Prof. Nicolai Petkov

(Institute of Mathematics and Computing Science University of Groningen )

"Contour detection by surround suppression of texture"

Various effects show that the visual perception of an edge or line can be influenced by other such stimuli in the surroundings. Such effects can be related to non-classical receptive field (non-CRF) inhibition, also called surround inhibition, that is found in 80% of the orientation selective neurons in the primary visual cortex.

A mathematical model of non-CRF inhibition is presented. Non-CRF inhibition acts as a feature contrast computation for oriented stimuli: the response to an edge at a given position is suppressed by other edges in the surround. Consequently, it strongly reduces the responses to texture edges while scarcely affecting the responses to isolated contours. The biological utility of this neural mechanism might thus be that of contour (vs. texture) detection.

The results of computer simulations based on the proposed model explain perceptual effects, such as orientation contrast pop-out, 'social conformity' of lines embedded in gratings, reduced saliency of contours surrounded by textures and decreased visibility of letters embedded in band-limited noise [Petkov and Westenberg, 2003 Biological Cybernetics 88: 236-246].

The insights into the biological role of non-CRF inhibition can be utilized in machine vision. The proposed model is employed in a contour detection algorithm. Applied on natural images it outperforms previously known such algorithms in computer vision [Grigorescu et al, 2003 IEEE Transactions on Image Processing 12 729-739] [Grigorescu et al, 2004 Image and Vision Computing 22 609-622].

A demonstration will be given of a Gabor filter augmented with surround inhibition of texture edges. (The operator is available on-line at matlabserver.cs.rug.nl/).

References: (downloadable from www.cs.rug.nl/~petkov/publications/journals.html):

N. Petkov and M. A. Westenberg: Suppression of contour perception by band-limited noise and its relation to non-classical receptive field inhibition, Biological Cybernetics, 88, 2003, 236-246. www.cs.rug.nl/~petkov/publications/2003bc.pdf

C. Grigorescu, N. Petkov and M. A. Westenberg: Contour detection based on nonclassical receptive field inhibition, IEEE Trans. on Image Processing, 12 (7), 2003, 729-739. www.cs.rug.nl/~petkov/publications/2003ieee-tip_contour.pdf

C. Grigorescu, N. Petkov and M. A. Westenberg: Contour and boundary detection improved by surround suppression of texture edges, Image and Vision Computing, 22 (8) 2004, 609-622. www.cs.rug.nl/~petkov/publications/2004ivc_contour.pdf

Zeit:	Donnerstag, 11.05.2006, 10.00 Uhr
Ort:	Gebäude 57, Rotunde