Selective attention to specific location cues: the peak and center of a patch are equally accessible as location cues

Combining cues in contour orientation discrimination

The perceived orientation of a Gabor-patch contour is determined, in part, by shifts in carrier phase between the patches [Popple, A. V. & Sagi, D., 2000. A Fraser illusion without local cues? Vision Research, 40, 873-878; Popple, A. V. & Levi, D. M., 2000a. A new illusion demonstrates long-range processing. Vision Research, 40, 2545-2549; Popple, A. V. & Levi, D. M., 2000b. Amblyopes see true alignment where normal observers see illusory tilt. Proceedings of the National Academy of Sciences of the United States of America, 97, 11667-11672]. Here we show that perceived orientation results from the combination of at least three stimulus cues: (1) patch orientation, (2) contour envelope orientation, and (3) between-patches orientation, which is a function of phase-shifts. In a series of three experiments, we investigated how these three cues were combined. The data are consistent with weighted cue combination.

Pulling the other one: 1st- and 2nd-order visual information interact to determine perceived location

We demonstrate that the 1st- and 2nd-order characteristics of a visual stimulus can have a profound influence on each other in terms of perceived position. We use the parameter of spatial separation to selectively manipulate the effect of one characteristic upon the other. 1st-order features have their largest effect upon the perceived position of 2nd-order structure when separation is small, whilst the reciprocal effect is maximal at large separations. Implications for models of 1st- and 2nd-order interaction are discussed.

Localization of shapes: eye movements and perception compared

The localization of spatially extended objects is thought to be based on the computation of a default reference position, such as the center of gravity. This position can serve as the goal point for a saccade, a locus for fixation, or the reference for perceptual localization. We compared perceptual and saccadic localization for non-convex shapes where the center of gravity (COG) was located outside the boundary of the shape and did not coincide with any prominent perceptual features. The landing positions of single saccades made to the shape, as well as the preferred loci for fixation, were near the center of gravity, although local features such as part boundaries were influential. Perceptual alignment positions were also close to the center of gravity, but showed configural effects that did not influence either saccades or fixation. Saccades made in a more naturalistic sequential scanning task landed near the center of gravity with a considerably higher degree of accuracy (mean error <4% of saccade size) and showed no effects of local features, constituent parts, or stimulus configuration. We conclude that perceptual and oculomotor localization is based on the computation of a precise central reference position, which coincides with the center of gravity in sequential scanning. The saliency of the center of gravity, relative to other prominent visual features, can depend on the specific localization task or the relative configuration of elements. Sequential scanning, the more natural of the saccadic tasks, may provide a better way to evaluate the "default" reference position for localization. The fact that the reference position used in both oculomotor and perceptual tasks fell outside the boundary of the shapes supports the importance of spatial pooling, in contrast to local features, in object localization.

Isolation of stimulus characteristics contributing to Weber's law for position

To examine the independent contribution of various stimulus characteristics to positional judgements, we measured vernier alignment performance for three types of Gabor stimuli. In one, only the contrast envelope of the upper and lower stimulus elements was offset, with the luminance-modulated carrier grating remaining in alignment. In the second, only the carrier grating was offset. In the third, both carrier and envelope were offset together. Performance was examined over a range of element separations. When both cues are available, thresholds for small separations are dominated by carrier offset information and are inversely proportional to carrier frequency. At large separations, thresholds are governed by the spatial scale characteristics of the envelope. For broad-band stimuli such as lines, bars or dots typically used for vernier acuity, their higher frequency content can be used when separations are small, but as separation increases a smooth transition between the scales that determine threshold results in the continuum known as Weber's law for position. That is, with increasing separation, larger scales must be used, and thresholds increase in direct proportion to 1/frequency.

Saccadic localization of occluded targets

Saccadic eye movements are able to localize spatially-extended targets, including patterns of random dots and simple shapes, with a high degree of precision [McGowan, Kowler, Sharma & Chubb (1998). Vision Research, 38, 895-909; Melcher & Kowler (1999). Vision Research, 39, 2929-2946]. This paper investigates the representations of object shape that guide saccades. We studied saccadic localization of partially-occluded triangles (two or three vertices removed) to find out whether saccades have access to a representation of the full shape, despite the missing portions. Targets were configured so that they could be seen either as triangles, which were partially occluded by polygons, or as fragments in front of the same polygons. Subjects tried to saccade to the inferred full triangle and a discrimination paradigm was used to evaluate their success. Occlusion cues were ineffective in that saccades directed to the occluded triangles landed near the center of the visible fragment, even when it was configured as a triangle behind occluders. Removing the occluders and leaving only three segments of the triangle (vertices removed) helped somewhat, but performance never resembled that achieved with either a full triangle or a 3-dot configuration. We conclude that the saccadic system is insensitive to at least some cues that can be used to infer the shape of objects. For occluded targets, the representation used by saccades may be closer to the configuration of the retinal image.