Stimulus over-selectivity is a phenomenon often displayed by individuals with many forms of developmental and intellectual disabilities, and also by individuals lacking such disabilities who are under cognitive strain. It occurs when only one of potentially many aspects of the environment controls behavior. Adult participants were trained and tested on a trial-and-error discrimination learning task, with a concurrent memory load task, and displayed over-selectivity. Participants' brain responses were also monitored and analyzed using an electroencephalogram (EEG). The results demonstrated a significant difference between the event-related potentials generated to the over-selected and under-selected stimuli. Although there was no difference in the P300a wave between over- and under-selected stimuli, the P300b wave was associated with over-selected, but not under-selected stimuli. This finding suggests that, in this context, the under-selected stimulus did not elicit ERP activity typically associated with novelty, which, in turn, suggests that over-selectivity may not be entirely an attention-based effect. (Contains 4 figures.)