Academic journal article Perception and Psychophysics

Hemispatial Asymmetries in Judgment of Stimulus Size

Academic journal article Perception and Psychophysics

Hemispatial Asymmetries in Judgment of Stimulus Size

Article excerpt

Recent research has demonstrated a leftward bias in judgments of size. In the present experiments, hemispatial size bias was measured through simultaneous presentation of a circle and an ellipse varying in horizontal or vertical extent. A consistent leftward bias of horizontal size judgments (but not vertical) was obtained; at the point of subjective equality, the width of the objects that were presented in left hemispace was smaller than the width of the objects that were presented in right hemispace. These data suggest that the horizontal extent of stimuli appear larger in left hemispace than in right hemispace. Results also indicated that symmetrical stimulus presentation, with respect to the vertical meridian, is required for the bias to emerge. Furthermore, increasing or decreasing stimulus eccentricity weakened the effect. Attenuation of this bias upon the manipulation of parameters indicates that this phenomenon is context specific and is affected by similar parameters that are known to influence the magnitude of error in pseudoneglect.

Asymmetries in perception are frequently demonstrated in patients suffering from hemispatial neglect. Neglect patients typically exhibit a perceptual deficit to the left, following a lesion to the right parietal cortex. Patients suffering from unilateral hemispatial neglect fail to attend to or report stimuli that are presented in the visual field contralateral to their lesion, despite intact visual processing and visual acuity. Neglect can manifest itself in various forms, and consequently, patients are often unaware of one half of space (Rapcsak, Watson, & Heilman, 1987) or of one side of an object (Driver & Halligan, 1991). For instance, in horizontal line bisection tasks that are commonly used in the screening of neglect, patients tend to place their midline mark to the ipsilesional side of true center because of a failure to respond to-or the neglect of-the leftward extent.

A similar pattern of response, termed pseudoneglect (Bowers & Heilman, 1980), has been demonstrated in normal observers. In this instance, however, observers tend to overestimate or over attend to objects presented to the left, resulting in comparative "neglect" of the right. Research suggests that this asymmetry is brought about by the dominance of the right hemisphere in mediating spatially directed attention (Foxe, McCourt, & Javitt, 2003; Heilman, Bowers, Valenstein, & Watson, 1987; Kinsbourne, 1970; Mattingley, Bradshaw, Nettleton, & Bradshaw, 1994). The effect is not as marked as the asymmetry found in neglect patients; however, findings consistently reveal the tendency for normal observers to overestimate the leftward extent of a stimulus and to bias their response toward the leftward features. For instance, in line-bisection tasks, nonclinical participants tend to overestimate the leftward extent of a horizontal line (underestimating the length of the right side in comparison) and bisect the line slightly to the left of its true center (see, e.g., Jewell & McCourt, 2000; Luh, 1995; McCourt & Jewell, 1999; Milner, Brechmann, & Pagliarni, 1992).

Reports of this phenomenon within normal observers have recently extended to judgments of stimulus luminosity, numerosity, and size (Nicholls, Bradshaw, & Mattingley, 1999). Nicholls and colleagues used the grayscales task (see Mattingley et al., 1994), in which participants are asked to make a relative judgment of the brightness of two left-right mirror-reversed luminance gradients. Each luminance gradient (or "grayscale") was presented as a horizontal bar that changed incrementally from white on one side to black on the other. Two luminance gradients were presented on each trial, one above the display screen's center and the other below. Researchers have previously shown that when asked to choose which stimulus is darker overall, participants tend to choose the grayscale that is dark on the left-hand side, even though both stimuli are actually of equal luminosity (Mattingley et al. …

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