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J. Zheng, B. Zhang, H. Bi, I. Watanabe, Y. Lin, E.L. Smith, III, Y. Chino; Effects of Receptive–Field (RF) Surround Stimulation on Orientation Tuning of V1 and V2 Neurons in Macaque Monkeys . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5365.
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It is a matter of considerable debate as to how the sensitivity of cortical neurons to stimulus orientation emerges. We previously reported that the response properties of V1 and V2 neurons are influenced by stimulating large areas surrounding their classic receptive–fields (Zhang et al, 2005). In this study we determined whether stimulation of RF surrounds improves the orientation selectivity of V2 neurons, and if so, whether the nature and degree of this sharpening effect in V2 are similar to those in V1.
Microelectrode recording experiments were conducted in two anesthetized and paralyzed normal adult monkeys. After optimizing the spatial frequency and orientation of sine wave gratings (diameter = 4.5, TF = 3.1 Hz, contrast = 80%) for each unit, we obtained area summation functions to determine the extent of unit's RF center and surround. The orientation/direction tuning functions were obtained for individual V1 and V2 neurons by drifting gratings in 12 directions over RF center with or without RF surround. The orientation selectivity of each unit was determined by calculating circular variances and by using a traditional method of calculating bandwidths of tuning functions.
Surround stimulation significantly improved the orientation tuning of the majority of both V1 and V2 neurons. The magnitude of this improvement was similar in each unit for the two analysis methods employed both in V1 and V2. However, the overall sharpening effects of surround stimulation were greater in V2 compared to V1.
These results suggest that the orientation tuning was narrower with surround stimulation primarily because the units responses to non–optimal orientations were suppressed by long–range signals via intrinsic or feedback connections and that these surround effects appear to be mediated by similar, but not identical, mechanisms in V1 and V2.
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