DISSOCIATING BRAIN RESPONSES TO IMPLICIT RULES OF VARYING COMPLEXITY

In this study, we used the Event-Related Optical Signal (EROS) to differentiate brain responses, in both time and space, related to implicit rules governing the stimulus sequence and varying in complexity. Participants, naive to the rules,responded to high or low tones by pressing buttons. On a given trial the tone was either the same as (repeat) or different from (alternate) that of the previous trial.Stimulus presentation followed 3 rules (in increasing order of complexity):1) Global Probability: 25% repeat, 75% alternate; 2) Single Repetition: there were never 3 identical tones in a row; 3) Local Probability: odd-numbered trials were 50% repeat/50% alternate, even-numbered trials were 100% alternate.Planned comparisons with orthogonal contrasts of different trial types were used to indicate the use of each rule. All implicit rules were used by the participants as shown by the behavioral results. EROS data revealed temporal activity at 200 –300 ms reflecting only the first two simpler rules. At later intervals (300 – 500 ms),a front o-parietal network was found to be sensitive to both the most simple and complex rules. Event-related brain potentials (ERP) data, recorded in a separate session, were consistent with the EROS results. This study suggests that the brain can simultaneously hold different models of the stimulus sequences within the information processing stream, but that these representations are held at different levels, both in terms of latency and location of the activity.