A High Spatial Frequency Mask Can Change The Shape Of Eccentric Spatial Summation Functions

Abstract

Abstract: : Purpose: Previously (ARVO 2001) we demonstrated that a contiguous luminance pedestal can alter the shape of chromatic spatial summation functions, revealing a second region of complete summation which we interpreted in terms of a second spatial mechanism revealed by the masking effects of the high spatial frequencies of the edge of the pedestal. Since the pedestal size varied with the stimulus size, it is difficult to exclude other factors such as the the effect of non-uniform adaptation state and/or the lower spatial frequency components of the pedestal. In the current experiment, we used noise masks which provide uniformly distributed adapting luminance and have fixed spatial frequency components. Methods: 500 msec equiluminant chromatic (L-M) and luminance pulses in an equal energy white (EEW) background were displayed on a 21" Sony Trinitron monitor driven by a VSG2/5 video controller. Spatial summation functions were measured at 9.5° eccentricity for stimuli ranging from 0.5° to 5.7° in diameter. The data were gathered under three conditions: with a uniform background, with a 2-D luminance white noise mask (pixels 2 min arc on a side) or with a 2-D L-M binary noise mask (pixels 6 min arc on a side). For the noise masks, each pixel was randomly selected around the EEW point along the either luminance axis or the L-M axis. The contrast for the luminance noise was 48% (in L+M units) and the contrast for the L-M noise was 16% (in L-M units). The background subtended 27° by 21° with a mean luminance of 50 cd/m₂. Two observers with normal vision served as subjects. Results: For the luminance pulses, the luminance noise mask could increase the linear summation region without altering the shape of the spatial summation functions. For the L-M pulses, with both the luminance and the L-M noise masks, the shape of the spatial summation functions was altered and a second region with complete summation was revealed. These results are similar to the results with the pedestal and are consistent with the hypothesis that multiple spatial mechanisms underlie the spatial summation functions. Conclusion: Multiple mechanisms tuned to different spatial frequencies may mediate peripheral spatial summation functions.

Keywords: 362 color vision • 347 chromatic mechanisms