Abstract
Alpha oscillations (8–12 Hz) are thought to inversely correlate with cortical excitability. Goal-oriented modulation of alpha has been studied extensively. In visual spatial attention, alpha over the region of visual cortex corresponding to the attended location decreases, signifying increased excitability to facilitate the processing of impending stimuli. In contrast, in retention of verbal working memory, alpha over visual cortex increases, signifying decreased excitability to gate out stimulus input to protect the information held online from sensory interference. According to the prevailing model, this goal-oriented biasing of sensory cortex is effected by top-down control signals from frontal and parietal cortices. The present study tests and substantiates this hypothesis by (a) identifying the signals that mediate the top-down biasing influence, (b) examining whether the cortical areas issuing these signals are task-specific or task-independent, and (c) establishing the possible mechanism of the biasing action. High-density human EEG data were recorded in two experimental paradigms: a trial-by-trial cued visual spatial attention task and a modified Sternberg working memory task. Applying Granger causality to both sensor-level and source-level data we report the following findings. In covert visual spatial attention, the regions exerting top-down control over visual activity are lateralized to the right hemisphere, with the dipoles located at the right frontal eye field (FEF) and the right inferior frontal gyrus (IFG) being the main sources of top-down influences. During retention of verbal working memory, the regions exerting top-down control over visual activity are lateralized to the left hemisphere, with the dipoles located at the left middle frontal gyrus (MFG) being the main source of top-down influences. In both experiments, top-down influences are mediated by alpha oscillations, and the biasing effect is likely achieved via an inhibition-disinhibition mechanism.
Highlights
It has been firmly established that posterior alpha oscillations can be modulated in a goal-oriented fashion by attention (Pfurtscheller et al, 1997; Shaw, 2003)
When attention is directed to external visual events, alpha power in visual cortex decreases with attention (Worden et al, 2000; Sauseng et al, 2005b; Rajagovindan and Ding, 2011); when attention is directed to internal representations, Top-Down Control of Visual Alpha Oscillations such as during visual imagery and retention of working memory, alpha power increases with attention (Klimesch et al, 1999; Jensen et al, 2002; Cooper et al, 2003; Tuladhar et al, 2007)
Goal-oriented modulations of visual alpha oscillations have been observed in numerous attention paradigms
Summary
It has been firmly established that posterior alpha oscillations can be modulated in a goal-oriented fashion by attention (Pfurtscheller et al, 1997; Shaw, 2003). Goal-oriented sensory biasing is thought to be effected by top-down signals propagating from higher-order brain areas in a topographic and modality specific manner via long-range projections (Kastner and Ungerleider, 2000; Pessoa et al, 2003; van Ede et al, 2010). Traditional approaches to the testing of this hypothesis involve observing changes in sensory cortex by (1) manipulating experimental instructions (Kastner and Ungerleider, 2000; Corbetta and Shulman, 2002; Pessoa et al, 2003; Woldorff et al, 2004), (2) stimulating frontal-parietal networks (Armstrong et al, 2006; Capotosto et al, 2009; Sauseng et al, 2011; Hsu et al, 2014; Jaegle and Ro, 2014), and (3) recording from stroke patients with frontal-parietal lesions (Knight et al, 1999; Barceló et al, 2000; Heilman et al, 2000). In the case of naturally occurring lesions such as stroke the extent of damage is not controllable
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