Early-phase of learning enhances communication between brain hemispheres.

In the somatosensory system, inputs from one side of the body are only transmitted to the contralateral primary somatosensory cortex, but both sides of the body representation can interact via interhemispheric connections. These interactions depend on the behavioural requirements of the animal and its level of arousal. During the process of learning, alertness and attention may modify the responsiveness of neuronal pathways. We functionally mapped the brains of mice by using [14C]2-deoxyglucose (2DG) autoradiography during the first and the third session of a classical conditioning paradigm, involving whiskers stimulation on one side of the muzzle paired with an aversive or appetitive unconditioned stimulus. During the first pairing session, an increased 2DG uptake was seen in the barrel cortex of both hemispheres, independently of the type of applied unconditioned stimulus. In the third session of the sensory pairing, activation of the barrel cortex was solely contralateral, as expected after unilateral whisker stimulation. Thus, sensory stimulation directed to one cerebral hemisphere during the initial stages of Pavlovian conditioning activates the primary sensory area in both hemispheres. These results suggest that during the early phase of conditioning, when alertness is presumably strongest, the interhemispheric interactions are enhanced.