The effects of visual object priming on brain activation before and after recognition

Background: Recognizing an object is improved by recent experience with tha t object even if one cannot recall seeing the object. This perceptual facil itation as a result of previous experience is called priming. In neuroimagi ng studies, priming is often associated with a decrease in activation in br ain regions involved in object recognition. It is thought that this occurs because priming causes a sharpening of object representations which leads t o more efficient processing and, consequently, a reduction in neural activi ty. Recent evidence has suggested, however, that the apparent effect of pri ming on brain activation may vary as a function of whether the neural activ ity is measured before or after recognition has taken place. Results: Using a gradual 'unmasking' technique, we presented primed and non -primed objects to subjects, and measured activation time courses using hig h-field functional magnetic resonance imaging (fMRI). As the objects were s lowly revealed, but before recognition had occurred, activation increased f rom baseline level to a peak that corresponded in time to the subjects' beh avioural recognition responses. The activation peak for primed objects occu rred sooner than the peak for non-primed objects, and subjects responded so oner when presented with a primed object than with a non-primed object. Dur ing this pre-recognition phase, primed objects produced more activation tha n non-primed objects. After recognition, activation declined rapidly for bo th primed and non-primed objects, but now activation was lower for the prim ed objects. Conclusions: Priming did not produce a general decrease in activation in th e brain regions involved in object recognition but, instead, produced a shi ft in the time of peak activation that corresponded to the shift in time se en in the subjects' behavioural recognition performance.