Detection and discrimination of simple and complex patterns at low spatial frequencies

Abstract These experiments examined the extent to which low spatial frequencies are processed independently. The assessment was carried out with respect to both detection and discrimination performance. For simple sinusoidal gratings, pairs of stimuli could be discriminated when their contrasts reached threshold, if the ratio of their spatial frequencies was 3:1 or larger, suggesting idependent processing in separate channels. For smaller frequency ratios, slightly more contrast was required for discrimination than for detection, suggesting that stimuli were not processed by entirely separate channels. The detection and discrimination thresholds of complex grating stimuli fell within the ranges which would be expected if probability summation effects and summation of different closely spaced harmonic frequencies within single channels are considered, supporting the hypothesis of independent processing of low-spatial frequency information. The single exception to this involved discrimination of a square wave and a square wave with its fundamental component removed. In this case, discrimination required considerably more contrast than detection, even when factors of probability summation and within-channel summation of harmonics are considered.