Visuocortical Activation during Mental Imagery Varies with the Source of the Image in Memory.

Abstract:
Are episodic memories experienced more vividly in the mind's eye than mental imagery generated from semantic knowledge of object appearances? Kosslyn and Thompson (2000) propose that primary visual cortex (PVC) will activate during imagery only when an image has a high level of visual detail or resolution. If episodic-based memories are indeed more vivid in the mind's eye, the Kosslyn and Thompson model predicts that PVC should activate during episodic- but not semantic-based visual imagery. To address this issue, subjects underwent fMRI scanning under two different imagery conditions. In the EPISODIC condition, imagery was generated by recalling recently-seen pictures of objects. In the SEMANTIC condition, imagery was generated by asking subjects to recall images of common objects. All imagery cues were auditory, and the baseline "rest" blocks interleaved with imagery blocks were identical between conditions. Although the group data showed PVC activation only during EPISODIC imagery, single-subject data revealed that most subjects had PVC activation during SEMANTIC imagery as well. These data have important implications for the debate over PVC activation during imagery, and the use of visuocortical activation as an indicator of visual memory retrieval.