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J. A. Movshon, J. R. Cavanaugh, W. Bair; The role of horizontal intracortical connections in “long-range” spatial interactions. Journal of Vision 2002;2(10):101. doi: 10.1167/2.10.101.
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© ARVO (1962-2015); The Authors (2016-present)
In primary visual cortex, as in other cortical areas, neurons are linked by a system of horizontal excitatory connections that extend over distances of 2–8 mm. These connections are said to carry signals outside the “classical” receptive field (CRF), and it is commonly thought that they are responsible for a variety of “long-range” and “feature-linking” effects observed psychophysically.
Previous studies have used a conservative definition of CRF size, the minimum response field (MRF). But MRF measurement misses parts of the CRF that are too insensitive to generate spikes when stimulated alone. We have measured the size of the CRF in macaque V1 neurons using a grating summation technique. On average the MRF underestimates the area of the CRF by a factor of 4 at high contrast. At low contrast, the suppressive surround is weakened and the area of summation increases by an additional factor of 6. Using published visuotopic maps, we projected our measured CRFs onto the cortical surface, and found that the majority have radii that correspond to horizontal cortical distances of 2–6 mm.
We conclude that horizontal intracortical connections do not link regions outside the CRF, but simply serve to construct the CRF itself. Such lateral linking connections are needed to allow convergence from the small and topographically precise RFs of cells in layer 4c to the larger RFs of cells in other cortical layers. Our results suggest that the circuits responsible for psychophysical long-range interactions lie outside primary visual cortex.
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