To the Editor:
A recent study by Tolias et al.1 demonstrated convincingly that neurons in visual area V4 can acquire direction selectivity after adaptation to coherent motion. This is a valuable result because it bears on the role of V4 in motion processing and may also reconcile an apparent discrepancy between neurophysiology and functional imaging of V4 (ref. 2). Moreover, it underscores how dynamic aspects of all types of selectivity can be missed without specific tests. However, the premise of the study, that V4 neurons “are generally not selective for direction of motion”, is not entirely accurate. At least three reports have assessed direction selectivity in V4 quantitatively3,4,5 and have found that roughly one-third of V4 neurons are direction selective (preferred:null response ratio ≥ 2.0 or d-prime ≥ 1.0). A direct comparison5 between V4, MT and area 7a in the same animals and using coherent random dot stimuli showed that MT neurons had a mean preferred:null ratio of 6.4, compared with 1.8 for V4 and 1.7 for area 7a. Thus, although direction selectivity in V4 is not nearly as pronounced as in MT, it is comparable to other areas in the dorsal pathway. One must also consider that in the macaque, V4 is many times larger than MT6 and may therefore contain a comparable number of highly direction-selective neurons even if the density of such neurons is much lower. Although the existence of conventional direction selectivity in V4 does not alter the authors' conclusions about the role of adaptation in stimulus selectivity, we feel that V4 should not be overlooked as a potentially reliable source of conventional motion signals outside of areas traditionally associated with motion processing.
Tolias, A.S., Keliris, G.A., Smirnakis, S.M. & Logothetis, N.K. Nat. Neurosci. 8, 591–593 (2005).
Tolias, A.S., Smirnakis, S.M., Augath, M.A., Trinath, T. & Logothetis, N.K. J. Neurosci. 21, 8594–8601 (2001).
Desimone, R. & Schein, S.J. J. Neurophysiol. 57, 835–868 (1987).
Mountcastle, V.B., Motter, B.C., Steinmetz, M.A. & Sestokas, A.K. J. Neurosci. 7, 2239–2255 (1987).
Ferrera, V.P., Rudolph, K.K. & Maunsell, J.H.R. J. Neurosci. 14, 6171–6186 (1994).
Felleman, D.J. & Van Essen, D.C. Cereb. Cortex 1, 1–47 (1991).
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Ferrera, V., Maunsell, J. Motion processing in macaque V4. Nat Neurosci 8, 1125 (2005). https://doi.org/10.1038/nn0905-1125a