Transcutaneous auricular vagus nerve stimulation and emotional responding in borderline personality disorder: a randomized single-blind, sham-controlled study.

Attention deficit hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder in children and adolescents and is accompanied by symptoms of inattention, hyperactivity, and impulsivity. In addition to core symptoms, impairments, such as emotional, social, and sensory processing, may also be observed at the neurophysiological level. The literature suggests that approximately 50–60% of children with ADHD present with sensory processing difficulties. Sensory processing problems can impair daily functioning, cognitive functions, emotion regulation and psychosocial development. Sensory processing is defined as the recognition, interpretation, and organization of sensations in order to generate appropriate responses. Sensory processing facilitates adaptive behaviour by enabling individuals to respond appropriately to situations and contributes to their overall functioning and psychophysical well-being. Sensory processing problems can affect children’s behaviour, cognitive functions, learning difficulties, emotions and attention regulation, psychosocial development and participation in social relationship in their daily lives. There are various types of atypical sensory processing, such as hyporeactivity, which is associated with a low neurological threshold, and hyperreactivity, which is associated with a high neurological threshold, sensory seeking and sensory discrimination difficulties. Dunn’s sensory processing model explains the nervous system’s stimulus perception thresholds and the response tendencies to these stimuli. This is closely related to habituation (reduced neural transmission once the stimulus becomes familiar) and sensitization (enhanced neural response when the stimulus is perceived as salient) in the central nervous system. When the balance between habituation and sensitivity is impaired, individuals exhibit maladaptive behaviours such as being overly excitable or hyperactive (too sensitive, low thresholds) or excessively lethargic and inattentive (excessive habituation, high thresholds). Sensory processing patterns are classified into four quadrants based on the interaction between individuals’ neurological threshold level and their behavioural response to stimuli. These quadrants are defined as: sensory sensitivity (associated with a low threshold and passive response), sensation avoiding (associated with a low threshold and active response), sensation seeking (associated with a high threshold and active response), and low registration (associated with a high threshold and passive response). Atypical sensory processing can be observed in children with ADHD in various levels.Several studies have reported that individuals with ADHD exhibit at least clinically/subclinically detectable hyposensitivity/hypersensitivity to auditory, visual, and tactile stimuli. Accordingly, atypical sensory processing has been suggested as a possible phenotype of ADHD. Behavioural and emotional problems have been linked to sensory processing difficulties in ADHD. Sensory processing abnormalities were found to be associated with internalizing and externalizing problems in children with ADHD. Since sensory processing difficulties can affect many areas of an individual’s life, assessing sensory processing is important in evaluating and improving a patient’s functionality. A growing body of literature has examined sensory processing problems. A recent meta-analysis examining the dimensions of sensory processing patterns revealed that individuals with ADHD have greater sensory sensitivity, sensory avoidance, sensory seeking, and low sensory registration. Additionally, greater difficulties in tactile, visual, oral, and auditory sensory discrimination have been reported in children with ADHD than in comparison children. Furthermore, ADHD increases the risk of hypersensitivity in children and adolescents. In a study conducted with adults, self-reported ADHD symptoms were associated with atypical sensory processing, such as low registration, sensation seeking, and sensory sensitivity. A similar study revealed that individuals with ADHD had an atypical sensory profile, with significant changes in low registration, sensory seeking, sensory sensitivity, and sensory avoidance, as well as differences across sensory modalities such as taste/smell, movement, vision, touch, and hearing.