Eccentric grouping by proximity in multistable dot lattices.

The Pure Distance Law predicts grouping by proximity in dot lattices that can be organised in four ways by grouping dots along parallel lines. It specifies a quantitative relationship between the relative probability of perceiving an organisation and the relative distance between the grouped dots. The current study was set up to investigate whether this principle holds both for centrally and for eccentrically displayed dot lattices. To this end, dot lattices were displayed either in central vision, or to the right of fixation with their closest border at 3 degrees or 15 degrees. We found that the Pure Distance Law adequately predicted grouping of centrally displayed dot lattices but did not capture the eccentric data well, even when the eccentric dot lattices were scaled. Specifically, a better fit was obtained when we included the possibility in the model that in some trials participants could not report an organisation and consequently responded randomly. A plausible interpretation for the occurrence of random responses in the eccentric conditions is that under these circumstances an attention shift is required from the locus of fixation towards the dot lattice, which occasionally fails to take place. When grouping could be reported, scale and eccentricity appeared to interact. The effect of the relative interdot distances on the perceptual organisation of the dot lattices was estimated to be stronger in peripheral vision than in central vision at the two largest scales, but this difference disappeared when the smallest scale was applied.

Synchronising horizontal arm movement with transparent motion.

The relationship between perception and motor performance was studied in a situation that required perceptual processing of a complex motion stimulus in which a target signal had to be segmented, selected, and tracked. Participants were asked to move their arm in synchrony with one surface of a transparent motion display in which two surfaces moved horizontally back-and-forth over each other. The quality of tracking performance was measured as a function of bottom-up and top-down perceptual cues and their interplay. Target signal strength was manipulated by lowering the relative amount of signal dots constituting the target, i.e., the coherence level (100%-50%-30%-10%; the distractor surface was always 100% coherent). A colour cue that distinguished the target from the distractor surface was either available or absent. In the presence of a colour cue, participants experienced little or no difficulties at coherence levels of 50%-100% but when surface formation was complicated by lowering the coherence level, synchronisation consistency decreased. This corresponds with continuous attempts, successful and unsuccessful, to correct inaccurate synchronisation. In the absence of a colour cue, difficulties were frequently observed in all coherence conditions, but they differed depending on the coherence level. Overall, these results suggest that colour can serve as a strong top-down cue for proper target selection and tracking, provided that bottom-up motion signals are sufficiently strong.