Biomedical subjects

F A Miles

Publications and source records attributed to F A Miles.

At least 37 records · Page 2Linked to original sources

PubMed1999

Short-latency vergence eye movements induced by radial optic flow in humans: dependence on ambient vergence level.

Radial patterns of optic flow, such as those experienced by moving observers who look in the direction of heading, evoke vergence eye movements at short latency. We have investigated the dependence of these responses on the ambient vergence level. Human subjects faced a large tangent screen onto which two identical random-dot patterns were back-projected. A system of crossed polarizers ensured that each eye saw only one of the patterns, with mirror galvanometers to control the horizontal positions of the images and hence the vergence angle between the two eyes. After converging the subject's eyes at one of several distances ranging from 16.7 cm to infinity, both patterns were replaced with new ones (using a system of shutters and two additional projectors) so as to simulate the radial flow associated with a sudden 4% change in viewing distance with the focus of expansion/contraction imaged in or very near both foveas. Radial-flow steps induced transient vergence at latencies of 80-100 ms, expansions causing increases in convergence and contractions the converse. Based on the change in vergence 90-140 ms after the onset of the steps, responses were proportional to the preexisting vergence angle (and hence would be expected to be inversely proportional to viewing distance under normal conditions). We suggest that this property assists the observer who wants to fixate ahead while passing through a visually cluttered area (e.g., a forest) and so wants to avoid making vergence responses to the optic flow created by the nearby objects in the periphery.

Adult↗

PubMed1999

Target selection for pursuit and saccadic eye movements in humans.

Eye movements were recorded from three subjects as they initiated tracking of a small circle ("target") moving leftward or rightward, above or below the horizontal meridian, either alone or in the presence of a small square ("distractor") moving leftward or rightward on the other side of the horizontal meridian. At the start of each trial, subjects were provided with either a "form" cue (always centrally positioned and having the circular shape and color of the upcoming moving target) or a "location" cue (a small white square positioned where the upcoming target would appear). The latency of pursuit increased in the presence of an oppositely moving distractor when subjects were provided the form cues but not when they were provided the location cues. The latency of saccades showed similar, but smaller, increases when subjects were given the form cues. On many trials with the form cues, pursuit started in the direction of the distractor and then reversed to follow the target. On these trials, the initial saccade often, but not always, also followed the distractor. These results indicate that the mechanisms of target selection for pursuit and saccades are tightly coordinated but not strictly yoked. The shared effects of the distractor on the latencies of pursuit and saccades probably reflect the common role of visual attention in filtering the inputs that guide these two types of eye movements. The differences in the details of the effects on pursuit and saccades suggest that the neural mechanisms that trigger these two movements can be independently regulated.

Adult↗

PubMed1998

Dependence of short-latency ocular following and associated activity in the medial superior temporal area (MST) on ocular vergence.

Motion of a large-field pattern elicits short-latency ocular following responses (OFR) in the monkey, which are mediated at least in part by the medial superior temporal area of the cortex (MST). The magnitude of the OFR is known to be inversely related to viewing distance, and we investigated the dependence of OFR and the associated neuronal activity in the MST on a major cue to viewing distance, ocular vergence, in alert monkeys (Macaca fuscata). The vergence angle, expressed in terms of the apparent viewing distance, ranged from infinity to 16.6 cm (0-6 m(-1)). The magnitude of the initial OFR increased monotonically with increases in convergence at a mean (+/-SD) rate of 19.6+/-4.5%/m(-1) in four monkeys (over the range 0-4 m(-1)). In two monkeys, we recorded the single unit activity of 160 MST neurons that responded to motion of a large-field pattern with directional selectivity. The mean latency (+/-SD) of the MST discharges elicited by large-field motion was 50+/-7.5 ms (n=115), which preceded the onset of OFR by an average of 10+/-9.9 ms. The discharge modulation elicited by large-field motion showed a significant dependence on vergence in 91/160 neurons (57%), 72 of which (79%) increased their firing rate with increasing convergence ("near" neurons), and the remainder increasing their firing rate with decreasing convergence ("far" neurons). However, on average, the sensitivity of these MST neurons to vergence was only about 30% of that shown by the OFR. It could be that only those neurons that are very sensitive to vergence angle contribute to the OFR, but it is also possible that much of the modulation of OFR with vergence occurs downstream from the MST or in alternative pathways (yet to be discovered) that contribute to OFR.

