Evaluating the motor slowing hypothesis of depression

The purpose of this study was to evaluate the hypothesis that processing speed deficits are the primary cognitive deficits in those with depression, consistent with the motor slowing hypothesis. Participants (n=223) were research volunteers who served in the US military since September 11, 2001, and denied a history of significant brain injuries. Depression was measured using a structured interview, the Personality Assessment Inventory (PAI), and the Beck Depression Inventory-II (BDI-II). Outcomes included performance on 10 processing speed variables. Invalid performance/report accounted for significant variance for 8 of 10 processing speed measures. There was not a consistent pattern of slowed processing speed in those with current depressive diagnoses compared to those without. However, depression symptom burden per the PAI Depression scale was significant for 7 of 10 processing speed tests. Only non-dominant fine motor dexterity was significantly slower in those with high versus low burden using BDI-II quartiles. Thus, the motor slowing hypothesis was supported, but only for depression burden and not diagnostic status or high versus low categorical classification. These results underscore the importance of validity assessment and consideration of how one measures psychiatric constructs when evaluating relations among symptoms and cognition.