Retinotopic encoding of the direction aftereffect.

Kohn and Movshon [Kohn, A., & Movshon, J. (2003). Neuronal adaptation to visual motion in area MT of the macaque. Neuron, 39, 681-691; Kohn, A., & Movshon, J. A. (2004). Adaptation changes the direction tuning of macaque MT neurons. Nature Neuroscience, 7(7), 764-772] measured the contrast response functions of single neurons in MT (V5) before and after adaptation to high contrast gratings. They found that when gratings were smaller than the MT receptive field, so that adapting and test regions could be either co-localised or non-overlapping, adaptation was spatially specific. This led to the hypothesis that grating adaptation occurs in V1, where receptive fields are small and retinotopically organized, and that MT merely inherits this adaptation. We predicted that spatial specificity would be less for dot stimuli that probably adapt MT cells directly. Also, given recent contradictory claims that hMT primarily exhibits both spatiotopy [d'Avossa, G., Tosetti, M., Crespi, S., Biagi, L., Burr, D., & Morrone, M. (2006). Spatiotopic selectivity of BOLD responses to visual motion in human area MT. Nature Neuroscience, 10, 249-255] and retinotopy [Gardner, J. L., Merriam, E. P., Movshon, J. A., & Heeger, D. J. (2008). Maps of visual space in human occipital cortex are retinotopic, not spatiotopic. The Journal of Neuroscience, 28, 3988-3999], we were interested in producing relevant psychophysical evidence using the direction aftereffect. In three experiments, we measured direction aftereffects (DAEs) induced and tested either with drifting gratings or drifting dots when stimulus location was changed both retinotopically and spatiotopically between adaptation and test; when retinotopic location only was changed; and when spatiotopic location only was changed. We predicted and found that spatial specificity was greater for gratings than for dots. We also found very small spatiotopic effects that call into question some recent claims that area MT exhibits a high degree of spatiotopicity.