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 resul ts obtained in the field of learning-induced plasticity. The review is focu sed on receptive Field and map changes observed in the auditory, somatosens ory and visual thalamo-cortical systems as a result of an associative train ing performed in waking animals. Receptive field (RF) plasticity, 2DG and map changes obtained in the audito ry 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 stimul us or map reorganizations in favour of the significant stimuli were observe d at the thalamic and/or cortical level. The analysis of the literature in each sensory modality indicates that rela tionships between learning-induced sensory plasticity and behaviourial perf ormance can, or cannot. be found depending on the tasks that were used. The involvement (i) of Hebbian synaptic plasticity in the described neurona l changes and (ii) of neuromodulators as "gating" factors of the neuronal c hanges, is evaluated. The weakness of the Hebbian schema to explain learnin g-induced changes and the need to better define what the word "learning" me ans are stressed. It is suggested that future research should focus on the dynamic of informa tion processing in sensory systems, and the concept of "effective connectiv ity" should be useful in that matter. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.