V1 mechanisms and some figure-ground and border effects.
V1 neurons have been observed to respond more strongly to figure than background regions. Within a figure region, the responses are usually stronger near figure boundaries (the border effect), than further inside the boundaries. Sometimes the medial axes of the figures (e.g., the vertical midline of a vertical figure strip) induce secondary, intermediate, response peaks (the medial axis effect). Related is the physiologically elusive "cross-orientation facilitation", the observation that a cell's response to a grating patch can be facilitated by an orthogonally oriented grating in the surround. Higher center feedbacks have been suggested to cause these figure-ground effects. It has been shown, using a V1 model, that the causes could be intra-cortical interactions within V1 that serve pre-attentive visual segmentation, particularly, object boundary detection. Furthermore, whereas the border effect is robust, the figure-ground effects in the interior of a figure, in particular, the medial axis effect, are by-products of the border effect and are predicted to diminish to zero for larger figures. This model prediction (of the figure size dependence) was subsequently confirmed physiologically, and supported by findings that the response modulations by texture surround do not depend on feedbacks from V2. In addition, the model explains the "cross-orientation facilitation" as caused by a dis-inhibition, to the cell responding to the center of the central grating, by the background grating. Furthermore, the elusiveness of this phenomena was accounted for by the insight that it depends critically on the size of the figure grating. The model is applied to understand some figure-ground effects and segmentation in psychophysics: in particular, that contrast discrimination threshold is lower within and at the center of a closed contour than that in the background, and that a very briefly presented vernier target can perceptually shine through a subsequently presented large grating centered at the same location.
Li
Zhaoping
Z
Department of Psychology, University College London, Gower Street, London WC1E 6BT, UK. z.li@ucl.ac.uk
D000818:Animals; D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D008959:Models, Neurological; D009474:Neurons / Q000502:physiology*; D014795:Visual Pathways / Q000502:physiology*