Neuroplasticity of attention: How brain stimulation and mental fatigue affect attentional performance

Attention allows us to focus on what is relevant and to ignore what is not. While we call upon attention at every waking moment, it is not static: we cannot sustain attention indefinitely, and often fall prey to distractions. This PhD thesis is a study of the short-term neuroplasticity of attentional processes: how susceptible is attention to change, and what processes in the brain (neuro-) give rise to changes in attention (-plasticity)? In Chapters 2–5, I examined whether attention can be improved with electrical stimulation of the brain, in the form of transcranial Direct Current Stimulation (tDCS). Previous studies that attempted to use tDCS to enhance attention have yielded promising, but inconsistent results (reviewed in Chapter 2). My attempt to enhance spatial attention with tDCS (Chapter 3) was unsuccessful, as stimulation of the frontal eye fields did not lead to changes in eye movements. Applying tDCS over the dorsolateral prefrontal cortex also did not enhance temporal attention (Chapters 4 and 5), as participants’ performance on an attentional blink task remained unchanged. In Chapter 6, I investigated the opposite effect: decreases in attention, when attention has to be sustained for a long time. Using EEG, I tracked whether similar decreases occurred in different attention-related signals in the brain. tDCS may one day be used to counteract these declines, or to relieve other deficits in attention. However, barring a deeper understanding of the technique and more large-scale studies of its efficacy, such practical applications of tDCS are not yet feasible

Transcranial electrical stimulation as a tool to enhance attention

Attention is a fundamental cognitive process—without it, we would be helplessly adrift in an overload of sensory input. There is considerable interest in techniques that can be used to enhance attention, including transcranial electrical stimulation (tES). We present an overview of 52 studies that have paired attention tasks with tES, mostly in the form of transcranial direct current stimulation (tDCS). In particular, we discuss four aspects of attention that have been most extensively targeted to date: visual search, spatial orienting (e.g., Posner cueing tasks), spatial bias (e.g., line bisection tasks), and sustained attention. Some promising results have been reported in each of these domains. However, drawing general conclusions about the efficacy of tES is at present hampered by a large diversity in study design and inconsistent findings. We highlight some pitfalls and opportunities and suggest how these may be addressed in future research aiming to use tES as a tool to enhance or test theoretical hypotheses about attention

No Differential Effects of Two Different Alpha-Band Electrical Stimulation Protocols Over Fronto-Parietal Regions on Spatial Attention

In a previous study using transcranial alternating current stimulation (tACS), we found preliminary evidence that phase coherence in the alpha band (8–12 Hz) within the fronto-parietal network may critically support top-down control of spatial attention (van Schouwenburg et al., 2017). Specifically, synchronous alpha-band stimulation over the right frontal and parietal cortex (0° relative phase) was associated with changes in performance and fronto-parietal coherence during a spatial attention task as compared to sham stimulation. In the current study, we firstly aimed to replicate these findings with synchronous tACS. Second, we extended our previous protocol by adding a second tACS condition in which the right frontal and parietal cortex were stimulated in a desynchronous fashion (180° relative phase), to test the specificity of the changes observed in our previous study. Participants (n = 23) were tested in three different sessions in which they received either synchronous, desynchronous, or sham stimulation over the right frontal and parietal cortex. In contrast to our previous study, we found no spatially selective effects of stimulation on behavior or coherence in either stimulation protocol compared to sham. We highlight some of the differences in study design that may have contributed to this discrepancy in findings and more generally may determine the effectiveness of tACS

No Evidence that Predictions and Attention Modulate the First Feedforward Sweep of Cortical Information Processing

Predictive coding models propose that predictions (stimulus likelihood) reduce sensory signals as early as primary visual cortex (V1), and that attention (stimulus relevance) can modulate these effects. Indeed, both prediction and attention have been shown to modulate V1 activity, albeit with fMRI, which has low temporal resolution. This leaves it unclear whether these effects reflect a modulation of the first feedforward sweep of visual information processing and/or later, feedback-related activity. In two experiments, we used electroencephalography and orthogonally manipulated spatial predictions and attention to address this issue. Although clear top-down biases were found, as reflected in pre-stimulus alpha-band activity, we found no evidence for top-down effects on the earliest visual cortical processing stage (<80 ms post-stimulus), as indexed by the amplitude of the C1 event-related potential component and multivariate pattern analyses. These findings indicate that initial visual afferent activity may be impenetrable to top-down influences by spatial prediction and attention