The effect of refractive error on central and peripheral motion sensitivity at various exposure durations

Pennsylvania State University, University Park, Pennsylvania 16802 The influence of refractive error on movement sensitivity was determined for a range of stimulus durations in the fovea and periphery. At short durations, threshold is determined by a constant displacement of the stimulus. At longer durations, a constant velocity is required. The correction of peripheral refractive error increases movement sensitivity in the displacement (short duration) component, but does not influence sensitivity in the velocity (long duration) component of the motion threshold. Implications concerning the mechanisms underlying motion perception are discussed.     

The relationships among time,distance, and intensity as determinants of motion discrimination

The threshold stimulus for visual motion discrimination was analyzed into the constituent parameters of velocity, i.e., time and distance, with both of these primary variables subject-determined. It was found that, given a constant stimulus luminance, motion threshold was characterized generally by a “trade-off” or inverse power relationship between time and distance of movement. Earlier reports of energy constancy at threshold (R. H. Brown, 1955, 1957, 1958), implying threshold relationships incompatible with these, were confirmed only for the atypical conditions of high-velocity/low-Iuminance stimuli and were attributed to absolute visibility requirements. Under more general and representative conditions, threshold was relatively insensitive to luminance. The present results were also contrasted with earlier findings (Graham. 1968) of distance or “displacement” constancy at threshold, pertaining to movement between stationary start and stop positions.     

Induced visual movement as nonveridical resolution of displacement ambiguity

Following the proposals made by Kinchla, it is argued that induced movement is a nonveridical resolution of stimulus ambiguity. The ambiguity derives from an identity between displacement of one element relative to another and displacement of the second relative to the first in a featureless field at velocities below the threshold for subject-relative movement. In such conditions, which element actually moves is perceptually unresolvable and veridical judgments therefore accord with chance. When a stationary field is introduced, perception is veridical, but when the field moves with the moving element, perception is nonveridical, i.e., induced movement of the stationary element and induced stationariness of the moving element occurs. The results from three experiments supported this interpretation and showed also that movement velocities above the subject-relative threshold contribute to the resolution of ambiguity.     

Sensitivity to relative and absolute motion.

The threshold of sensitivity to movement could be governed by mechanisms that are sensitive either to change in spatial position, or directly to the movement itself. The use of spatially complex patterns (random-dot patterns) has been suggested to eliminate the former strategy allowing examination of the movement detecting mechanisms in isolation. By means of such a technique, thresholds for directional judgements were determined for patterns which underwent either a simple displacement or a shearing displacement. Thresholds for shearing motion were found to be around one half of those for simple motion, suggesting that relative, rather than absolute, motion governs performance for small displacements. This contrasts with previous experiments which showed that absolute motion governs performance for much larger displacements.     

An indirect measure of perceived distance from oculomotor cues

The sensitivity of an indirect method of measuring perceived distance was compared in two experiments with the direct procedure of eliciting verbal reports of distance. Perceived distance was varied by varying the oculomotor cues to object distance. The indirect method, called the “adjustable pivot method,” uses an apparatus that physically moves the stimulus object laterally concomitantly with the lateral motion of the head. The magnitude and direction of this concomitant motion determines the distance of the point around which the direction of gaze to the object rotates (the pivot distance) as the head is moved. The pivot distance at which the object appears stationary with head movement measures the apparent distance of the object. Both types of measures were found to vary systematically with the oculomotor distance of the object for points of light (Experiment 1) and extended objects (Experiment 2). A previous study has shown that the adjustable pivot method avoids cognitive errors that can distort verbal reports of distance. The present study, by demonstrating the discriminative capability of this method under conditions in which differences in perceived distance were expected to occur, provides clear evidence that the adjustable pivot method is a sensitive and useful procedure for measuring perceived distance.     

There is no induced motion at near-threshold velocities

Duncker's classic experiments on induced motion at near-threshold velocities were based on the assumption that movement of one stimulus relative to the other (object-relative) is perfectly detected while it remains completely impossible to identify which of the two stimuli is moving (subject-relative). In the present experiments it is shown that the threshold areas of object-relative and subject-relative movement are largely overlapping. Consequently Dunker's assumption cannot be fulfilled: whenever object-relative movement is well detectable accuracy of detecting subject-relative motion will also be over 50%. In four additional experiments it is shown that the well-established effects of fixation, stimulus size and enclosure on induced motion are to be interpreted as effects of these variables on the detection of subject-relative movement: the so-called induction phenomena occured only when accuracy was over 50%.     

