Direction Of Target Motion Research Articles

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.

Object-motion sensitivity loss due to motion in the peripheral visual field

The effect of peripheral-field movement on detection of object motion was investigated in two experiments. In each experiment, 24 subjects judged the movement of targets while elements in the subject’s peripheral visual field were moving and while they were stationary. Targets were stationary or were moving at speeds between 6.2 and 23.1 minutes of visual arc per second. Target locations were to the left or right of (Experiment 1), or above or below (Experiment 2), a central fixation stimulus. Peripheral-field condition, direction of target movement, and speed of target movement significantly affected motion detection. The interaction between direction of target movement and speed of target movement was found to be significant in the analyses of both experiments. Target location was found to be a significant factor in Experiment 2, and sensitivity differences were found between horizontal and vertical target movement. The decrease in sensitivity due to peripheral-field motion did not depend upon subjects’ experiencing vection.

Smooth pursuit dysfunction in Alzheimer's disease.

Smooth ocular pursuit was measured by magnetic search coil oculography in 13 patients with Alzheimer's disease and compared with control subjects. Smooth eye movement gain was uniformly reduced in Alzheimer's disease at all target velocities for several frequencies of sinusoidal target motion, signifying impairment of steady-state gain. Normal phase relationships between the target and eyes indicated an intact predictor mechanism for smooth pursuit. When peak target velocity was held constant, pursuit gain decreased markedly in response to small increments of target acceleration, indicating involvement of an acceleration saturating nonlinear element that limits smooth pursuit. Large-amplitude saccadic intrusions, in the direction of target motion, often disrupted pursuit; smooth eye movements continued in response to target velocity despite large position errors of the fovea from its target. These disorders of smooth eye movement control can quantify motor dysfunction in Alzheimer's disease.

The role of striate cortex in the guidance of eye movements in the monkey

We studied the effect of unilateral striate cortical ablations on smooth pursuit and saccadic eye movements in the monkey. The monkeys made quite accurate saccades to stationary stimuli in the field contralateral to the lesion, and they readily pursued foveal targets moving in all directions. However, when visual stimuli were stepped into the field contralateral to the lesion and then began to move, thus insuring that the moving stimulus was confined to the impaired visual hemifield, several oculomotor abnormalities emerged. Saccades to moving stimuli presented in the impaired field consistently undershot targets that moved away from the central fixation point after the step, and overshot targets that moved back towards the central fixation point. There was little or no smooth pursuit eye velocity generated in any direction to moving stimuli in the impaired field, and the monkeys could not generate smooth pursuit to stimuli maintained a few degrees from the fovea in the impaired field, although they were able to pursue such stimuli held in the normal field. Ablation of striate cortex also affected the latencies of saccades. When step-ramp stimuli were presented in the normal field, the monkeys delayed the initiation of saccades to targets moving towards the central fixation point, and hastened the initiation of saccades to targets moving away from the central fixation point. By contrast, changes in the direction of target movement did not affect the latencies of saccades into the impaired field. The deficits seemed permanent, lasting as long as the monkeys were tested--over 2 years in one case--but they were not total. Each monkey could use stimuli moving into the affected field to develop some eye velocity, although this residual ability had a much longer latency and lower gain than that provided by the intact visual system. These results show that striate cortex is intimately involved in the estimation of stimulus velocity critical to the genesis of smooth pursuit and saccadic eye movements.

Attention to central and peripheral visual space in a movement detection task. III. Separate effects of auditory deprivation and acquisition of a visual language

We employed event-related brain potentials (ERPs) and measures of signal detectability to compare attention to peripheral and central visual stimuli in normal hearing subjects who were born to deaf parents (HD Ss) and whose first language was American Sign Language (ASL). The results were compared with those obtained from normal hearing Ss 42 and congenitally deaf Ss 43 in the same paradigm. Task performance and ERPs during attention to the foveal region were similar in the 3 groups. In contrast, with attention to the peripheral stimuli the deaf Ss displayed attention effects over the occipital regions of both hemispheres that were several times larger than those in the hearing and the HD Ss. However, both HD and deaf Ss displayed lateral asymmetries in behavior and ERPs that were opposite in direction to those of the hearing Ss. Whereas hearing Ss detected the direction of target motion better when it occurred in the left visual field, deaf and HD Ss performed better for right visual field targets. Consistent with these results, the amplitude of the attention-related increases in the ERPs were larger from temporal and parietal regions of the right than the left hemisphere in hearing Ss, but were larger from the left than the right hemisphere in both the HD and the deaf Ss. These results suggest that auditory deprivation and the acquisition of a visual language have marked and different effects on the development of cortical specializations in humans.

Residual vision in patients with retrogeniculate lesions of the visual pathways.

