Nudging the N170 forward with prior stimulation – Bridging the gap between N170 and recognition potential

Some neural responses are classified by the stimulus conditions leading up to that response while other neural responses are also classified by the morphology of the responses themselves. However, morphology-based classification may not be appropriate if one can nudge a neural response into looking like another neural response. Morphology-based classification occurs with the N170 and RP (Recognition Potential), ERP components that are studied in separate literatures and yet share much in common in terms of functionality. In this study, we demonstrate a gradual transformation in the morphology of the N170 to the RP using a simple parametric manipulation of forward masks that is unlikely to cause a change in the underlying processing. Both the N170 and RP are N1 components, meaning that they are the first negative deflection of the evoked response. However, the RP is often measured with a forward mask that ends at stimulus onset whereas the N170 is often measured with no masking at all. This study investigates how ISI may delay and distort the N170 into an RP by manipulating the temporal gap (ISI) between forward mask and target. The results revealed reverse relationships between the ISI on the one hand, and the N170 latency, single-trial N1 jitter (an approximation of N1 width) and reaction time on the other hand. Importantly, we find that scalp topographies have a unique signature at the N1 peak across all conditions, from the longest gap (N170) to the shortest (RP). These findings prove that the mask-delayed N1 is still the same N170, even under conditions that are normally associated with a different component like the RP. In general, our results suggest that greater caution should be taken to interpret the time course of a measured effect when forward masks are employed.