Vernier acuity with compound gratings: the whole is equal to the better ofits parts

To evaluate the relative importance of local feature and spatial filter mod els for hyperacuity, vernier thresholds were determined for gratings consis ting of a fundamental (F-o) and third harmonic (3F) presented alone, and ad ded together in compound gratings where the relative phase offsets of 3F to F-o was 0, 90, 180 and 270 degrees. Thresholds were determined for a range of spatial frequencies of F-o (0.5-16 c deg(-1)) for abutting and non-abut ting stimuli. Compound grating vernier performance was found to be: (i) inv ariant with relative phase offset for the abutting and non-abutting conditi ons; and (ii) predictable from the vernier thresholds for the individual gr ating components making up the compound stimulus. The results support a vie w that supra-threshold components in a multi-frequency stimulus act indepen dently and it is the spatial frequency content, not the local feature chara cteristics, which limit vernier performance. (C) 1999 Elsevier Science Ltd. All rights reserved.