Evidence for a threshold in experience-dependent long-term changes of kitten visual cortex

In order to determine the role of unstructured and of spontaneous activity in the developmental plasticity of the visual cortex, we blocked in 7 kittens the retinal activity of one eye completely by intraocular injections of tetrodotoxin (TTX) while the other eye was left open. Littermates of these kittens were monocularly deprived by lid suture and served as controls. In 3 additional kittens one eye was inactivated with TTX and the other closed by lid suture to assess whether mere asymmetries in the activation levels of the two eyes were sufficient to induce ocular dominance changes. Despite some variability between the matched pairs, the ocular dominance shift towards the normal eye did not differ significantly in the kittens with monocular lid suture and monocular TTX injection (P greater than 0.3/chi 2). In the TTX-treated kittens there was a trend for neurones to be less sharply tuned for orientation but it was not significant (P greater than 0.08/chi 2). In the kittens which had one eye injected with TTX and the other sutured closed no shift of ocular dominance was apparent. Separate analysis of both hemispheres revealed only a small bias towards the respective contralateral eye. We conclude that the process mediating activity-dependent long-term changes of ocular dominance has a threshold. Afferent activity that fails to excite the postsynaptic neuron above this threshold plays neither a significant role in the stabilization nor in the disruption of functional connections.