The global shape detection deficit in strabismic amblyopia: Contribution of local orientation and position

We extended a previous study (Hess et al. (1999). A deficit in strabismic amblyopia for global shape detection. Vision Research, 39, 901–914) where it was claimed that strabismic amblyopes exhibit a deficit for the detection of continuous radial frequency patterns, a task that is thought to involve global processing and in particular, a contribution of extra-striate area V4. We confirm this previous report using a novel Gabor-sampled stimulus and show that the deficit for the amblyopic eye occurs across a range of circular contour frequencies, that is the number of radial cycles per circular contour length in degrees. By arranging the Gabor-samples to coincide with either the peaks/troughs or zero-crossings of the radial modulation, we were able to tease apart the relative contributions of local position and local orientation respectively to the shape processing deficit. The deficit for the amblyopic eye involves both anomalous position and orientation coding with the latter being more affected than the former. While this suggests that ventral extra-striate processing is anomalous, it leaves open the possibility that the striate input may be responsible.