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PubMed1996

Further evidence for monocular determinants of perceived plaid direction.

This report adds to existing evidence that a monocular, feature-sensitive motion mechanism is involved in two-dimensional (2-D) motion processing, and also accounts for an earlier, unexplained result [Alais et al.(1994) Vision Research, 34, 1823-1834]. The central finding is that the perceived direction of a monocularly viewed type II plaid changes over a period of continuous exposure such that post-adaptation direction judgements exhibit more of the component-direction bias known to occur with these stimuli than pre-adaptation judgements. These adaptation effects are confined to the adapted eye: when the adapting stimulus is presented to one eye, pre- and post-adaptation direction judgements made with the other, non-adapted eye are identical. These results strongly suggest the involvement of a monocular motion mechanism in two-dimensional motion processing, in addition to the more commonly presumed binocular mechanisms.

Adaptation, Ocular↗
PubMed1996

Direction-selective adaptation and simultaneous contrast induced by stereoscopic (cyclopean) motion.

Across four experiments, this study investigated direction-specific adaptation and simultaneous contrast induced by moving binocular disparity information (stereoscopic motion). The stimuli were moving arrays of stereoscopic dots created from dynamic random-element stereograms. Experiments 1 and 2 examined the effects of adaptation to motion in a given direction on the apparent direction of test motion. Results showed that the direction of test motion appeared repulsed away from the direction of adapting motion (repulsion aftereffect) by as much as 20 deg or more when directions of adapt and test were similar. Experiment 3 investigated transfer of the repulsion aftereffect across the stereoscopic and luminance domains by employing stereoscopic adapting motion and luminance test motion or vice versa. Results showed that the repulsion aftereffect transferred across the two stimulus domains. Experiment 4 investigated direction-specific contrast by measuring the perceived direction of two stereoscopic arrays presented simultaneously and moving in different directions. Results showed that the directions of the arrays appeared repulsed away from one another when their directions were similar. Taken together, these results suggest that the direction of stereoscopic motion is coded in the activity of directionally selective mechanisms, as is the case for luminance-domain motion. Transfer of the repulsion aftereffect between stereoscopic and luminance domains indicates the two kinds of motion perception are mediated by a common substrate.

Adaptation, Ocular↗
PubMed1996

Responses to opposed directions of motion: continuum or independent mechanisms?

Opponency between opposite directions of motion is a characteristic of many models of movement detection and is commonly invoked in explanations of the motion after-effect. If detection of opposite directions is mediated by a single mechanism, then a single, smooth psychometric function for the discrimination of global direction in random-dot kinematograms should be found as a function of the percent of directional coherence of dots in the display, ranging from 100% coherence leftwards through 0% coherence to 100% coherence rightwards. Moreover, after rightward motion adaptation, a single psychometric function should still be observed if adaptation affects the perceptual system prior to opponent interactions. If, however, leftward and rightward detectors operate independently, then the slopes of the leftward and rightward halves of the function may differ, particularly after unidirectional adaptation. We measured the probability of a "rightward" direction response for nine values of motion coherence for five observers with and without prior rightward motion adaptation. Although a smooth psychometric function was found without motion adaptation, after adaptation, the rightward half of the psychometric function was flattened whereas the leftward half remained unchanged. Such results indicate that movement direction analysers operate in a non-opponent manner.

Adaptation, Ocular↗
PubMed1996

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.

Adaptation, Ocular↗
PubMed1996

Temporal and spatial frequency tuning of the flicker motion aftereffect.

The motion aftereffect (MAE) was used to study the temporal and spatial frequency selectivity of the visual system at supra-threshold contrasts. Observers adapted to drifting sine-wave gratings of a range of spatial and temporal frequencies. The magnitude of the MAE induced by the adaptation was measured with counterphasing test gratings of a variety of spatial and temporal frequencies. Independently of the spatial or temporal frequency of the adapting grating, the largest MAE was found with slowly counterphasing test gratings (at approximately 0.125-0.25 Hz). The largest MAEs were also found when the test grating was of similar spatial frequency to that of the adapting grating, even at very low spatial frequencies (0.125 c/deg). These data suggest that MAEs are dominated by a single, low-pass temporal frequency mechanism and by a series of band-pass spatial frequency mechanisms. The band-pass spatial frequency tuning even at low spatial frequencies suggests that the "lowest adaptable channel" concept [Cameron et al. (1992). Vision Research, 32, 561-568] may be an artifact of disadvantaged low spatial frequencies using static test patterns.

Adaptation, Ocular↗
PubMed1995

The effect of red and neutral density filters on the degree of eccentric fixation.

The reported shift of fixation to a more central position when a red filter is placed before an eye which fixates eccentrically, has never been satisfactorily explained. To further investigate this phenomenon, 14 subjects with eccentric fixation were recruited. A Wratten No. 92 filter was placed before the eccentrically fixating eye, and any change of fixation was assessed by the after-image transfer test. A neutral density (ND) filter was then used, to mimic the reduced retinal illumination caused by the red filter. The results suggest that there is a significant reduction in the eccentricity of fixation, when placing the red filter before an eccentrically fixating eye, and also when placing the ND filter. No significant difference in the effects produced by either filter was found. This suggests that it is the reduction in retinal illuminance caused by the red filter which contributes towards such improvements in fixation.

