Displacement Discrimination Research Articles

Residual vision in patients with retrogeniculate lesions of the visual pathways.

Twenty-five subjects have been studied who, as a result of damage to the striate cortex, were 'blind' in extensive areas (scotomata) of the visual fields. Of these 25 subjects, 5 exhibited residual vision in response to transient lights presented within the scotoma, which enabled them to locate the stimuli by hand-reaching or by eye movements; the latter have been measured by electro-oculography. The residual vision underlying their responses was characterized by low flicker-fusion and by sensitivity in detection of movement which increased as target speed was raised. Discrimination for the direction of target movement was poor, but spatial resolution in the discrimination of target displacement was essentially normal. The subjects were unable to recognize or discriminate the spatial structure of targets located within the 'blind' field, and the observed dissociation between spatial discrimination of displacement and pattern is examined in relation to the 'two systems' hypothesis of visual function. There is no obvious correlation between the extent of neuronal damage as revealed by CT scans and the existence of residual vision.

Scale invariant features of differential spatial displacement discrimination

For a configuration of three blobs, with Gaussian spatial contrast profiles, at threshold luminance contrast, differential spatial displacement discrimination thresholds were determined. The blobs were arranged one above the other, at equal spatial intervals. Thresholds were determined for displacements of the middle blob both orthogonal to and along the axis joining the outer two blobs. Thresholds for both tasks were obtained as a function of both the resolution and the separation of the blobs. The thresholds for both tasks increased linearly with decreasing resolution (increasing blur), for a constant ratio of the resolution parameter and the separation of the outer two blobs. At all levels of resolution there are two blob-separation regimes, in which different strategies are used to compute differential spatial displacements. Independent of the level of resolution, transition between those regions occurs when the separation of the outer two blobs is a constant multiple (approx. 25) of their blur parameter. We interpret these results as further evidence for a scale-invariant mechanism for differential spatial displacement computation.

Mechanisms of displacement discrimination with and without perceived movement.

A nearby visual reference point improves displacement discrimination. This effect occurs regardless of the delay between successive displays. The delay can be varied from short delays (50 ms), where observers report perceived movement, to long delays (2,000 ms), where observers report no movement. The similarity of reference effects for such different delay conditions was investigated by testing the independence of reference and delay effects. The critical independence property is whether the spatial variables (displacement and separation from the visual reference) combine independently of the temporal variable (delay). This kind of independence did not hold for displacement discrimination at delays of 50 ms and 2,000 ms. A further experiment tested for the independence property at delays of 500 ms and 4,000 ms, where observers reported no perceived movement. For these longer delays, the property was satisfied. These results are discussed using the general concept of a psychophysical mechanism. Such a mechanism combines the effects of several stimulus variables into a single mediating representation. By this analysis, at least two mechanisms must mediate the reference effect on displacement discrimination, one for delays of 200 ms and less and another for delays of 500 ms and more.

Mechanisms of displacement discrimination with a visual reference

A nearby visual reference point facilitates displacement discrimination. For example, a nearby stationary point of light improves discriminating left or right displacement of a point by several fold. This reference effect interacts with the temporal characteristics of displacement. Discrimination thresholds rise with increases in the delay between the offset of the initial stimulus and the onset of the displaced stimulus. Moreover, the effect of a nearby reference point is larger with a long delay than with a short delay. This interaction was investigated using two nonexclusive hypotheses of the mediating mechanisms. A single-signal hypothesis specifies a single mediating mechanism that aggregates the effect of displacement and delay. A single-noise hypothesis specifies a single mediating mechanism that aggregates the effect of reference and delay. Each of these hypotheses predicts an equivalence property similar to the equivalent background principle of adaptation research. The equivalence required by the single-signal hypothesis was not satisfied, disconfirming the hypothesis. In contrast, the equivalence required by the single-noise hypothesis was satisfied. The equivalence was extended to delays where the reported judgment was of perceived position and not of perceived movement. The second result is compatible with displacement discrimination mechanisms that are the same function of displacement, whatever their other differences.

Mechanisms of displacement discrimination with and without perceived movement.

Mechanisms of displacement discrimination with and without perceived movement.

Detection of changes in spatial position: Short-term visual memory or motion perception?

The limiting conditions for short-term visual memory were explored for materials which are not readily codable. Two fields of random dots were presented successively in time. On half of the observations, the second display was identical to the first, and on the other half, a fraction of the dots was displaced relative to the first display. The task of the S was to indicate if any of the dots had moved (displacement detection), and in separate tests, if any of the dots had not moved (stationarity detection). Stationarity and displacement detection are largely related to the same variables, although performance is somewhat better with displacement detection. Discrimination of small displacements is critically dependent upon the interval between displays, with maximum accuracy in the region of 64 msec for briefly presented displays. Maximum accuracy is obtained under conditions which yield good apparent motion. By contrast, displacement discrimination is, within limits, relatively independent of the number of dots displayed, of the fraction of displaced dots.