Twenty-five subjects have been studied who, as a result of damage to the striate cortex, were 'blind' in extensive areas (scotomata) of the visual fields. Of these 25 subjects, 5 exhibited residual vision in response to transient lights presented within the scotoma, which enabled them to locate the stimuli by hand-reaching or by eye movements; the latter have been measured by electro-oculography. The residual vision underlying their responses was characterized by low flicker-fusion and by sensitivity in detection of movement which increased as target speed was raised. Discrimination for the direction of target movement was poor, but spatial resolution in the discrimination of target displacement was essentially normal. The subjects were unable to recognize or discriminate the spatial structure of targets located within the 'blind' field, and the observed dissociation between spatial discrimination of displacement and pattern is examined in relation to the 'two systems' hypothesis of visual function. There is no obvious correlation between the extent of neuronal damage as revealed by CT scans and the existence of residual vision.

Visual motion processing for the initiation of smooth-pursuit eye movements in humans.

We have used the initiation of pursuit eye movements as a tool to reveal properties of motion processing in the neural pathways that provide inputs to the human pursuit system. Horizontal and vertical eye position were recorded with a magnetic search coil in six normal adults. Stimuli were provided by individual trials of ramp target motion. Analysis was restricted to the first 100 ms of eye movement, which precedes the onset of corrective feedback. By recording the transient response to target motion at speeds the pursuit motor system can achieve, we investigated the visual properties of images that initiate pursuit. We have found effects of varying the retinal location, the direction, the velocity, the intensity, and the size of the stimulus. Eye acceleration in the first 100 ms of pursuit depended on both the direction of target motion and the initial position of the moving target. For horizontal target motion, eye acceleration was highest if the stimulus was close to the center of the visual field and moved toward the vertical meridian. For vertical target motion, eye acceleration was highest when the stimulus moved upward or downward within the lower visual field. The shape of the relationship between eye acceleration and initial target position was similar for target velocities ranging from 1.0 to 45 degrees/s. The initiation of pursuit showed two components that had different visual properties and were expressed early and late in the first 100 ms of pursuit. In the first 20 ms, instantaneous eye acceleration was in the direction of target motion but did not depend on other visual properties of the stimulus. At later times (e.g., 80-100 ms after pursuit initiation), instantaneous eye acceleration was strongly dependent on each property we tested. Targets that started close to and moved toward the position of fixation evoked the highest eye accelerations. For high-intensity targets, eye acceleration increased steadily as target velocity increased. For low-intensity targets, eye acceleration was selective for target velocities of 30-45 degrees/s. The properties of pursuit initiation in humans, including the differences between the early and late components, are remarkably similar to those reported by Lisberger and Westbrook (12) in monkeys. Our data provide evidence that the cell populations responsible for motion processing are similar in humans and monkeys and imply that the functional organization of the visual cortex is similar in the two species.

Control of eye movement with peripheral vision: implications for training of eccentric viewing.

A review of peripheral visual control of eye movements suggests that such control may be easier if the direction of the extrafoveal locus used is orthogonal rather than radial to the direction of target motion. When the extrafoveal locus lies parallel (radial) to the direction of target or eye motion, artificial feedback is usually required to avoid reflexive foveation. These findings have important implications for training eccentric viewing in low vision patients with central scotomata.

Improved Localization of Moving Targets with Prior Knowledge of Direction of Target Motion

In dynamic environments in which many stimulus elements are in motion, visual search may depend upon specific characteristics of target motion that are known in advance. When stimulus elements move in various directions but each element can move in only one direction, prior knowledge of the target's direction of motion reduces search time.

Optokinetic backgrounds affect perceived velocity during ocular tracking.

The perceived velocity of visually tracked moving objects may depend on interactions between reflexive and voluntary oculomotor mechanisms. To investigate this hypothesis, subjects were required to compare sequentially the velocity of a standard target with that of a test target moving at one of five velocities. The standard target was viewed against a plain field (no optokinetic stimulation), and the test target was viewed against (1) a plain field, (2) a stationary grating, (3) a grating drifting in the direction of target motion, or (4) a grating drifting against target motion. In one condition, subjects tracked the target; in the other, a stationary point was fixated. The tracking group overestimated velocity when backgrounds drifted against target motion, but underestimated velocity when gratings drifted with target motion. Subjects not required to track experienced no such misestimations. The results are discussed in relation to the interactive mechanisms of the two eye-movement systems.

