Electrical activity was amplified 10,000 times using SynAmps2 amplifiers (Compumedics USA, Charlotte, NC) and was recorded continuously over a bandwidth between near-DC (0 Hz) and 100.0 Hz on a Neuroscan system (Versions 4.3–4.5; Compumedics USA, Charlotte, NC) at a sampling rate of 500 Hz. The EEG data were resampled offline to 256 Hz for the analyses. Then the waveforms were bandpass filtered offline with 0.05 Hz (low pass) and 55 Hz (high pass). EOG correction procedures were not applied. However, the trials with waveforms exceeding ±100 µV (primarily due to eye movement artifacts) and other artifacts (e.g., low frequency (DC) drifts and shifts and high frequency noise above 50 Hz) were excluded from the analyses. EROs were extracted from the trial epochs of outcome stimuli (1000 ms post stimulus) which contained the feedback of either loss or gain condition (i.e., the epochs following colored frames in Figure 1). The ERO data for the subjects whose ERP waveforms were morphologically aberrant were also further removed from the analyses. Only the trials containing loss and gain conditions for the bigger amount (50¢) were analyzed in the current study, as our previous work showed topographic similarity of theta power for both amounts within
