The effect of body and environment-centred coordinates on free-viewing perceptual asymmetries for vertical and horizontal stimuli

Leftward and upward perceptual biases are commonly reported for horizontal and vertical stimuli, respectively. It is unclear, however, whether these biases are based upon body or environment-centred coordinates. Two experiments examined the contribution of these coordinate systems to free-viewing vertical and horizontal perceptual biases. In Experiment 1, normal participants (n = 35) made forced-choice luminance judgments on two mirror-reversed luminance gradients (the 'greyscales' task) presented in vertical and horizontal orientations. Body and environment-based coordinates were dissociated by tilting participants' heads to the left or right. A leftward and upward bias, which was observed in the horizontal and vertical conditions (respectively) when the head was upright, was extinguished when the head was tilted. Results indicated a dual reliance on body and environmental coordinates with some suggestion that the upward bias was more dependent on environmental coordinates. In Experiment 2 the same stimuli were presented as participants (n = 24) adopted an upright or supine pose. Once again, leftward and upward biases were observed in the upright condition. The leftward bias persisted in the supine condition whereas the upward bias was eliminated. Results demonstrate that the leftward bias is based predominantly on body coordinates whereas the upward bias is reliant on environmental/gravitational coordinates. The possibility that the neural basis for the biases lies in the inter-modal centres of the intraparietal region of the right hemisphere is discussed.