With the purpose of mapping those anatomical structures participating in the processing and analysis of form, colour and disparity information, we have measured, with positron emission tomography and [15O]butanol, regional cerebral blood flow (rCBF) as an indicator of regional cerebral metabolic activity in 13 right-handed male volunteers during visual discrimination of colour, form and disparity information. The brain images were anatomically standardized using a computerized brain atlas and statistically significant changes were localized by cluster analysis. The changes in rCBF between specific activation and reference states were measured and the volumes of changes were determined, as were the loci and volumes of areas commonly activated by two or three different tasks. Each of the tasks activated over a dozen distinct and separate fields in the cortex--in the occipital, parietal, temporal and frontal lobes as well as the cerebellum. A number of overlapping fields were commonly activated in two tasks (four in the form and colour tasks, five in the form and disparity tasks, and eleven in the colour and disparity tasks), and two field overlaps were present in all three tasks (in the right superior frontal and left lingual gyri). These findings indicate that, in a visual discrimination task, the processing and analysis of single visual submodalities take place in a number of cortical fields in the human brain. As the same visual submodality is processed and analysed by numerous fields and the same field may participate in the processing of different submodalities, a divergence-convergence pattern of information processing is present in the human brain. This observation supports a hypothesis based on earlier studies in primates, namely that information processing in the visual system requires the concerted activation of a relatively large number of fields of functional networks in the brain.