Effect of Noninvasive Electrical Brain Stimulation on Memory at Different Times of Day in Younger and Older Adults

This study will investigate the extent to which tDCS to dorsolateral prefrontal cortex (or dlPFC) impacts memory performance as a function of time-of-day in younger and older adults.

Transcranial Direct Current Stimulation (tDCS) is the safest and most accessible, non-invasive brain stimulation technique available for testing causal links between different brain regions and functions, by manipulating cognitive abilities. By identifying key experimental factors that can improve the reliability and robustness of stimulation effects on cognitive performance in different age groups, this project should lead to the widespread adoption of these design features in future applications. This study will investigate the extent to which tDCS to dorsolateral prefrontal cortex (or dlPFC) impacts recollection accuracy and working memory performance as a function of time-of-day in younger and older adults. Moreover, this study will test the extent that tDCS to dlPFC impacts memory performance by impacting information-specific processes and/or cognitive control processes that operate across different types of information, thereby informing basic theories of how dlPFC contributes to memory in younger and older adults.

Eligibility Criteria:
Inclusion Criteria:

* Right-handed (according to the Edinburgh Handedness Inventory)
* Normal or corrected vision
* Fluent in English (started learning by age 6)
* Ability to understand and provide informed consent for study procedures, and to comply - with study procedures for the entire length of the study.
* For individuals in the 'younger adults' group, must be between 18 and 30 years of age
* For individuals in the 'older adults' group, must be between 60 and 75 years of age
* A score of 23 or above on the Montreal Cognitive Assessment (out of 30, education-corrected) is required. This is to minimize the inclusion of suspected mild cognitive impairment (MCI) or dementia, targeting individuals that score in the normal range according to the recent meta-analysis of MoCA's ability to differentiate normal aging from MCI in Carson et al. (2018, Int. J of Geriatric Psychiatry).
* Performance above threshold on the episodic memory task during the baseline session. The threshold is defined as having a hit rate that is at least 5% greater than the false alarm rate, where hit rate is defined as the number of studied items identified as studied, divided by the total number of studied items, and false alarm rate is defined as the number of new items identified as studied, divided by the total number of new items. We don't anticipate this threshold to exclude many, if any subjects.

Exclusion Criteria:

* Neuropsychological conditions associated with cognitive decline or seizure
* Cochlear implants or metal in the brain/skull (except titanium)
* Psychoactive medications, or diagnosis of depression, bipolar disorder, or any psychotic diagnoses
* History of excessive use (clinically treated) alcohol or narcotics
* Hospitalization for head trauma (e.g. concussions) in the past 5 years
* Individuals above a threshold score on an assessment of depression, specifically, a score of 10 or above on the Patient Health Questionnaire (PHQ-9)(Manea et al., 2012)
* Risk of pregnancy
* Low tolerance of skin irritation
* Prior brain stimulation experience (self-report)
* Ongoing cognitive or sensory deficits/symptoms from a previous (or current) COVID-19 infection.