Differential processing: Towards a unified model of direction and speed perception

Abstract

In two experiments, we demonstrate a misperception of the velocity of a random-dot stimulus moving in the presence of a static line oriented obliquely to the direction of dot motion. As shown in previous studies, the perceived direction of the dots is shifted away from the orientation of the static line, with the size of the shift varying as a function of line orientation relative to dot direction (the statically-induced direction illusion, or 'SDI'). In addition, we report a novel effect - that perceived speed also varies as a function of relative line orientation, decreasing systematically as the angle is reduced from 90° to 0°. We propose that these illusions both stem from the differential processing of object-relative and non-object-relative component velocities, with the latter being perceptually underestimated with respect to the former by a constant ratio. Although previous proposals regarding the SDI have not allowed quantitative accounts, we present a unified formal model of perceived velocity (both direction and speed) with the magnitude of this ratio as the only free parameter. The model was successful in accounting for the angular repulsion of motion direction across line orientations, and in predicting the systematic decrease in perceived velocity as the line's angle was reduced. Although fitting for direction and speed produced different best-fit values of the ratio of underestimation of non-object-relative motion compared to object-relative motion (with the ratio for speed being larger than that for direction) this discrepancy may be due to differences in the psychophysical procedures for measuring direction and speed.

Diverted by dazzle: perceived movement direction is biased by target pattern orientation

“…The effect of rigid stripe orientation on direction perception in our experiment is a surprising new finding. Several previous studies have shown that rigid orientation cues within elongated objects or in the background are able to affect direction perception [27][28][29][30][31][32][33]. However, our study is the first to show that orientation cues within the stimulus (as opposed to elongation of the stimulus, or cues placed outside the stimulus) can affect perceived direction.…”

“…For example, the perceived direction of a moving line [27] or a group of moving lines [28,29] can be influenced by line orientation, and the trajectory of a dot moving in the visual periphery can be influenced by the orientation of lines in the background [30]. Similarly, static line cues placed near the stimulus have been shown to influence the perceived direction of random-dot kinematograms [31,32] and the motion of a 'barber pole' stimulus [33]. However, the effect of the orientation of a rigid striped pattern within a target, a type of stimulus that is highly relevant for the study of motion dazzle, has not been investigated.…”

Local biases drive, but do not determine, the perception of illusory trajectories

“…In another view, the perpendicularity bias is a result of Bayesian inference [11][12][13][14] , based on the fact that perpendicular angles are more likely to occur in the visual environment (for instance, trees on the horizon). Lastly, in the differential processing account 15,16 , a perpendicularity bias observed with random dot kinematograms in the context of a single tilted line was successfully modelled on the psychophysical finding that object-relative motion components (perpendicular to the line) contribute more to the perceived dot speed than non-object-relative motion components (parallel to the line) 17 .…”

“…The results showed that the proportional underestimation was similar across different trajectory amplitudes, although significantly larger for the smallest amplitude. The underestimation of amplitudes in the Blank condition can potentially be viewed in the light of the differential processing view on dot motion 15,16 , whereby sensitivity to non-object relative velocities is decreased.…”