Animals↗

PubMed1998

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.

Animals↗

PubMed1998

Role of the oculomotor vermis in generating pursuit and saccades: effects of microstimulation.

We studied the eye movements evoked by applying small amounts of current (2-50 microA) within the oculomotor vermis of two monkeys. We first compared the eye movements evoked by microstimulation applied either during maintained pursuit or during fixation. Smooth, pursuitlike changes in eye velocity caused by the microstimulation were directed toward the ipsilateral side and occurred at short latencies (10-20 ms). The amplitudes of these pursuitlike changes were larger during visually guided pursuit toward the contralateral side than during either fixation or visually guided pursuit toward the ipsilateral side. At these same sites, microstimulation also often produced abrupt, saccadelike changes in eye velocity. In contrast to the smooth changes in eye velocity, these saccadelike effects were more prevalent during fixation and during pursuit toward the ipsilateral side. The amplitude and type of evoked eye movements could also be manipulated at single sites by changing the frequency of microstimulation. Increasing the frequency of microstimulation produced increases in the amplitude of pursuitlike changes, but only up to a certain point. Beyond this point, the value of which depended on the site and whether the monkey was fixating or pursuing, further increases in stimulation frequency produced saccadelike changes of increasing amplitude. To quantify these effects, we introduced a novel method for classifying eye movements as pursuitlike or saccadelike. The results of this analysis showed that the eye movements evoked by microstimulation exhibit a distinct transition point between pursuit and saccadelike effects and that the amplitude of eye movement that corresponds to this transition point depends on the eye movement behavior of the monkey. These results are consistent with accumulating evidence that the oculomotor vermis and its associated deep cerebellar nucleus, the caudal fastigial, are involved in the control of both pursuit and saccadic eye movements. We suggest that the oculomotor vermis might accomplish this role by altering the amplitude of a motor error signal that is common to both saccades and pursuit.

Animals↗

PubMed1997

Radial optic flow induces vergence eye movements with ultra-short latencies.

An observer moving forwards through the environment experiences a radial pattern of image motion on each retina. Such patterns of optic flow are a potential source of information about the observer's rate of progress, direction of heading and time to reach objects that lie ahead. As the viewing distance changes there must be changes in the vergence angle between the two eyes so that both foveas remain aligned on the object of interest in the scene ahead. Here we show that radial optic flow can elicit appropriately directed (horizontal) vergence eye movements with ultra-short latencies (roughly 80 ms) in human subjects. Centrifugal flow, signalling forwards motion, increases the vergence angle, whereas centripetal flow decreases the vergence angle. These vergence eye movements are still evident when the observer's view of the flow pattern is restricted to the temporal hemifield of one eye, indicating that these responses do not result from anisotropies in motion processing but from a mechanism that senses the radial pattern of flow. We hypothesize that flow-induced vergence is but one of a family of rapid ocular reflexes, mediated by the medial superior temporal cortex, compensating for translational disturbance of the observer.

Convergence, Ocular↗

PubMed1997

Vergence eye movements in response to binocular disparity without depth perception.

