BDNF and depressive symptom measurement: Alterations in the expression or signalling of BDNF are implicated in the pathophysiology of mental disorders such as depression (Yu & Chen, 2011) and schizophrenia (Favalli et al., 2012), execution of motor functions (He et al., 2013) and LTP and consequent memory formation (Cunha et al., 2010; Loprinzi & Frith, 2019).