MEG study of early cortical plasticity following multiple digit frequency discrimination training in humans

Animal studies have provided direct evidence for constant modification of cortical maps of an adult modulated by training [1, 2]. This representational remodeling was further studied by Wang et al. [3] showing that temporal coincident multiple digit stimuli resulted in integrated multiple receptive fields (fusion). Recent advances in human brain imaging techniques have made it possible to study cortical plasticity during learning using human subjects. Three recent MEG studies have reported plasticity changes after somatosensory stimulation. The first MEG study [4] used the protocol of Wang et al. and showed that extensive training produced a reversible decrease in the strength of equivalent current dipoles after perceptual learning, although fusion was predicted. The second MEG study [5] showed that passive tactile finger co-activation for 40 min reduced the Euclidean distance between median and ulnar nerve somatosensory evoked filed. The third MEG study [6] reported that sequences of tactile stimulation affect the organization of the primary somatosensory cortex after a brief training series. In this work, we used MEG to study early plasticity following multiple digit frequency discrimination training. Human subjects were trained to discriminate small changes in the frequency from a 21 Hz comparison steady-state stimulus applied for 1 s simultaneously to the fingertips of digits 2+3+4 of the right hand. After long training, synapses of fingertips (digits 2+3+4) receiving training would be strengthened by the simultaneous firing of their inputs through a process of Hebbian learning: stimulating digit 3 after the training would elicit a response from neurons of digits 2 and 4. Synaptic modifications are however unlikely immediately after short training, yet within a few trials subjective and behavioral measures show that a change has already taken place. This study produces evidence for changes over widely distributed brain areas as a concomitant of this early cortical plasticity. 2 Methods