The Neural Basis of Implicit Short-Term Memory: TMSInvestigations of Visual Priming.

Repeated presentation of the same or similar visual stimuli can improve the speed of detection or discrimination of such stimuli. This phenomenon, called visual priming, is due to an implicit short-term memory mechanism that can operate at the level of object features and by means of the activity of functionally specialised low-level visual areas. Despite a number of studies attempted to uncover the neural basis of visual priming, only recent transcranial magnetic stimulation (TMS) experiments could establish a causal role of specific extrastriate cortical areas in the generation of priming. A new TMS paradigm was implemented for interfering with the functional integrity of circumscribed portions of cerebral cortex during the inter-stimulus interval; that is, when the subject was basically idle. The rationale was that, if a specific targeted area is responsible for holding in memory the priming information from one trial to the next one, such information could be cancelled by TMS delivered during the inter-stimulus interval and therefore priming would not take place. In this way it was possible to show that area V5/MT, known to be involved in motion perception, is also crucial for the generation of priming of motion direction (Campana et al., 2002, 2006). On the other hand, it has been shown that left frontal eye field (lFEF), known to be involved in spatial cognition and saccade preparation, is also responsible for priming of spatial position (Campana et al., 2007). These findings, together with psychophysical, human neuroimaging and neuropsychological studies, converge in suggesting that priming results from a low-level perceptual representation of the memory trace of simple stimulus attributes. Indeed, as postulated by the Perceptual Representation System hypothesis (PRS: Tulving & Schacter, 1990), we maintain that the same visual areas involved in the representation of simple stimulus attributes such as colour, orientation, motion direction or spatial position, are also responsible for the implicit short-term memory expressed by visual priming.