Jane E. Raymond

Similarity Determines the Attentional Blink

When participants are required to respond to a target letter imbedded in a stream of rapid serially presented letters, perception of a 2nd target letter is impaired if the interval between the 2 targets is less than about 450 ms. This attentionally based posttarget suppression in visual processing, referred to as the attentional blink (AB), is not found when there is a brief pause in the stream immediately after the 1st target. To investigate the importance of posttarget stimulation in AB production, the categorical, featural, and spatial similarity of the immediate posttarget item to other items in the stream was manipulated. Although featural and spatial dissimilarity produced significant attenuation of the AB effect, categorical dissimilarity did not. Significant AB effects were found in all conditions, suggesting that the presentation of any patterned stimulus in close temporal proximity to the target provokes the AB.

Directional anisotropy of motion sensitivity across the visual field

There is an implicit assumption in most models of movement direction perception that sensitivity to motion is directionally isotropic, i.e. equal for movement in all directions. However, most previous research on directional effects on human motion perception have measured responses (e.g. manual reaction times) to suprathreshold stimuli rather than motion sensitivity. I investigated this possibility by measuring monocular sensitivity to motion coherence in small field random-dot kinematograms with global leftwards or rightwards motion as a function of eccentricity along the horizontal meridian in each eye of six observers. Although foveal motion sensitivity was isotropic, small but significant differences (about 0.1 log units) in sensitivity in favour of centripetal motion were observed at eccentricities between 5.0 and 12.5 deg. Motion anisotropy was significantly larger in the temporal than the nasal visual hemifield. In a second experiment, motion coherence thresholds for upwards and downwards movement were measured foveally and at +/- 5 deg on the vertical meridian. At the fovea, no difference in sensitivity to upwards vs downwards motion was observed but these vertical axis thresholds were significantly higher than foveal thresholds for movement along the horizontal axis. Directional anisotropy in favour of centripetal motion was present in the inferior visual field but directional isotropy was found for the superior visual field. Perhaps this generally heightened sensitivity to centripetal directions may function normally to facilitate figure/ground segmentation in the presence of centrifugally-moving background contours that typically accompany forward locomotion.

Complete interocular transfer of motion adaptation effects on motion coherence thresholds.

The binocularity of visual mechanisms in humans can be investigated by measuring the interocular transfer (IOT) of visual aftereffects. Cells in extrastriate visual areas of macaque, e.g. the middle temporal (MT) area, are uniformly binocular, whereas cells in striate area V1 vary in their degree of binocularity. Therefore, IOT of aftereffects mediated by extrastriate cortex should be nearly complete compared to the partial transfer (about 70%) found for aftereffects thought to be mediated by V1. If MT and other extrastriate areas play a significant role in motion perception, then IOT of motion adaptation aftereffects on the perception of moving stimuli should be nearly complete. After motion adaptation, the perception of global movement direction in partially coherent random dot kinematograms (RDKs) is temporarily impaired if the predominant direction of dots in the test stimulus matches that of the adaptation stimulus. I measured the IOT of this motion incoherence aftereffect in four observers. Post-adaptation motion coherence thresholds were elevated equally for interocular and intraocular adaptation, indicating complete transfer of the aftereffect. Measurement of the classical motion aftereffect using the same stimuli and conditions showed partial or absent transfer. These data support the idea that extrastriate areas play a key role in motion perception and suggest that the motion incoherence aftereffect and the classical motion aftereffect may involve different mechanisms.

Movement direction analysers : independence and bandwidth

Opponent, or ratio, models of movement direction perception propose that pairs of analysers, sensitive to opposite directions, are linked in an opponent fashion. Alternatively, distribution models posit independence of movement direction analysers. To investigate analyser independence, a direction-selective adaptation experiment was conducted in which motion coherence thresholds for random-dot apparent motion stimuli were measured for upward, downward, leftward and rightward motion with and without prior adaptation to rightward motion. Coherence thresholds for rightward motion were elevated on average by 0.64 log units after adaptation. Thresholds for motion in the other directions remained unchanged. Since opponent models predict enhanced sensitivity after adaptation for the direction opposite that of adaptation, these results suggest that movement direction analysers operate independently. In a second experiment subjects adapted to a bidirectional stimulus containing an equal number of leftward and rightward moving dots. Leftward and rightward coherence thresholds were elevated even though no motion aftereffects were produced. A third experiment in which coherence thresholds for test directions within a narrow range were measured with and without prior adaptation to motion in the direction of the ranges mean showed that threshold elevation was maximal when test and adaptation direction were matched and fell off as the difference in adaptation and test direction increased. The resulting functions indicate that the bandwidth of movement direction analysers is between +/- 35 and +/- 40 deg, a value consistent with reported mean directional tuning functions of movement sensitive units in the middle temporal area but smaller than previously reported psychophysical values.

