Siobhán Harty

Psychobiotics and the Manipulation of Bacteria–Gut–Brain Signals

Psychobiotics were previously defined as live bacteria (probiotics) which, when ingested, confer mental health benefits through interactions with commensal gut bacteria. We expand this definition to encompass prebiotics, which enhance the growth of beneficial gut bacteria. We review probiotic and prebiotic effects on emotional, cognitive, systemic, and neural variables relevant to health and disease. We discuss gut–brain signalling mechanisms enabling psychobiotic effects, such as metabolite production. Overall, knowledge of how the microbiome responds to exogenous influence remains limited. We tabulate several important research questions and issues, exploration of which will generate both mechanistic insights and facilitate future psychobiotic development. We suggest the definition of psychobiotics be expanded beyond probiotics and prebiotics to include other means of influencing the microbiome.

Transcranial Direct Current Stimulation over Right Dorsolateral Prefrontal Cortex Enhances Error Awareness in Older Age

The ability to detect errors during cognitive performance is compromised in older age and in a range of clinical populations. This study was designed to assess the effects of transcranial direct current stimulation (tDCS) on error awareness in healthy older human adults. tDCS was applied over DLPFC while subjects performed a computerized test of error awareness. The influence of current polarity (anodal vs cathodal) and electrode location (left vs right hemisphere) was tested in a series of separate single-blind, Sham-controlled crossover trials, each including 24 healthy older adults (age 65–86 years). Anodal tDCS over right DLPFC was associated with a significant increase in the proportion of performance errors that were consciously detected, and this result was recapitulated in a separate replication experiment. No such improvements were observed when the homologous contralateral area was stimulated. The present study provides novel evidence for a causal role of right DLPFC regions in subserving error awareness and marks an important step toward developing tDCS as a tool for remediating the performance-monitoring deficits that afflict a broad range of populations.

Neural evidence accumulation persists after choice to inform metacognitive judgments

The ability to revise one’s certainty or confidence in a preceding choice is a critical feature of adaptive decision-making but the neural mechanisms underpinning this metacognitive process have yet to be characterized. In the present study, we demonstrate that the same build-to-threshold decision variable signal that triggers an initial choice continues to evolve after commitment, and determines the timing and accuracy of self-initiated error detection reports by selectively representing accumulated evidence that the preceding choice was incorrect. We also show that a peri-choice signal generated in medial frontal cortex provides a source of input to this post-decision accumulation process, indicating that metacognitive judgments are not solely based on the accumulation of feedforward sensory evidence. These findings impart novel insights into the generative mechanisms of metacognition. DOI: http://dx.doi.org/10.7554/eLife.11946.001

Older adults have diminished awareness of errors in the laboratory and daily life.

Poor recognition of the degree or scope of ones own cognitive deficits is a common feature of many neurological conditions, including diseases of aging, but little is known about the impact the natural aging process has on this aspect of self-awareness (SA). Here, a group of 45 healthy older adults and a comparison group of 45 young adults completed a multidomain assessment of SA. Awareness of daily functioning was evaluated based on discrepancies between self- and informant ratings on questionnaire measures of attentional control, memory functioning, and socioemotional functioning. Online error awareness was also assessed using a variant of the Go/No-Go Error Awareness Task (EAT) in which participants are required to signal commission errors via a separate manual response. Whereas younger participants tended to underestimate their attentional control and memory functioning relative to informant reports, older adults significantly overestimated their abilities. The older adults also exhibited substantially poorer online error awareness compared with young adults, despite the fact that the two groups were matched for overall accuracy. Levels of online error awareness were significantly correlated with discrepancy scores for daily attentional and memory functioning, and with performance of a sustained attention task. These novel findings suggest that an important aspect of the neuropsychology of healthy aging has hitherto been overlooked.

The Influence of Prebiotics on Neurobiology and Behavior.

Manipulating the intestinal microbiota for the benefit of the brain is a concept that has become widely acknowledged. Prebiotics are nondigestible nutrients (i.e., fibers, carbohydrates, or various saccharides) that proliferate intrinsic, beneficial gut bacteria, and so provide an alternative strategy for effectively altering the enteric ecosystem, and thence brain function. Rodent studies demonstrating neurobiological changes following prebiotic intake are slowly emerging, and have thus far revealed significant benefits in disease models, including antiinflammatory and neuroprotective actions. There are also compelling data showing the robust and favorable effects of prebiotics on several behavioral paradigms including, anxiety, learning, and memory. At present, studies in humans are limited, though there is strong evidence for prebiotics modulating emotional processes and the neuroendocrine stress response that may underlie the pathophysiology of anxiety. While the mechanistic details linking the enteric microbiota to the central nervous system remain to be elucidated, there are a number of considerations that can guide future studies. These include the modulation of intestinal endocrine systems and inflammatory cascades, as well as direct interaction with the enteric nervous system and gut mucosa. Our knowledge of gut microbiome-brain communication is steadily progressing, and thorough investigations validating the use of prebiotics in the treatment of neuropsychiatric disorders would be highly valued and are encouraged.

