Sharpening and formation: two distinct neuronal mechanisms of repetition priming.

The repetition of an object stimulus results in faster and better recognition of this object (repetition priming). This phenomenon is neuronally associated with a reduced firing rate of neurons (repetition suppression). It has been interpreted as a sharpening mechanism within the cell assembly representing the object. In the case of an unfamiliar stimulus for which no object representation exists, the repetition of the stimulus results in an increase in the firing rate (repetition enhancement).It has been hypothesized that this increase reflects the formation of a cortical object representation. We aimed to investigate cortical object representations as well as repetition suppression and enhancement by means of the steady-state visual evoked potential (SSVEP) in the healthy human brain. To that end, we used a repetition paradigm with familiar and unfamiliar objects, each presented with 12-Hz flicker, producing an oscillatory brain response at the same frequency (i.e. an SSVEP). Results showed significantly smaller SSVEP amplitudes for repeated familiar objects compared to their first presentation (repetition suppression). For unfamiliar objects, SSVEP amplitudes increased with stimulus repetition (repetition enhancement). Source reconstruction revealed inferior temporal regions as generators for the repetition suppression effect, probably reflecting a sharpening mechanism within the cortical representations of the constituting features of an object. In contrast, repetition enhancement was localised in the superior parietal lobe, possibly reflecting the formation of a structural object representation. Thus, the mechanisms underlying repetition priming (i.e. sharpening and formation) depend on the semantic content of the incoming information.