Color Blindness Research Today is a free monthly online journal that collates and summarizes the latest research about Color Blindness, including details on causes, classification, red-green and blue-yellow color blindness, diagnosis.

Causal visual interactions as revealed by an information theoretic measure and fMRI.

Hinrichs H, Heinze HJ, Schoenfeld MA

Department for Neurology II, University of Magdeburg, Leipziger Str. 44, D-39120 Magdeburg, Germany. Hermann.Hinrichs@medizin.uni-magdeburg.de

In the present study, we evaluated the direction of the effective connectivity between fMRI activations in neural structures mediating preserved visual function in a patient with homonymous hemianopsia due to a posterior cerebral artery stroke. Although the lesion affected the primary visual cortex, the visual abilities of this patient included above-chance verbal reports of movement and color change as well as the discrimination of movement direction in his hemianopic field. These abilities were coupled with awareness (Riddoch syndrome). The strength and the direction of the interactions between visual regions were assessed by applying directed transinformation (T), a nonparametric information theoretic causal measure sensitive to linear as well as to nonlinear interactions. In the healthy hemisphere, T identified a strong flow of information from visual area V1 to V5 during stimulation by visual movement and from V1 to V4/V8 during stimulation by color change. In addition, during color change stimulation, a bi-directional flow was observed between V4/V8 and V5, suggesting crosstalk between these regions. In the lesioned hemisphere, the color change stimulation evoked a stronger flow from V5 to V4/V8 and a flow from V4/V8 to V2. These observations provide support for the hypothesis that visual information is mediated via subcortical pathways that bypass V1 and project first to higher-tier visual areas V5 and V4/V8 then subsequently to lower-tier area V2.

Published 16 June 2006 in Neuroimage, 31(3): 1051-60.
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