An fMRI study of the cerebral macro network involved in ‘cue invariant’ form perception and how it is influenced by stimulus complexity

We investigated the influence of stimulus complexity on the macro network of visual areas involved in ‘cue invariant’ form perception. Functional MRI imaging on 14 healthy, adult volunteers was performed during a two alternative forced choice (2-AFC) form discrimination task. The functional load imposed onto the visual system was varied by using simple and complex shapes. The figures were defined using a luminance, a chromatic or a motion contrast cue. The three cues activated the same visual areas in the ventral pathway, including area ‘LO’. Activation of visual area ‘V3v’ but not area ‘KO’ in the dorsal pathway was observed to the motion contrast cue. The simple shapes induced a larger BOLD response in BA18 than the complex shapes, reflecting the selectivity of this region for the features in the stimuli such as edges and vertices. The brain regions yielding a larger BOLD signal to the complex shapes were areas know to be selective to the orthographic content of our complex stimuli. The processing requirement was assessed by comparing the subjects’ reaction time. We found no significant difference in the reaction times to the simple and complex shapes. The reaction times to the luminance contrast cue and the chromatic contrast cue were identical but that to the motion contrast cue were 200 ms longer. This finding concurs with neurophysiological studies, reporting a longer onset latency for motion contrast stimuli. It further lends support to the idea that the motion contrast cue requires auxiliary processing by the visual areas of the dorsal pathway before entry into the ventral pathway.