Mean responses to dark ((A) and (E)) and bright ((B) and (F)) circles (blue traces, (A) and (B)) and annuli (red traces, (E) and (F)) of different sizes presented for 3 s on an intermediate illumination level background, around identified RF centers. Shading here and in ((G) and (H) denotes +/− 1 SEM. (C and D) Mean amplitudes of responses to dark (C) and bright (D) circles, as a function of the radius, R. ** a significant difference between the two means by one-way anova, according to Tukey’s honestly significant difference criterion (D). Error bars denote +/− 1 SEM. (G and H) Comparing predicted responses (black continuous traces) to measured responses (blue continuous traces) to the presentation of a 15° circle (top) and a 20° circle (bottom), assuming linearity, as a sum of responses to circles and annuli of appropriate sizes. Dark stimuli responses (G), bright stimuli responses (H). See also Figure S2.