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Anita M. Schmid, Ferenc Mechler, Ifije Ohiorhenuan, Keith P. Purpura, Jonathan D. Victor; Processing of orientation discontinuities in space and time in V1 and V2. Journal of Vision 2008;8(6):228. doi: 10.1167/8.6.228.
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© ARVO (1962-2015); The Authors (2016-present)
Neurons in primary visual cortex (V1) respond to luminance borders. Neurons in secondary visual cortex (V2) also respond to texture borders, but the extent of the differences between V2 and V1 and the ways in which new properties arise in V2 are as yet unclear.
To address these questions, we probed spatial and temporal nonlinearities in single neurons in V1 and V2 of anesthetized monkeys. The stimulus was a 4 by 5 grid of adjacent rectangular regions, covering the classical and non-classical receptive field. Each region contained sinusoidal gratings with one of two orthogonal orientations, controlled by an m-sequence. To disambiguate second-order interactions, we used a novel approach: on interleaved runs, regions were assigned differently-spaced “taps” into the m-sequence.
In response to this stimulus, first-order kernels of neurons in V1 are monophasic. In contrast, V2 neurons have temporally biphasic responses: first positive, then negative. That is, a higher firing rate is elicited by the preferred grating following an orthogonal grating than by continuous presentation of the preferred grating.
A second-order interaction driven by a change of orientation in time was present, but this signal was opposite in V1 and V2: in V1, responses were augmented when the same orientation was presented on successive frames, but in V2, responses were augmented when orientation changed. A spatial second-order interaction was also present: in V1, it was monophasic and corresponded to cross-orientation suppression; in V2, this component was present but in addition a later opposing component existed, indicating response augmentation by orientation differences.
Thus, V2 provides a signal for orientation discontinuities over time as well as space. Whereas all of the interactions in V1 can be explained by threshold nonlinearities, the opposing interaction components in V2 require a nonlinearity that follows a spatial and temporal differencing operation in orientation space.
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