From neuron to perception: The role of long-range interactions in grouping collinear textures

Purpose. Long-range facilitation and inhibition is found psychophysically (Polat & Sagi, Vis. Res., 1994) and neurophysiologically in humans (Polat & Norcia, Vis. Res., In press) as well as in single cells of cat Area 17 (Mizobe et al, ARVO 1996). We examined the functional organization of long-range interactions in humans and in cat Area 17, paying particular attention to the role of the global configuration of local stimulus elements. Methods. Visual evoked potentials (VEPs) in humans and single cell discharges elicited by elongated Gabor patches were studied using frequency domain stimulation and analysis methods. The Gabor patches were temporally modulated (2-6.6 Hz) and responses were recovered using spectral analysis (see Norcia et al, ARVO 1996). For the elongated Gabors we varied the orientation of the carrier grating inside the Gabor envelope (θ_l) relative to the envelope orientation (θ_g) and produced elongated patches in which θ_g ≠ θ_l. For single cells, we used optimally oriented patches centered on the calsical receptive field (CRF). The patch size and elongation were varied parametrically, so as to stimulate both within CRF and outside the CRF. Results. Responses were maximal for co-axial and co-oriented target/mask combinations (θ_g = θ_l, collinear) in both single-cell and VEP recordings. Perceptually, the elongated collinear patches were perceived as having uniform contrast over the entire patch (e.g. 0.5° by 3°, at 3c/deg) while the patches with orthogonal local/global orientation combinations were not. Conclusions. The present results are consistent with the idea of configuration specific, long-range non-linear interactions, possibly related to the existence of unique mechanisms which underlie our ability to bind collinear textures. These grouping mechanisms may be based on a second-order non-classical elongated interactions field.