Biomedical subjects
M T Swanston
Publications and source records attributed to M T Swanston.
Visual motion aftereffects: critical adaptation and test conditions.
The visual motion aftereffect (MAE) typically occurs when stationary contours are presented to a retinal region that has previously been exposed to motion. It can also be generated following observation of a stationary grating when two gratings (above and below it) move laterally: the surrounding gratings induce motion in the opposite direction in the central one. Following adaptation, the centre appears to move in the direction opposite to the previously induced motion, but little or no MAE is visible in the surround gratings [Swanston & Wade (1992) Perception, 21, 569-582]. The stimulus conditions that generate the MAE from induced motion were examined in five experiments. It was found that: the central MAE occurs when tested with stationary centre and surround gratings following adaptation to surround motion alone (Expt 1); no MAEs in either the centre or surround can be measured when the test stimulus is the centre alone or the surround alone (Expt 2); the maximum MAE in the central grating occurs when the same surround region is adapted and tested (Expt 3); the duration of the MAE is dependent upon the spatial frequency of the surround but not the centre (Expt 4); MAEs can be observed in the surround gratings when they are themselves surrounded by stationary gratings during test (Expt 5). It is concluded that the linear MAE occurs as a consequence of adapting restricted retinal regions to motion but it can only be expressed when nonadapted regions are also tested.
A general model for the perception of space and motion.
The perception of space and motion involves successive transformations of signals with respect to different reference systems. The visual input is coded in terms of retinal coordinates. The retinocentric values from each eye require to be unified, and to be combined with signals for eye position and movement. This egocentric reference provides a signal for the angular size, motion, or orientation of the stimulus with respect to the observer. The egocentric signals are transformed to a coordinate system that is three-dimensional-the geocentric frame of reference. Further transformations can occur at earlier levels owing to patterncentric interactions within the visual field. When the geocentric signal corresponds to the physical dimensions of space and motion, this is referred to as perceptual constancy.
Frames of reference and motion aftereffects.
Evidence concerning the origin of the motion aftereffect (MAE) is assessed in terms of a model of levels of representation in visual motion perception proposed by Wade and Swanston. Very few experiments have been designed so as to permit unambiguous conclusions to be drawn. The requirements for such experiments are identified. Whereas retinocentric motion could in principle give rise to the MAE, data are not available which would enable a conclusion to be drawn. There is good evidence for a patterncentric origin, indicating that the MAE is primarily the result of adaptation in the systems responsible for detecting relative visual motion. There is evidence for a further contribution from the process that compensates retinocentric motion for eye movements, in the form of nonveridical information for eye movements. There may also be an effect at the level at which perceived distance and self-movement information are combined with egocentric motion to give a geocentric representation which provides the basis for reports of phenomenal experience. It is concluded that the MAE can be caused by changes in activity at more than one level of representation, and cannot be ascribed to a single underlying process.
Monocular and dichoptic interactions between moving and stationary stimuli.
Visual motion of a physically stationary stimulus can be induced by the movement of adjacent stimuli. The frequencies of motion reports and the angular separations required to induce motion were determined for a number of stimulus configurations. A stationary stimulus was fixated in the centre of the display and the point at which induced motion was initially reported was measured. In the first experiment either one or two stationary stimuli were presented in the centre of a display and either one or two similar stimuli moved horizontally towards them. The percentage of trials on which motion was induced varied with the display configuration, being greatest with two moving and one stationary stimuli. The angular separations at which motion was reported were about 2 deg for all conditions. In the second experiment the binocular interaction of such induced motion was examined. A single static fixation stimulus was presented binocularly and a range of monocular or dichoptic conditions was examined: a single moving stimulus to one eye, two moving stimuli to one eye, or two moving stimuli dichoptically. Induced motion was reported on about 90% of the trials for the monocular and dichoptic conditions with two moving stimuli. Motion was first induced at similar angular separations by two moving stimuli, whether presented monocularly or dichoptically. Binocular interaction was further examined with a display that induced motion in the stimulus presented to one eye but not in that presented to the other: this resulted in the apparent motion in depth of the binocularly fixated stimulus.
