No abstract

Human neuroimaging and animal studies have recently implicated the ventromedial prefrontal cortex (vmPFC) in memory schema, particularly in facilitating new encoding by existing schemas. In humans, the most conspicuous memory disorder following vmPFC damage is confabulation; strategic retrieval models suggest that aberrant schema activation or reinstatement plays a role in confabulation. This raises the possibility that beyond its role in schema-supported memory encoding, the vmPFC is also implicated in schema reinstatement itself. If that is the case, vmPFC lesions should lead to impaired schema-based operations, even on tasks that do not involve memory acquisition. To test this prediction, ten patients with vmPFC damage, four with present or prior confabulation, and a group of twelve matched healthy controls made speeded yes/no decisions as to whether words were closely related to a schema (a visit to the doctor). Ten minutes later, they repeated the task for a new schema (going to bed) with some words related to the first schema included as lures. Last, they rated the degree to which stimuli were related to the second schema. All four vmPFC patients with present or prior confabulation were impaired in rejecting lures and in classifying stimulus belongingness to the schema, even when they were not lures. Nonconfabulating patients performed comparably to healthy adults with high accuracy, comparable reaction times, and similar ratings. These results show for the first time that damage to the human vmPFC, when associated with confabulation, leads to deficient schema reinstatement, which is likely a prerequisite for schema-mediated memory integration.

Prior knowledge, such as schemas or semantic categories, influences our interpretation of stimulus information. For this to transpire, prior knowledge must first be reinstated and then instantiated by being applied to incoming stimuli. Previous neuropsychological models implicate the ventromedial prefrontal cortex (vMPFC) in mediating these functions for schemas and the anterior/lateral temporal lobes and related structures for categories. vMPFC, however, may also affect processing of semantic category information. Here, the putative differential role of the vMPFC in the reinstatement and instantiation of schemas and semantic categories was examined by probing network-level oscillatory dynamics. Patients with vMPFC damage (n = 11) and healthy controls (n = 13) were instructed to classify words according to a given schema or category, while electroencephalography was recorded. As reinstatement is a preparatory process, we focused on oscillations occurring 500 msec prior to stimulus presentation. As instantiation occurs at stimulus presentation, we focused on oscillations occurring between stimulus presentation and 1000 msec poststimulus. We found that reinstatement was associated with prestimulus, theta and alpha desynchrony between vMPFC and the posterior parietal cortex for schemas, and between lateral temporal lobe and inferotemporal cortex for categories. Damage to the vMPFC influenced both schemas and categories, but patients with damage to the subcallosal vMPFC showed schema-specific deficits. Instantiation showed similar oscillatory patterns in the poststimulus time frame, but in the alpha and beta frequency bands. Taken together, these findings highlight distinct but partially overlapping neural mechanisms implicated in schema- and category-mediated processing.