Clinical and Neurophysiological Effects of tDCS on Depression in Parkinson's Disease

The Beck Depression Inventory-II (BDI-II) is a 21-item, self-report rating inventory that measures characteristic attitudes and symptoms of depression. Each item is scored on a scale from 0 to 3, with total scores ranging from 0 to 63. Higher scores indicate more severe depressive symptoms.

The Montgomery-Åsberg Depression Rating Scale (MADRS) is a ten-item diagnostic questionnaire which mental health professionals use to measure the severity of depressive episodes in patients with mood disorders. Each item is rated from 0 to 6, leading to a total score range of 0 to 60. Higher scores indicate more severe depression.

The Visual Analog Scale (VAS) for mood is a widely used tool to assess a person's emotional state. It consists of a straight line, usually 100 mm in length, with one end representing the most negative mood state (e.g., very sad, depressed) and the other end representing the most positive mood state (e.g., very happy, elated). The individual marks a point on the line that corresponds to their current mood, and the distance from the "negative" end to the mark is then measured in millimeters, providing a quantitative measure of the individual's emotional state. Scores range from 0 to 10, with higher scores indicating a more positive mood. This scale is easy to use and provides a subjective yet reliable way to track mood changes over time.

The State-Trait Anxiety Inventory (STAI) is a psychological inventory consisting of 40 self-report items on a 4-point Likert scale. The STAI measures two types of anxiety - state anxiety and trait anxiety. Total scores range from 20 to 80 for each subscale, with higher scores indicating greater anxiety.

The Apathy Evaluation Scale (AES) is a 18-item self-report questionnaire assessing apathy in behavioral, cognitive, and emotional domains. Each item is rated on a 4-point Likert scale (1 = not at all, 4 = a lot). Total scores range from 18 to 72, with higher scores indicating greater levels of apathy.

The Parkinson's Disease Questionnaire-8 (PDQ-8) is a self-report measure derived from the PDQ-39. It assesses health-related quality of life in individuals with Parkinson's disease across eight key domains, providing a brief yet effective evaluation of disease impact. It consists of 8 items, each rated on a 5-point Likert scale (0 = never, 4 = always). The total score is transformed to a 0-100 scale, where higher scores indicate worse health-related quality of life.

The Pittsburgh Sleep Quality Index (PSQI) is a 19-item self-report questionnaire assessing sleep quality over the past month. It evaluates seven components (e.g., sleep duration, disturbances, latency, and daytime dysfunction), each scored from 0 to 3, yielding a global score ranging from 0 to 21. Higher scores indicate poorer sleep quality

The EEG peak frequency in the beta band (13-30 Hz) reflects cortical activity associated with cognitive processing, alertness, arousal, and emotional regulation. Changes in peak beta frequency may indicate alterations in neurophysiological functioning. Higher values are generally linked to increased cortical activation, while lower values may reflect reduced cortical excitability.

The blink reflex (BR) area quantifies the overall amplitude of the reflexive eye closure response to external stimuli. It provides information about brainstem excitability and neuromuscular response intensity.

The blink reflex (BR) duration represents the total time from the initiation to the completion of the reflexive eye closure. Prolonged duration may reflect altered neural conduction or motor control.

The blink reflex (BR) latency is the time interval between the stimulus and the onset of the eye closure response. It serves as an indicator of neural transmission efficiency within the trigeminal-facial circuitry.

The startle saccadic reflex (SSR) area measures the magnitude of the rapid eye movement response to a sudden stimulus, providing insight into sensorimotor integration and brainstem responsiveness.

The startle saccadic reflex (SSR) duration refers to the total time span of the reflexive saccadic eye movement. Prolonged duration may reflect changes in neuromuscular coordination and sensorimotor processing.

The startle saccadic reflex (SSR) latency is the time interval between the presentation of a sudden stimulus and the initiation of the saccadic eye movement. It provides a measure of neural conduction speed in reflex pathways.