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

Changes in synaptic efficacy, including long-term potentiation (LTP) and long-term depression (LTD), provide mechanisms for experience-induced plasticity and play a key role in learning processes. Some types of learning (e.g., motor learning, fear conditioning) result in LTP and/or LTD-like changes at synapses. Here, rats learned to discriminate two visual stimuli, P+ and P-, indicating the presence and absence, respectively, of a hidden escape platform in a Y-shaped water maze. Following task acquisition, trained rats showed larger amplitude of visually evoked potentials (VEPs) in V1 to both stimuli encountered during training relative to novel stimuli. Training also resulted in a facilitation of LTP induced by theta-burst stimulation (TBS) of thalamic afferents to V1 with no effect on depression induced by low-frequency stimulation (LFS). Visual VEP enhancement and increased LTP both required that visual stimuli carried some significance to the animal, as both effects were absent in control rats exposed to the same visual stimuli in the absence of pairing with platform location. Together, these experiments show that visual experience can result in a stimulus-selective response enhancement and an increase in the synaptic modification range of V1 synapses, providing a novel example of metaplasticity in circuits of the adult cortex.