Cite as:
Cornelissen F W, Vladusich T, 2006, "Modelling fMRI edge responses in early visual cortex" Perception 35 ECVP Abstract Supplement

Modelling fMRI edge responses in early visual cortex

F W Cornelissen, T Vladusich

Neurons in the early visual cortex (V1/V2) tend to have relatively small receptive field sizes. The line-spread function of the BOLD signal has been estimated at about 3.5 mm full-width-at-half-maximum (FWHM). For these reasons, one would expect BOLD responses to be confined to the immediate vicinity of the neural source. Yet, we have found that the fMRI responses in these areas to 1 Hz contrast modulating luminance and colour edges are very wide. Here we show that such fMRI edge responses can be accurately modeled by the linear addition of an extended (FWHM ~16 - 20‚mm) edge response in addition to a much narrower local one (~1 mm FWHM). Moreover, the narrow and extended-edge responses can be fitted with the same intercepts and slopes without affecting model performance. This latter result suggests that the local and extended-edge responses arise from highly correlated, if not identical, neural sources. The extended response may be related to 'non-classical' receptive field properties. Our findings have implications for fMRI studies of the role of V1 in perception. The use of regions-of-interest that are situated next to physically stimulated regions of the cortex may cause 'spill-over' of BOLD activity into non-stimulated regions, leading to erroneous inferences about perceptually related activity.

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