Stimulus paradigm. 1. Low rate (mean ISI; 2s) D2 stimulation (black bars) alone. 2. High rate (mean ISI: 0.33s) D2 stimulation alone. 3. Ipsilateral D3; 4. ipsilateral D4; 5. contralateral D2 as the standard stimulus respectively (white bars), with D2 as the deviant stimulus (black bars) in the digit mismatch paradigm. RD 3, 4 and LD 2 were used as standards with RD2 as deviants; LD 3, 4 and RD2 were used as standards with LD2 as deviants.