Neuronal Plasticity in the Associative Visual Cortex of the Cat after Callosatomy

Abstract

Changes in receptive field (RF) properties and connectivity of neurons in cat associative visual cortex were studied after callosatomy. Callosatomy and skull trepanation at area 21 were performed in cats weighing 2.5 – 4 kg (with nembutal 40 mg kg⁻¹). A total of 120 neurons in area 21 were studied. In experiments performed a short time after the operation, RF sizes were found to be smaller on average, and more equal between the two hemispheres compared with intact brains [mean RF sizes previously found in intact brains: 39 deg (left hemisphere) and 28 deg (right hemisphere); after operation: 23 (left hemisphere) and 24 deg (right hemisphere)]. At this stage, all investigated neurons lacked ipsilateral input and were selective for the orientation of the stimulus, whereas we have previously found that 70% of the neurons in the left and 30% of the neurons in the right hemisphere of the intact brain are not selective for orientation. Two months after the operation, RF sizes had increased, associated with the appearance of extensive inhibitory zones including input from the ipsilateral hemifield (in 40% of the neurons). Connectivity was studied by injecting horseradish peroxidase (HRP) in area 21. In intact animals, this has been found to result in a characteristic pattern of staining of neurons in area 17 in groups of 2 – 15. In animals studied two months after callosatomy, the resulting HRP staining in area 17 was more uniform. Further, in area 7 of operated animals large numbers of cells were stained, compared with only single cells in intact brains. More than 20% of the stained area 7 neurons of operated animals were inhibitory interneurons, which might correlate with the large inhibitory zones apparent in area 21 RFs. We conclude that extensive compensatory reorganisations occur in the associative visual cortex after callosatomy, involving changing connections in at least areas 7 and 17.