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The Journal of Neuroscience, May 1989, g(5): 1749-l 763 Mechanisms of Contour Perception in Monkey Visual Cortex. II. Contours Bridging Gaps Esther Peterhans and Riidiger von der Heydt Department of Neurology, University Hospital Zurich, 8091 Zurich, Switzerland We have studied the mechanism of contour perception by recording from neurons in the visual cortex of alert rhesus monkeys. We used stimuli in which human observers per- ceive anomalous contours: A moving pair of notches in 2 bright rectangles mimicked an overlaying dark bar. For con- trol, the notches were closed by thin lines so that the anom- alous contours disappeared or half of the figure was blanked, with a similar effect. Orientation-selective neurons were studied. With the receptive fields centered in the gap, 23 of 72 (32%) neurons tested in area V2 responded to the moving “bar” even though the stimulus spared their response fields, and when the notches were closed, their responses were reduced or abolished. Likewise, when half of the figure was removed, the neurons usually failed to respond. Neurons with receptive fields within 4” of the fovea signaled anom- alous contours bridging gaps of lo-3.5”. The anomalous- contour responses were compared quantitatively with re- sponse field profiles and length-summation curves and found to exceed the predictions by linear-summation and sum- mation-to-threshold models. Summation models also fail to explain the effect of closing lines which add only negligible amounts of light. In Vl, only one of 26 neurons tested showed comparable responses, and only with a narrow gap. The others responded only when the stimulus invaded the re- sponse field and did not show the effect of closing lines, or failed to respond at all. The contour responses in V2, the nonadditivity, and the effect of closure can be explained by the previously proposed model (Peterhans et al., 1986), as- suming that the corners excite end-stopped fields orthog- onal to the contour whose signals are pooled in the contour neurons. This is the second of 2 papers in which we try to relate contour perception to neuronal signals by usinganomalous contours. In the preceding paper (von der Heydt and Peterhans, 1989a), we showed that neurons exist in area V2 ofthe monkey visual cortex that signalorientation and position of a line of discontinuity in a grating as though it were a real line or light-dark edge.Since in this case the anomalous contour and the linesinducing it had Received Sept. 29, 1988; accepted Oct. 24, 1988. We wish to thank Vappu Furrer-Isoviita and Bernadette Disler for technical assistance, and Elisabeth R. Strickler for histological work. Giinter Baumgartner suggested the use of illusory-contour figures and the effect of closure for studying cortical processing at the neuronal level. The manuscript benefited from comments from Peter 0. Bishop, Walter H. Ehrenstein, Stephen Grossberg, and 2 anonymous referees. This work was supported by the Swiss National Foundation Grant 3.939.84.
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