Abstract:

PURPOSE: The N170 and the RP are both negative-going deflections in ERP that exhibit preferential activity to visually recognizable objects; occurring at ~170 and ~250 ms, respectively. Despite their similarities, there is very little overlap in the study of these two components. Also, the paradigms associated with each are different: The RP is usually obtained by a rapid stream stimulation procedure (RSS) (Rudell, 1992), while the N170 is observed with a single presentation of visual stimuli. Studies of RP often claim that RSS can enhance activity that is preferential to recognizable objects. However, no study has ever demonstrated that RSS results in stronger preferential activity than the N170 paradigm. By testing the same subjects in RSS and N170 paradigms that are designed to be comparable, we show the first evidence in favor of RSS enhancements of preferential activity. 
METHODS: This study comprises two experiments. The RP has a longer latency. Therefore, in the first experiment, we determine if the N170 and RP signals are, in fact, comparable. A parametric increase in the temporal gap between a forward mask and target produces a continuum between RSS and N170 paradigms. The second experiment addresses the main question: Does RSS enhance preferential activity to recognizable objects, compared to the N170 paradigm? In that experiment, object recognizability was manipulated by image inversion. And for generality, we test 2 different categories of recognizable object: faces, and Chinese characters.
RESULTS and CONCLUSIONS: Experiment 1 shows that, along our gap-continuum, there is also a gradual shift between RP and N170 waveforms: systematic relations between potentials evoked by mask and target, topographic similarities across gap, and associations with the vertex positive potential. Our Experiment 2 results clearly show that the RSS paradigm does enhance preferential activity. For both categories of object, inversion effects for both characters and faces were greatly enhanced.

Key words: N170, Recognition potential, Rapid stream stimulation procedure, Inversion effect