Sensory Disorders

Participants for the current study were pairs of Caucasian (including Hispanic) and African-American twins (monozygotic, dizygotic same-sex, and dizygotic opposite-sex) between the ages of 15 and 20 (M=17.25 years, SD=1.3) residing in the mid-Atlantic region of the United States (see Table 1). The majority of the current sample (97.7%) were recruited from the Mid-Atlantic Twin Registry (MATR), a database of twins and families who have previously expressed interest in participating in research studies (Lilley & Silberg, 2013 (link)). MATR staff informed twins of the appropriate age of the current study and provided contact information of those who expressed interest and met inclusion criteria. The study coordinator then contacted participants to describe the study in more detail and schedule an appointment. A subset of the current sample (2.3%) was recruited directly by study staff through advertisements (e.g., radio, newspaper) in the Richmond, Virginia area. Twins or parents of twins who were interested in the advertisements contacted the study coordinator directly. The study coordinator screened twins and scheduled an appointment if twins were eligible.
The sample size of T1 (N=430 twin pairs) achieved over 80% power to detect statistical significance of additive genetic effects that explain at least 40% of the variance for the proposed quantitative (endo)phenotypes (Verhulst, 2016 ).
Inclusion criteria included: a) being an identical or fraternal twin, b) being between the ages of 15 and 20, and c) living primarily at home. Exclusion criteria were: a) current use of psychotropic medications (e.g., antidepressants) or medications with psychotropic effects (e.g., beta-adrenergic blockers), b) diagnosis of an autism spectrum disorder, c) diagnosis of an intellectual disability, d) diagnosis of a spatial learning disorder, or prior testing indicating an IQ below 70, e) seizure without a clear and resolved etiology, f) current or past episodes of psychosis, g) serious, not stabilized illness (e.g., liver, kidney, gastrointestinal, respiratory, cardiovascular, endocrinologic, neurologic, immunologic, or blood disease), h) inadequate production of human growth hormone, i) sensory integration disorder, j) congenital adrenal hyperplasia, k) adrenal inefficiency, l) deaf with bicochlear implants, m) cancer (current or past diagnosis), and n) pregnancy (current or lifetime). A family was excluded if either twin met any exclusionary criteria. Exclusionary criteria were established to reduce interference with physiological data (e.g., psychotropic medications) or participants’ ability to complete laboratory tasks.

The original CTSIB is a clinical version of the SOT.^{18 (link),19 (link)} It uses a stop-watch to measure duration of independent standing under 6 sensory conditions, substituting a compliance foam surface for the sway-referenced surface and a visual conflict doam for the sway referenced visual surround.^{18 (link)} As previously reported,^{19 (link),20 (link)} the CTSIB has good test-retest reliability in community-dwelling older adults. Using SOT as the criterion measure, the CTSIB has also been reported to correlate moderately with 90% sensitivity and 95% specificity.^{19 (link)} The CTSIB was later modified to eliminate the visual conflict doam and includes 4 increasingly challenging balance conditions: C1 = firm surface eyes open, C2 = firm surface eyes closed, C4= eyes open foam surface, and C5 eyes closed foam surface. Participants stand upright with feet together and arms across the chest for 30 seconds, first on a firm surface with and without vision, then on an Airex, medium density foam pad (18 in × 18 in × 5 in) with and without vision.^{18 (link)} Each condition is scored on an ordinal scale based on performance in time and sway 0–3 (0= unable, 1=˂30s, 2=30s unstable, 3=30s stable). To instrument the mCTSIB, 1 wireless tri-axial body-worn inertial sensor (Opal by APDM, Inc) was placed at L-5 with an elastic belt (Figure 1b). The system included a set of Opals with a docking station and an access point for wireless data transmission, from which the anterior-posterior (AP) and medio-lateral (ML) directions of change were used to quantify postural sway after wirelessly transmitted to a laptop using Mobility Lab software from APDM (Figure 1a). Postural sway metrics, were quantified during each stance condition.^{13 (link)} Although the system automatically calculates amplitude, velocity, frequency and jerkiness of postural sway in the ML direction, AP direction as well as an average of both directions, we used the amplitude measure in the AP direction (range) as the primary metric since it is equivalent to the peak-to-peak measure used to calculate the SOT equilibrium score by NeuroCom. This AP range measure, which has equal intervals and an absolute zero, is defined as the range of acceleration [m] ([m/s2]).^{12 (link)} In PD, the Opal range measure has been shown to be more responsive than clinical measures^{13 (link),21 (link)} and have good test-retest reliability (ICC of 0.82) and significantly correlate with the clinical postural stability score r = 0.56.^{12 (link)}