The relation between mental ability and the ability to detect violations of an abstract, third-order conjunction rule was examined using event-related potential measures, specifically mismatch negativity (MMN). The primary objective was to determine whether the extraction of invariant relations based on abstract conjunctions between two first-order sounds varied as a function of mental ability. Fifty-five female volunteers were presented with standard stimuli that varied across a range of co-joined frequency and intensity levels following the rule that the higher the frequency, the higher the intensity. An MMN was elicited by infrequent deviant stimuli following the rule that the higher the frequency, the lower the intensity. Higher ability (HA) was associated with larger MMN amplitude than lower ability (LA) at fronto-central electrode sites for patterns incorporating the extremes of the co-joined frequency and intensity levels. These effects demonstrate that the greater facility of discrimination for HA than LA that develops prior to consciousness is extant at a third-order of discrimination featuring the detection of an abstract conjunction rule governing the regularity of conjunction between first-order stimulus features. (Contains 2 tables and 3 figures.)