Contrast Transfer Characteristics of Visual Short-term Memory

A two interval forced choice constant stimuli method was used to determine: (i) the point of subjective equality (PSE); and (ii) the just-noticeable-difference (JND) in contrast for two luminance gratings, one held in short-term visual memory. Psychometric functions for delayed contrast discrimination were determined as a function of spatial frequency from 1 to 8 c/deg, reference contrast from 5 to 60% and inter-stimulus interval from 1 to 10 sec. The PSE for remembered contrast was invariant with spatial frequency and inter-stimulus interval for the three reference contrast levels tested. The JND contrast plotted against spatial frequency followed a U-shaped function with lowest thresholds at around 4 c/deg. The threshold function translates parallel to the sensitivity axis with an increase in either the reference contrast or the inter-stimulus interval. However, the bandpass shape of the threshold function is invariant with both reference contrast and inter-stimulus interval. At 1, 3 and 10 sec inter-stimulus intervals, contrast JNDs increase with reference contrast according to a power law with an average exponent of 0.70. Contrast JNDs also increase as a power function of the inter-stimulus interval with an average exponent of 0.38 for the three reference contrasts tested.