Visual bisection of freely-viewed asymmetrical stimuli

When performing line bisection tasks, neurologically normal individuals exhibit leftward biases. Similarly, normals also exhibit leftward biases during judgements of brightness, numerosity, and size. However, when these same judgement stimuli are visually bisected, this results in rightward bisection biases. These apparently contradictory results are complicated by the fact that bisection tasks are typically performed using single stimuli, whereas judgements of brightness, numerosity, and size are performed on pairs of stimuli. The present study examined the effects of visual bisection on pairs of stimuli. A sample of 34 undergraduate psychology volunteers exhibited leftward biases when making judgements of brightness, numerosity, and size. However, these same participants did not exhibit leftward bisection biases with the same stimuli. Instead, overall rightward bisection errors were observed. These results indicate that although these two types of tasks elicit similar perceptual biases, they are probably the result of different perceptual mechanisms.