The figure shows a series of receptive fields mapped with dark (OFF) and light stimuli (ON) and the ON–OFF receptive field difference. The last receptive field on the right for each row shows the average of all receptive fields across 0.8 mm of cortical distance. The plot on the right shows the retinotopy of the ON (red) and OFF (blue) receptive fields. Cortical regions where OFF retinotopy rotated around ON retinotopy were more difficult to find than regions where ON retinotopy rotated around OFF retinotopy. To estimate the relative frequency of ON and OFF retinotopy rotations, we measured the distance between the retinotopic centre of mass of single horizontal penetrations for each ON or OFF receptive field (81 penetrations with receptive field measurements from at least five recording sites per penetration). We then calculated a ratio of the average distances, as (ON − OFF)/(ON + OFF), and used a ratio of 0.5 as an arbitrary threshold to classify a penetration as OFF-anchored (ON rotates around OFF) or ON-anchored (OFF rotates around ON). Based on this criterion, there were 3.75 more OFF-anchored than ON-anchored penetrations (15 versus 4 penetrations, respectively; n = 17 animals). Each square box framing a receptive field has a side of 19.4°.