Stages of Orientation Coding Assessed by the Tilt Aftereffect

We investigated orientation coding via the spatial-frequency tuning of the tilt aftereffect (TAE). In the single-adaptation condition, subjects adapted to single gratings of 1 or 8 cycles deg−1, 40% contrast, tilted 15° clockwise or anticlockwise from vertical; in two double-adaptation conditions the 1 and 8 cycles deg−1 gratings were superimposed at opposite orientations (‘plaid’ condition) or at the same orientation (‘parallel’ condition). Test gratings of 1, 2, 4, and 8 cycles deg−1, 20% contrast, were presented for 150 ms in an interleaved staircase procedure that measured the TAE by nulling it, hence making a tilted test grating appear vertical. Initial adaptation was for 3 min, topped up for 2 s between test presentations. Results from the single-grating condition indicated broad spatial-frequency tuning of the TAE, since the effect was still strong when tested three octaves away from the adapter. In the parallel condition, the TAEs were around the average of those reported in the single condition. Negligible TAEs were found in the 1+8 cycles deg−1 plaid condition, indicating that opposing adaptations had effectively cancelled each other out. These findings strengthen the suggestion of Olzak and Thomas (1992 Vision Research32 1885 – 1898) that orientation is encoded via an integrative mechanism which pools or sums the outputs of different spatial-frequency channels, and further imply that much of the adaptation responsible for the TAE occurs at this later broad-band stage.