Subject: Effects of grouping in contextual modulation From: Jont Allen <jba(at)RESEARCH.ATT.COM> Date: Wed, 23 Jan 2002 22:40:51 -0500http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v415/n6870/abs/415433a_r.html&dynoptions= Effects of grouping in contextual modulation MICHAEL H. HERZOG* AND MANFRED FAHLE*? * Human Neurobiology, University of Bremen, Argonnenstrasse 3, 28211 Bremen, Germany ? City University, Department of Optometry and Visual Science, Tait Building, Northampton Square, London EC1V 0HB, UK Correspondence and requests for materials should be addressed to M.H.H. (e-mail: mherzog(at)uni-bremen.de). Perception of a visual target and the responses of cortical neurons can be strongly influenced by a context surrounding the target. This observation relates to the fundamental issue of how cortical neurons code objects of the external world. In high-contrast regimes, embedding a target in an iso-oriented context reduces neural responses and deteriorates performance in psychophysical experiments. Performance from orthogonal surrounds is better than that from iso-oriented ones. This contextual interference is often postulated to be caused by long- or short-range interactions between neurons tuned to orientation. Here we show, using a new illusion called 'shine-through' as a sensitive psychophysical probe, that the orientation difference between target and context does not determine performance. Instead, contextual modulation depends on the overall spatial structure of the context. We propose that contextual suppression vanishes if the contextual elements are grouped to an independent and coherent object.