On interocular transfer of motion aftereffects.
A brief history of quantitative assessments of interocular transfer (IOT) of the motion aftereffect (MAE) is presented. Recent research indicates that the MAE occurs as a consequence of adapting detectors for relative rather than retinal motion. When gratings above and below a stationary, fixated grating are moved in an otherwise dark field the central, retinally stationary grafting appears to move in the opposite direction; when tested with stationary gratings an MAE is almost entirely confined to the central grating. The IOT of such an MAE was measured in experiment 1: the display was presented to one eye with a black field in the other. The IOT was about 30% of the monocular MAE. Similar values were found in experiment 2, in which the contralateral eye received an equivalent central stationary grating during adaptation and test. The dichoptic interaction of the processes involved in the MAE was examined by presenting the central gratings to both eyes and a single flanking grating above in one eye and below in the other (experiment 3). The MAE was tested with either the same or the contralateral pairing. Oppositely directed MAEs were found for the central and flanking gratings, but they were confined mainly to the conditions in which the configurations presented during adaptation were present in the same eyes during test. In experiment 4, the surround MAEs were compared after adaptation with two moving gratings in one eye or with a similar dichoptic configuration, and they were of similar duration. In a final experiment the MAE was tested either monocularly or binocularly after alternating adaptation of the left and right eyes and was found to be of the same duration. It is concluded that the MAE is a consequence of adapting relational-motion detectors, which are either monocular or of the binocular OR class.
Motion over the retina and the motion aftereffect.
The motion aftereffect (MAE) was measured with retinally moving vertical gratings positioned above and below (flanking) a retinally stationary central grating (experiments 1 and 2). Motion over the retina was produced by leftward motion of the flanking gratings relative to the stationary eyes, and by rightward eye or head movements tracking the moving (but retinally stationary) central grating relative to the stationary (but retinally moving) surround gratings. In experiment 1 the motion occurred within a fixed boundary on the screen, and oppositely directed MAEs were produced in the central and flanking gratings with static fixation; but with eye or head tracking MAEs were reported only in the central grating. In experiment 2 motion over the retina was equated for the static and tracking conditions by moving blocks of grating without any dynamic occlusion and disclosure at the boundaries. Both conditions yielded equivalent leftward MAEs of the central grating in the same direction as the prior flanking motion, ie an MAE was consistently produced in the region that had remained retinally stationary. No MAE was recorded in the flanking gratings, even though they moved over the retina during adaptation. When just two gratings were presented, MAEs were produced in both, but in opposite directions (experiments 3 and 4). It is concluded that the MAE is a consequence of adapting signals for the relative motion between elements of a display.
The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head.
A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in perceived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.
Depth-coded motion signals in plaid perception and optokinetic nystagmus.
When two sine-wave gratings drift in different directions at the same speed behind a circular window, a single coherent plaid is seen rather than one grating sliding over the other. We find that as the stereo depth separation of the two component gratings increases, the probability of seeing a plaid declines. The gain of the slow phase of vertical optokinetic nystagmus (OKN) also falls as the separation of the components increases. When the two grating components are in the same depth plane, the vertical eye velocity is greater than that of either component. This shows that the OKN is being driven by the plaid, whose vertical speed is roughly twice as fast as the components. We conclude that both perception and OKN are fed by the same motion signal, which arises after binocular combination and after plaid synthesis.
Induced rotary motion and ocular torsion.
When a large patterned annulus rotates around a stationary sectored disc the latter appears to rotate in the opposite direction. Such induced rotary motion was examined with central discs subtending 5, 20 and 40 deg at the eye, with the surround filling the remainder of the visual field. The annular surround or the central disc could be oscillated sinusoidally around the fixation point through 20 deg at 0.2 Hz. In each case, subjects estimated the angles through which the moving and stationary parts of the display appeared to rotate on one half-cycle. Subjects also estimated the angle of rotation of an oscillating display that filled the visual field. Induced rotation of the centre was around 100% of the inducing amplitude for all disc sizes, but there was no induced motion of the surround when the centre rotated. Ocular torsion was measured under the same conditions, using the scleral search-coil technique. The amplitude of ocular torsion was a function of the size of the stationary or rotating field. Thus, variations in stimulus conditions affected induced rotary motion and ocular torsion in different ways. The implications of the results for theories of induced motion in terms of underregistered eye movements are discussed.