Mind the Brain: The Mediating and Moderating Role of Neurophysiology

Most studies involving experimental manipulations or interventions tailored to modulate behavior do not account for variability in the critical antecedent of behavior, the brain. Here, we describe elegant approaches to model the role that neurophysiology can play in mediating or moderating relationships in this context. We highlight the capacity for these approaches to improve the inferential power of research, bridge the gap between neural and behavioral levels of analysis, and bolster the prospects for reproducibility.

Parsing the neural signatures of reduced error detection in older age

Abstract Recent work has demonstrated that explicit error detection relies on a neural evidence accumulation process that can be traced in the human electroencephalogram (EEG). Here, we sought to establish the impact of natural aging on this process by recording EEG from young (18–35 years) and older adults (65–88 years) during the performance of a Go/No‐Go paradigm in which participants were required to overtly signal their errors. Despite performing the task with equivalent accuracy, older adults reported substantially fewer errors, and the timing of their reports were both slower and more variable. These behavioral differences were linked to three key neurophysiological changes reflecting distinct parameters of the error detection decision process: a reduction in medial frontal delta/theta (2–7 Hz) activity, indicating diminished top‐down input to the decision process; a slower rate of evidence accumulation as indexed by the rate of rise of a centro‐parietal signal, known as the error positivity; and a higher motor execution threshold as indexed by lateralized beta‐band (16–30 Hz) activity. Our data provide novel insight into how the natural aging process affects the neural underpinnings of error detection. HighlightsOlder adults exhibit reduced error detection relative to young adults.Neural signals reflecting the strength of error evidence are diminished in older adults.Older adults also exhibit a higher response threshold in decision‐related motor signals.A conventional modeling approach only partially captures the neural data.

Neurocognitive Effects of tDCS in the Healthy Brain

This chapter provides an overview of the literature concerning the effects of tDCS on high-level cognitive functions in young healthy adults. tDCS has been found to modulate a multitude of components of cognition, but here we place a particular focus on studies that have examined working memory, attention, language, numerical cognition, general learning and memory. We additionally devote latter portions of the chapter to evaluating two other pertinent topics: the neurocognitive effects of tDCS in the healthy older brain and individual differences in the context of tDCS outcomes. Based on the studies reviewed, we conclude that tDCS holds substantial promise as a tool for exploring novel theoretical hypotheses, as well as for improving cognitive functions in both young and older healthy adults. However, the coherence of the evidence base and the translational potential of these findings is currently constrained by a number of factors, including pervasive inter-individual differences in response to tDCS, heterogeneity of tDCS protocols across studies and inadequate knowledge about the longevity of the effects.

The Microbiome in Psychology and Cognitive Neuroscience

Psychology and microbiology make unlikely friends, but the past decade has witnessed striking bidirectional associations between intrinsic gut microbes and the brain, relationships with largely untested psychological implications. Although microbe-brain relationships are receiving a great deal of attention in biomedicine and neuroscience, psychologists have yet to join this journey. Here, we illustrate microbial associations with emotion, cognition, and social behavior. However, despite considerable enthusiasm and potential, technical and conceptual limitations including low statistical power and lack of mechanistic descriptions prevent a nuanced understanding of microbiome-brain-behavior relationships. Our goal is to describe microbial effects in domains of cognitive significance and the associated challenges to stimulate interdisciplinary research on the contribution of this hidden kingdom to psychological processes.

Modulating hemispheric lateralization by brain stimulation yields gain in mental and physical activity

Imagery plays an important role in our life. Motor imagery is the mental simulation of a motor act without overt motor output. Previous studies have documented the effect of motor imagery practice. However, its translational potential for patients as well as for athletes, musicians and other groups, depends largely on the transfer from mental practice to overt physical performance. We used bilateral transcranial direct current stimulation (tDCS) over sensorimotor areas to modulate neural lateralization patterns induced by unilateral mental motor imagery and the performance of a physical motor task. Twenty-six healthy older adults participated (mean age = 67.1 years) in a double-blind cross-over sham-controlled study. We found stimulation-related changes at the neural and behavioural level, which were polarity-dependent. Specifically, for the hand contralateral to the anode, electroencephalographic activity induced by motor imagery was more lateralized and motor performance improved. In contrast, for the hand contralateral to the cathode, hemispheric lateralization was reduced. The stimulation-related increase and decrease in neural lateralization were negatively related. Further, the degree of stimulation-related change in neural lateralization correlated with the stimulation-related change on behavioural level. These convergent neurophysiological and behavioural effects underline the potential of tDCS to improve mental and physical motor performance.