Effects of modafinil and methylphenidate on visual attention capacity: a TVA-based study

Theory of visual attention (TVA; Bundesen 1990) whole report tasks allow the independent measurement of visual perceptual processing speed and visual short-term memory (vSTM) storage capacity, unconfounded by motor speed. This study investigates how cognitive enhancing effects of psychostimulants depend on baseline performance and individual plasma levels. Eighteen healthy volunteers (aged 20-35 years) received single oral doses of either 40 mg methylphenidate, 400 mg modafinil or placebo in a counterbalanced, double-blind crossover design. A whole report of visually presented letter arrays was performed 2.5-3.5 h after drug administration, and blood samples for plasma level analysis were taken. Methylphenidate and modafinil both enhanced perceptual processing speed in participants with low baseline (placebo) performance. These improvements correlated with subjective alertness. Furthermore, we observed differential plasma level-dependent effects of methylphenidate in lower and higher performing participants: higher plasma levels led to a greater improvement in low-performing participants and to decreasing improvement in high-performing participants. Modafinil enhanced visual short-term memory storage capacity in low-performing participants. This is the first pharmacological investigation demonstrating the usefulness of a TVA task for high-resolution and repeated cognitive parameter estimation after cognitive-enhancing medication. Our results confirm previous findings of attentional capacity improvements in low performers and extend the baseline dependency model to methylphenidate. Plasma level-dependent effects of psychostimulants can be modelled on an inverted U-shaped dose-response relationship, which is highly relevant to predict cognitive enhancing and detrimental effects of psychostimulants in patients with cognitive deficits (e.g., attention deficit hyperactivity disorder) and healthy volunteers (e.g., self-medicating academics).