Primates use vergence eye movements to align their two eyes on the same object and can correct misalignments by sensing the difference in the positions of the two retinal images of the object (binocular disparity). When large random-dot patterns are viewed dichoptically and small binocular misalignments are suddenly imposed (disparity steps), corrective vergence eye movements are elicited at ultrashort latencies. Here we show that the same steps applied to dense anticorrelated patterns, in which each black dot in one eye is matched to a white dot in the other eye, initiate vergence responses that are very similar, except that they are in the opposite direction. This sensitivity to the disparity of anticorrelated patterns is shared by many disparity-selective neurons in cortical area V1, despite the fact that human subjects fail to perceive depth in such stimuli. These data indicate that the vergence eye movements initiated at ultrashort latencies result solely from locally matched binocular features, and derive their visual input from an early stage of cortical processing before the level at which depth percepts are elaborated.

Convergence, Ocular↗

PubMed1997

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.

Animals↗

PubMed1996

A role for stereoscopic depth cues in the rapid visual stabilization of the eyes.

Primates have visual tracking systems that help stabilize the eyes on the surroundings by responding to retinal image motion at ultra-short latencies. However, as the observer moves through the environment, the image motion on the retina depends on the three-dimensional structure of the scene. We report here that the very earliest of these tracking responses is elicited only by objects moving in the immediate vicinity of the plane of fixation: objects nearer or farther are ignored. This selectivity is achieved by means of a stereoscopic depth mechanism which uses the fact that the two eyes have differing viewpoints, so only objects in the plane of fixation have images that occupy corresponding positions on the two retinae. Such behaviour is readily explained by the known binocular properties of some motion-selective neurons in the visual cortex. Some (stereoanomalous) subjects showed highly specific tracking deficits as though lacking one subtype of these neurons.

Animals↗

PubMed1996

Short-latency compensatory eye movements associated with a brief period of free fall.

The vertical eye movements induced by a brief period of free fall were recorded from three monkeys (Macaca mulatta) using the electromagnetic search coil technique. Free fall was initiated in total darkness immediately following binocular fixation of one of six target lights located at viewing distances ranging from 20 to 107 cm. Responses consisted of an initial transient downward eye movement (anticompensatory direction) with a latency of a few milliseconds at most followed by a sustained upward (compensatory) eye movement. The early transient was independent of viewing distance and attributed to an artifact, whereas the later component was a linear function of the inverse of the prior viewing distance and attributed to the translational vestibulo-ocular reflex (TVOR). Response latencies for the four nearer viewing distances were determined from the individual eye velocity traces using a computerized algorithm: after removing the initial transient by subtracting the mean response obtained with the most distant viewing, a regression line was fitted to the initial rising phase of the residual response and then extrapolated back to the baseline to determine the onset. When so determined, median latencies for the nearest viewing ranged from 16.4 to 18.5 ms, values appreciably shorter than any in the literature.

Acceleration↗

PubMed1996

Decreases in the latency of smooth pursuit and saccadic eye movements produced by the "gap paradigm" in the monkey.

The initiation of both pursuit and saccades was affected by the presence of a temporal gap between the disappearance of a fixated visual target and the appearance of a second, eccentric, target. For pursuit, the gap paradigm produced a modest (20 msec) decrease in latency. For saccades, the gap paradigm produced a similar modest decrease in the latency of some saccades, but also revealed a population of very short latency "express" saccades. The modest changes in the latency of pursuit and regular saccades displayed a similar dependence on gap duration, with the largest decreases produced by gaps of 200-300 msec. The gap paradigm did not produce "express" pursuit, even though express saccades could be elicited on interleaved trials.

Animals↗

PubMed1996

Short-latency disparity vergence responses and their dependence on a prior saccadic eye movement.