The binocular representation of uniform motion.
In the model of motion perception proposed by Swanston, Wade, and Day (1987, Perception 16 143-159) it was suggested that retinocentric motion and eye movement information are combined independently for each eye, to give left and right orbitocentric representations of movement. The weighted orbitocentric values are then added, to give a single agocentric representation. It is shown that for a physical motion observed without pursuit eye movements this formulation predicts a reduction in the perceived extent of motion with monocular as opposed to binocular viewing. This prediction was tested, and shown to be incorrect. Accordingly, a modification of the model is proposed, in which the left and right retinocentric signals are weighted according to the presence or absence of stimulation, and combined to give a binocular retinocentric representation. In a similar way left-eye and right-eye position signals are combined to give a single binocular eye movement signal for version. This is then added to the binocular retinocentric signal to give the egocentric representation. This modification provides a unified account of both static visual direction and movement perception.
The perception of visual motion during movements of the eyes and of the head.
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The effect of orientation and motion of enclosed texture on induced movement.
Optical expansion or contraction of a vertical grating induces rotation of an oblique line or bar superimposed upon it. Three experiments are reported in which the oblique (45 deg clockwise) bar was defined by parallel line segments that were varied in orientation and motion. In Experiment 1 subjects estimated the degrees through which the boundaries of the bar appeared to rotate during 2 sec expansion of the vertical grating background. The induced rotation declined with increasing relative orientation between the texture elements and the background; virtually no effect was reported when they were orthogonal. The texture elements were stationary in Experiment 1 whereas they expanded or contracted in Experiment 2: when their motion was in-phase with the background induced rotation of the boundary occurred irrespective of relative orientation. Expansion or contraction of the texture elements alone did not result in induced bar rotation (Experiment 3). It was concluded that induced movement at the boundaries between two patterns is reduced when they differ in static orientation and in the directions of motion due to zooming.
The representation of uniform motion in vision.
For veridical detection of object motion any moving detecting system must allocate motion appropriately between itself and objects in space. A model for such allocation is developed for simplified situations (points of light in uniform motion in a frontoparallel plane). It is proposed that motion of objects is registered and represented successively at four levels within frames of reference that are defined by the detectors themselves or by their movements. The four levels are referred to as retinocentric, orbitocentric, egocentric, and geocentric. Thus the retinocentric signal is combined with that for eye rotation to give an orbitocentric signal, and the left and right orbitocentric signals are combined to give an egocentric representation. Up to the egocentric level, motion representation is angular rather than three-dimensional. The egocentric signal is combined with signals for head and body movement and for egocentric distance to give a geocentric representation. It is argued that although motion perception is always geocentric, relevant registrations also occur at the three earlier levels. The model is applied to various veridical and nonveridical motion phenomena.
The representation of nonuniform motion: induced movement.
Induced motion occurs when there is a misallocation of nonuniform motion. Theories of induced motion are reviewed with respect to the model for uniform motion recently proposed by Swanston, Wade, and Day. Theories based on single processes operating at one of the retinocentric, orbitocentric, egocentric, or geocentric levels are not able to account for all aspects of the phenomenon. It is therefore suggested that induced motion is a consequence of combining two different types of motion signals: one provides information by registering the motion with respect to the retina, orbit, and egocentre; the other provides information only on the relational motions between the pattern elements. Simple rules are given for defining a frame of reference for the relational motion process, which can result in a reallocation of the motion signals. It is proposed that the two signals in combination are weighted differentially, with the greater influence coming from the relational signals. Procedures for determining the weighting factors are described, and predictions from the model are examined.
Perceived size and motion in depth from optical expansion.
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Binocular interaction in induced line rotation.
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Factors affecting the speed of acquisition of tabulated information from visual displays.
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