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Michael Kubovy, Martin Berg; Oblique effects in grouping: Surprising individual differences. Journal of Vision 2002;2(7):480. doi: 10.1167/2.7.480.
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In our work on grouping by proximity we noticed effects of lattice orientation on grouping by proximity. In a systematic study of this phenomenon, 25 observers (Os) were presented with 30 repetitions of a hexagonal lattice (in which dots are equidistant along three orientations, 120 degrees apart) at all possible orientations. In a phenomenological 4AFC, Os chose one of four grouping orientations. The 900 trials of each O fell by and large into one of three categories: vertical orientations preferred over all others (V), horizontal orientations preferred over all others (H), and vertical and horizontal orientations preferred over oblique ones (VH). In a more detailed analysis we noted that although some Os' choices were stable, some varied considerably, gradually moving from category V to category H, and back. Our findings are consistent with the operation of two independent mechanisms of roughly equal strength involved in the processing of vertical information: (a) A vertical sensitivity mechanism (VSM), which makes us exquisitely sensitive to deviations from the vertical or to misalignments along the vertical. Increasing the activity of VSM would increase the likelihood V trials. (b) A vertical emphasis mechanism (VEM), which makes vertical lines look longer (and produces the V–H illusion). Increasing the activity of VEM would increase the apparent distance between dots along the vertical and decrease the likelihood of V trials. When VSM is stronger than VEM, V-trials will occur; when VEM is stronger than VSM, H-trials will occur; when VSM and VEM are in equilibrium, small and momentary fluctuations in their strengths could either lead to VH data — or to fluctuations over time. [Supported by NEI grant No. R01 EY 12926 to the University of Virginia.]
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