A Search for the Optimal Stimulus

Abstract

How do stimulus size and item number relate to the magnitude and direction of error on center estimation and line cancellation tests? How might this relationship inform theories concerning spatial neglect? These questions were addressed by testing twenty patients with right hemisphere lesions, eleven with left hemisphere lesions and eleven normal control subjects on multiple versions of center estimation and line cancellation tests. Patients who made large errors on these tests also demonstrated an optimal or pivotal stimulus value, i.e., a particular size center estimation test or number of lines on cancellation that either minimized error magnitude relative to other size stimuli (optimal) or marked the boundary between normal and abnormal performance (pivotal). Patients with right hemisphere lesions made increasingly greater errors on the center estimation test as stimuli were both larger and smaller than the optimal value, whereas those with left hemisphere lesions made greater errors as stimuli were smaller than a pivotal value. In normal subjects, the direction of errors on center estimation stimuli shifted from the right of true center to the left as stimuli decreased in size (i.e., the crossover effect). Right hemisphere lesions exaggerated this effect, whereas left hemisphere lesions diminished and possibly reversed the direction of crossover. Error direction did not change as a function of stimulus value on cancellation tests. The demonstration of optimal and pivotal stimulus values indicates that performances on center estimation and cancellation tests in neglect are only relative to the stimuli used. In light of other studies, our findings indicate that patients with spatial neglect grossly overestimate the size of small stimuli and underestimate the size of large stimuli, that crossover represents an "apparent" shift in error direction that actually results from normally occurring errors in size perception, and that the left hemisphere is specialized for one aspect of size estimation, whereas the right performs dual roles.

Bias in magnitude estimation following left hemisphere injury

“…Patient 2 showed that magnitude estimation is biased following damage to temporo-parietal regions of the left hemisphere even in the absence of behavioral symptoms of neglect. Her data are consistent with previous studies of magnitude estimation in patients without neglect following right hemisphere injury (MacKay, 1963;Mennemeier et al, 1998;Mennemeier et al, 2002;Mennemeier et al, 2005). Patient 2 had an expressive aphasia but we do not think deficits in comprehension or writing account for bias in magnitude estimation because her data fit power functions, indicating response consistency, and her r 2 values were not appreciably lower than either patient 1 or patients with right hemisphere injury who do not have aphasia.…”

“…Power function relationships are altered by right hemisphere injury, particularly in association with unilateral neglect (Chatterjee, Mennemeier, & Heilman, 1992b;Chatterjee, Dajani, & Gage, 1994;Chatterjee, Mennemeier, & Heilman, 1994;Chatterjee, 1995;Mennemeier, Rapcsak, Dillon, & Vezey, 1998;Mennemeier, Vezey, Lamar, & Jewell, 2002;Mennemeier et al, 2005) which follows damage to a neural system involving frontal, cingulate, temporal, and parietal cortices in both hemispheres (Heilman, Watson, & Valenstein, 1994;Mesulam, 1981;Vallar & Perani, 1987;Karnath, Ferber, & Himmelbach, 2001;Karnath, Himmelbach, & Rorden, 2002). When data from bedside tests for neglect are fit to power functions, a restricted range of perception is revealed in neglect patients signaled by a decrease in the power function exponent (a flatter slope) and an increase in the constant (an elevated y-intercept) relative to normal subjects (Chatterjee et al, 1992b;Chatterjee, Mennemeier, & Heilman, 1992a;Chatterjee, 1995;Mennemeier et al, 2002;Mennemeier et al, 2005).…”

Biases in Attentional Orientation and Magnitude Estimation Explain Crossover: Neglect is a Disorder of Both

“…Accuracy varies with line length-short lines being overestimated and long lines underestimated. The point of crossover is also a point at which bisections are most accurate and, as a result, least consistently bisected to one side of true center (Mennemeier, Rapcsak, Dillon, et al, 1998). Replicating previous experiments (Mennemeier, Rapcsak, Pierce, et al, 2001;Mennemeier, Rapcsak, Dillon, et al, 1998), normal subjects in this study were least accurate when bisecting the shortest lines and the direction of errors on short lines was very consistent.…”

“…Crossover, Orientation Bias, and Magnitude Estimation-Crossover has been observed on line bisection and length estimation tasks in both young and old normal subjects (Mennemeier, Vezey, Lamar, et al, 2002;Mennemeier, Rapcsak, Pierce, & Vezey, 2001;McCourt & Jewell, 1999;Mennemeier, Rapcsak, Dillon, & Vezey, 1998;Manning, Halligan, & Marshall, 1990). Crossover also occurs in patients without neglect following right or left brain injury (Mennemeier, Vezey, Lamar, et al, 2002;Mennemeier, Rapcsak, Pierce, et al, 2001;Mennemeier, Rapcsak, Dillon, et al, 1998).…”

“…Crossover also occurs in patients without neglect following right or left brain injury (Mennemeier, Vezey, Lamar, et al, 2002;Mennemeier, Rapcsak, Pierce, et al, 2001;Mennemeier, Rapcsak, Dillon, et al, 1998).…”