Reverse effects of conditioning produced by two different unconditioned stimuli on thalamocortical transmission

Experience-dependent changes in cortical whisker representation in the adult mouse: a 2-deoxyglucose study

Sensory experience and learning can modify cortical body maps. We have previously reported that 3 days of classical conditioning, in which stimulation of a row of whiskers was paired with tail shock, produced an expansion of the cortical representation of the "trained row" labeled with 2-deoxyglucose (2DG), in layer IIIb and IV of the barrel cortex. The present study examined plastic remodelling of the vibrissal cortical representation after pairing whisker stimulation with a drop of sweet water. Cortical representations of rows of whiskers were mapped by 2DG autoradiography after 3 days and 2 months of training. The training resulted in enlargement of the cortical representation of vibrissae involved in the stimulus pairing compared with the contralateral representation of a row of whiskers, that were not touched during the training. This modification of whisker representation was different after short-term and long-term appetitive training. After three pairing sessions, changes in the width of cortical representation were visible in layers II/IIIa (29%) and layers V/VI (28%). After 2 months of training, significant changes in the width of cortical representation row B were found only in layer IV (41%). The changes were not observed in animals, that received whisker stimulation alone or in those who were subjected to training with unpaired stimuli. The results demonstrate that stimulus-pairing-induced changes in cortical whisker representation appeared with different time courses at different levels of cortical columnar information processing.

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.