Memory complaints are a common symptom among Subjective Cognitive Decline, Mild Cognitive Impairment and Alzheimer disease. Indeed, episodic memory and Alzheimer disease are highly sensitive to aging. Many brain areas become active when we remember an event from the past. The precuneus, part of the posterior system of default mode network, has been identified as a central node of episodic memory retrieval and a central node in various psychiatric and neurological disorders, such as Alzheimer disease. Age-related changes have been consistently found in the default mode network connectivity, occurring within its posterior areas and with the reduction of connectivity between anterior and posterior subnetwork. It has also been found that aging is associated with decreased functional connectivity in the precuneus and posterior cingulate of the default mode network. Furthermore, γ-Aminobutyric acid (GABA), which plays an essential role in regulating brain activity and modulating the default mode network, shows a decrease throughout the normal course of aging, in Mild Cognitive Impairment and Alzheimer disease. It was also verified a link between cognitive decline and the dysfunction of the inhibitory activity of GABAergic interneurons, particularly parvalbumin-positive interneurons, that play a role in control local circuitry, brain networks and memory processing. Parvalbumin interneurons are the fast-spiking interneurons that generate and maintain gamma frequency. Gamma band has been associated with the encoding and retrieval of episodic memory. Impairments of gamma oscillations have been observed in healthy aging, Mild Cognitive Impairment and Alzheimer disease.

By identifying these functional connectivity changes, non-invasive neuromodulation techniques may offer a new strategy for improving cognitive functions. Transcranial Alternating Current Stimulation (tACS) is a form of non-invasive brain stimulation that modulates cortical activity by applying weak, oscillating electrical currents to the cortex via two or more scalp electrodes. By using alternating currents, it is possible to simulate the rhythmic pattern of electrophysiological activity of the brain. tACS can interact with neural firing at behaviorally relevant frequencies linked to memory.

In this randomized, double-blind, sham-controlled, cross-over study, the investigators will evaluate whether stimulation with gamma tACS on the precuneus can improve episodic memory in healthy older adults.

Memory complaints are a common symptom among Subjective Cognitive Decline, Mild Cognitive Impairment and Alzheimer disease. Indeed, episodic memory and Alzheimer disease are highly sensitive to aging. Many brain areas become active when we remember an event from the past. The precuneus, part of the posterior system of default mode network, has been identified as a central node of episodic memory retrieval and a central node in various psychiatric and neurological disorders, such as Alzheimer disease. Age-related changes have been consistently found in the default mode network connectivity, occurring within its posterior areas and with the reduction of connectivity between anterior and posterior subnetwork. It has also been found that aging is associated with decreased functional connectivity in the precuneus and posterior cingulate of the default mode network. Furthermore, γ-Aminobutyric acid (GABA), which plays an essential role in regulating brain activity and modulating the default mode network, shows a decrease throughout the normal course of aging, in Mild Cognitive Impairment and Alzheimer disease. It was also verified a link between cognitive decline and the dysfunction of the inhibitory activity of GABAergic interneurons, particularly parvalbumin-positive interneurons, that play a role in control local circuitry, brain networks and memory processing. Parvalbumin interneurons are the fast-spiking interneurons that generate and maintain gamma frequency. Gamma band has been associated with the encoding and retrieval of episodic memory. Impairments of gamma oscillations have been observed in healthy aging, Mild Cognitive Impairment and Alzheimer disease.

By identifying these functional connectivity changes, non-invasive neuromodulation techniques may offer a new strategy for improving cognitive functions. Transcranial Alternating Current Stimulation (tACS) is a form of non-invasive brain stimulation that modulates cortical activity by applying weak, oscillating electrical currents to the cortex via two or more scalp electrodes. By using alternating currents, it is possible to simulate the rhythmic pattern of electrophysiological activity of the brain. tACS can interact with neural firing at behaviorally relevant frequencies linked to memory.

In this randomized, double-blind, sham-controlled, cross-over study, the investigators will evaluate whether stimulation with gamma tACS on the precuneus can improve episodic memory in healthy older adults.

Participants will be randomized into two groups in a 1:1 ratio, and the order of administration of either intervention (i.e., gamma-tACS or sham) will be counterbalanced across all participants. Stimulation will be inverted (crossover phase) after 1 week (wash-out). Participants will be evaluated with neuropsychological tests (AVLT and FNMT) and neurophysiological measures of functional connectivity (rs EEG).

Inclusion Criteria:

• Older adults aged 65 or over;
• Normal general cognition: score within the normative values for the Portuguese population in the Addenbrooke's Cognitive Examination

Exclusion Criteria:

• Neurological disorders (stroke, tumours, significant head trauma, and epilepsy), psychiatric conditions (such as autism, schizophrenia, depression), or uncontrolled medical illness (hypertension, metabolic, endocrine, toxic, and infectious diseases) able to interfere with cognition;
• Sensory deficits likely to interfere with neuropsychological assessment;
• Impairment in activities of daily living;
• Chronic alcohol or drug abuse;
• Non-native Portuguese speakers;
• Metal implants on the head or implanted electronic devices.

All data, including outcome measure results, study protocol and statistical analysis plan, will be shared upon reasonable request

Data will be shared after the study completion indefinitely.

Reasonable request