Induced motion and oculomotor capture

Three experiments investigating the basis of induced motion are reported. The proposition that induced motion is based on the visual capture of eye-position information and is therefore a subject-relative, rather than object-relative, motion was explored in the first experiment. Observers made saccades to an invisible auditory stimulus following fixation on a stationary stimulus in which motion was induced. In the remaining two experiments, the question of whether perceived induced motion produces a straight ahead shift was explored. The critical eye movement was directed to apparent straight ahead. Because these saccades partially compensated for the apparent displacement of the induction stimulus, and saccades to the auditory stimulus did not, we conclude that induced motion is not based on oculomotor visual capture. Rather, it is accompanied by a shift in the judged direction of straight ahead, an instance of the straight ahead shift. The results support an object-relative theory of induced motion.     

Detection of moving local density differences in dynamic random patterns by human observers

The way in which movement enhances target visibility has been investigated by measuring the detectability of the direction of motion of a dot pattern added to a background of dynamic visual noise. When the positions of all the dots were changed randomly from frame to frame, so that there was no dot configuration to define the target area (experiments 1 and 2), the threshold density difference necessary was for direction of motion detection less than 3 dots/frame (between 20% and 50% density difference). The spatial displacement (S) at which optimal detection occurs increased when a target elongated in the direction of motion was used. If S was either larger or smaller than its optimal value, thresholds rose progressively. The rise in threshold when S was smaller than 0.25 deg (the width of the target area) decreased when the target dots had a fixed spatial arrangement (experiment 3). It is suggested that in both fixed and random target configurations there is a grouping of dots with similar trajectories via a global directionally-selective process. The strength of the overall motion signal is greater in the fixed-dot configuration because each target dot has associated with it a vector precisely aligned in the direction of the target motion.     

Changes in the perception of spatial location: A test of potentiation vs. recalibration theory

Two experiments tested recalibration and muscle potentiation theories of adaptation to prismatic displacement of the visual field. Each experiment included a condition in which only recalibration could occur and another condition in which only potentiation was possible. In one experiment, the displacement was simulated on a computer-driven cathode ray tube, and in the other, the displacement was produced by actual prisms. In both experiments, significant adaptation occurred only in the potentiation condition. Implications of this finding for recent criticisms of potentiation theory are discussed.     

The role of action plans and other cognitive factors in motion extrapolation: A modelling study

When observers are asked to remember the final location of an object undergoing apparent or implied motion, a forward displacement is observed. The magnitude of this form of motion extrapolation is known to depend on various factors including stimulus attributes, action plans, and other cognitive cues. Here we present a modelling approach that aims at bridging different existing theories of displacement within a single theoretical framework. A network model consisting of interacting excitatory and inhibitory cell populations coding for stimulus attributes like position or orientation is used to study the response to motion displays. The intrinsic network dynamics can be modulated by additional information sources representing action plans directed at the moving target or cognitive cues such as prior knowledge about the trajectory. These factors decide the extent to which the dynamic representation overshoots the final position. The model predictions are quantitatively compared with the experimental findings. The results are discussed in relation to theoretical ideas about processing principles underlying motion extrapolation and a comparison with neurophysiological findings linked to movement prediction is made.     

Control of a Virtual Avatar Influences Postural Activity and Motion Sickness

The influence of vehicular control on motion sickness has implications for theories of motion sickness etiology. We asked whether motion sickness susceptibility might also be related to the control of non-vehicular locomotion. Participants were exposed to a console video game that featured ambulatory locomotion of a virtual avatar. In a yoked control design, individuals either played the game (players) or watched another participant's recorded game play (viewers). Viewers were more likely than players to report motion sickness. During exposure to the video game players moved more than viewers, and the movement of players was more predictable or self-similar than the movement of viewers. Coupling of movement within player-viewer pairs was greater for pairs in which the viewer later reported motion sickness than for pairs in which both participants stated that they were not motion sick. The results reveal that motion sickness incidence can be influenced by the control of stimulus motion, in general, and is not limited to control of vehicular motion. We discuss implications of these findings for theories of motion sickness etiology.     

