The neural basis of implicit short-term memory: TMS investigations of visual priming

Repeated presentation of the same or similar visual stimuli can improve the speed of detection or discrimination of those 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 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 cortex during the inter-stimulus interval, so when the subject was basically idle. The rationale was that, if the targeted area was responsible for holding in memory the priming information from one trial to the next one, that information could be cancelled by TMS 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 end, 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). Finally, monkey studies showed that lesions of visual areas V4 and TEO, that have a major role in colour processing, impair colour priming (Walsh et al., 2000). Indeed, as postulated by the Perceptual Representation System hypothesis (PRS: Tulving & Schacter, 1990), these data suggest that the same visual areas involved in the representation of simple stimulus attributes such as colour, orientation, motion or spatial position, are also be responsible for the implicit short-term memory expressed by visual priming.