Relative localization in the human fovea: radial-tangential anisotropy

A distinction is drawn between two intrinsic directions within a simple spa tial configuration. The line joining two elements is the radial direction a nd orthogonal to it is the tangential direction. Separation and bisection d iscrimination and just-detectable differences in line length are examples o f radial thresholds. Vernier and alignment detection are tangential thresho lds. Neural processing along these two intrinsic directions differs. There is a strong 'oblique effect' for tangential thresholds and virtually none f or radial thresholds. Flank interaction impairs tangential but not radial t hresholds.