Hubel and Wiesel1,2 have described the responses of cells in the cat's visual cortex to precise stimulation of the retina. They say: “A comparison of visual responses in the anaesthetized animal with those in the unansesthetized unrestrained preparation1 shows that the main differences lie in the frequency and firing patterns of maintained activity and in the vigour of responses, rather than in basic receptive field organization.” They found that responses declined in vigour with anaesthesia.