Smooth frequency discrimination functions for foveal, high-contrast, mid spatial frequencies

Intensive testing of spatial-frequency discrimination thresholds for 21% contrast gratings between 2.9 and 7.8 cycles/degree (c/deg) reveals, on average, smooth functions conforming to Weber's law. Discrimination overall is very sensitive, indicating highly skilled performance, but there are no consistent peaks or troughs. Individual, single-run functions do vary, resulting in candidate peaks and troughs that might be proposed to reflect true variations in an observer's sensory discrimination capacities. However, these features do not replicate from session to session nor from observer to observer. We do find consistent differences in overall sensitivity level between testing methods, with randomized presentation of reference frequencies giving significantly higher thresholds than blocked reference frequencies.