Optokinetic backgrounds affect perceived velocity during ocular tracking

The perceived velocity of visually tracked moving objects may depend on interactions between reflexive and voluntary oculomotor mechanisms. To investigate this hypothesis, subjects were required to compare sequentially the velocity of a standard target with that of a test target moving at one of five velocities. The standard target was viewed against a plain field (no optokinetic stimulation), and the test target was viewed against (1) a plain field, (2) a stationary grating, (3) a grating drifting in the direction of target motion, or (4) a grating drifting against target motion. In one condition, subjects tracked the target; in the other, a stationary point was fixated. The tracking group overestimated velocity when backgrounds drifted against target motion, but underestimated velocity when gratings drifted with target motion. Subjects not required to track experienced no such misestimations. The results are discussed in relation to the interactive mechanisms of the two eye-movement systems.

Training of efficient oculomotor strategies enhances skill acquisition

The present study investigated the hypothesis that efficient oculomotor behaviours can be acquired through practice on a series of simple tasks and can be transferred subsequently to a complex visuomotor task, such as a video game. Each of two groups of subjects were exposed to a different set of simple tasks, or drills. One group, the efficient eye movement experimental group, received training designed to minimize eye movements and optimize scan path behaviours, whereas a second group of subjects, the inefficient eye movement experimental group, received training designed to increase the frequency of eye movements. Oculomotor training was interspersed with practice on the video game. Performance of these two experimental groups in the video game was compared to a control group playing the video game but receiving no specific training and matched for total time in the experiment. The group receiving efficient oculomotor training exhibited significantly superior performance in the video game and fewer foveations than either the inefficient or control groups, which did not differ from each other. Overall there was a significant inverse correlation between the number of foveations in the game and game score. The results of this study are discussed in terms of their implications for the importance of oculomotor training in the acquisition of any complex perceptual motor task.

THE EFFECT OF LOCAL LUMINANCE CONTRAST ON INDUCED MOTION

Recently it has been suggested that the magnocellular, as opposed to the parvocellular, subsystem of the primary visual pathway of primates subserves motion perception. This suggestion is partly based on the observation that both the visual responses of magnocellular neural units and certain motion perception phenomena have high contrast sensitivity and are only dependent on luminance contrast for a narrow range of low contrasts. Parvocellular units have low contrast sensitivity and are dependent on contrast for a wide range of values. In the present experiment, the effect of local luminance contrast on induced motion was measured using a nulling procedure to quantify the magnitude, of illusory motion perceived in a centre grating which was viewed against a moving surround grafting. Centre grating contrast was either matched to the surround or maintained at a low (2.5%) or high (60%) value. Surround contrast ranged from 2.5% to 60%. It was found that (1) centre contrast had no observable effect on the magnitude of the illusion, (2) induced motion was marginal or absent with low contrast but detectable surrounds, and (3) induced motion increased as contrast in the surround increased for the range of contrasts tested. This contrast response function is more similar to that of parvocellular than magnocellular units and therefore suggests that the parvocellular stream may play a role in some aspects of motion processing.

The interaction of target size and background pattern on perceived velocity during visual tracking

The effect of target size and background contour on the perceived velocity of a visually tracked target was determined using a psychophysical comparison procedure. For targets viewed against a uniform field, an increase in target size produced a modest underestimation of target velocity. For small test targets, a background of contours moving in the same direction as target motion also produced an underestimation of target velocity. A background of contours moving in the direction opposite to that of target motion produced a small, but consistent, overestimation of target velocity. As target size increased, the underestimation effect seen with striped backgrounds moving in the same direction was not enhanced. The overestimation effect seen with a striped background moving in the opposite direction was nulled by an increase in target size.

Viewing distance and the sustained detection of high spatial frequency gratings

Although conventional visual acuity tests measure the ability to resolve fine detail, many visual tasks demand prolonged detection. By evaluating the ability to maintain visual resolution, we observed that detection of stimuli subtending a small but constant visual angle depends on viewing distance and is probably determined by accommodative control. Detection was longest when viewing distance corresponded to the individual observers accommodative resting, or tonus, position and fell off markedly for further distances. Since most visual assessment procedures involve a momentary detection criterion, they will not predict the ability to sustain accommodation and may thus overestimate visual resolution for prolonged viewing tasks.

Viewing distance affects reaction time to discriminate letters of a constant small size

Continuous physiological fluctuations in visual accommodation may affect the ability to sustain resolution of small-letter stimuli. To investigate this possibility, the experimenter had subjects view a small letter that was subtly transformed into another letter at a random time during a 12-sec viewing interval. Reaction time (RT) to detect the letter change was measured for stimuli of a constant size and brightness viewed at different distances. RTs were shortest at and around an individually determined optical distance and rose to over 1 sec as the difference between the viewing distance and this point increased, for both far and near optical distances. The data suggest that the ability to sustain an accommodative response during a 12-sec interval varies as a function of the optical distance of the stimuli and that transient accommodative inaccuracies are sufficient to impair letter recognition.