Learning-induced physiological plasticity in the thalamo-cortical sensory systems: a critical evaluation of receptive field plasticity, map changes and their potential mechanisms.

The goal of this review is to give a detailed description of the main results obtained in the field of learning-induced plasticity. The review is focused on receptive field and map changes observed in the auditory, somatosensory and visual thalamo-cortical system as a result of an associative training performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the auditory and somatosensory system are reviewed. In the visual system, as there is no RF and map analysis during learning per se, the evidence presented are from increased neuronal responsiveness, and from the effects of perceptual learning in human and non human primates. Across sensory modalities, the re-tuning of neurons to a significant stimulus or map reorganizations in favour of the significant stimuli were observed at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neuronal changes and (ii) of neuromodulators as "gating" factors of the neuronal changes, is evaluated. The weakness of the Hebbian schema to explain learning-induced changes and the need to better define what the word "learning" means are stressed. It is suggested that future research should focus on the dynamic of information processing in sensory systems, and the concept of "effective connectivity" should be useful in that matter.