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Simulated spatial summation functions for single spatial mechanisms with the 2 cpd DN filters (a), 2 cpd DoG filters (b), and the weakly orientation-tuned D1 filters with four different peak spatial frequencies (c). Sensitivities for the smallest stimulus were normalized to –3.0. The functions could be described by a line with a slope of 1 for small stimuli and with lines of varying slopes for large stimuli.
