Regional and hemispheric differences have been observed in the scalp distribution of N4 (Curran et al., 1993). Studies have shown that N4 was maximally negative at the centroparietal region and exhibited larger amplitude at locations in the right hemisphere (Kutas and Hillyard, 1982; Kutas and Van Petten, 1988; Hill et al., 2002; Franklin et al., 2007). These topographic effects were robust across differences in presentation rate as well as in proportion of congruous to incongruous sentences (Kutas and Van Petten, 1988). In contrast to the above findings, we have observed predominantly more negative N4 at frontocentral regions in the left hemisphere for both primed and unprimed word conditions (Figure 3). This may be due to the different semantic priming paradigm adopted in this study that involved antonym-pairs. However, there were no statistically significant differential priming effects with respect to the brain regions and hemispheres. In general, the nature and objectives of this study were more clinical, and aimed at studying whether HR subjects have deficits in semantic priming, rather than to study the differential contribution of automatic spreading activation or controlled processes on semantic priming or the differential activation pattern with respect to the scalp region and hemisphere.
