Recognizing an object is easier if one has seen the object before -- even if one cannot recall having seen the object earlier. This facilitation of perception is called priming. In neuroimaging studies, priming is often associated with a decrease in activation in brain regions involved in object recognition. It is thought that this occurs because priming causes a sharpening of object representations which leads to more efficient processing and, consequently, a reduction in neural activity. A recent 4T-fMRI study in my laboratory (led by my former Ph.D. student, Tom James, now at Indiana University) has shown, however, that the apparent effect of priming on brain activation varies as a function of whether the neural activity is measured before or after recognition. By slowing down the process of recognition by gradually unmasking an object, Tom has been able to show that the activation peak for primed objects in the fusiform object area occurs sooner than the peak for non-primed objects. After recognition, activation declines rapidly for both primed and non-primed objects. This experiment shows that priming does not produce a general decrease in activation in the brain regions involved in object recognition but, instead, produces a shift in the time of peak activation that corresponds to the shift in time seen in the subjects' recognition performance.
More recently, we have used a priming paradigm with fMRI to show that object-related activity in the dorsal (action) stream is viewpoint-dependent whereas object-related activity in high-order regions of the ventral (perception) stream is viewpoint-independent.
James, T.W., Humphrey, G.K., Gati, J.S., Menon, R.S., & Goodale, M.A. (2000). The effects of visual object priming on brain activation before and after recognition. Current Biology, 10, 1017-1024. Download pdf
James, T.W., Humphrey, G.K., Gati, J.S., Menon, R.S., & Goodale, M.A. (2002). Differential effects of viewpoint on object-driven activation in dorsal and ventral streams. Neuron, 35, 793-801. Download pdf