112 research outputs found
The role of part structure in the perceptual localization of a shape
No full textThe process of object localization may be accomplished with respect to a particularreference location, such as the center of gravity, COG (eg Vishwanath and Kowler, 2003 VisionResearch 43 1637-1653). Here, we investigated how part structure affects an object's referencelocation. The reference location was evaluated with a measure of the illusory displacement of an internal target element embedded within a larger object (Morgan et al, 1990 Vision Research 30 1793-1810). To examine whether the reference location is different for shapes with part structure, two shapes were tested: circle (small and large; no part structure) and bell (shape with two parts, one larger than the other). Results were examined with respect to two predictions: either the location of an object is based on its shape as a whole, disregarding part structure (ie a single, overall COG), or the parts are processed separately (different COGs).With the circles, the results showed a systematic illusory displacement of the internal target toward the COG. With the bell, the illusion was significantly weaker than with both circles--even though the main part of the bell had the same size as the small circle, and its horizontal axis had the same extent as the large circle. Moreover, the distance judgments for the bell were consistent with a (weaker) reference point being located at the COG of the larger part, rather than at the COG of the entire bell. These results show that the part structure of a shape plays a role in the representation of its location, and that for complex shapes the perceived location of an embedded element depends more on the parts within which it is embedded, rather than on the whole shape.Supported by the Air Force Office of Scientific Research, Grant AF 49620- 02-1-0112, Life Sciences Directorate to Eileen Kowler, and by NSF, Grant BCS-0216944 to Manish Singh.AF 29620-02-1-0112; to Eileen KowlerNSF BCS-0216944; to Manish SinghDenisova, Kristina, Manish Singh, Eileen Kowler, 2006. The definitive, peer-reviewed and edited version of this article is published in Perception, 35, 1073-1087, DOI:10.1068/p5518
The relationship between spatial pooling and attention in saccadic and perceptual tasks
No full textAbstractSaccades aimed at spatially extended targets land reliably at central locations determined by pooling information across the target shape [Melcher, D., & Kowler, E. (1999). Shape, surfaces and saccades. Vision Research, 39, 2929–2946; Vishwanath, D., & Kowler, E. (2003). Localization of shapes: Eye movements and perception compared. Vision Research, 43, 1637–1653]. Previous findings of saccadic errors when attempting to look at a target in the midst of distractors encouraged suggestions that pooling occurs indiscriminately, with little or no influence of a selective filter to eliminate the influence of nearby distractors. To determine the effectiveness of filtering, saccadic localization was studied for saccades made to a set of target elements (discs) interleaved with an equivalent set of distractors of a different color. With such interleaved elements, selection and spatial pooling are constrained to occur over the same spatial region. The results showed that filtering was effective and saccadic landing position was determined mainly by the target elements. Concurrent perceptual judgments made about the same stimuli (estimating the mean size of either target or distractor discs) showed better performance for the target discs than distractors, confirming that perceptual attention was allocated to the set of target elements. These results: (1) support the role of attention in setting the input to the spatial pooling process that guides saccades to spatially extended targets, and (2) show that perceptual judgments of mean value, often thought to impose modest attentional demands, are not immune to the constraints of this pre-saccadic filter
Saccadic localization in the presence of cues to three-dimensional shape
No full textSaccades directed to simple two-dimensional (2D) target shapes under instructions to look at the target as a whole land near the center of gravity (COG) of the shape with a high degree of precision (He Kowler, 1991; Kowler Blaser, 1995; McGowan, Kowler, Sharma, Chubb, 1998; Melcher Kowler, 1999; Vishwanath, Kowler, & Feldman, 2000). This pattern of performance has been attributed to the averaging of visual signals across the shape. Natural objects, however, are three-dimensional (3D), and the shape of the object can differ dramatically from its 2D retinal projection. This study examined saccadic localization of computer-generated perspective images of 3D shapes. Targets were made to appear either 2D or 3D by manipulating shading, context, and contour cues. Average saccadic landing positions (SDsimilar to10% eccentricity) fell at either the 2D or 3D COG, and occasionally in between, depending on the nature of the 3D cues and the subject. The results show that saccades directed to objects are not compelled to land at the 2D COG, but can be sensitive to other visual cues, such as cues to 3D structure. One way to account for these results, without abandoning the averaging mechanism that has accounted well for performance with simple 2D shapes, is for saccadic landing position to be computed based on averaging across a weighted representation of the shape in which portions projected to be located at a greater distance receive more weight.</p
Attention during sequences of saccades along marked and memorized paths
No full textAbstractNatural scenes are explored by combinations of saccadic eye movements and shifts of attention. The mechanisms that coordinate attention and saccades during ordinary viewing are not well understood because studies linking saccades and attention have focused mainly on single saccades made in isolation. This study used an orientation discrimination task to examine attention during sequences of saccades made through an array of targets and distractors. Perceptual measures showed that attention was distributed along saccadic paths when the paths were marked by color cues. When paths were followed from memory, attention rarely spread beyond the goal of the upcoming saccade. These different distributions of attention suggest the involvement of separate processes of attentional control during saccadic planning, one triggered by top-down selection of the saccadic target, and the other by activation linked to visual mechanisms not tied directly to saccadic planning. The concurrent activity of both processes extends the effective attentional field without compromising the accuracy, precision, or timing of saccades
Eye movements during multiple readings of the same text
No full textAbstractPeople often read the same text more than once. Studying eye movements during multiple readings of the same texts provides a unique opportunity to observe the consistency of saccadic landing positions. Eye movements were recorded while 5 people read the same 4 texts more than 40 times, no more than 4 times/day, and never on consecutive trials. Other texts, read only once, were interspersed. Comprehension questions and a change-detection task helped maintain attention in the face of the repetition. There were two main findings: (1) repeated reading produced significant, but modest, changes in global saccadic patterns. The only change found in all readers was a reduction in the proportion of regressions. (2) Saccadic landing positions fell into clusters located at a variety of places with respect to word boundaries, and often across word boundaries. A mixed-strategy model of saccadic guidance (look to the center of words, while trying to maintain fairly uniform saccade lengths), could account for the overall strength of clustering, but not for the variability among cluster locations, suggesting that saccadic landing sites are selected in part on the basis of local text characteristics. The reliable clustering of saccadic landing positions found during multiple readings of the same text opens the way for cluster patterns to be used to study eye movement strategies during reading and overcome at least some of the variability associated with traditional global single-text measures
Supplemental figures and tables for "Bayesian approaches to smooth pursuit of random dot kinematograms: effects of varying RDK noise and the predictability of RDK direction"
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