We evaluated processing-speed and shift-cost measures in adults with depression or attention-deficit hyperactivity disorder (ADHD) and monitored the effects of treatment. We hypothesised that cognitive-speed and shift-cost measures might differentiate diagnostic groups.

Colour, form, and colour–form stimuli were used to measure naming times. The shift costs were calculated as colour–form-naming time minus the sum of colour- and form-naming times. Measurements were done at baseline and end point for 42 adults with depression and 42 with ADHD without depression. Patients with depression were treated with transcranial pulsed electromagnetic fields and patients with ADHD with methylphenidate immediate release.

During depression treatment, reductions in naming times were recorded weekly. One-way analysis of variance indicated statistical between-group differences, with effect sizes in the medium range for form and colour–form. In both groups, naming times were longer before than after treatment. For the ADHD group, shift costs exceeded the average–normal range at baseline but were in the average–normal range after stabilisation with stimulant medication. For the depression group, shift costs were in the average–normal range at baseline and after treatment. Baseline colour–form-naming times predicted reductions in naming times for both groups, with the largest effect size and index of forecasting efficiency for the ADHD group.

The cognitive-processing-speed (colour–form) and shift-cost measures before treatment proved most sensitive in differentiating patients with depression and ADHD. Reductions in naming times for the depression group were suggested to reflect improved psychomotor skills rather than improved cognitive control.