Data Analysis

Scalp potential and current density topographic maps were generated on a color graphics terminal using a two-dimensional spherical spline interpolation (Perrin, Pernier, Bertrand, & Echallier, 1989; Perrin, Pernier, Bertrand, & Giard, 1987) and a radial projection from Oz (back views) or from T3 or T4 (lateral views), along the length of the meridian arcs. The topographies were color coded and were normalized to the peak voltage value (positive or negative) of the recording montage. As described in detail at the beginning of the Results section, the electrophysiological manifestations at different levels of processing word were assessed by first calculating the statistical validity of the difference between the mean amplitude of the ERP elicited by each stimulus type in each decision task. The means were calculated for a visually determined epoch during which the waveforms seemed to be modulated by each task and for electrodes symmetrically located across the right and the left hemispheres.

The onset of this difference was statistically determined as the first latency at which the difference between waveforms was significant using a series of point-by-point t tests. In addition, a series of negative potentials was associated with the different stimulus conditions in each task. The mean amplitude of each component was calculated for an epoch comprising 24 points (98-msec) 12 before and 12 after its visually determined peak. To allow the comparison of the scalp distributions of each component (and hence help distinguishing among them), these values were calculated at the same 14 electrodes that covered the temporal and superior temporal areas, symmetrically located over each hemisphere. Finally, the latency of each peak was defined as the latency of the most negative point during the relevant epoch. Statistical differences among the ERP components for different stimulus types in each experimental session (visual, phonetic, lexical, and semantic sessions) were tested with repeated-measures ANOVAs. For all repeated measures with more than 1° of freedom, the more conservative Greenhouse-Geisser adjusted df-values were used. ANOVAs were followed by post hoc Tukey-A tests or univariate □□contrasts (Greenhouse & Geisser, 1959).

Figure 10. Thirty-two-channel electrode montage including all standard sites in the 10–20 system. The ground was located on the forehead between FP1 and FP2 and the nose was used as the reference.