Sources of position-perception error for small isolated targets

Abstract

It has often been reported that, in the presence of static reference stimuli, briefly presented visual targets are perceived as being closer to the fixation point than they actually are. The first purpose of the present study was to investigate whether the same phenomenon can be demonstrated in a situation without static reference stimuli. Experiment 1, with position naming as the task, showed that such a central shift is also observed under these conditions. This finding is of importance because it completes an explanation for central near-location errors in the partial-report bar-probe task. The second purpose of the present study was to provide an explanation for these central shifts. For this explanation information about the exact size of the central shift is required. In Exps. 2, 3, and 4, with cursor setting as the task, it was attempted to assess more precisely the size of the central shifts. These experiments revealed that two different factors determine the results in cursor setting tasks; a factor "target position" and a factor "cursor position." Experiment 5 showed that it is the point of fixation, not the fixation point, that serves, at least in part, as the reference point in this type of task. All the findings together allow us to conclude that the target positions are underestimated by about 10%. From vision research it is known that saccadic eye movements, performed for bringing a target in the fovea, also show an undershoot of about 10%. It is therefore concluded that the system in charge of saccadic eye movements also provides the metric in visual space within a single eye fixation.

The locus of “memory displacement” is at least partially perceptual: Effects of velocity, expectation, friction, memory averaging, and weight

“…Because in the corner fixation condition, the observers looked at a position slightly below the target's trajectory, the reduction of O-displacement also indicates localization toward the fovea. The bias to localize peripheral stimuli toward the fovea has been reported before (e.g., Van der Heijden et al, 1999). In sum, the instruction to look at a corner of the friction surface had strong effects on the pattern of localization.Thus, presentation of a friction surface may have induced the observers in Hubbard (1995aHubbard ( , 1998 to follow the target less accurately than in TO trials.…”

Weighted integration of visual position information

“…Higher foveal weights might have been expected given earlier reports of foveal biases in localization tasks (Mateeff & Gourevich, 1983; O’Regan, 1984; van der Heijden, van der Geest, de Leeuw, Krikke, & Musseler, 1999), and lower foveal detection thresholds (Johnson, Keltner, & Balestrery, 1978). We hypothesized initially that the three subjects with higher peripheral weights may have placed particular emphasis on the dots along the boundary of the stimulus (Findlay, Brogan, & Wenban-Smith, 1993).…”

“…This bias is evident for both single point stimuli (Mateeff & Gourevich, 1983; O’Regan, 1984; van der Heijden, van der Geest, de Leeuw, Krikke, & Musseler, 1999) and for spatially extended stimuli (Stork, Musseler, & van der Heijden, 2010). Notably, this bias increases in magnitude for more spatially extended stimuli (Müsseler, van der Heijden, Mahmud, Deubel, & Ertsey, 1999; Ploner, Ostendorf, & Dick, 2004; but see Kowler & Blaser, 1995), and the gradient of this effect across increasing eccentricities is steeper for spatially extended stimuli compared to single point stimuli (Müsseler et al,1999).…”

Spatial distortions in localization and midline estimation in hemianopia and normal vision

“…Although studies of spatial biases in hemianopia have focused primarily on line bisection tasks, numerous other paradigms have been developed to study peripheral localization (Adam, Ketelaars, Kingma, & Hoek, 1993; Fortenbaugh & Robertson, 2011; Fortenbaugh, Sanghvi, Silver, & Robertson, 2012; Müsseler, van der Heijden, Mahmud, Deubel, & Ertsey, 1999; Temme, Maino, & Noell, 1985; van der Heijden, van der Geest, de Leeuw, Krikke, & Müsseler, 1999) and the application of these paradigms may help to provide further insight into the perceptual processes leading to the HLBE. In particular, given the existence of perceptual biases in neurologically healthy individuals that may be object-based (Orr & Nicholls, 2005), it is of interest to employ other paradigms that assess perceived location in the absence of external objects to determine whether the HLBE represents an expansion of central visual space beyond that observed in neurologically healthy participants under similar experimental conditions.…”