Visual discrimination of direction changes based upon two types of angular motion.
(33/3995)
We address the question of how the visual system analyses changes in direction. Using plaid stimuli, we define type O direction changes which entail a change in the orientations of the plaid components, and type V direction changes in which the orientations of the components remain constant, relative to the observer but their relative speeds change. Lower thresholds for discriminating type O and type V direction changes were compared. Type O thresholds for clockwise/anticlockwise direction change were very low (0.2-0.5 degree), were resistant to directional noise, and showed a low-pass relationship with drift velocity. Type V thresholds on the other hand were higher (1-5 degrees), and exhibited a bandpass relationship with drift velocity. Type O direction changes gave low thresholds at short inter-stimulus intervals (ISI) (< 160 ms) and higher thresholds (successive orientation discrimination) at long ISI (240 ms-12.8 s). Type V thresholds, on the other hand, exhibited no short-range process and performance at short ISI, was no better than for successive direction discrimination at long ISI. A two-stage rotary motion model is sufficient to explain the discrimination of type O direction changes and results rule out a model based on velocity discrimination. For type V direction changes, a two-stage mechanism is insufficient and results are consistent with a minimum of three computational stages. (+info)
Effects of a benzodiazepine, lorazepam, on motion integration and segmentation: an effect on the processing of line-ends?
(34/3995)
Previous studies have shown that the perceptual integration of component motions distributed across space is inhibited whenever segmentation cues, such as line-ends, are salient. Herein, we investigate to what extent enhanced inhibition induced by lorazepam, a benzodiazepine facilitating the fixation of GABA on GABAA receptors, modifies the balance between motion integration and motion segmentation at the behavioural level. Motion integration was tested in 16 healthy volunteers taking a single and oral dose of either placebo or lorazepam (0.038 mg kg-1). The stimulus consisted of an outlined diamond presented behind four, otherwise invisible, apertures and translating along a circular trajectory (Lorenceau & Shiffrar (1992). Vision Research, 32, 263-273). Under these conditions, recovering the global diamond direction requires the integration of the component motions available within each aperture. The observers were asked to discriminate the global, clockwise or counter-clockwise, diamond direction under difficult--at high luminance contrasts--or easy--at low luminance contrasts--conditions. Overall, reaction times and error rates increased in the lorazepam group as compared to the placebo group, suggesting strong non-specific effects. However, the changes in performance in the lorazepam group are not homogeneous across conditions, suggesting that lorazepam also induces specific effects that modulate the integration/segmentation balance. Additional experiments performed with visible apertures or visible diamond vertices indicate that the effects of lorazepam are unlikely to reflect a deficit of motion processing or motion integration mechanisms since performance is only slightly impaired in the lorazepam as compared to the placebo group under these conditions. These results suggest that lorazepam might specifically modulate the saliency of line-ends, presumably because processing these features involves inhibitory mechanisms using GABA as a neuromediator, and in turn modify the balance between motion integration and segmentation. (+info)
Eye movements of rhesus monkeys directed towards imaginary targets.
(35/3995)
Is the presence of foveal stimulation a necessary prerequisite for rhesus monkeys to perform visually guided eye movements? To answer this question, we trained two rhesus monkeys to direct their eyes towards imaginary targets defined by extrafoveal cues. Independent of the type of target, real or imaginary, the trajectory of target movement determined the type of eye movement produced: steps in target position resulted in saccades and ramps in target position resulted in smooth pursuit eye movements. There was a tendency for the latency of saccades as well as pursuit onset latency to be delayed in the case of an imaginary target in comparison to the real target. The initial eye acceleration during smooth pursuit initiation elicited by an imaginary target decreased in comparison to the acceleration elicited by a real target. The steady-state pursuit gain was quite similar during pursuit of an imaginary or a real target. Our results strengthen the notion that pursuit is not exclusively a foveal function. (+info)
The effects of ageing on reaction times to motion onset.
