Remembering is the ability to bring back to mind episodes from one's past and is presumably accomplished by multiple, interdependent processes. In the present functional magnetic resonance imaging study, neural correlates of three hypothesized components of remembering were explored, including those associated with control, perceived oldness, and retrieved content. Levels of each component were separately manipulated by varying study procedures and sorting trials by subject response.
Neuroimaging experiments in humans suggest that regions in parietal cortex and along the posterior midline are functionally connected to the medial temporal lobe and are active during memory retrieval. It is unknown whether macaques have a similar network. We examined functional connectivity in isoflurane-anesthetized macaques to identify a network associated with posterior parahippocampal cortex (PPHC). Functional connectivity was observed between the PPHC and retrosplenial, posterior cingulate, superior temporal gyrus, and inferior parietal cortex.
The contribution of medial temporal lobe structures to memory is well established. However recent brain-imaging studies have indicated that frontal cortex may also be involved in human memory formation. Specific frontal areas are recruited during a variety of procedures that promote memory formation, and the laterality of these areas is influenced by the type of information contained in the memory. Imaging methods that capture momentary changes in brain activity have further shown that the likelihood of memory formation correlates with the level of activity in these areas.
Previous studies using PET and fMRI to examine memory retrieval have been limited by the requirement to test different types of items in separate blocks and to average data across items and response types within blocks. We used recently developed procedures for analyzing event-related mixed trial data from fMRI experiments to compare brain activity during true recognition of previously studied words and false recognition of semantic associates.
Spatiotemporal maps of brain activity based on magnetoencephalography were used to observe sequential stages in language processing and their modification during repetition priming. Subjects performed word-stem completion and produced either novel or repeated (primed) words across trials. Activation passes from primary visual cortex (activated at approximately 100 msec after word presentation), to left anteroventral occipital ( approximately 180 msec), to cortex in and near Wernicke's ( approximately 210 msec) and then Broca's ( approximately 370 msec) areas.
Neuroimaging techniques that allow the assessment of memory performance in healthy human volunteers while simultaneously obtaining measurements of brain activity in vivo may offer new information on the neural correlates of particular forms of memory retrieval and their association with consciousness and intention. We consider evidence from studies with positron emission tomography and functional magnetic resonance imaging indicating that priming, a form of implicit retrieval, is associated with decreased activity in various cortical regions.
V1 is a canonical cortical area with clearly delineated architectonic boundaries and a continuous topographic representation of the visual hemifield. It thus serves as a touchstone for understanding what new mapping methods can tell us about cortical organization. By parcellating human cortex using local gradients in functional connectivity, Wig et al. (2014--in this issue) detected the V1 border with V2.
Functional connectivity MRI (fcMRI) has been widely applied to explore group and individual differences. A confounding factor is head motion. Children move more than adults, older adults more than younger adults, and patients more than controls. Head motion varies considerably among individuals within the same population. Here we explored the influence of head motion on fcMRI estimates.
In fMRI studies of language processing, it would be extremely useful to obtain high-quality images during tasks requiring spoken output. Recent studies have suggested that this may be possible, particularly if event-related fMRI methods are used. This study assesses the feasibility of acquiring interpretable images during speech by applying event-related methods to visual word stem completion, a task that has been studied extensively.
Imaging is widely used in the evaluation of Alzheimer's disease (AD). In addition to imaging's traditional role of aiding in the exclusion of diseases that may be confused with AD, structural and functional imaging are being explored for potential use in the early detection of AD and as surrogate markers of treatment outcome. Volumetric measurements of the hippocampal and entorhinal regions using modern magnetic resonance imaging (MRI) and computed tomography are most widely studied in the discrimination of AD from nondemented aging and other dementias.