1. A dichoptic viewing arrangement was used to study the initial vergence eye movements elicited by brief horizontal disparity steps applied to large textured patterns in three rhesus monkeys. Disconjugate steps (range, 0.2-10.9 degrees) were applied to the patterns at selected times (range, 13-303 ms) after 10 degrees leftward saccades into the center of the pattern. The horizontal and vertical positions of both eyes were recorded with the electromagnetic search coil technique. 2. Without training or reinforcement, disparity steps of suitable amplitude consistently elicited vergence responses at short latencies. For example, with 1.8 degrees crossed-disparity steps applied 26 ms after the centering saccade, the mean latency of onset of convergence for each of the three monkeys was 52.2 +/- 3.8 (SD) ms, 52.3 +/- 5.2 ms, and 53.4 +/- 4.1 ms. 3. Experiments in which the disparity step was confined to only one eye indicated that each eye was not simply tracking the apparent motion that is saw. For example, when crossed-disparity steps were confined to the right eye (which saw leftward steps), the result was (binocular) convergence in which the left eye moved to the right even though that eye had seen only a stationary scene. This movement of the left eye cannot have resulted from independent monocular tracking and indicates that the vergences here derived from the binocular misalignment of the two retinal images. 4. The initial vergence responses to crossed-disparity steps had the following main features. 1) They were always in the correct (i.e., convergent) direction over the full range of stimuli tested, the initial vergence acceleration increasing progressively with increases in disparity until reaching a peak with steps of 1.4-2.4 degrees and declining thereafter to a nonzero asymptote as steps exceeded 5-7 degrees. 2) They showed transient postsaccadic enhancement whereby steps applied in the immediate wake of a saccadic eye movement resulted in much higher initial vergence accelerations than the same steps applied some time later. The response decline in the wake of a saccade was roughly exponential with time constants of 67 +/- 5 (SD) ms, 35 +/- 2 ms, and 54 +/- 4 ms for the three animals. 3) That the postsaccadic enhancement might have resulted in part from the visual stimulation associated with the prior saccade was suggested by the finding that enhancement could also be observed when the disparity steps were applied in the wake of (conjugate) saccadelike shifts of the textured pattern. However, this visual enhancement did not reach a peak unit 17-37 ms after the end of the "simulated" saccade, and the peak enhancement averaged only 45% of that after a "real" saccade. 4) Qualitatively similar transient enhancements in the wake of real and simulated saccades have also been reported for initial ocular following responses elicited by conjugate drifts of the visual scene. We replicated the enhancement effects on ocular following to allow a direct comparison with the enhancement effects on disparity vergence using the same animals and visual stimulus patterns and, despite some clear quantitative differences, we suggest that the enhancement effects share a similar etiology. 5. Initial vergence responses to uncrossed-disparity steps had the following main features. 1) They were in the correct (i.e., divergent) direction only for very small steps (< 1.5-2.5 degrees), and then only when postsaccadic delays were small; when the magnitude of the steps was increased beyond these levels, responses declined to zero and thereafter reversed direction, eventually reaching a nonzero (convergent) asymptote similar to that seen with large crossed-disparity steps; convergent responses were also seen with larger vertical disparity steps, suggesting that they represent default responses to any disparity exceeding a few degrees. 2) As the postsaccadic delay was increased, responses to small steps (1.8 degrees) declined to zero and thereafter re

Animals↗

PubMed1996

Transitions between pursuit eye movements and fixation in the monkey: dependence on context.

