed animals and, we suggest, acts as a backup to the TVOR to deal primarily with translational disturbances of gaze. Frontal-eyed animals with good binocular vision must be able to keep both eyes directed at the object of regard irrespective of proximity, and in order to achieve this during translational disturbances, the output of the TVOR is modulated inversely with the viewing distance. OKNe shares this sensitivity to absolute depth, consistent with the idea that it is synergistic with the TVOR and shares some of its central pathways. There is evidence that OKNe is also sensitive to relative depth cues such as motion parallax, which we suggest helps the system to segregate the object of regard from other elements in the scene. However, there are occasions when the global optic flow cannot be resolved into a single vector useful to the oculomotor system (e.g., when the moving observer looks towards the direction of heading). We suggest that on such occasions a third independent tracking mechanism, the smooth pursuit system, is deployed to stabilize gaze on the local feature of interest. In this scheme, the pursuit system has an attentional focusing mechanism that spatially filters the visual motion inputs driving the oculomotor system. The major distinguishing features of the 3 visual tracking mechanisms are summarized in Table 1.

Induced motion of a fixated target: influence of voluntary eye deviation

Induced motion (IM) was observed in a fixated target in the direction opposite to the real motion of a moving background. Relative to a fixation target located straight ahead, IM decreased when fixation was deviated 10 degrees in the same direction as background motion and increased when fixation was deviated 10 degrees opposite background motion. These results are consistent with a "nystagmus-suppression" hypothesis for subjective motion of fixated targets: the magnitude of illusory motion is correlated with the amount of voluntary efference required to oppose involuntary eye movements that would occur in the absence of fixation. In addition to the form of IM studied, this explanation applies to autokinesis, apparent concomitant motion, and the oculogyral illusion. Accounts of IM that stress visual capture of vection, afferent mechanisms, egocenter deviations, or phenomenological principles, although they may explain some forms of IM, do not account for the present results.

Induced motion: isolation and dissociation of egocentric and vection-entrained components

Induced motion (IM) is illusory motion of a stationary test target opposite to the direction of the real motion of the inducing stimulus. We define egocentric IM as an apparent motion of the test target relative to the observer, and vection-entrained IM as an apparent motion of a stationary object along with an apparent motion of the self (vection) induced by the same stimulus. These two forms of IM are often confounded, and tests for distinguishing between them have not been devised. We have devised such tests. Our test for egocentric IM relies on evidence that this form of IM is due mainly to a misregistration of eye movements when optokinetic nystagmus (OKN) is inhibited, and on evidence that OKN is evoked only by stimuli in the plane of convergence. Our test for vection-entrained IM relies on evidence that vection is evoked only by the more distant of two superimposed inducing stimuli. Thus we found egocentric IM to be induced without vection or vection-entrained IM when subjects converged on a foreground moving display with a stationary display in the background, and vection-entrained IM to be induced without egocentric IM when subjects converged on a stationary-foreground display with a moving display in the background. The two types of IM were evoked in opposite directions at the same time when subjects converged on a foreground moving display while a background display moved in the opposite direction. The two forms of IM showed no signs of interaction, and we conclude that they rely on independent motion mechanisms that operate within distinct frames of reference. A control experiment suggested that the depth adjacency effect in IM is determined by the depth adjacency of the inducing stimulus to convergence, not just to the test target.

OKN asymmetries and binocular function in amblyopia

Asymmetrical optokinetic nystagmus (OKN) means that OKN has a lower gain (slow-phase eye velocity/stimulus velocity) for monocular temporalward than nasalward visual field motion. OKN tends to be asymmetric in amblyopia, leading to suggestions of a link between OKN asymmetry and binocularity in the literature. The present study measured OKN in 13 amblyopes and five normal subjects. In an attempt to identify those binocular cells used in the OKN response, the degree of OKN asymmetry was compared with binocularity assessed by two different techniques: (1) stereopsis and (2) interocular transfer of threshold elevation (IOT). Horizontal monocular OKN was recorded for three different stimulus velocities in each subject. All the amblyopes were found to be stereoblind, although three amblyopes showed OKN asymmetries close to those found for the normal group. More association was seen between interocular transfer of the threshold elevation and OKN asymmetry; not all amblyopes demonstrated reduced IOT, but those amblyopes with no IOT all had OKN asymmetries more than 125% of the mean of the normal group. However, no association was seen between the amount of OKN asymmetry and the degree of IOT. The results are discussed in terms of the role of different groups of binocular neurones for OKN and the effect of the sensitive periods of development on these binocular neurones.

Human optokinetic nystagmus: competition between stationary and moving displays

We reported earlier that occlusion of the central retina and stationary edges have highly interactive effects on the gain of optokinetic nystagmus (OKN; Murasugi, Howard, & Ohmi, 1986). In this study, we explored this effect in more detail. A central occluding band of variable height, flanked by vertical bars, was superimposed onto an array of dots moving at 30 degrees per second. The height of the occluding band required to abolish OKN increased with the separation of the vertical bars. For bars 3.5 degrees apart, OKN was abolished in most subjects when a band only 6' high ran between them. For bars 75 degrees apart, a band at least 20 degrees in height was required to abolish the response. The effects of the stationary figure depended to some extent on the subject's attention, but only at intermediate values of bar separation. Both low- and high-level mechanisms are proposed to account for the results.