Attentional load as a modulator of theta burst-driven plasticity in the adult human primary motor cortex

Neural plasticity is a fundamental attribute of the brain thought to underpin processes such as learning, memory, skill refinement and recovery from injury. Previous research has suggested that the cognitive process of attention may modulate plasticity, but few studies have empirically addressed this relationship. The aim of the present study was to investigate how the availability of attentional resources can influence plasticity within the human primary motor cortex (M1). To manipulate attentional demands, participants completed a visual detection task that across different sessions varied only in task difficulty and therefore attentional demands (load). To induce plasticity, a transcranial magnetic stimulation protocol known as intermittent theta-burst stimulation (iTBS) was used. This protocol increases the amplitude of motor-evoked potentials (MEPs), as measured by surface electromyography, for a period outlasting the stimulation. It was expected that there would be a greater increase in MEP amplitude after iTBS when the procedure was delivered during the low attentional load task relative to the high load task due to the availability of fewer attentional resources under high load. Consistent with predictions, MEP amplitudes showed a significantly greater increase under conditions of low attentional load, suggesting that M1 plasticity is influenced by the attentional demands of a concurrent visual task. These findings have important implications for furthering understanding of attentional and plastic processes in the brain, and for informing the development of better rehabilitation procedures for individuals recovering from acquired motor impairments