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PubMed2004

Visual mechanisms of motion analysis and motion perception.

Psychophysical experiments on feature tracking suggest that most of our sensitivity to chromatic motion and to second-order motion depends on feature tracking. There is no reason to suppose that the visual system contains motion sensors dedicated to the analysis of second-order motion. Current psychophysical and physiological data indicate that local motion sensors are selective for orientation and spatial frequency but they do not eliminate any of the three main models-the Reichardt detector, the motion-energy filter, and gradient-based sensors. Both psychophysical and physiological data suggest that both broadly oriented and narrowly oriented motion sensors are important in the early analysis of motion in two dimensions.

Humans↗
PubMed2005

Chromatic sensitivity of neurones in area MT of the anaesthetised macaque monkey compared to human motion perception.

We recorded activity from neurones in cortical motion-processing areas, middle temporal area (MT) and middle posterior superior temporal sulcus (MST), of anaesthetised and paralysed macaque monkeys in response to moving sinewave gratings modulated in luminance and chrominance. The activity of MT and MST neurones was highly dependent on luminance contrast. In three of four animals isoluminant chromatic modulations failed to activate MT/MST neurones significantly. At low luminance contrast a systematic dependence on chromaticity was revealed, attributable mostly to residual activity of the magnocellular pathway. Additionally, we found indications for a weak S-cone input, but rod intrusion could also have made a contribution. In contrast to the activity of MT and MST neurones, speed judgments and onset amplitude of evoked optokinetic eye movements in human subjects confronted with equivalent visual stimuli were largely independent of luminance modulation. Motion of every grating (including isoluminant) was readily visible for all but one observer. Similarity with the activity of MT/MST cells was found only for motion-nulling equivalent luminance contrast judgments at isoluminance. Our results suggest that areas MT and MST may not be involved in the processing of chromatic motion, but effects of central anaesthesia and/or the existence of intra- and inter-species differences must also be considered.

Adaptation, Physiological↗
PubMed2001

A coordinate system for visual motion perception.

The purpose of this research was to determine the reference axes used by the visual system to specify direction of motion of objects by the visual system at the perceptual level. Ten young adults aligned motion of a moving luminous dot on a computer display to body-fixed and external vertical and horizontal plane axes while operating in a dark room. Accuracy of aligning dot motion to earth-fixed vertical, a displayed luminous line (external visual axis) of varied orientations, and head and trunk longitudinal axes was tested in one experiment with the display in the vertical frontal plane. In a second experiment, dot motion was aligned to head and trunk anterior/posterior (A-P) axes and to an external visual axis presented on a horizontal computer screen. Head and trunk orientations were varied in the frontal plane (left/right tilt) when testing vertical plane axes and by rotation of the head and/or trunk about a vertical axis when testing horizontal plane axes. Perceptual errors were lowest when aligning to earth-fixed vertical in the vertical plane and to an external oblique line in the horizontal plane when head and trunk orientations were varied. Perceptions of horizontal plane motion direction were accurate relative to the trunk-fixed A-P axis when only head orientation was varied, but large errors were made when trunk orientation was varied. Proprioceptive influences on visual perceptions of motion direction were shown by a dependence of perceptual errors on trunk and neck orientations when aligning to all axes. Furthermore, when aligning motion to an external line, the errors depended on orientation of the line in addition to trunk and neck orientations, but not when aligning to intrinsic axes or earth-fixed vertical in the presence of an external line. We conclude that the visual motion system defines direction relative to earth-fixed vertical and an external horizontal reference axis when available. The trunk-fixed A-P axis can be used to accurately define motion direction when operating without an external reference if a neutral trunk orientation is maintained.

Adult↗
PubMed1993

Different parameters control motion perception above and below a critical density.

The maximum displacement for the detection of apparent motion (Dmax) is measured using stimuli made up of Gabor function micro-patterns randomly distributed across the stimulus field. Previous studies using high densities of micro-patterns have demonstrated Dmax to be dependent on the spatial frequency content of the stimulus and not the size of the stimulus elements. Here we report that Dmax increases suddenly when the number of micro-patterns in the visual field is reduced beyond some critical point. The number of micro-patterns at which the transition in Dmax occurs is found to be inversely proportional to the width of the micro-patterns along the axis of motion. Beyond this transition, for low density stimuli, Dmax is found to be dependent on both the number and size of micro-patterns in the stimulus field. These results are suggestive of the operation of different motion mechanisms under conditions of low vs high micro-pattern density.

Female↗
PubMed1999

Position-based motion perception for color and texture stimuli: effects of contrast and speed.

