Spatial Extent of Masking by Sine-Wave Gratings

The contrast threshold for detection of a target consisting of 1.5 periods of a sine wave was determined as a function of the number of cycles in a sinusoidal mask with the same spatial frequency and orientation. The test frequencies were 2, 4, and 8 cycles deg −1 . The masks were spatial-frequency modulated so as to equate their spectral extent. Stimuli were seen monocularly in Maxwellian view at a mean luminance of 10 cd m −2 . The contrast threshold in a backward masking paradigm was determined by a 2AFC staircase. Data were obtained from three subjects with normal vision. It was found that as the number of cycles in the mask was increased, the contrast threshold fell, but only to a certain level. The full range of the threshold decrement was about 2 dB. At all the spatial frequencies tested, the final threshold level was reached with 3 cycles in the mask and then remained unaffected by a further increase in the number of cycles. The results implicate frequency-tuned mechanisms of very restricted spatial extent. It is suggested that these may underlie processing of spatially distributed information at the post-striate stages.