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Luke E. Hallum, J. Anthony Movshon; Second-order selectivity of single units in macaque primary visual cortex (V1) and V2. Journal of Vision 2011;11(11):1198. https://doi.org/10.1167/11.11.1198.
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The way humans perceive texture-defined form suggests that 2nd-order stimuli are encoded by mechanisms that receive rectified input from orientation-selective neurons like those in V1. Direct physiological evidence for 2nd-order neurons in primate visual cortex is elusive. The responses of orientation-selective neurons are often modulated by stimuli falling in the receptive field (RF) surround; a variant 2nd-order model uses inhomogeneities in the RF center and surround to create selectivity for form defined by variations in contrast and texture (Tanaka & Ohzawa, 2009). We tested this model by measuring the responses of single units in V1 and V2 of paralyzed, opiate-anaesthetized macaques. We stimulated the RF center and surround with large, contrast-modulated gratings made from a sinusoidal grating carrier modulated by a second, drifting sinusoidal grating of lower spatial frequency. The carrier was of optimal spatial frequency, orientation, and drift rate; the modulator varied across 8 drift directions and 5 or 6 spatial frequencies. We found cells in both V1 and V2 that were selective for modulator orientation and spatial frequency. We used a 2nd-order difference-of-Gaussians model to capture the spatial arrangement of the contrast-responsive center and surround regions of the RF. Cells unselective for modulator structure had classical circular centers with annular surrounds, while others - with modulator selectivity - showed diverse spatial arrangements of the RF center and surround. In many selective cells, the surround was arranged anisotropically relative to the RF center, and explained the modulator selectivity. In others, selectivity appeared to arise from an elongated RF center, in the absence of an anisotropic, suppressive surround. Selectivity for 2nd-order form may not depend on pure 2nd-order neurons, but might arise instead by selective modulation of neuronal responses to conventional 1st-order stimuli.
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