Click on the image to see a PDF version (for zooming in)

Fig. 2.3. Measuring cortical maps. Optical imaging techniques allow neuronal preferences to be measured for large numbers of neurons at once (Blasdel and Salama 1986). In such experiments, part of the skull of a laboratory animal is removed by surgery, exposing the surface of the visual cortex. Visual patterns are then presented to the eyes, and a video camera records either light absorbed by the cortex or light given off by voltage-sensitive fluorescent chemicals that have been applied to it. Depending on the neural activity, there will be small differences in the emitted or reflected light, and these differences can be amplified by repeated presentations and averaging. The results are an indirect measure of the average two-dimensional pattern of neural activity resulting from a particular stimulus. Measurements can then be compared between different stimulus conditions, e.g. different orientations, determining which stimulus is most effective at activating each small patch of neurons. Figure 2.4 and later figures in this chapter will show maps of orientation preference computed using these techniques. Adapted from Weliky et al. (1995).