Processing ordinality and quantity: ERP evidence of separate mechanisms.

We report an event-related potential (ERP) experiment of ordinal processing exploring the relationship between ordinal and numerical information. ERPs were recorded from healthy adults while making ordered/non-ordered judgments on 3 non-symbolic numerical stimuli (arrays of dots). Three main variables were manipulated: (1) Ordinality (ordered vs. non-ordered groups of dots), tapping the quick "gist" estimation of ordinality. (2) Direction (ascending vs. descending order), tapping the symbolic, culturally influenced aspect of ordinality, and (3) Ratio between the group of dots, tapping the processing of the basic numerosity information. Behavioral results showed independent effects for each variable, replicating our previous findings with this paradigm. ERP effects differentiated between three cognitive processes for estimating ordinality, processing numerosity, and direction. This differentiation was found both in terms of timing and topography: Order estimation was associated with early scalp parietal and lateral occipital positivity (80-130ms) originating in the left Middle Temporal Gyrus; numerical ratio was associated with a later scalp medial posterior positivity (130-200ms); and direction was associated with a late and widespread scalp right frontal and scalp right parietotemporal positivity and a corresponding scalp left frontal and scalp left parietotemporal negativity (300-600ms). A theoretical model is suggested, stressing an early and basic ordinal-specific mechanism.