An fMRI study on memory discriminability for complex visual scenes.

The fan effect represents an increase in reaction time for the recognition of an item as a function of the amount of information associated with that item in memory. The present study used fMRI to study the neural correlates of the fan effect for complex visual scenes. We used a test in which landscape pictures were divided vertically into three equal segments. In the high discriminability condition only one segment was presented during encoding, whereas in the low discriminability condition two different segments from the same picture were presented. During a subsequent forced-choice recognition test, reaction times were significantly faster for the high discriminability condition. Increase in brain activity for the low relative to high discriminability condition was observed in the right prefrontal cortex, several regions of parietal cortex bilaterally, and several late visual processing areas, including the occipito-temporal regions, precuneus, and cuneus. These results support the hypothesis that a region of the prefrontal cortex is involved in the control of memory interference at retrieval elicited by the amount of related information in memory, and further suggests that this involvement is right-lateralized for nonverbal material. The high versus low discriminability contrast showed an increase in activity principally in the bilateral medial temporal gyrus, including the enthorinal cortex/hippocampus and in several bilateral prefrontal cortex regions mostly located in BA 10. These activations were associated with a condition, in which the stimuli were more salient in memory and thus could represent the perceptual salience of items in memory.