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Noah Z. Schwartz, Bosco S. Tjan; Spatial summation zone for gratings in natural scenes. Journal of Vision 2004;4(8):541. doi: 10.1167/4.8.541.
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Spatial summation has been used to psychophysically assess the size of a functional receptive field. Physiological evidence has shown that the receptive field size of neurons in IT decrease in size when a complex target is presented against a cluttered background compared to the same target presented in isolation (Rolls et al 2003). In the current study, we sought to determine if low-level receptive fields for sine wave gratings are likewise affected by a similar background condition. We measured the spatial summation zone for a gabor patch added to natural scenes using a contrast detection task in a 2IAFC paradigm. Stimuli consisted of an 8cpd grating added to natural, woodland scenes. RMS contrast for all scenes was 23.95% at a mean luminance of 20.5 cd/m^2. Threshold contrast energy of the target gabor at 79% correct detection was measured for 7 space constants (sizes) of the gabor, from 0.5 to 17.7 cycles in equal log steps, and for two conditions: target alone, and target with background. Two subjects (one author) participated. When the 8cpd target gabor was added to a natural scene, contrast detection threshold was elevated by a factor of 6 relative to the target-alone condition for all space constants. The size of the spatial summation zone for both conditions, as determined by a contrast energy vs. space constant curve, was about 0.9 cycles of the sine wave carrier, and revealed no significant difference between the two conditions for our subjects. It would appear from this study that natural scene backgrounds do not affect the functional size of a low-level receptive field. However, this result needs to be reconciled with the previous results that, for a similar detection task, a white noise background was found to cause a reduction in the size of spatial summation zone by a factor of 4 (Kersten 1984), similar to that found by Rolls et al.
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