DETECTING THE ORIENTATION OF SHORT LINES IN THE PERIPHERY

M. P. Davey and J. M. Zanker

Visual information processing in human cortex is based on a highly ordered representation of the surrounding world. In addition to the retintopic mapping of the visual field, systematic variations of the orientation tuning of neurones are described electrophysiologically for the first stages of the visual stream. As a step to understanding the relationship between position and orientation representation, we investigated the minimum spatial requirements for the determination of orientation. In psychophysical experiments the orientation of short lines had to be determined at various positions in the visual field. From earlier experiments we know that the minimum physical length of a line whose orientation can barely be resolved varies with eccentricity. It covaries inversely with the cortical magnification factor, so that the critical length at which a line is perceived as oriented corresponds to slightly less than 0.2 mm of the cortical surface. Looking more closely at the data, it is apparent that along the horizontal meridian horizontal lines are detected with higher precision than vertical or oblique lines. We tested whether this is a preference for horizontal lines as such, of for lines which are oriented radially away from the fovea. Four subjects were tested with lines positioned at two horizontal, two oblique and one vertical meridian at eccentricities between 5 degrees and 25 degrees. While one subject showed no clear pattern of preference, the other three were most sensitive for lines oriented parallel to the meridian of representation. This indicates that the visual system ahs the highest resolution in directions radiating from the fovea, which may be particularly useful for the analysis of flow fields resulting from forward translation.