The temporal dynamics of visual object priming

Abstract

Priming reflects an important means of learning that is mediated by implicit memory. Importantly, priming occurs for previously viewed objects (item-specific priming) and their category relatives (category-wide priming). Two distinct neural mechanisms are known to mediate priming, including the sharpening of a neural object representation and the retrieval of stimulus-response mappings. Here, we investigated whether the relationship between these neural mechanisms could help explain why item-specific priming generates faster responses than category-wide priming. Participants studied pictures of everyday objects, and then performed a difficult picture identification task while we recorded event-related potentials (ERP). The identification task gradually revealed random line segments of previously viewed items (Studied), category exemplars of previously viewed items (Exemplar), and items that were not previously viewed (Unstudied). Studied items were identified sooner than Unstudied items, showing evidence of item-specific priming, and importantly Exemplar items were also identified sooner than Unstudied items, showing evidence of category-wide priming. Early activity showed sustained neural suppression of parietal activity for both types of priming. However, these neural suppression effects may have stemmed from distinct processes because while category-wide neural suppression was correlated with priming behavior, item-specific neural suppression was not. Late activity, examined with response-locked ERPs, showed additional processes related to item-specific priming including neural suppression in occipital areas and parietal activity that was correlated with behavior. Together, we conclude that item-specific and category-wide priming are mediated by separate, parallel neural mechanisms in the context of the current paradigm. Temporal differences in behavior are determined by the timecourses of these distinct processes.

Keywords: Event-related potentials; Implicit memory; Neural suppression; Priming.

Figure 1

(A) A depiction of the behavioral task. First, participants made size judgments while viewing a sequence of pictures (top left) followed by a size comparison task (bottom left). Participants then performed a fragmented picture identification task (on right), viewing a sequence of picture fragments that gradually become a complete picture, and making a speeded response once they could identify the item. Participants then made a size judgment to the identified item. The identification task presented Studied items, Exemplar items, and Unstudied items. (B) Behavioral results. The mean number of frames required to make a response, or response threshold, is depicted on the horizontal axis with error bars depicting the standard error of the mean. The threshold is plotted in parallel with an example sequence to illustrate its relationship with the stimulus. *p < .01, **p < .001

Figure 2

(A) Waveforms for stimulus-locked ERPs in each spatial region-of-interest (ROI). Regions exhibiting priming-related activity are framed with bold lines. (B) Difference waves illustrating neural suppression in parietal regions. The black lines depict the mean difference at each time point and the gray regions span the standard deviation around the mean. The top row depicts difference waves for item-specific suppression (Studied – Unstudied) in regions CPS (left) and RPS (right), while the bottom row depicts differences waves for category-wide suppression (Exemplar – Unstudied) in regions CPS and RPS. (C) Averaged ERP magnitudes for ROIs showing significant differences between conditions. Note that the activity in the parietal regions is plotted on a different scale than the left occipital region. Legend: + = studied different from unstudied (p < .05); ^ = exemplar different from unstudied (p < .05); * studied different from exemplar (p < .05). (D) The spatial ROIs located across the scalp.

Figure 3

(A) Scatterplot depicting the priming/suppression correlation in RPS during the 100–300 ms epoch. The behavioral priming effect (threshold for Exemplar-Unstudied items) is plotted on the y-axis and the neural suppression effect in region RPS (ERP magnitude for Exemplar-Unstudied items) is plotted on the x-axis. The data appear with the linear trend line (y = 1.28x + 0.571). (B) Scatterplot depicting the priming/suppression correlation in CPS during the 300–500 ms epoch. The behavioral priming effect (threshold for Exemplar-Unstudied items) is plotted on the y-axis and the neural suppression effect in region CPS (ERP magnitude for Exemplar-Unstudied items) is plotted on the x-axis. The data appear with the linear trend line (y = 1.83x + 0.625).

Figure 4

(A) Waveforms for response-locked ERPs in each spatial ROI. Regions exhibiting priming related activity are framed with bold lines. (B) Averaged ERP magnitudes for regions CPS and RPI. *p < .05 (C) Scatterplot depicting the priming/enhancement correlation in CPS during the −300 to 0 ms epoch. The memory effect (Studied – Unstudied) is plotted on the y-axis against the ERP effect (Studied – Unstudied) on the x-axis. The data appear with the linear trend line (y = −1.48x −0.77).