Abstract

It is generally believed that the signal-to-noise ratio at the detection threshold is constant for noisy stimuli. We tested whether this is also true when the spatial structure of noise varies. We measured contrast thresholds for vertical cosine gratings embedded in two-dimensional spatial noise. Stimuli were generated under computer control on a RGB monitor using a super VGA graphics board and a video summation device. Gratings were produced by using a constant pixel size of 0.042 × 0.042 cm². The spatial structure of noise was varied by changing the width and/or height of the noise picture element (pel). However, noise spectral density, equal to pel area (pel_x × pel_y) multiplied by noise RMS-contrast squared (cn²), was kept constant. Noise was white in the frequency range of the gratings used, and its spectral density was always great enough to allow us to ignore the internal noise of our visual system. The contrast thresholds were determined by a two-alternative forced-choice algorithm. According to our results, neither pel area nor shape had any effect on contrast thresholds or efficiency as long as noise spectral density did not change. Our experiments showed that, irrespective of noise picture element area or shape, the signal-to-noise ratio at contrast threshold was constant for gratings in spatial noise.

© 1992 Optical Society of America