Training-induced plasticity in the visual cortex of adult rats following visual discrimination learning

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Hager, Audrey
LTP , plasticity
Synaptic plasticity plays a key role in processes of learning and memory. Long-term potentiation (LTP) is a relatively stable enhancement of synaptic transmission following specific patterns of electrical stimulation. Some types of learning (e.g. motor learning, fear conditioning) result in LTP-like changes at synapses. However, no studies have examined LTP-like plasticity in the visual cortex as a result of visual discrimination learning. A visual discrimination task was used to examine changes in LTP in the primary visual cortex (V1) of adult rats. Rats were placed in a Y-shaped water maze and required to swim to one choice arm containing a hidden platform. Distinct visual cues indicated the presence (CS+) and absence (CS-) of the platform. Rats learned to reliably discriminate the visual cues to successfully navigate the maze. Control rats received the same procedure, but the visual cues did not have a predictive relation with the platform. Following training, trained, control, and task-naïve rats were anesthetized and visual evoked potentials (VEPs) in V1 were recorded in response to CS+, CS-, and novel stimuli. Results indicate that, in both task-naïve and control animals, all visual stimuli elicit VEPs of similar (p > 0.05) amplitude. In contrast, trained animals show significantly larger amplitude VEPs to stimuli encountered during training relative to novel stimuli, regardless of whether stimuli act as CS+ or CS-. In addition, trained animals show 71% and controls 47% potentiation (p < .05) that was induced by electrical (theta-burst) stimulation of the lateral geniculate nucleus (LGN) indicating greater plasticity of thalamocortical synapses following training. There were no differences between controls and task-naïve animals. This facilitation of LTP was shown to allow past visual experience to influence the efficiency of encoding novel visual features. These experiments demonstrate that visual discrimination learning might involve stimulus-selective facilitation of neuronal responses at early stages of visual processing (LGN, V1). The effect requires that stimuli carry some significance to the animal, while exposure to stimuli with no significance does not result in the same level of neuronal enhancement. Further, visual experience alters the plasticity properties of V1 (metaplasticity) by facilitating LTP along thalamocortical sensory fibers.
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