Observations on stroboscopic induced motion

Subject-relative explanations of motion induction state that induced motion is the result of a misperceived shift of the median plane of the visual field of the subject. This theory does not require relative motion of the spot and frame, in the classical spot-and-frame condition, only asymmetrical stimulation. Three experiments are reported in which stroboscopic induced motion was investigated. The experimental arrangement was unconventional in that the induced object (spot) was presented only during the interstimulus interval between the exposures of the inducing object (frame). This allowed differentiation of the duration of the induced movement and that of the inducing one. In the first experiment it was demonstrated that perception of induced motion depends upon the duration of the interstimulus interval between the presentations of the inducing frame. In the second experiment it was shown that the perceived velocity of the induced movement can be different from that of the inducing one and depends on the duration of exposure of the induced object. In the third experiment a stimulus display was created in which the apparent displacement of an object and its induced motion are incongruous. The results are incompatible with subject-relative displacement as the sole determining factor of motion induction and they present some difficulties for the hypothesis that induced motion is the result of the apportionment of the objective displacement of the frame.     

Motion parallax driven by head movements: conditions for visual stability, perceived depth, and perceived concomitant motion

Yoking the movement of the stimulus on the screen to the movement of the head, we examined visual stability and depth perception as a function of head-movement velocity and parallax. In experiment 1, for different head velocities, observers adjusted the parallax to find (a) the depth threshold and (b) the concomitant-motion threshold. Between these thresholds, depth was seen with no perceived motion. In experiment 2, for different head velocities, observers adjusted the parallax to produce the same perceived depth. A slower head movement required a greater parallax to produce the same perceived depth as faster head movements. In experiment 3, observers reported the perceived depth for different parallax magnitudes. Perceived depth covaried with smaller parallax without motion perception, but began to decrease with larger parallax and concomitant motion was seen. Only motion was seen with the larger parallax.     

Temperature reinforcement for visual discrimination training in baby chicks

A fully automated training apparatus, using thermoregulation as performance incentive, was constructed to train baby chicks on visual discrimination and reversal problems. In the base condition, the S is bathed in cold air in a small training cubicle. Two stimuli are back-projected onto two display windows according to a pseudorandom balanced schedule. A peck at the positive stimulus gives convective as well as radiant heat reward with adjustable temperature and duration. A peck at the negative stimulus merely advances the program to the next trial. Permanent printout records give trial-to-trial information on response choice and latency. Performance data of 64 Ss are reported.     

Second-order reversed phi.

In a first-order reversed-phi motion stimulus (Anstis, 1970), the black-white contrast of successive frames is reversed, and the direction of apparent motion may, under some conditions, appear to be reversed. It is demonstrated here that, for many classes of stimuli, this reversal is a mathematical property of the stimuli themselves, and the real problem is in perceiving forward motion, which involves the second- or third-order motion systems or both. Three classes of novel second-order reversed-phi stimuli (contrast, spatial frequency, and flicker modulation) that are invisible to first-order motion analysis were constructed. In these stimuli, the salient stimulus features move in the forward (feature displacement) direction, but the second-order motion energy model predicts motion in the reversed direction. In peripheral vision, for all stimulus types and all temporal frequencies, all the observers saw only the reversed-phi direction of motion. In central vision, the observers also perceived reversed motion at temporal frequencies above about 4 Hz, but they perceived movement in the forward direction at lower temporal frequencies. Since all of these stimuli are invisible to first-order motion, these results indicate that the second-order reversed-phi stimuli activate two subsequent competing motion mechanisms, both of which involve an initial stage of texture grabbing (spatiotemporal filtering, followed by fullwave rectification). The second-order motion system then applies a Reichardt detector (or equivalently, motion energy analysis) directly to this signal and arrives at the reversed-phi direction. The third-order system marks the location of features that differ from the background (the figure) in a salience map and computes motion in the forward direction from the changes in the spatiotemporal location of these marks. The second-order system's report of reversed movement dominates in peripheral vision and in central vision at higher temporal frequencies, because it has better spatial and temporal resolution than the third-order system, which has a cutoff frequency of 3-4 Hz (Lu & Sperling, 1995b). In central vision, below 3-4 Hz, the third-order system's report of resolvable forward movement of something salient (the figure) dominates the second-order system's report of texture contrast movement.     

Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

Extent-of-motion thresholds under subject-relative and object-relative conditions

The experiment was designed to discover the threshold extent of motion at medium speeds amounting to 41, 82, and 164 min./sec., and to compare the perception of motion arising from subject-relative displacement with the perception of motion arising from object-relative displacement. Extent thresholds were found while velocity was kept constant. Different groups of ten Ss were used for each displacement velocity, and for each S the extent threshold was twice obtained by the method of constant stimuli, once under subjectrelative and once under object-relative displacement conditions. Sensitivity to brief displacements of a continuously visible target was high; average thresholds ranged from 1.0 to 4.4 min. under the various conditions employed. The thresholds were higher for subject-relative conditions and the slower displacement velocities and lower for objectrelative conditions and faster displacements.     

The design and operation of an improved olfactometer for behavioral and physiological investigation1

An improved olfactometer is described, and its range of application for the study of the olfaction in man is discussed. The apparatus is based on air dilution; an odorant is diluted in a constant-flowing stream of air. The odor-containing air flows to S, who is seated in an enclosed partition with his head in a Plexiglas hood. A visual signal system allows communication between E and S. Precision flowmeters, solenoid valves, one-way floats, and an automated control system permit E to select rapidly a large range of stimuli, stimulus concentrations, and trial lengths. The apparatus has been used in a variety of situations ranging from absolute threshold detection to adaptation and recovery.     

Thresholds for movement direction: Two directions are less detectable than one

Thresholds for detecting movement direction were measured for two different types of dynamic dot display; first, one in which all dots moved upwards, and secondly, one in which half the dots moved upwards and half moved downwards. Direction sensitivity was found to be worse for the stimulus containing two simultaneous directions of motion than for the stimulus in one direction. These data are taken as evidence of some form of competition, or AND-NOT gating, between the outputs of direction-specific analysers during threshold determination.     

Second-order reversed phi

In a first-order reversed-phi motion stimulus (Anstis, 1970), the black-white contrast of successive frames is reversed, and the direction of apparent motion may, under some conditions, appear to be reversed. It is demonstrated here that, for many classes of stimuli, this reversal is a mathematical property of the stimuli themselves, and the real problem is in perceiving forward motion, which involves the second- or third-order motion systems or both. Three classes of novel second-order reversed-phi stimuli (contrast, spatial frequency, and flicker modulation) that are invisible to first-order motion analysis were constructed. In these stimuli, the salient stimulus features move in theforward (feature displacement) direction, but the second-order motion energy model predicts motion in thereversed direction. In peripheral vision, for all stimulus types and all temporal frequencies, all the observers saw only the reversed-phi direction of motion. In central vision, the observers also perceived reversed motion at temporal frequencies above about 4 Hz, but they perceived movement in the forward direction at lower temporal frequencies. Since all of these stimuli are invisible to first-order motion, these results indicate that the second-order reversed-phi stimuli activate two subsequent competing motion mech-anisms, both of which involve an initial stage of texture grabbing (spatiotemporal filtering, followed by fullwave rectification). The second-order motion system then applies a Reichardt detector (or equiva-lently, motion energy analysis) directly to this signal and arrives at the reversed-phi direction. The third-order system marks the location of features that differ from the background (the figure) in a salience map and computes motion in the forward direction from the changes in the spatiotemporal location of these marks. The second-order system’s report of reversed movement dominates in peripheral vision and in central vision at higher temporal frequencies, because it has better spatial and temporal resolu-tion than the third-order system, which has a cutoff frequency of 3–4 Hz (Lu & Sperling, 1995b). In cen-tral vision, below 3–4 Hz, the third-order system’s report of resolvable forward movement of something salient (the figure) dominates the second-order system’s report of texture contrast movement.