Motion can be perceived either through low-level, motion-energy detection or through tracking the change in position of features. Previously we have shown that, while luminance-based motion likely is detected with velocity-sensitive motion-energy units, patterns defined by texture or binocular disparity ('second-order' stimuli) were tracked by a position-sensitive mechanism (Seiffert & Cavanagh (1998) Vision Research, 38, 3569-3582). Here, we use the same technique, measuring motion amplitude thresholds of oscillating gratings over a range of temporal frequencies and find that the motion of low-contrast equiluminant red/green gratings is also detected with position tracking. In addition, we find that as contrast or speed increases these results change: high-contrast or high-speed equiluminant color or texture-based motion is detected by velocity-sensitive mechanisms. These results help resolve the dispute over the processes which detect the motion of non-luminance based stimuli. Both systems are available, but their relative efficiency changes as a function of contrast and speed. A position-tracking process is more sensitive at low contrasts and low speeds whereas a motion-energy system is more sensitive at high contrasts and high speeds.

Color Perception↗
PubMed1989

The effects of dichoptic and binocular viewing on bistable motion percepts.

Two competitive percepts are produced from a bistable stroboscopic motion display. In this display two frames, each containing three horizontally arrayed elements are presented alternately for several cycles. At short interstimulus intervals (ISIs) element or end-to-end motion responses are obtained when the two inner, spatially overlapping elements are seen as stationary and the third element moves back and forth from one end to the other end. Group motion responses are obtained at longer ISIs when the three elements are seen to move back and forth as a group. The dominance of these two percepts across ISIs was controlled by the manipulation of (1) element size, (2) frame duration, and (3) viewing conditions. Under both binocular and dichoptic viewing, element motion responses increase as element size and frame duration decrease. By maximizing pattern persistence substantial element motion responses were obtained dichoptically as well as binocularly. Instead of supporting the existence of two separate, low-level and high-level, motion systems, our data suggest that there is a single, high-level mechanism for motion whose output can be modulated by pattern persistence.

Afterimage↗
PubMed1998

Insights into motion perception by observer modeling.

The statistical efficiency of human observers performing a simplified version of the motion detection task of Salzman and Newsome [Science 264, 231 (1994)] is high but not perfect. This reduced efficiency may be caused by noise internal to the observers or by the observers' using strategies that are different from that used by an ideal machine. We therefore investigated which of three simple models best accounts for the observers' performance. The models compared were a motion detector that uses the proportion of dots in the first frame that move coherently (as would an ideal machine), a model that bases its decision on the number of dots that move, and a model that differentially weights motions that occur at different locations in the visual field (for instance, differentially weights the point of fixation and the periphery). We compared these models by explicitly modeling the human observers' performance. We recorded the exact stimulus configuration on each trial together with the observer's response, and, for the different models, we found the parameters that best predicted the observer's performance in a least-squares sense. We then used N-fold cross validation to compare the models and hence the associated hypotheses. Our results show that the performance of observers is based on the proportion, not the absolute number, of dots that are moving and that there was no evidence of any differential spatial weighting. Whereas this method of modeling the observers' response is demonstrated only for one simple psychophysical paradigm, it is general and can be applied to any psychophysical framework in which the entire stimulus can be recorded.

Algorithms↗
PubMed2001

Regional cerebral correlates of global motion perception: evidence from unilateral cerebral brain damage.

We used a psychophysical task to measure sensitivity to motion direction in 50 stroke patients with unilateral brain lesions and 85 control subjects. Subjects were asked to discriminate the overall direction of motion in dynamic stochastic random dot displays in which only a variable proportion of the spots moved in a single direction while the remainder moved randomly. Behavioural and neurophysiological evidence shows that the middle temporal (MT/V5) and middle superior temporal (MST) areas in the macaque monkey are indispensably involved in the perception of this type of motion. In human subjects too, lesions in the same region disrupt performance on this task. Here we assessed more extensively the correlation between direction sensitivity for global motion and the anatomical locus of the lesion. Thresholds for perceiving the direction of global motion were impaired in the visual field contralateral to the lesion in patients with lesions in the occipitoparietal and parietotemporal areas involving the human analogue of areas MT/V5 and MST, but not by lesions in the occipito-temporal or anterior frontal areas. Patients with lesions involving the anterior temporal or parietal lobes displayed poor performance for stimuli presented in either visual field, which is consistent with the large and bilateral receptive fields in these areas in monkeys. The perception of global motion was also more impaired in the centripetal than the centrifugal direction in the hemifield contralateral to the MT/V5 lesion. Surprisingly, thresholds were normal in all patients when the displays contained static but not dynamic visual noise, suggesting that their deficit reflects an inability to filter out dynamic noise. Although frequent repeated testing of some patients whose lesion involved the human homologue of MT was accompanied by an improvement in performance, this was no greater than in other patients who received training on different motion tasks.

Adult↗
PubMed2006

Effects of depth, eccentricity and size of additional static stimulus on visually induced self-motion perception.

Static visual stimulus presented behind a moving pattern inhibits vection. On the other hand, the same static stimulus facilitates vection if it is located in front of the moving pattern. In this study, the effects of depth, eccentricity and size of the additional static stimulus on inhibition and facilitation of horizontal linear vection were investigated. Results indicated that both inhibition and facilitation become conspicuous with increasing the size of the static stimulus. Furthermore, the inhibition caused by the static foreground is dominated by the central stimulus, while the facilitation caused by the static background is more effective in the peripheral stimulus.

Depth Perception↗