Autonomic arousal and cognitive performance in seasonal affective disorder

Seasonal affective disorder (SAD) has generally been seen to be at the extreme end of a continuum of seasonal variation to mood and associated behavioural symptoms in the general population. SAD is currently categorised in the DSM IV (American Psychiatric Association, 1994) as Seasonal Specifier, a type of recurrent disorder, which may be applied to (non-seasonal) Major Depressive Disorder (NSD). On the other hand, evidence is accumulating for SAD as a disorder distinct from NSD. Hence the thesis investigates the nature of SAD in relation to the seasonality of mood and behaviour in the general population. An epidemiological survey was conducted by administering the Seasonal Pattern Assessment Questionnaire (SPAQ) (Rosenthal, 1989) to Tasmanian Psychology 1 participants (N = 502) showing up to 9% and 24 % may have SAD at clinical (C-SAD) and subsyndromal (S-SAD) levels respectively. Statistical analyses on SPAQ variables further confirmed the short day aetiology of SAD and also implicated metabolic factors. Two longitudinal studies were conducted, each using three groups (control, subsyndromal SAD, and clinical SAD) and six bimonthly testing sessions. Participants were screened using the SP AQ and selected according to Rosenthal's (1989) criteria. In each longitudinal study measures of depression, behavioural symptoms and emotion were also obtained at each testing session. Longitudinal Study 1 (n = 23 control, 21 S-SAD, and 18 C-SAD) investigated the proposition that the symptoms of SAD may represent an increased parasympathetic or decreased sympathetic arousal. Longitudinal Study 2 (n = 22 control, 21 S-SAD, and 17 C-SAD) studied cognitive processing across the twelve-month period. Digit Span and Visual Memory Span subtests from the Wechsler Memory Scale measured memory processes and a Mental Rotation task as well as verbal and spatial versions of a Hemispheric Asymmetry Task determined cognitive efficiency, spatial processing, and any differential hemispheric specialisation effects that may be involved in SAD. Distinctions between SAD and NSD were shown from the autumn/ winter specificity of the atypical vegetative behavioural symptoms accompanying decreases in mood and an underlying hypo-arousal showing similarities with hibernation. Impairments to cognitive processing include deficits specific to the spatial tasks that may have implications for differentiating between subsyndromal and clinical levels of SAD and also in understanding vulnerability to SAD. Implications for SAD theories are presented with findings indicative of dual underlying mechanisms consisting of a seasonal component, as well as a depression component in vulnerable SAD participants. Circannual rhythms were documented and several variables were shown to vary with the seasons in control participants, thus extending current knowledge.