a, ON and OFF receptive fields were mapped with light (ON) and dark (OFF) sparse noise and calculated from the response to the stimulus onset (grey shaded area). b, Horizontal penetrations that ran for more than 1.2 mm through a monocular band were assumed to be nearly parallel to ocular dominance columns (top) and those that alternated between monocular responses for left and right eyes were assumed to be nearly orthogonal to ocular dominance columns (bottom). Receptive fields normalized for ocular dominance. Icons on the left illustrate ocular dominance columns for contralateral (C) and ipsilateral (I) eyes (arrow illustrates horizontal penetration). Each receptive field box has a side of 27°.