1. We compared the visuomotor processing underlying the onset and offset of pursuit by recording the eye movements of three monkeys as they smoothly tracked a target that was initially at rest, started to move suddenly at a constant velocity along the horizontal meridian, and then stopped. We presented this sequence of target motions in two different contexts. In the first context the target sometimes stopped after 500 ms, but on other interleaved trials the target either continued moving at a constant velocity, slowed down, speeded up, or reversed direction. In the second context the target always stopped, but the duration of the preceding constant velocity was randomized from 500 to 700 ms. 2. The dynamics of the eye velocity during the offset of pursuit were markedly different in the two experiments. When the target stopped only sometimes, the decrease in eye velocity at the offset of pursuit often overshot zero, producing a brief, small reversal in the direction of pursuit before eye speed settled to zero. When the target always stopped, the decrease in eye velocity at the offset of pursuit followed a more gradual transition toward zero with no overshoot. Thus the eye velocity profiles obtained in the first experiment contradict, whereas those obtained in the second experiment confirm, previous characterizations of the offset of pursuit as an exponential decay toward zero eye speed. 3. To investigate the basis of the different eye velocity profiles obtained in the two experiments, we probed the state of transmission along the visuomotor pathways for pursuit with the use of small perturbations in the motion of the target. We used perturbations consisting of 1 degree step changes in target position superimposed on the constant velocity motion of the target, on the basis of previous findings that such perturbations elicit saccades during fixation but smooth changes in eye speed during maintained pursuit. Single perturbations were imposed at regularly spaced intervals on separate interleaved trials during the onset, maintenance, and offset of pursuit. 4. Perturbations imposed during the onset and maintenance of pursuit had similar effects regardless of whether the target stopped only sometimes or always. In both experiments, perturbations that stepped the target in the direction opposite to the constant velocity of the target produced decreases in eye speed; perturbations in the same direction produced negligible or inconsistent changes in eye speed. The changes in eye speed caused by perturbations were largest for those perturbations introduced within the first 100 ms after the start of target motion, before the onset of the smooth eye movement, and became progressively smaller as target motion continued. The largest changes in eye speed were therefore caused by those perturbations imposed during periods of large retinal slip and by those perturbations whose direction opposed that slip. 5. Perturbations imposed during the offset of pursuit had different effects depending on whether the target stopped only sometimes or always. When the target stopped only sometimes, forward perturbations produced large increases in eye speed, whereas backward perturbations produced negligible or inconsistent changes in eye speed. Thus the visuomotor processing underlying the offset of pursuit in this experiment strongly resembled that underlying the onset of pursuit: in both cases, those perturbations in the direction opposing large retinal slip produced the largest effects. In contrast, when the target always stopped, neither forward nor backward perturbations imposed during the offset of pursuit produced large changes in eye speed. This indicates that the visuomotor processing underlying the offset of pursuit in this experiment was different from the processing underlying the onset of pursuit. 6. Perturbations also produced changes in the frequency of saccades, although these effects were less consistent than the changes in pursuit eye speed

Animals↗

PubMed1996

Release of fixation for pursuit and saccades in humans: evidence for shared inputs acting on different neural substrates.

1. In three human subjects, we measured the latency of pursuit and saccadic eye movements made to an eccentric target after a fixated central target was extinguished. In one set of experiments, we varied the time interval between the extinction of the central target and the appearance of the eccentric target ("gap duration"). In a second set of experiments, we varied the eccentricity at which the second target appeared. 2. Varying the gap duration produced similar changes in the latencies of pursuit and saccades. Gaps as short as 30 ms caused significant decreases in latency; progressively longer gaps produced shorter latencies, reaching a minimum for gaps of 150-200 ms. Over the range of gap durations used, the latencies of pursuit and saccades displayed the same dependence on gap duration. 3. Varying the eccentricity of the second target produced different effects on the latencies of pursuit and saccades. Saccade latencies increased when the eccentricity of the second target was decreased from 4 degrees to 0.5 degree, whereas pursuit latencies were not consistently altered. Despite these differences in the dependence on retinal eccentricity between pursuit and saccades, imposing a 200-ms gap between the extinction of the fixation point and appearance of the second target still reduced the latency of both. 4. Our results are consistent with the idea that the mechanisms underlying the release of fixation for pursuit and saccades have shared inputs but a different neural substrate. The common dependence on gap duration may indicate that a single preparatory input coordinates both types of movements. The different dependence on retinal eccentricity indicates that there are differences in the spatial organization of the premotor circuits that trigger the onset of the two types of movements.

Adult↗

PubMed1996

Initiation of saccades during fixation or pursuit: evidence in humans for a single mechanism.

