Abstract
V1 neurons have been observed to respond more strongly to figure than
background regions. The responses are stronger near figure boundaries (the border effect)
than within figure surfaces further inside the boundaries
(the surface effect). Sometimes the medial axes of the figures induce a secondary,
intermediate, response peak (the medial axis effect). Feedbacks from higher visual areas
have been suggested as responsible for these figure-ground phenomena. Using a model, I
show that the phenomena are caused by intra-cortical interactions within V1 to serve
pre-attentive visual segmentation, particularly, object boundary detection.
Furthermore, whereas the border effect is robust, the surface and the medial axis effects
are by-products of the border effect and are predicted to diminish to zero for larger
figures. This understanding also explains why contextual ``cross-orientation
facilitation" from background to figure is elusive physiologically. Relation to the
medial axis effect in psychophysics is discussed.