The rise and fall of visual priming

Seeing a picture of an object primes its cortical representation and consequently facilitates its recognition in later encounters. Such priming has been linked to the physiological finding that repeated stimuli elicit relatively reduced activation. By showing that the behavioral development of priming under various conditions correlates with the dynamic of fMRI signal change, we provide a critical support for this link. Twelve subjects participated in this study. Objects appeared first for one of six possible durations (40, 150, 250, 350, 500 or 1900ms), followed by a mask and a blank interval (total 2000ms). All stimuli were presented once again in subsequent blocks for a fixed duration of 500ms each. Subjects were required to decide whether each of the individual objects was natural or man-made. Repetition priming (measured by difference in reaction time between recognizing repeated and novel objects) and repetition suppression (measured by the corresponding reduction in fMRI signal) peaked by 150-250ms from stimulus onset. An intuitive prediction would be that an increased presentation duration in the first encounter will result in an increased, or at least an equal amount of priming. However, our findings indicate that presenting the stimulus for a longer duration did not contribute further to these effects, but rather resulted in less priming and less repetition suppression. In other words, objects that were initially presented for 350-1900ms showed significantly less priming and less signal reduction compared with objects that were initially presented for 150-250ms. We will discuss the implications of these findings on our understanding of long-term object representations and the possible effect of top-down processes on efficient coding of visual information.