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Oliver Braddick, Justin O'Brien, Geraint Rees, John Wattam-Bell, Janette Atkinson, Robert Turner; Linear and non-linear responses to form coherence in extra-striate cortical areas. Journal of Vision 2003;3(9):149. doi: 10.1167/3.9.149.
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Sensitivity to global form coherence provides a possible psychophysical measure of ventral stream function, both in normal vision and in neurodevelopmental anomalies. We have identified extrastriate cortical areas which show a differential fMRI response to form coherence, tested by the contrast between arrays of line segments aligned to concentric circles, vs randomly oriented segments (Braddick et al, Current Biology, 2000). To understand the role of these areas in detection of coherent form, we need to know how their activity varies quantitatively with form coherence.
We have tested, in a blocked fMRI design, BOLD responses to graded levels of form coherence (0, 33, 66 and 100%). Foci where signal varied quantitatively with % coherence were found bilaterally in the middle occipital gyrus (MOG), the lingual/fusiform area (LF) and the posterior intraparietal sulcus (IPS). In MOG and the more lateral part of LF, the response showed a monotonic, approximately linear increase with stimulus coherence, analogous to the results of Rees at al (Nature Neuroscience, 2000) for varying motion coherence in area V5. However, IPS and a medial part of the LF region, showed non-monotonic responses, with a higher BOLD response for 66% than for 100% coherence. Both results were consistent bilaterally.
The linear responses imply that MOG and LF may be involved in encoding and transmitting the information used to detect form coherence, either in parallel or in a sequence which does not involve any non-linear transformation. Non monotonic signals must play a different role and may reflect a contribution of attentional processes which need to be engaged more strongly when the coherent form becomes less clear. The anatomical relationship between IPS and medial LF suggests that such processes may include descending modulation of relatively early stages in the form processing pathway.
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