The Cognitive Neuroscience of Working Memory., 2014 Sep 19.
Annual review of psychology. Abstract
For more than 50 years, psychologists and neuroscientists have recognized the importance of a working memory to coordinate processing when multiple goals are active and to guide behavior with information that is not present in the immediate environment. In recent years, psychological theory and cognitive neuroscience data have converged on the idea that information is encoded into working memory by allocating attention to internal representations, whether semantic long-term memory (e.g., letters, digits, words), sensory, or motoric. Thus, information-based multivariate analyses of human functional MRI data typically find evidence for the temporary representation of stimuli in regions that also process this information in non-working memory contexts. The prefrontal cortex (PFC), on the other hand, exerts control over behavior by biasing the salience of mnemonic representations and adjudicating among competing, context-dependent rules. The "control of the controller" emerges from a complex interplay between PFC and striatal circuits and ascending dopaminergic neuromodulatory signals. Expected final online publication date for the Annual Review of Psychology Volume 66 is November 30, 2014. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Distinct mechanisms in visual category learning., 2007 Sep.
Cognitive, affective & behavioral neuroscience. 7:251-9. Abstract
The ways in which visual categories are learned, and in which well-established categories are represented and retrieved, are fundamental issues of cognitive neuroscience. Researchers have typically studied these issues separately, and the transition from the initial phase of category learning to expertise is poorly characterized. The acquisition of novel categories has been shown to depend on the striatum, hippocampus, and prefrontal cortex, whereas visual category expertise has been shown to involve changes in inferior temporal cortex. The goal of the present experiment is to understand the respective roles of these brain regions in the transition from initial learning to expertise when category judgments are being made. Subjects were explicitly trained, over 2 days, to classify realistic faces. Subjects then performed the categorization task during fMRI scanning, as well as a perceptual matching task, in order to characterize how brain regions respond to these faces when not explicitly categorizing them. We found that, during face categorization, face-selective inferotemporal cortex, lateral prefrontal cortex, and dorsal striatum are more responsive to faces near the category boundary, which are most difficult to categorize. In contrast, the hippocampus and left superior frontal sulcus responded most to faces farthest from the category boundary. These dissociable effects suggest that there are several distinct neural mechanisms involved in categorization, and provide a framework for understanding the contribution of each of these brain regions in categorization.
Functional plasticity in ventral temporal cortex following cognitive rehabilitation of a congenital prosopagnosic., 2007 Nov.
Journal of cognitive neuroscience. 19:1790-802. Abstract
We used functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) to measure neural changes associated with training configural processing in congenital prosopagnosia, a condition in which face identification abilities are not properly developed in the absence of brain injury or visual problems. We designed a task that required discriminating faces by their spatial configuration and, after extensive training, prosopagnosic MZ significantly improved at face identification. Event-related potential results revealed that although the N170 was not selective for faces before training, its selectivity after training was normal. fMRI demonstrated increased functional connectivity between ventral occipital temporal face-selective regions (right occipital face area and right fusiform face area) that accompanied improvement in face recognition. Several other regions showed fMRI activity changes with training; the majority of these regions increased connectivity with face-selective regions. Together, the neural mechanisms associated with face recognition improvements involved strengthening early face-selective mechanisms and increased coordination between face-selective and nonselective regions, particularly in the right hemisphere.
Network changes in the transition from initial learning to well-practiced visual categorization., 2009.
Frontiers in human neuroscience. 3:44. Abstract
Visual categorization is a remarkable ability that allows us to effortlessly identify objects and efficiently respond to our environment. The neural mechanisms of how visual categories become well-established are largely unknown. Studies of initial category learning implicate a network of regions that include inferior temporal cortex (ITC), medial temporal lobe (MTL), basal ganglia (BG), premotor cortex (PMC) and prefrontal cortex (PFC). However, how these regions change with extended learning is poorly characterized. To understand the neural changes in the transition from initially learned to well-practiced categorization, we used functional MRI and compared brain activity and functional connectivity when subjects performed an initially learned categorization task (100 trials of training) and a well-practiced task (4250 trials of training). We demonstrate that a similar network is implicated for initially learned and well-practiced categorization. Additionally, connectivity analyses reveal an increased coordination between ITC, MTL, and PMC when making category judgments during the well-practiced task. These results suggest that category learning involves an increased coordination between a distributed network of regions supporting retrieval and representation of categories.
Activity in fusiform face area modulated as a function of working memory load., 2001 Jan.
Brain research. Cognitive brain research. 10:355-64. Abstract
Previous fMRI results suggest that extrastriate visual areas have a predominant role in perceptual processing while the prefrontal cortex (PFC) has a predominant role in working memory. In contrast, single-unit recording studies in monkeys have demonstrated a relationship between extrastriate visual areas and visual working memory tasks. In this study we tested whether activity in both the PFC and fusiform face area (FFA) changed with increasing demands of an n-back task for gray-scale faces. Since stimulus presentation was identical across conditions, the n-back task allowed us to parametrically vary working memory demands across conditions while holding perceptual and motor demands constant. This study replicated the result of PFC areas of activation that increased directly with load n of the task. The novel finding in all subjects was FFA activation that also increased directly with load n of the task. Since perceptual demands were equivalent across the three task conditions, these findings suggest that activity in both the PFC and the FFA vary with face working memory demands.
Dissecting contributions of prefrontal cortex and fusiform face area to face working memory., 2003 Aug 15.
Journal of cognitive neuroscience. 15:771-84. Abstract
Interactions between prefrontal cortex (PFC) and stimulus-specific visual cortical association areas are hypothesized to mediate visual working memory in behaving monkeys. To clarify the roles for homologous regions in humans, event-related fMRI was used to assess neural activity in PFC and fusiform face area (FFA) of subjects performing a delay-recognition task for faces. In both PFC and FFA, activity increased parametrically with memory load during encoding and maintenance of face stimuli, despite quantitative differences in the magnitude of activation. Moreover, timing differences in PFC and FFA activation during memory encoding and retrieval implied a context dependence in the flow of neural information. These results support existing neurophysiological models of visual working memory developed in the nonhuman primate.
Functional MRI studies of spatial and nonspatial working memory., 1998 Jul.
Brain research. Cognitive brain research. 7:1-13. Abstract
Single-unit recordings in monkeys have revealed neurons in the lateral prefrontal cortex that increase their firing during a delay between the presentation of information and its later use in behavior. Based on monkey lesion and neurophysiology studies, it has been proposed that a dorsal region of lateral prefrontal cortex is necessary for temporary storage of spatial information whereas a more ventral region is necessary for the maintenance of nonspatial information. Functional neuroimaging studies, however, have not clearly demonstrated such a division in humans. We present here an analysis of all reported human functional neuroimaging studies plotted onto a standardized brain. This analysis did not find evidence for a dorsal/ventral subdivision of prefrontal cortex depending on the type of material held in working memory, but a hemispheric organization was suggested (i.e., left-nonspatial; right-spatial). We also performed functional MRI studies in 16 normal subjects during two tasks designed to probe either nonspatial or spatial working memory, respectively. A group and subgroup analysis revealed similarly located activation in right middle frontal gyrus (Brodmann’s area 46) in both spatial and nonspatial [working memory-control] subtractions. Based on another model of prefrontal organization [M. Petrides, Frontal lobes and behavior, Cur. Opin. Neurobiol., 4 (1994) 207-211], a reconsideration of the previous imaging literature data suggested that a dorsal/ventral subdivision of prefrontal cortex may depend upon the type of processing performed upon the information held in working memory.