A SEARCH FOR THE OPTIMAL STIMULUS

How do stimulus size and item number relate to the magnitude and direc tion of error on center estimation and line cancellation tests? How mi ght this relationship inform theories concerning spatial neglect? Thes e questions were addressed by testing twenty patients with right hemis phere lesions, eleven with left hemisphere lesions and eleven normal c ontrol subjects on multiple versions of center estimation and line can cellation tests. Patients who made large errors on these tests also de monstrated an optimal or pivotal stimulus value, i.e., a particular si ze center estimation test or number of lines on cancellation that eith er minimized error magnitude relative to other size stimuli (optimal) or marked the boundary between normal and abnormal performance (pivota l). Patients with right hemisphere lesions made increasingly greater e rrors on the center estimation test as stimuli were both larger and sm aller than the optimal value, whereas those with left hemisphere lesio ns made greater errors as stimuli were smaller than a pivotal value. I n normal subjects, the direction of errors on center estimation stimul i shifted from the right of true center to the left as stimuli decreas ed in size (i.e., the crossover effect). Right hemisphere lesions exag gerated this effect, whereas left hemisphere lesions diminished and po ssibly reversed the direction of crossover. Error direction did not ch ange as a function of stimulus value on cancellation tests. The demons tration of optimal and pivotal stimulus values indicates that performa nces on center estimation and cancellation tests in neglect are only r elative to the stimuli used. In light of other studies, our findings i ndicate that patients with spatial neglect grossly overestimate the si ze of small stimuli and underestimate the size of large stimuli, that crossover represents an ''apparent'' shift in error direction that act ually results from normally occurring errors in size perception, and t hat the left hemisphere is specialized for one aspect of size estimati on, whereas the right performs dual roles, (C) 1998 academic Press.