An investigation of the relationship between free-viewing perceptual asymmetries for vertical and horizontal stimuli.

Two experiments examine the relationship between free-viewing vertical and horizontal perceptual biases. In Experiment 1, normal participants (n=24) made forced-choice luminance judgments on two mirror-reversed luminance gradients (the 'grayscales' task). The stimuli were presented in vertical, horizontal and oblique (+/-45 degrees ) orientations. Leftward and upward biases were observed in the horizontal and vertical conditions, respectively. In the oblique conditions, leftward and upward biases combined to produce a strong shift of attention away from the lower/right space toward the upper/left. Regression analyses revealed that the oblique biases were the combined product of the vertical and horizontal biases. A lack of correlation between the vertical and horizontal biases, however, suggests they reflect the operation of independent cognitive/neural mechanisms. In Experiment 2, the same stimuli were given to right-hemisphere-lesioned patients with spatial neglect (n=4). Rightward and upward biases were observed for horizontal and vertical stimuli, respectively. The biases combined to produce a strong shift of attention away from the lower/left space toward the upper/right. While our research demonstrates that vertical and horizontal attentional biases are additive, it also appears that they reflect the operation of independent cognitive/neural mechanisms. Potential applications of these findings to the remediation of spatial neglect are discussed.