Both analyses yielded significant group main effects, with patients having reduced P3 source (at Pz, controls vs. patients, 1.95 ±1.31 vs. 1.22 ±1.31). Across groups, tones resulted in a greater P3 source than syllables at Pz (1.69 ±1.38 vs. 1.48 ±1.34). Also across groups, significant response mode main effects in both analyses stemmed from greater P3 source to button press (at Pz; for right press, 1.76 ±1.29; for left press, 1.72 ±1.44) than silent count (1.27 ±1.30; simple contrasts, left/right press vs. silent count, both F[1,44] ≥ 18.2, both p ≤ .0001; left vs. right press, F[1,44] < 1.0, n.s.), and this interacted with hemisphere at medial and lateral centroparietal sites (Figure 5). Whereas left-greater-than-right P3 source was associated with right press (LH vs. RH, 1.02 ±1.26 vs. 0.72 ±1.17), the opposite asymmetry was seen for left press (0.73 ±1.17 vs. 0.90 ±1.22), with no hemisphere difference for silent count (0.62 ±0.97 vs. 0.66 ±1.00). These response-related P3 source asymmetries were indirectly caused by superimposed anterior sinks contralaterally to the response hand, which are evident for both groups over left
