ECVP 2010 Abstract

Cite as:
Schipper M, Ernst U, Stecher H I, Pawelzik K, Fahle M, 2010, "A horizontal bias in contour integration: Evidence from psychophysics and electrophysiology" Perception 39 ECVP Abstract Supplement, page 26

A horizontal bias in contour integration: Evidence from psychophysics and electrophysiology

M Schipper, U Ernst, H I Stecher, K Pawelzik, M Fahle

Perception of shapes in natural scenes is strongly modulated by global context interacting with local integration processes, to construct a representation from local image features. An example is contour integration, grouping nearly collinear, aligned edge elements into coherent forms. We investigate how global and local integration processes interact, and whether they are separate or combined processes. We combined EEG recordings with psychophysical experiments. Observers had to detect contours of locally aligned edge elements, embedded in a background of randomly oriented distractors. The global form of the contours was either a horizontal or a vertical ellipse. For horizontal ellipses, both reaction times and detection thresholds are smaller than for vertical ellipses (horizontal bias). Ellipses aligned radially to the fixation spot are easier detected than tangentially aligned ellipses. The relative strengths of these effects depend on the eccentricity of the contours. The horizontal bias is confirmed by electrophysiological data, showing a significant difference in the occipital and parietal ERPs between horizontal versus vertical ellipses starting 150 ms after stimulus onset. ERP differences in frontal electrodes appear later starting 220 ms after stimulus onset. Hence the horizontal bias emerges already during contour integration, suggesting that local and global form integration interact very early.
[Supported by the BMBF (Bernstein Group Bremen, Förderkennzeichen 01GQ0705)]

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