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Visual neurobiology

Colouring the cortex

Nature volume 388, pages 23–24 (1997)Cite this article

On page 68 of this issue, Engel et al.3 report that, in sharp contrast to the single-cell recordings, most of the activity that is detected by functional magnetic resonance imaging (fMRI) of visual cortical areas V1 and V2 in humans is due to the activity of colour-opponent cells. Until now, most fMRI studies have only shown that there is a change in activity in a defined brain region when a particular stimulus is used. But Engel et al. have measured differential responses to subtle changes in colour and contrast, so the levels of activity in the fMRI signal can be used to produce response contours for the whole range of colours. By comparing these responses with behavioural measurements obtained for the same stimuli and observers, they have found that the activity of the red-green and blue-yellow colour-opponent mechanisms seems to underlie both sets of data.

Functional MRI is based on blood-oxygen-level-dependent (bold) changes in the magnetic resonance signal. The bold assumption is that the blood oxygen level is an indicator for local cortical activity. Because there is activity everywhere — even in the resting brain — only the activity changes that are induced by stimuli differing in the attribute of interest are measured. The resulting difference in the fMRI signal is typically about five per cent of the total signal, and it depends on the stimulus that is used. For example, if a region responds differentially to a moving stimulus, it is tagged as the ‘motion centre’ in the brain. Several such processing centres have been found, and the trend is going towards a ‘grandmother centre’. This is analogous to the trend towards increasingly specialized processing units6, which came about from single-cell recordings during the 1970s. Clearly, it is implicitly assumed that information processing is managed by specialized brain regions — distributed processing goes unnoticed in this standard approach.

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Figure 1: In human colour vision, the information from the three different types of cone photoreceptors, short-, middle- and long-wavelength-. sensitive (S, M, L), is combined by retinal ganglion cells and cells in the lateral geniculate nucleus (LGN) into two chromatic channels and a luminance channel1.
Figure 2: Spatial and temporal resolution of some of the most common experimental methods that are used to study sensory processing.

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  1. Max-Planck-Institut fr biologische Kybernetik, 38, Spemannstrae, Tbingen, 72076, Germany

    Karl R. Gegenfurtner

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  1. Karl R. Gegenfurtner
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Gegenfurtner, K. Colouring the cortex. Nature 388, 23–24 (1997). https://doi.org/10.1038/40275

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