Encoding tasks moderated the reward effect on brain activity during memory retrieval

Abstract

Previous studies have explored the effects of retrieval reward and depth of processing in encoding on recognition, but it remains unclear whether and how reward and depth of processing during encoding influence recognition. We investigated the effect and neural mechanisms of encoding reward and processing depth on recognition using event-related potentials (ERPs) in this study. In the study phase, participants were asked to perform two encoding tasks: congruity-judgment (deep processing) and size-judgment (shallow processing) in reward and no-reward conditions. The test phases included object (item) and background (source) tests. The results of item retrieval showed that the accuracy of rewarded items was higher than that of unrewarded items only in the congruity-judgment task, and the reward effect (the average amplitudes in the reward condition were significantly more positive than those in the no-reward condition) in the 300-500 and 500-700 ms were greater in the congruity-judgment task than in the size-judgment task. The results of source retrieval showed that the accuracy of rewarded items was higher than that of unrewarded items, that the difference in the size-judgment task was significantly larger, and that the reward effect in the 300-500 and 500-700 ms were greater in the size-judgment task than in the congruity-judgment task. In conclusion, the encoding task moderated the reward effect in item and source memory.

Figure 1

Schematic representations of a trial in the study phase (left), object test phase (middle) and background test phase (right). See text for details. Due to copyright restrictions all original images have been replaced with images taken by the authors.

Figure 2

Behavioral performance across conditions for item memory and source memory. CJT, congruity-judgment task; SJT, size-judgment task. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3

Amplitude distribution and topographic maps of ERPs in the different conditions in the study phase. (a) Comparison of ERP amplitudes in the four conditions. (b) Topographic maps of encoding effects ERPs (congruity-judgment task minus size-judgment task) both in reward and no-reward conditions.

Figure 4

Amplitude distribution and topographic maps of ERPs in the different conditions in the object test phase. (a) Amplitudes of the old-rewarded images, the old-unrewarded images and the new images were compared in the congruity-judgment and size-judgment tasks. (b) Topographic maps of reward-effect ERPs in the two encoding conditions. Note, CJT, congruity-judgment task; SJT, size-judgment task.

Figure 5

Amplitude distribution and topographic maps of ERPs in the different conditions in the background test phase. (a) Amplitudes of the rewarded images and the unrewarded images were compared in the congruity-judgment and size-judgment tasks. (b) Topographic maps of reward-effect ERPs in the two encoding conditions. Note, CJT, congruity-judgment task; SJT, size-judgment task.