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Research concerning the perception of apparent motion is not easy to conduct: it is hard to obtain quantitative results that can be easily interpreted. A solution to this problem is the use of motion aftereffects (MAEs). Adapting subjects to a specific type of motion leads to apparent motion in the opposite direction when the stimulus is removed. However, subjects are aware of the change in stimulus conditions. A new dynamic test stimulus is proposed in order to avoid artefacts introduced by the awareness of the conditions by the subject. A model, derived from earlier observations, is described which includes contributions from monocular and binocular systems. Results from an experiment in which the dynamic test stimulus was used show that they do not necessarily reproduce the results obtained with a static test stimulus. Central monocular systems are added to the model to account for this discrepancy. The ‘pooling hypothesis’, which states that the MAE is a weighted mean of the processes involved, permits the estimation of the weights of the individual subsystems. The results of the experiments are explained in terms of this hypothesis by the new model.

Contains fulltext : 29107.pdf (Publisher’s version ) (Open Access) Assimilation and contrast have opposite effects: Contrast leads to an increase of perceived differences between neighbouring fields, whereas assimilation leads to a reduction. It is relatively easy to demonstrate these effects, but the precise localisation of these effects in the perceptual system is not yet possible. In an experiment the strength of assimilation effects was modified by adding spatial noise. By varying the localisation in perceived space of the added noise (by presentation of the noise pattern with different binocular disparities) the masking effect of this noise can be influenced. Masking caused by binocularly disparate noise is less than masking caused by binocularly non-disparate noise. It is concluded that the effect at least partly occurs beyond the (binocular) locus of separation in different depth planes. A similar approach, involving moving noise, is also presented. Finally, several demonstrations show that images that are peripherally similar can give rise to differences in the perceived amount of assimilation. These effects further indicate that a central mechanism is involved in assimilation.

There has been evidence for the existence of a purely binocular system in human vision that acts as an AND gate on information from the two eyes. There also has been evidence for the nonexistence of such a purely binocular system, indicating only the existence of an OR-type binocular system that responds to input from one or both eyes. As a result there are a number of possible explanations for the differing experimental results: the binocular system is an OR-type system only, it is a facilitating OR system that has AND-type characteristics, or it consists of independent OR and AND subsystems. Monocular adaptation, alternating monocular adaptation, or binocular adaptation were used to demonstrate the existence of the different systems, but in none of the previous experiments was the AND-type binocular system activated directly, and the existence of this AND system was deduced mostly because of differences in aftereffect strengths between monocular and binocular test conditions. Experiments are reported in which stimuli that activate the AND-type binocular system explicitly have been used, and the results show that we need the existence of such an AND-type binocular system to account for the results.

According to Kanizsa (1979), “The modification of brightness in the typical subjective contour stimuli is not the cause, but a consequence of the particular spatial configuration that the perceptual field is forced to assume” (p. 204). Recent data from Cavanagh (1986) expresses an alternative viewpoint. Although Cavanagh is not particularly arguing against Kanizsa’s point of view, his finding that isoluminant stimuli do not evoke subjective contours can be used as a valid counter argument against the view of brightness change as a consequence rather than a cause. The lack of strong subjective contours with isoluminant stimuli has been reported several times (Brigner & Gallagher, 1974; Brussell, Stober, & Bodinger, 1977; Frisby & Clatworthy, 1975; Gregory, 1977).