Projects
Task-dependent changes in cortical effective connectivity
It has been suggested that both attending to and remembering different kinds of visual information (e.g., an object's color or shape) depends on task-specific functional interactions between regions implicated in executive control functions, such as the prefrontal cortex, and posterior visual areas representing specific sensory features. However, most of the work exploring this issue has relied on correlational methods such as functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). Although these methods are very useful for detecting changes in brain activity that are correlated with the performance of particular cognitive tasks, by themselves they are insufficient to demonstrate the causal necessity of such activity for a given cognitive function. Additionally, they can tell us relatively little about how activity within interconnected cortical areas is coordinated, or about causal interactions among co-activated brain regions.
To overcome these limitations, our research utilizes a combination of noninvasive brain stimulation (TMS), EEG, and EEG source modeling techniques, which make it possible to explore causal interactions among activated brain networks both at rest and during the performance of cognitive tasks. Specifically, one line of research currently under way in the lab uses these methods to 1) test the hypothesis that the active retention of information in WM relies on causal interactions between the prefrontal cortex (PFC) and regions of the posterior cortex involved in the representation of sensory information; and 2) to provide evidence for selective changes in patterns of causal interaction between the PFC and the posterior cortex as a function of task demands (e.g., during WM for color versus motion).
The role of neural oscillations in working memory
A third line of work in the lab focuses on the contribution of alpha-band oscillations to VWM. Prior work suggests that increased alpha-band power during the delay period of VWM tasks reflects the inhibition of task-irrelevant visual areas-e.g., of dorsal stream areas during WM tasks engaging the ventral stream. Contrary to this prominent proposal, research conducted by Dr. Johnson (Johnson, Sutterer, et al., 2011) revealed elevated delay-period alpha-band power over posterior electrodes both during the retention of abstract shapes and when subjects were required to remember the shape and the spatial location of perceived targets (i.e., when all of the information in WM was task relevant). This suggests that elevated alpha-band power observed in WM tasks may be related to the selection and maintenance of object representations in VWM rather than, or in addition to, the inhibition of task-irrelevant information.
Ongoing projects in the lab extend this line of research to explore whether spontaneous fluctuations in alpha-band power during the delay interval of feature WM tasks predict the brain's response to irrelevant distractor probes. If delay-period alpha-band oscillations serve an inhibitory gating function, protecting WM representations by suppressing activity in task-irrelevant visual areas, the brain's response to task-irrelevant probe stimuli would be expected to vary with the strength of alpha-band oscillations.
A dynamic field approach to working memory
Finally, theoretical work in the lab is focused on the development and testing of neurally-plausible models of WM. With his collaborators, Dr. John Spencer and Dr. Gregor Schöner, Dr. Johnson developed a neural field model of WM and change detection that addressed the encoding and maintenance of information in WM and the process by which the contents of WM are compared to current perceptual inputs (Johnson, Spencer, & Schöner, 2008, 2009; Johnson, Spencer, Luck, & Schöner, 2009). Within this framework, maintenance is mediated by locally excitatory and laterally inhibitory interactions among neurons, and change detection is realized through interactions among the model's layers. The proposed model has generated several predictions that have been confirmed in behavioral experiments (Johnson, Dineva, & Spencer, in progress; Johnson, Spencer, Luck, & Schöner, 2009). In collaboration with Dr. Vanessa Simmering, current work within this framework is focused on examining common assumptions regarding the nature of capacity limits observed in WM experiments, and current debates regarding the nature of the representations maintained in WM.