Rapid cortical plasticity underlying novel word learning

Abstract

Humans are unique in developing large lexicons as their communication tool. To achieve this, they are able to learn new words rapidly. However, neural bases of this rapid learning, which may be an expression of a more general cognitive mechanism, are not yet understood. To address this, we exposed our subjects to familiar words and novel spoken stimuli in a short passive perceptual learning session and compared automatic brain responses to these items throughout the learning exposure. Initially, we found enhanced activity for known words, indexing the ignition of their underlying memory traces. However, just after 14 min of learning exposure, the novel items exhibited a significant increase in response magnitude matching in size with that to real words. This activation increase, as we would like to propose, reflects rapid mapping of new word forms onto neural representations. Similar to familiar words, the neural activity subserving rapid learning of new word forms was generated in the left-perisylvian language cortex, especially anterior superior-temporal areas. This first report of a neural correlate of rapid learning suggests that our brain may effectively form new neuronal circuits online as it gets exposed to novel patterns in the sensory input. Understanding such fast learning is key to the neurobiological explanation of the human language faculty and learning mechanisms in general.

Figure 1.

Electric brain responses, recorded early and late in the learning session, to rarely presented critical word and pseudo-word stimuli and their corresponding frequently presented standard stimuli. Responses recorded from the vertex (electrode Cz) and time-locked to stimulus onsets are presented here; waveforms and spectrograms of critical stimulus items are overlaid on the brain response plots (see also supplemental Fig. 1, available at www.jneurosci.org as supplemental material for complete stimulus information). Note that the responses to frequent and rare stimuli in each set are similar until the divergence point at 345 ms when the final stop occurs. Note also, that following the divergence point the temporal dynamics of word and pseudo-word responses differ: larger responses to words early in the session (left) and more similar responses at the end, after pseudo-word responses had increased (right).

Figure 2.

Electric brain response registered at vertex (Cz) for word and pseudo-word stimuli early and late in the learning session. Responses are time-locked to the stimulus divergence points (final plosion onsets) when the final phoneme could be perceived and the word or pseudo-word stimulus first be recognized. Note the larger word response early in the session (left) and the similar responses at the end, after the pseudo-word response had increased (right). Cortical source distributions (L2 minimum-norm topographies at 120 ms after the divergence point, left view) are displayed in the insets. Note that known words elicited perisylvian language-area activation early and late in the session. In contrast, novel pseudo-words elicited anterior-superior-temporal activation only at the end of the session (brighter colors indicated higher dipole moment, activation thresholded at top 20%; for full unthresholded activations landscape, see supplemental Fig. 2, available at www.jneurosci.org as supplemental material).

Figure 3.

Statistical assessment of response change through the exposure session. Note the absence of statistical change in the word response and significant increase in the pseudo-word response confirmed by both factorial comparison of early vs late 25% of trials (left) and linear regression over consecutive 10% sub-blocks (right). Individual subjects' data points and regression lines in black, group average amplitudes and regression lines in color.