Echolocation of moving targets by the fish-catching bat,Noctilio leporinus

1. Behavioral measures of target velocity discrimination, Doppler shift discrimination, and Doppler shift compensation are described for the fish-catching bat,Noctilio leporinus. These bats echolocate using a short constant frequency/frequency modulated (short-CF/FM) sonar pulse. 2. Two bats, trained by operant conditioning, performed an in-flight discrimination between sequentially-presented targets which differed in the velocity of movement in the line of the bat's flight path. Both bats discriminate target velocity differences as low as 40 cm/s. This discrimination ability is independent of the absolute speed or direction of target movement. 3. These same bats were subsequently trained by operant conditioning to discriminate between the presence or absence of an upward frequency (simulated Doppler) shift in artificial echoes of their sonar pulses. The smallest frequency shift that can be discriminated is 570 Hz, corresponding to the Doppler shift produced by a relative velocity of 170cm/s. 4. Since these bats cannot detect the Doppler shift associated with moving targets at the threshold of target velocity discrimination, we conclude that Doppler information is not used to make velocity measurements. Target velocity discrimination must instead be based upon range-rate information obtained from the wide band FM component of the sonar pulse. 5. Three of four bats partially compensate for Doppler shifts of their echoes produced by their flight velocity. During flight, the frequency of their sonar emissions is lowered enough to compensate for about 50% of the flight-induced Doppler shift. None of the bats compensate for target-induced Doppler shifts. 6. The functional significance of short CF signals and the echolocation of moving targets byNoctilio and other bats are discussed.

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.

A limitation of position constancy.

When the eyes track a moving object, the image of a stationary target shifts on the retina colinearly with the eye movement. A compensation process called position constancy prevents this image shift from causing perceived target motion commensurate with the image shift. The target either appears stationary or seems to move in the direction opposite to the eye movement, but much less than the image shift would warrant. Our work is concerned with the question of whether position constancy operates when the image shift and the eye movement are not colinear. That can occur when, during the eye movement, the target undergoes a motion of its own. Evidence is reported that position constancy fails to operate when the direction of the target motion forms an angle with the direction of the eye movement.

Hyperacuity for luminance phase angle in the human visual system

A moving target with identical trajectories in the two eyes appeared shifted in depth if flickered with a phase difference between the eyes. The direction of the depth shift was the same as if the phase-lagging eye had been seeing the target with a spatial advance in the direction of target motion. Maximum acuity for this effect was in the region of 7 deg of phase angle, which may be expressed as a spatial difference between corresponding luminance points of about 13 sec arc. Acuity was approximately constant over a wide range of modulation periods from 50 to 150 msec when expressed in terms of phase angle, but fell off rapidly at higher and at lower frequencies. The results cannot be explained simply by either conventional disparity theory or by spatio-temporal interpolation, but suggest that inter-ocular correspondence is influenced by dynamic luminance information.

Cues reduce direction uncertainty and enhance motion detection.

Previous work has shown that detectability of motion is better when the observer knows ahead of time the direction of that motion (“certainty”) than when he does not know the direction (“uncertainty”). We now report attempts to reduce this performance decrement associated with direction uncertainty. In these experiments, a briefly flashed, oriented line cued the observer to the direction of motion that might occur. When the cue appeared before the moving target, detectability increased; when the cue appeared after the moving target, performance dropped below that for no cue at all. In addition, we examined the effect of cue reliability, varying the relation between cue orientation and actual direction of target motion. The more accurate the cue is, the larger the performance increment. When the cue indicated a direction more than 90 deg from the actual target direction, performance was worse than when there was no cue. Results are discussed with regard to the feasibility of reducing uncertainty in real-world situations.

The spatial and temporal organisation of motion perception units in human vision

Measurements of threshold illumination levels for detection of retinally non-localised moving targets show that detection of a moving target is influenced by both the spatial and the temporal modulation of the background field. The temporal response characteristics obtained from these measurements are similar to those obtained from experiments on detection of temporal flicker. Experiments with spatially modulated background fields reveal visual mechanisms with spatial properties which are essentially independent of many stimulus parameters. The response amplitude of the spatial filter which characterises these mechanisms increases linearly as a function of background contrast and is independent of the relative orientation between the background structure and the direction of target movement. These properties are used to compute the two-dimensional spatial characteristics of mechanisms involved in the detection of moving targets.

The effect of expectations on slow oculomotor control—III. Guessing unpredictable target displacements

Previously, we had shown that expectations about the direction of future target motion produce involuntary anticipatory smooth eye movements in the direction of the expected target motion ( Kowler and Steinman, 1979, b). The present experiments extend these results to expected target motions in unpredictable directions. Subjects showed anticipatory smooth eye movements while tracking an unfamiliar pattern of right and left-going target steps while they were guessing the direction of the expected steps. Eye velocity increased when subjects became certain that they knew what the pattern was. Guesses also produced anticipatory smooth eye movements for both expected target steps and ramps in one of 12 unknown directions. Anticipatory smooth eye movements produced by guesses and by certain knowledge of target direction were not affected when subjects performed a distracting task (mental arithmetic). These results show that the effect of expectations on slow eye movements cannot be removed simply by making target motions unpredictable. Models of the slow oculomotor subsystems, to be complete, require development of techniques to predict the direction and certainty of human expectations about unpredictable patterns of target motion. A technique, which may serve this purpose, is described.

