The coding of spatial position by the human visual system: effects of spatial scale and retinal eccentricity

In this study we investigate the nature of the computations that underlie the encoding of spatial position by the human visual system. Specifically, we explore the relationship between alignment accuracy and retinal eccentricity for stimuli where local luminance, local contrast, and orientation cues do not underlie performance. Spatial scale is especially important for such a comparison because of the well documented spatial inhomogeneity of the human visual field. The results suggest that the relationship between spatial localization and eccentricity is invariant with spatial scale if accuracy and eccentricity are expressed in terms of the stimulus envelope size. We show that the photoreceptor disarray does not determine the limit to performance for this task, the limit is post-receptoral and can be modelled in terms of a positional uncertainty within the early filters located before the response envelope has been extracted. This uncertainty varies with eccentricity in a similar way within each spatial array.