Afterimage↗
PubMed1992

Evaluation of the visual system in multiple sclerosis: a comparative study of diagnostic tests.

In 22 patients with clinically definite multiple sclerosis (MS) who were without visual symptoms and had a visual acuity of at least 1.0 in both eyes at the time of measurement, the following tests were performed to detect subclinical lesions in the visual system: visual evoked potential (VEP), contrast sensitivity test (CS), flight of colours test (FOC), colour vision test (Ishihara plates) (CV) and the pupillary light reflex (PLR). VEP was abnormal in 81.8%, CS in 72.7%, FOC in 36.4%, CV in 31.8%, and PLR in 52.3% of the patients. VEP and CS together were most sensitive: combining these techniques subclinical lesions of the visual system were detected in 90.9% (20/22) of these asymptomatic patients.

Adult↗
PubMed1991

Visual perception--a hypothesis.

Based on the results of after-image experiments, it can be concluded that the retinal perception of image position is linked to head position information, since this affects after-image positioning. Why actual perception of object position is not affected is hypothesised as a resetting of the retinal horizon, from information received on head position. Tilting of the after-image is thus explained as superimposition of the former image on a reset (tilted) horizon (axis).

Afterimage↗
PubMed2003

Unconscious adaptation: a new illusion of depth induced by stimulus features without depth.

Here, we show a new illusion of depth induced by psychophysical adaptation to dynamic random-dot stereograms (RDS) that are interocularly anticorrelated (i.e., in which the images for the two eyes have reversed contrast polarity with each other). After prolonged viewing of anticorrelated RDS, the presentation of uncorrelated RDS (i.e., in which two images are mutually independent random-dot patterns) produces the sensation of depth, although both anticorrelated and uncorrelated RDSs are perceptually rivalrous with no consistent depth by themselves. Contrary to other aftereffects demonstrated in a number of visual dimensions, including motion, orientation, and disparity, this illusion results from unconscious adaptation; observers are not aware of what they are being adapted to during the process of adaptation. We further demonstrate that this illusion can be predicted from the simulated responses of disparity-selective neurons based on a local filtering model. Model simulations indicate that the inspection of anticorrelated RDS causes the adaptation of all disparity detectors except one sensitive to its disparity; therefore, those selectively unadapted detectors show relatively strong activation in response to the subsequent presentation of uncorrelated RDS and produce depth perception.

Adaptation, Psychological↗
PubMed2003

Collinear contextual suppression.

The context of a target can modulate behavioral as well as neural responses to that target. For example, target processing can be suppressed by iso-oriented surrounds whereas it can be facilitated by collinear contextual elements. Here, we present experiments in which collinear elements exert strong suppression whereas iso-oriented contextual surrounds yield no contextual modulation--contrary to most studies in this field. We suggest that contextual suppression depends strongly on the spatial arrangement of the context pointing to the influence of Gestalt factors in contextual modulation.

Afterimage↗
PubMed2004

Tuning properties of radial phantom motion aftereffects.

Motion aftereffects are normally tested in regions of the visual field that have been directly exposed to motion (local or concrete MAEs). We compared concrete MAEs with remote or phantom MAEs, in which motion is perceived in regions not previously adapted to motion. Our aim was to study the spatial dependencies and spatiotemporal tuning of phantom MAEs generated by radially expanding stimuli. For concrete and phantom MAEs, peripheral stimuli generated stronger aftereffects than central stimuli. Concrete MAEs display temporal frequency tuning, while phantom MAEs do not show categorical temporal frequency or velocity tuning. We found that subjects may use different response strategies to determine motion direction when presented with different stimulus sizes. In some subjects, as adapting stimulus size increased, phantom MAE strength increased while the concrete MAE strength decreased; in other subjects, the opposite effects were observed. We hypothesise that these opposing findings reflect interplay between the adaptation of global motion sensors and local motion sensors with inhibitory interconnections.

Adaptation, Ocular↗
PubMed2006

Predicting the motion after-effect from sensitivity loss.

The widely accepted disinhibition theory of the motion after-effect (MAE) proposes that the balance point of an opponent mechanism is changed by directional adaptation. To see if the post-adaptation balance point could be predicted from contrast adaptation, we measured threshold-vs-contrast (i.e., T-vs-C or dipper) functions, before and after adaptation to moving gratings. For test stimuli moving in the same direction, adaptation shifted the point of maximum facilitation (i.e., the dip) upwards and rightwards. For tests moving in the opposite direction, adaptation produced a similar, but smaller, shift. These shifts are consistent with a change in divisive gain control. They are also consistent with subtractive inhibition followed by half-wave rectification. We attempted to use transducer functions derived from these data to predict the strength of the MAE. When combined, gratings moving in the adapted and opposite directions appeared perfectly balanced (i.e., counterphasing) when the latter was given approximately 2% more contrast than was predicted on the basis of the derived transducers. This small under-prediction may be indicative of sensory recalibration. Finally, we found that adaptation did not alter the fact that low-contrast stimuli could be detected and their direction identified with similar accuracy. We conclude that both static and dynamic forms of MAE are primarily caused by a decreased sensitivity in directionally tuned mechanisms, as proposed by the disinhibition theory.

Adaptation, Biological↗