1. In four human subjects, we measured the latency of saccadic eye movements made to a second, eccentric target after an initial, foveated target was extinguished. In separate interleaved trails, the targets were either both stationary ("fixation") or both moving with the same velocity ("pursuit"). For both fixation and pursuit trials, we extinguished the first target at randomized times during maintained fixation or pursuit and varied the time interval ("gap duration") before the appearance of the second target. 2. During both fixation and pursuit, the presence of a 200-ms gap reduced the latencies of saccades, compared with those obtained with no gap. For two subjects, we imposed additional, intermediate gap durations and found that saccade latencies varied as a function of gap duration. Furthermore, the latencies of saccades elicited during pursuit displayed the same dependence on gap duration as those elicited during fixation. 3. Our results demonstrate that the "gap effect" observed for saccades made during fixation also occurs for saccades made during pursuit. To the extent that the gap effect on saccade latency reflects a mechanism underlying the release of fixation, our results suggest that the same mechanism is invoked for saccades made during pursuit. From the viewpoint of initiating saccades, the existence of separate fixation and pursuit systems may be irrelevant.

Fixation, Ocular↗

PubMed1994

Human ocular responses to translation of the observer and of the scene: dependence on viewing distance.

Recent experiments on monkeys have indicated that the eye movements induced by brief translation of either the observer or the visual scene are a linear function of the inverse of the viewing distance. For the movements of the observer, the room was dark and responses were attributed to a translational vestibulo-ocular reflex (TVOR) that senses the motion through the otolith organs; for the movements of the scene, which elicit ocular following, the scene was projected and adjusted in size and speed so that the retinal stimulation was the same at all distances. The shared dependence on viewing distance was consistent with the hypothesis that the TVOR and ocular following are synergistic and share central pathways. The present experiments looked for such dependencies on viewing distance in human subjects. When briefly accelerated along the interaural axis in the dark, human subjects generated compensatory eye movements that were also a linear function of the inverse of the viewing distance to a previously fixated target. These responses, which were attributed to the TVOR, were somewhat weaker than those previously recorded from monkeys using similar methods. When human subjects faced a tangent screen onto which patterned images were projected, brief motion of those images evoked ocular following responses that showed statistically significant dependence on viewing distance only with low-speed stimuli (10 degrees/s). This dependence was at best weak and in the reverse direction of that seen with the TVOR, i.e., responses increased as viewing distance increased. We suggest that in generating an internal estimate of viewing distance subjects may have used a confounding cue in the ocular-following paradigm--the size of the projected scene--which was varied directly with the viewing distance in these experiments (in order to preserve the size of the retinal image). When movements of the subject were randomly interleaved with the movements of the scene--to encourage the expectation of ego-motion--the dependence of ocular following on viewing distance altered significantly: with higher speed stimuli (40 degrees/s) many responses (63%) now increased significantly as viewing distance decreased, though less vigorously than the TVOR. We suggest that the expectation of motion results in the subject placing greater weight on cues such as vergence and accommodation that provide veridical distance information in our experimental situation: cue selection is context specific.

Animals↗

PubMed1993

The sensing of rotational and translational optic flow by the primate optokinetic system.

In primates, 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 semi-circular canals and otolith organs, respectively. Two independent visual tracking systems deal with any residual disturbances of gaze (global optic flow) and are manifest in the two components of the optokinetic response: the indirect or delayed component (OKNd) and the direct or early component (OKNe). I 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 residual 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 seems to have evolved much more recently in frontal-eyed animals and, I suggest, acts as a backup to the TVOR--also recently evolved?--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. This sensitivity to absolute depth is also shared by OKNe, consistent with the idea that OKNe 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 and disparity, which I suggest help the system to segregate the object of regard from other elements in the scene. I also suggest that highly complex optic flow patterns (such as those experienced by the moving observer who looks a little off to one side of his direction of heading) are dealt with by a third visual tracking mechanism--the smooth pursuit system--that spatially filters visual motion inputs so as to exclude all but the motion of the object of interest (local optic flow).

Animals↗

PubMed1992

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.

Animals↗