The late posterior negativity in episodic memory. A correlate of stimulus retrieval?

We investigated whether the late posterior negativity (LPN) is a component linked to stimulus retrieval or rather to complex, higher-order stimulus evaluation processes or response preparation processes. Participants performed three separate tasks across separate sessions: an encoding task, a memory recognition task, and a visual discrimination task. In the visual discrimination task, the difficulty of stimulus evaluation was manipulated via stimuli varying in complexity (easy vs. moderately difficult) and duration of stimulus presentation (short vs. long). Three indices of the LPN peak were examined: amplitude, latency, and width. The LPN was present in all three tasks, with maximum amplitudes at occipital sites. Results of the visual discrimination task showed that the LPN amplitude is modulated by task difficulty. No latency differences were observed between short and long presentations, suggesting that the LPN is not related to response preparation. Consequently, we compared the LPN associated with short presentations of easy and difficult stimuli with the LPN of the encoding and memory task. The LPN amplitude was more negative in the memory task compared to the other tasks. Latency and width of the LPN were modulated by stimulus complexity, with increased latency and width in the encoding and memory task relative to the visual discrimination task. Overall, these findings suggest that the LPN is not a component linked to stimulus retrieval and response preparation, but rather to complex, higher-order stimulus evaluation processes, which are modulated by task difficulty.