Stimuls-Present Modulation.
One of first research goals of the lab, and still a major focus, was to study top-down modulation of neural activity in visual regions of the brain while relevant and irrelevant visual stimuli were presented in the context of a working memory task. Distinct measures of
enhancement and
suppression were obtained utilizing a paradigm developed by Dr. Gazzaley. The experiment consisted of three tasks in which participants observed sequences of two faces and two natural scenes presented in a randomized order. The tasks differed based on the instructions, which informed the participants how to process the stimuli: 1) remember faces and ignore scenes, 2) remember scenes and ignore faces, or 3) passively view faces and scenes without attempting to remember them. In each task, the time period when the cue stimuli were presented contained the same overall visual information, thus any activity differences between the tasks in visual cortex must reflect the influence of goal-directed decisions on neural activity (i.e., top-down modulation).
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The initial series of experiments with this paradigm utilized fMRI and EEG during different sessions to record neural activity while participants performed the task. The fMRI and EEG data revealed top-down modulation of both activity magnitude and speed of synchronization above and below the passive viewing baseline depending on task instructions, thus defining functional markers of enhancement and suppression (Gazzaley et al., JOCN 2005) [pdf]. Recent functional connectivity analysis of the fMRI data suggests that connections from the prefrontal cortex bias activity in the visual cortex in accordance with the relevance of the stimuli to the task (Gazzaley et al., Cerebral Cortex 2007) [pdf].
This paradigm has now been used to explore changes in top-down modulaiton that occur with
normal aging. Current experiments in the lab are using newly designed versions of the paradigm to study other aspects of stimulus-present modulation; e.g., modulation associated with attention to
faces and scenes presented simultaneously and attention to
motion and color.
Stimulus- Absent Modulation.
In addition to exploring mechanisms involved in modulation when a stimulus is present in the environment, the lab also focuses on top-down modulation when a stimulus is absent. Applying a recently developed functional connectivity method, the network associated with maintaining a representation of a face in mind over a brief period of time was described (Gazzaley et al. CABN 2005) [pdf]. Specifically, the brain regions whose activity significantly correlated with the visual association cortex during a period of visual working memory maintenance included the dorsolateral and ventrolateral prefrontal cortex, premotor cortex, intraparietal sulcus, caudate nucleus, thalamus, hippocampus, and occipitotemporal regions. These findings support the notion that the coordinated functional interaction between nodes of a widely distributed network underlies the active maintenance of a representation.
Research in the lab is now directed at exploring how this memory network changes with aging and with different task demands. Further studies have been initiated to characterize stimulus-absent, top-down modulation involved in mental imagery and when an ensuing stimulus is anticipated.
> Cognitive Aging
