PubMed · 10481187
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M Mon-Williams. 1999. Time to decrease the weight attached to looming?. https://doi.org/10.1016/s0166-2236(99)01460-5
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Functions reliably describing perception of motion in depth have been established experimentally by using psychophysical methods of size and distance estimations and threshold measurements. The stimuli were generated with a new hybrid technique yielding an image refresh rate of 1667 Hz. In this way it was possible to generate rapid expansions and contractions of the moving checkerboard pattern constituting the stimulus for depth motion perception. The results showed that perceived size constancy as well as depth impression varied with oscillation frequency. Under the conditions of slow motions (oscillation frequencies around 2 Hz), perfect size constancy was obtained. Above that limit, size constancy systematically decreased, and with oscillation frequencies of about 5 Hz the perceived size constancy was close to zero when small-sized patterns were used. Under the conditions of wide field stimulation (when the pattern subtended 66 degrees of visual angle), the cut-off limit increased to 16 Hz. Since the perception of depth motion amplitudes as well as perceived velocities of the visual object are related to perceived size constancy, the findings have certain implications for theoretical explanations of depth motion perception.
We measured the maximum disparity grating amplitude (d(max)) for seeing cyclopean surface shape, using stereograms made from dense arrays of micropatterns, whose luminance characteristics were manipulated. In Experiment 1, we used disparity gratings made from Gabor micropatterns. D(max) was found to vary inversely both with luminance spatial frequency and with Gabor size, but was constant for a constant bandwidth (frequency times size). To test whether this was due to changes in bandwidth per se or to changes in the number of local features, in Experiment 2 we manipulated the local feature content with a range of micropatterns that we termed 'edgels'. The results supported neither hypothesis. In Experiment 3 we varied the phases of the Fourier components of square wave edgels, thereby introducing more features, and we found that this did not change d(max). Taken together, our results show that d(max) decreases with an increase in the number of local luminance cycles at each luminance scale. D(max) is mainly limited by false target matching between similar components of the micropatterns. Stereopis, in terms of surface shape perception, is served only by first order mechanisms, and only by luminance filters that are broadband.
Previous work [Prince, S. J. D, & Eagle, R. A. (1999). Size-disparity correlation in human binocular depth perception. Proceedings of the Royal Society: Biological Sciences, 266, 1361-1365] has demonstrated that disparity sign discrimination performance in isolated bandpass patterns is supported at disparities much larger than a phase disparity model might predict. One possibility is that this extended performance relies on a separate second-order system [Hess, R. F., & Wilcox, L. M. (1994). Linear and non-linear filtering in stereopsis. Vision Research, 34, 2431-2438]. Here, a 'weighted directional energy' model is developed which explains a large body of crossed versus uncrossed disparity discrimination data with a single mechanism. This model assumes a population of binocular complex cells at every image point with a range of position disparity shifts. These cells sample a local energy function which is weighted so that energy at large disparities is relatively attenuated. Disparity sign is determined by summing and comparing energy at crossed and uncrossed disparities in the presence of noise. The model qualitatively predicts matching data for one-dimensional Gabor stimuli. This scheme also predicts DMax in Gabor stimuli and filtered noise. Moreover, a range of 'non-linear' phenomena, in which disparity is perceived from contrast envelope information alone, can be explained. The weighted directional energy model presents a biologically plausible, parsimonious explanation of matching behaviour in bandpass stimuli for both 'first-order' and 'second-order' stimuli which obviates the need for multiple mechanisms in stereo correspondence.