The effects of eccentricity and separation on positional localization bias

Previous work has shown that outside one degree of fixation, positional accuracy depends on the ratio of stimulus separation to eccentricity. Here, we examined the effects of this ratio on bias. The task was free localization comprising dual bisection and alignment of stimuli positioned on an isoeccentric arc. Stimuli were viewed peripherally at 32 locations (4 eccentricities x 8 axes), and judgements were made in the diagonal, horizontal and vertical direction relative to the reference, yielding three separation/eccentricity ratios (0.78, 1.85, 2.00). Absolute error (accuracy), and signed error (bias) in Cartesian and polar coordinates were measured. Results: The ratio of separation to eccentricity influenced both accuracy and bias, with stronger effects on bias, particularly on the bisection judgement. Stimulus separation determined the sign and magnitude of bias, with eccentricity affecting bias only at large separations.The combined spatial map revealed a pattern of bias in which responses at large absolute separations were pushed toward the map centre and those at small separations were pushed away. Polar angle bias resembled reference repulsion effects and varied with separation. The results suggest that positional accuracy and bias covary with the separation/eccentricity ratio, with an increasing interdependence at large ratios.