Prey-localizing behaviour of the nocturnal desert scorpion, Paruroctonus mesaensis: Orientation to substrate vibrations

A nocturnal desert scorpion, Paruroctonus mesaensis, uses information that propagates through sand to locate prey as far as 50 cm away. Visual and air-borne auditory stimuli from nearby prey do not influence the scorpion's localizing behaviour. Adult scorpions accurately determine both the direction and distance of a standard vibration source less than 15 cm away. At greater distances, only direction of the vibration source is determined. Some qualities of the substrate disturbances (intensity, temporal pattern, and direction of target movement) are also sensed from a distance, while others (geometric size and composition of the disturbing object) do not influence the orientation response.

Role of primate flocculus during rapid behavioral modification of vestibuloocular reflex. I. Purkinje cell activity during visually guided horizontal smooth-pursuit eye movements and passive head rotation

1. Extracellular recordings were obtained from 124 Purkinje cells (P-cells) in the flocculus of alert monkeys. P-cell simple spike-firing rate was analyzed quantitatively during various combinations of smooth-pursuit eye movement and passive head rotation. 2. During sinusoidal smooth eye movements, 80% of the P-cells displayed increased firing rate during ipsilateral and 20% during contralateral eye movement. Over the frequency range 0.3--1.4 Hz, firing-rate modulation was proportional to and in phase with maximum eye velocity. During the steady state of triangle-wave tracking, firing rate increased monotonically as a function of eye velocity. Since firing rate was uncorrelated with retinal-error velocity, one component of P-cell firing rate was related to eye velocity. 3. During the transient phase of triangle-wave tracking, when an instantaneous change in the direction of target movement caused a large retinal-error velocity, 40% of the P-cells were related only to eye velocity. Sixty percent of the P-cells displayed an overshoot or undershoot in firing rate, indicating a relationship to either retinal-error velocity or eye acceleration as well as to eye velocity. 4. During the vestibuloocular reflex (VOR), evoked by head rotation in the dark, P-cell firing rate was only weakly modulated. In contrast, when the monkey suppressed the VOR by fixating a target that rotated with him, P-cell rate was deeply modulated. Since the modulation was proportional to and in phase with maximum head velocity, another component of P-cell firing rate was related to head velocity. 5. Of 36 P-cells tested, 35 displayed firing-rate modulation during both suppression of the VOR and smooth-pursuit eye movement. P-cells that reached peak firing rate during ipsilateral head rotation also reached peak firing rate during ipsilateral smooth eye rotation. Average population sensitivitites to head velocity and eye velocity were equal. In three conditions in which eye and head velocity were elicited simultaneously, P-cell firing rate could be predicted by the linear, vector addition of the separate eye and head velocity components of firing rate. Therefore, the relatively weak modulation of P-cell firing rate during the VOR in the dark can be accounted for by the cancellation of equal but opposite head and eye velocity components. 6. The connections of flocculus P-cells to interneurons in the brain stem VOR pathways have been established in other mammals. In the context of those connections, P-cell firing patterns were appropriate to facilitate the eye movements the monkey was required to make. We conclude that the flocculus is important for sustaining any smooth eye movements that are different from those evoked by head rotation in the dark. The eye velocity component may represent an efference copy signal that sustains ongoing eye velocity during smooth pursuit.

The organization of visual processing in the pigeon cerebellum.

1. Visual responses were sought in the cerebella of decerebrate pigeons using extracellular micro-electrodes, and were found in folia VIc-IXb, especially folia VII and VIII. The responses were mainly, but not exclusively, from the ipsilateral eye. Four binocular units were recorded.2. In the anterior and posterior walls of folium VII the organization was clearly, though rather crudely, retinotopic. The temporal field was represented laterally, on the ipsilateral side, and the nasal field medially; the superior field was represented superficially, and the inferior field towards the base of the folium. In the lateral wall of folium VII there was a small anomalous region innervated by the contralateral eye.3. The visual input arrived via the mossy fibre system.4. Units exhibited a strong preference for moving targets, 20-60 degrees /sec being the range of optimal speeds. About three quarters of the units responded most strongly to a particular direction of target motion. The preferred direction was most frequently upwards or backwards.5. Units in the granule layer were to some extent clustered according to their direction-preference, which tended to change gradually as the electrode advanced along the granule layer.6. Units were classified as Gr-units (granule cells or mossy fibre rosettes) or P-units (Purkinje cells) or were left unclassified. Receptive fields of Gr-units were usually 5-30 degrees across; they were much larger for P-units in folium VIII, but not obviously so for those in folium VII. Gr-units were more frequently direction-sensitive than were P-units.