Ipsilateral cortical activation during finger sequences of increasing complexity: representation of movement difficulty or memory load?

Objective: To investigate, if increasing ipsilateral cortical activation during sequential finger movements of increasing complexity relates to the difficulty of transitions (‘sequence complexity’) or to increasing motor memory load (‘sequence length’). Methods: Pre-learned, memorized sequences (M ) of different complexities (S =e.g., 2-2-2-2-2-2-2-2-2-2-2-2-2-2-2-2; S =e.g., 2-5-4-3-2-5-4-3-2-5-4-3-2-5-4-3; and C =e.g., 5-3-2-4-3-4-2-5-4-4-2-3-5-2-4-3; 2=index, 5=pinky) were randomly alternated with visually instructed, novel sequences (N ) of matched complexity. In this design, memory load co-varied with complexity during M because of increasing length of the memorized sequences. In N , memory load was eliminated because each sequence element was prompted by an instructive visual cue. Cortical activation was measured by spectral power analysis of 28-channel electroencephalogram (EEG) in 15 healthy, right-handed subjects. Results: The increases of ipsilateral sensorimotor activation from S over S to C in N were linearly correlated with the corresponding pattern in M (P<0.01). No significant differences were found between M and N (analysis of variance, n.s.). Conclusions: The similar dynamics of cortical activation patterns across movement sequences during M and N indicate that increasing ipsilateral activation primarily reflects processing of increasingly difficult transitions between movements, and not motor memory load. Significance: Function of ipsilateral sensorimotor areas during complex motor behavior.