(36/3995)
We have measured reaction time (RT) to motion onset in two groups of subjects (average ages: 70 and 29 years), for horizontal gratings of 1 c deg-1, modulated in either luminance or colour (equiluminant red-green), for various contrasts and speeds. For both old and young subjects, RTs depended on both speed and contrast, being faster at high speeds and high contrasts, and showed a stronger contrast dependency for chromatic gratings. The older subjects were systematically slower than the younger subjects. The difference between old and young RTs varied with condition, being 30-40 ms more at the slow than at the fast speed. The relative difference in RTs in different stimulus conditions shows that at least some of the increase in response time with age has a sensory origin. The results relate well to previous work on visual evoked potentials. (+info)
Axis-of-motion affects direction discrimination, not speed discrimination.
(37/3995)
The motion of an object can be described by a single velocity vector, or equivalently, by direction and speed separately. Similarly, our ability to see subtle differences in the motion of two objects could be constrained by either a velocity-based sensory response, or separate sensory responses to direction and speed. To distinguish between these possibilities we investigated whether direction discrimination and speed discrimination were differentially affected by changes in the axis-of-motion. Psychophysical data from 12 naive observers indicated that direction discrimination depended on axis-of-motion, but speed discrimination did not. The difference suggests that a velocity-based sensory response is not the limiting factor on the two tasks. Instead, the results imply that the sensory response which constrains speed discrimination is at least partially independent from the sensory response which constrains direction discrimination. (+info)
The perception and discrimination of speed in complex motion.
(38/3995)
Random dot kinematograms were used to simulate radial, rotational and spiral optic flow. The stimuli were designed so that, while dot speed increased linearly with distance from the centre of the display, the density of dots remained uniform throughout their presentation. In two experiments, subjects were required to perform a temporal 2AFC speed discrimination task. Experiment 1 measured the perceived speed of a range of optic flow patterns against a rotational comparison stimulus. Radial motions were found to appear faster than rotations by approximately 10%, with a smaller but significant effect for spirals. Experiment 2 measured discrimination thresholds for pairs of similar optic flow stimuli identical in all respects except mean speed. No consistent differences were observed between the speed discrimination thresholds of radial, rotational and spiral motions and a control stimulus with the same speed profile in which motion followed fixed random trajectories. The perceived speed results are interpreted in terms of a model satisfying constraints on motion-in-depth and object rigidity, while speed discrimination appears to be based upon the pooled responses of elementary motion detectors. (+info)
Enhanced motion aftereffect for complex motions.
(39/3995)
We measured the magnitude of the motion after effect (MAE) elicited by gratings viewed through four spatial apertures symmetrically positioned around fixation. The gratings were identical except for their orientations, which were varied to form patterns of global motion corresponding to radiation, rotation or translation. MAE magnitude was estimated by three methods: the duration of the MAE; the contrast required to null the MAE and the threshold elevation for detecting an abrupt jump. All three techniques showed that MAEs for radiation and rotation were greater than those for translation. The greater adaptability of radiation and rotation over translation also was observed in areas of the display where no adapting stimulus had been presented. We also found that adaptation to motion in one direction had equal effects on sensitivity to motion in the same and opposite directions. (+info)
Global-motion detection with transparent-motion signals.
(40/3995)
A number of experiments were conducted to compare the ability of observers to extract unidirectional and bidirectional (transparent) global-motion signals. In the unidirectional condition, the noise signal consisted of purely randomly-moving dots while in the bidirectional condition, a number of the randomly moving dots were replaced by the same number of dots moving in a specific (secondary-signal) direction. The threshold measure was the minimum number of signal dots required to determine the global-motion direction. For the bidirectional condition, parameters varied were the angular separation between the global-motion and secondary-signal directions and the strength of the secondary signal. Thresholds for unidirectional and bidirectional conditions were the same when the angular difference between global-motion and secondary-signal directions were 90 degrees or greater, i.e. the ability of observers to extract a transparent signal was the same as their ability to extract a unidirectional one. Similarly, with motion-in-depth signals, thresholds for extracting a centripetal signal were not elevated by replacing a number of the randomly-moving noise dots with the same number centrifugally-moving dots. The results are interpreted as indicating that motion signals moving between 90 and 180 degrees to the global-motion direction provide uniform masking of the global-motion signal. For angular separations less than 90 degrees, a suprathreshold secondary signal resulted in threshold elevation. This result could be due, to stronger inhibition from motion units tuned to similar (< 90 degrees) directions, broad directional-tuning of the underlying motion units (changing the task from signal detection to a signal discrimination) or a combination of the two. (+info)