NEUROIMAGING

Much of the cognitive neuroscience of memory has taken advantage of lesion-behavior studies, which assess behavioral deficits that ensue from damage to targeted parts of the brain. For example, lesions to the medial temporal lobe, buried deep in the brain, often produce profound anterograde amnesia.

In the mid-1980s functional neuroimaging began to play an increasingly important role in enhancing our understanding of the organization of memory in the human brain. Two of the main functional neuroimaging methods, positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), measure signals related to blood flow or oxygenation. Both methods are based on the correlation of blood flow and neuronal activity: as is the case with muscles, blood flows more briskly in those parts of the brain that are the most active neuronally. PET can capture a single time-lapse image of forty seconds of blood flow. Methodological necessity dictated that PET studies of memory be done with blocked designs, which involve a comparison of the blood flow—and hence neuronal activity—resulting from several trials of a task performed in two separate blocks. fMRI can take images much more rapidly, usually producing a picture of the relative oxygenation level in blood vessels every two to three seconds. Although early fMRI studies still use blocked designs similar to those used in PET, the finer temporal capability of fMRI has also allowed event-related studies, in which trials of different types can be intermixed, much as in a standard cognitive psychological experiment. Researchers interested in the cognitive neuroscience of memory have developed ingenious applications of event-related designs for this technology. Although many imaging studies have confirmed the importance of medial temporal cortex to memory, the studies have also highlighted important contributions of other cortical areas, particularly the frontal cortex.

Encoding

A pair of papers, appearing in the same issue of the journal Science, highlighted the utility of event-related designs. In these studies participants were presented with either words (Wagner et al., 1998) or indoor/outdoor scenes (Brewer et al., 1998) and then were asked to make judgments about the items while undergoing imaging testing. These tasks were designed to be good incidental encoding tasks. Following the imaging sessions, the researchers administered recognition memory tasks for the studied items, noting which items the subjects remembered and which they forgot. They then sorted the images related to the encoding of the remembered or forgotten items. In both frontal and medial temporal regions, in the left hemisphere for the words and the right hemisphere for the scenes, activity was greater at encoding for the remembered than for the forgotten items. In other words, the level of activity at the time of encoding in these frontal and medial temporal regions predicted, on average, whether an item would be remembered or forgotten. Several further studies have replicated this "subsequent memory effect" in a number of encoding conditions.

Retrieval

Studies of memory retrieval have made several further associations between memory and cortical areas outside the medial temporal lobe. Starting with early PET studies of memory and continuing through many fMRI studies, right frontal activation was common when people retrieved information from episodic memory (Squire, 1992; Squire et al., 1992; Tulving et al., 1994). Considerable debate has focused on whether this right-frontal activation relates to the effortful state accompanying retrieval, to the successful retrieval of items or to postretrieval processes. Researchers are striving to develop new, more complex designs that aim to separate sustained, modelike processes from the transient processing of items (Donaldson, Petersen, Ollinger, and Buckner, 2001).

Other, left-lateralized cortical regions in parietal and frontal cortex have been more directly associated with retrieval success. Event-related studies, mainly of recognition memory tests, show greater activation in these regions for studied items than for previously unstudied, new words. Researchers have found these old-new effects in several kinds of experiments with different types of material, but always in similar regions (see Figure 1).

Finally, several studies (e.g., Nyberg, Habib, McIntosh, and Tulving, 2000) have suggested that recollection of, say, visual memories entails the reactivation of visual perceptual-processing mechanisms. In these studies, as people recall previously studied visual information, extra-primary visual regions are activated, even in the absence of a visual cue. It seems that some top-down process is reactivating perceptual mechanisms to retrieve specific perceptual content.

Other Studies

Other studies have focused on myriad other aspects of learning and memory, such as procedural learning (Karni et al., 1995), priming (Schacter and Buckner, 1998), and so on. Each of these lines of study seems to elucidate further neural mechanisms related to distinct memory processes. Such studies have yielded insights into cognitive architectures as well. For example, the earlier-noted right-prefrontal activation in many episodic retrieval tasks suggests the presence of a specific retrieval mode. Advancing neuroimaging technology promises to shed still more light on the neural mechanisms that underlie human memory and cognition.

See also:EPISODIC MEMORY; PROCEDURAL LEARNING

Bibliography

Brewer, J. B., Zhao, Z., Desmond, J. E., Glover, G. H., and Gabrieli, J. D. (1998). Making memories: Brain activity that predicts how well visual experience will be remembered. Science 281, 1,185-1,187.

Donaldson, D. I., Petersen, S. E., Ollinger, J. M., and Buckner, R. L. (2001). Dissociating state and item components of recogni tion memory using fMRI. Neuroimage 13, 129-142.

Karni, A., Meyer, G., Jezzard, P., Adams, M. M., Turner, R., and Ungerleider, L. G. (1995). Functional MRI evidence for adult motor cortex plasticity during motor skill learning. Nature 377, 155-158.

Nyberg, L., Habib, R., McIntosh, A. R., and Tulving, E. (2000). Reactivation of encoding-related brain activity during memory retrieval. Proceedings of the National Academy of Sciences of the United States of America 97, 11,120-11,124.

Schacter, D. L., and Buckner, R. L. (1998). Priming and the brain. Neuron 20, 185-195.

Squire, L. R. (1992). Memory and the hippocampus: A synthesis from findings with rats, monkeys, and humans. Psychological Review 99, 195-231.

Squire, L. R., Ojemann, J. G., Miezin, F. M., Petersen, S. E., Videen, T. O., and Raichle, M. E. (1992). Activation of the hippocampus in normal humans: A functional anatomical study of memory. Proceedings of the National Academy of Sciences of the United States of America 89, 1,837-1,841.

Tulving, E., Kapur, S., Markowitsch, H. J., Craik, F. I. M., and Habib, R. (1994). Neuroanatomical correlates of retrieval in episodic memory: Auditory sentence recognition. Psychology 91, 2,012-2,015.

Wagner, A. D., Schacter, D. L., Rotte, M., Koustaal, W., Maril, A., Dale, A. M., Rosen, B. R., and Buckner, R. L. (1998). Building memories: Remembering and forgetting of verbal experiences as predicted by brain activity. Science 281, 1,188-1,191.

Steven E.Petersen