Plasma level-dependent effects of methylphenidate and modafinil on processing speed and short term memory capacity parameters of the theory of visual attention (TVA) task

Abstract

Bundesen's theory of visual attention (TVA) task allows the adaptive and independent measurement of storage capacity and processing speed in visual short term memory. This study investigates how cognitive enhancing effects of psychostimulants depend on baseline performance and individual plasma levels. 18 healthy volunteers (aged 20–35 years) received single oral doses of either methylphenidate 40mg, modafinil 400mg or placebo in a double-blind crossover design. A semi-computerised version of the TVA task requiring whole report of visually presented letter arrays with ultra-short presentation times between 22 and 86ms was performed 2.5–3.5h after drug administration. Single doses of both psychostimulants enhanced visual perceptual processing speed. Differenzial plasma-dependent effects of methylphenidate were observed in low- and high-performing participants: Higher plasma levels resulted in greater improvement of processing speed in low-performing participants and decreasing improvement in high-performing participants. Modafinil enhanced visual short term memory storage capacity in low performing subjects. Processing speed improvements after both psychostimulants correlated with increases of subjective alertness (all effects significant at p<0.05). Our results confirm previous findings of cognitive enhancing effects of modafinil in lower performing (or sleep-deprived) participants and extent the model of baseline dependency to cognitive effects of methylphenidate.