Experience-dependent Changes in the Visual Processing of Letters: Evidence from Electroencephalography Decoding.

Learning to read involves the formation and tuning of letter representations, but it is unknown whether this orthographic tuning influences very early visual processing or only later processing. This study tested the hypothesis that experience increases the extraction of sensory information about letters by comparing the EEG activity elicited by upright and inverted letters. In a set of conventional univariate analyses, we found that inverted letters elicited larger P1 amplitudes (starting ca. 110 msec) and larger N170 amplitudes (starting ca. 160 msec) compared with upright letters. These larger amplitudes could reflect enhanced processing, but they might instead reflect degraded processing. We therefore performed multivariate pattern classification (decoding) to assess the amount of information about letter identity in the neural signal. Specifically, we decoded which individual letter was presented from the pattern of voltage across the scalp at each time point. We found that decoding accuracy was greater for upright letters than for inverted letters during the P1 latency range (starting ca. 90 msec), particularly in electrodes over the left hemisphere. This provides evidence for enhanced tuning for upright letters in early visual processing. By contrast, we found higher decoding accuracy for inverted letters than for upright letters during and after the N170 component (starting ca. 140 msec). These results demonstrate that massive experience with upright letters influences sensory processing, leading to enhanced feature extraction for highly familiar (upright) letter forms at an early stage, followed by enhanced neural discriminability for less familiar (inverted) letter forms at a later stage.