This randomized, double-blind, sham-controlled clinical trial evaluates the effects of transcranial direct current stimulation (tDCS) using the Sooma Oy device on cognitive function in older adults with Subjective Cognitive Decline (SCD). The study investigates changes in memory, executive function, attention, and global cognition after a structured intervention consisting of 22 intensive sessions and 8 maintenance sessions. Assessments occur at baseline, Week 4, Week 8, and Week 12.

This study uses the Sooma tDCS system (Sooma Oy, Finland) to deliver a current of 2 mA for 30 minutes per session. The anode is positioned over F3 (left dorsolateral prefrontal cortex) and the cathode over F4 (right dorsolateral prefrontal cortex), according to the international 10-20 system. Participants receive 22 intensive sessions (five days per week) followed by 8 weekly maintenance sessions. The sham condition mimics the stimulation procedure through a 30-second ramp-up and ramp-down period without sustained active stimulation.

Primary and secondary outcomes focus on changes in cognitive function, assessed using validated neuropsychological instruments.

In addition, the study includes the analysis of molecular biomarkers associated with calcium (Ca²⁺) homeostasis, as age-related changes disrupt multiple electrophysiological processes within the hippocampus. These alterations have been proposed as potential biomarkers of brain aging and are closely linked to cognitive decline. Biological samples will be obtained through buccal swabs using sterile cotton applicators from participating older adults. Total RNA will then be extracted using the TRIzol method, followed by the design of specific primer oligonucleotides and amplification using a commercial RT-PCR kit.

This clinical trial aims to generate evidence on the therapeutic utility of tDCS in individuals with Subjective Cognitive Decline, supporting early intervention strategies and contributing to the identification of molecular correlates associated with cognitive changes.

Inclusion Criteria:

1. Right-handed (for MRI purposes)
2. Age 60 to 75 years
3. 6 years or more of schooling
4. Proficiency in Spanish
5. Amnestic Mild Cognitive Impairment established by clinical examination
6. Adequate visual and auditory acuity to perform neuropsychological tests and undergo cognitive rehabilitation
7. If receiving psychotropic medication, to have started it at least 12 weeks prior to the start of the study, remain at stable doses, or have discontinued it for at least 4 weeks
8. Good general health without medical or psychiatric illnesses that would interfere with the study
9. An informant capable of responding to clinimetric assessments throughout the study and willing to accompany the patient for at least 10 hours per week

Exclusion Criteria:

• 1. Any neurological disease other than Alzheimer's disease that raises suspicion of cognitive impairment, such as Parkinson's disease, multiple infarct dementia, Huntington's disease, hydrocephalus, brain tumor, progressive supranuclear palsy, seizure disorder, subdural hematoma, multiple sclerosis, or a history of traumatic brain injury with loss of consciousness.

  2. Participants with a history of severe psychiatric disorders according to the DSM-5 (bipolar disorder, schizophrenia, chronic depression) or with psychotic features, agitation, or behavioral problems in the last three months that could lead to difficulties in complying with the protocol.

  3. History of psychoactive substance abuse and current alcohol consumption with a pattern of abuse or dependence in the last two years.

  4. Participants with abnormalities on a conventional electroencephalogram (paroxysmal phenomena identified by a neurophysiologist). 5. Participants with pacemakers, intracranial metallic objects or a history of brain surgery, aneurysm clips, artificial heart valves, ear implants, metal fragments or foreign objects in the eyes, skin or body.

  5. Participants with pacemakers, intracranial metallic objects or a history of brain surgery, aneurysm clips, artificial heart valves, ear implants, metal fragments, or foreign objects in the eyes, skin, or body.

  6. Participation in clinical studies with neuropsychological measures taken more than once a year.

  7. Having received prior treatment.

This study represents the first time the research team has registered a clinical trial; therefore, the process of defining and classifying outcome measures has been carried out with particular care to ensure methodological clarity and coherence.

In addition to clinical and neuropsychological outcomes, the study includes an exploratory measurement of molecular biomarkers related to calcium (Ca²⁺) homeostasis, as age-related changes disrupt multiple electrophysiological processes, particularly at the hippocampal level. These alterations have been proposed as potential biomarkers of cognitive decline.

The inclusion of this measurement is complementary and exploratory in nature, and its purpose is to provide additional information that allows for a more comprehensive interpretation of the cognitive changes observed following transcranial direct current stimulation (tDCS). The collection of biological samples through buccal swabbing constitutes a non-invasive, safe, and appropriate proced