Dissociation between the activity of the right middle frontal gyrus and the middle temporal gyrus in processing semantic priming

Abstract

The aim of this event-related functional magnetic resonance imaging (fMRI) study was to test whether the right middle frontal gyrus (MFG) and middle temporal gyrus (MTG) would show differential sensitivity to the effect of prime-target association strength on repetition priming. In the experimental condition (RP), the target occurred after repetitive presentation of the prime within an oddball design. In the control condition (CTR), the target followed a single presentation of the prime with equal probability of the target as in RP. To manipulate semantic overlap between the prime and the target both conditions (RP and CTR) employed either the onomatopoeia "oink" as the prime and the referent "pig" as the target (OP) or vice-versa (PO) since semantic overlap was previously shown to be greater in OP. The results showed that the left MTG was sensitive to release of adaptation while both the right MTG and MFG were sensitive to sequence regularity extraction and its verification. However, dissociated activity between OP and PO was revealed in RP only in the right MFG. Specifically, target "pig" (OP) and the physically equivalent target in CTR elicited comparable deactivations whereas target "oink" (PO) elicited less inhibited response in RP than in CTR. This interaction in the right MFG was explained by integrating these effects into a competition model between perceptual and conceptual effects in priming processing.

Figure 1. Experimental design: schematic diagram of sequences used in RP (upper panel) and CTR (lower panel).

The example here pertains to the case when “pig” served as the target (onomatopoeia “oink” primes pig: OP), but an identical scheme of stimulation (not shown) was used when “oink” served as the target and “pig” (PO) as either the repetitive prime (RP) or the single prime preceding the target (CTR). The stimuli “duck” and “quack” served as “filler” stimuli and were used to prevent immediate proximity between “oink” and “pig” within the varying segments in CTR so that the target would have the same probability of occurrence as in RP (∼17%). A complete run was divided into eight 54 msec segments separated by a 2 sec interval, each containing 54 stimuli (targets, primes and in CTR also fillers) with an SOA of 1 sec. Stimulus duration was 330 msec. For each condition (RP, CTR) there were two different sequences, one for each target-type. The four sequences were first ordered according to a Latin-square design to minimize carry-over effects and then mirror-imaged for the second part of the run. Each run started and ended with a period of 30 sec duration of white noise that served as the baseline. Note that the study used auditory stimuli (not visual), i.e., naturally produced speech stimuli (the words: “oink”, “pig”, “duck” and “quack”) and that there were three exemplars of each word.

Figure 2. The MFG (A) and MTG (B) ROIs.

The figure presents the selected ROIs in three orthogonal views (sagittal, coronal, axial). ROIs were extracted from a contrast map between CTR targets vs. CTR primes. The x,y,z Talairach coordinates for the right MFG and MTG are set on the center of mass coordinates of each ROI as follows: x = 36, y = 30, z = 21; x = 59, y = −50, z = 5, respectively. The left ROIs were achieved by flipping the right ones. The functional ROIs were superimposed on a reference anatomical image (Holmes et al., 1998). Display follows radiological convention.

Figure 3. Responses to targets in RP and CTR within each of the ROIs.

Upper panel: the Condition (RP, CTR)×Stimulus (oink”, “pig”) interaction in the right MFG. Note the different responses to “oink” in RP relative to CTR and the more similar activation levels for “pig” primes in these conditions. Lower panels: the condition effect (RP>CTR) in the left and right MTG (left and right panels, respectively). Note the deactivation in CTR in the right MTG and the positive increased activation in RP relative to CTR in the left MTG. R = right; L = left. Error bars depict the standard error. *p<0.05; **p<0.01.