The effect of a chronic lesion in cortical area 17 on the visual responses of units in area 18 of the cat.

1. Lesions were made in cortical Area 17 (Visual I) of eight cats which were then allowed to recover. 2. During acute experiments between 1 and 11 weeks after the lesion the activity of Area 18 (Visual II) units was recorded and the results were compared with those obtained in normal cats. 3. The receptive fields were similar in size and distribution in the two groups but the lesioned animals had a much higher proportion of units unaffected by a visual stimulus and a higher proportion of the visually responsive units lacked specific direction or orientation preference. 4. Of six units which were tested in Area 18 of cats with lesions five showed variability of their direction or orientation preference with time. 5. The effects described above are most probably due to destruction of the corticocortical pathway which connects Areas 17 and 18. Some units in Area 18 appear to be driven by visual stimuli via their geniculate input alone. The corticocortical (Area 17 to 18) pathway may normally confer additional specificity on some of these units.