The discrimination of blur in peripheral coloured borders.

Our interest is in measuring the threshold for blur, for long borders at well-defined retinal eccentricities between large fields of colour. Our method is to precisely match a centrally fixated sharp-edged disk of one colour against a surround of another, using the minimum distinct border criterion. We then measure the amount of added Gaussian blur that makes the sharp step-edged border just noticeably different. The selected colours are small fixed chromatic deviations of 500 msec duration from an adaptive low photopic or mesopic white. The effects of retinal eccentricity are striking. Blur thresholds are large for yellow-blue borders centrally (at 1.25 degrees eccentricity) and extremely large for red-green borders peripherally (at 20 degrees). Blur thresholds are generally larger for isoluminous chromatic borders than for a low contrast achromatic border. Accurate Rovamo-Virsu M-scaling is limited to photopic achromatic borders.