Positron emission tomography (PET) has enabled researchers to gain great insight into the working of the brain. The study of memory, in particular, has gained much from the use of PET in studies. A review by Nyberg, Cabeza, and Tulving (1996) examined the contributions PET has made to the development and understanding of the HERA model of memory.

The hemispheric encoding/retrieval asymmetry (HERA) model describes the differential involvement of regions of the prefrontal cortex in the process of memory (Kapur, Craik, et al., 1994). The model states that the left prefrontal cortex is involved in the retrieval of information from semantic memory, as well as encoding any novel aspects of this retrieved information into episodic memory. On the other hand, the right prefrontal cortex is more involved in the retrieval of information from episodic memory. This model is theorized to apply even when different kinds of information are being encoded or recalled.

Nyberg, Cabeza, and Tulving's review found that PET studies provided good support for this model. With a single exception, all of the PET studies found significant evidence of left prefrontal cortex involvement with episodic memory encoding tasks, and no evidence of right prefrontal cortex involvement. An overwhelming majority of the PET studies also found that the opposite was true during nonverbal episodic memory retrieval tasks.

The authors of this review warned that the HERA model only provides a rough description of the areas of the brain involved in different memory processes. They also note that the numerous other PET activations found in the reviewed studies were ignored, and that they brain areas they discuss are large and contain many different, specific regions. However, the PET studies have provided support for a the HERA model, which provides a broad, organizational framework for examining which areas of the brain are involved in which memory tasks.

References
Kapur, S., Craik, F.I.M., Tulving, E., Wilson, A.A., Houle, S., & Brown, G.M. (1994). Neuroanatomical correlated of encoding in episodic memory: Levels of processing effect. Proceedings of the National Academy of Science, 91, 2008-2011.

Nyberg, L., Cabeza, R., & Tulving, E. (1996). PET studies of encoding and retrieval: The HERA model. Psychonomic Bulletin & Review, 3, 135-148.

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