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 (MEM) of different complexities (SIMPLE=e.g., 2-2-2-2-2-2-2-2-2-2-2-2-2-2-2-2; SCALE=e.g., 2-5-4-3-2-5-4-3-2-5-4-3-2-5-4-3; and COMPLEX=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 (NOV) of matched complexity. In this design, memory load co-varied with complexity during MEM because of increasing length of the memorized sequences. In NOV, 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 SIMPLE over SCALE to COMPLEX in NOV were linearly correlated with the corresponding pattern in MEM (P<0.01). No significant differences were found between MEM and NOV (analysis of variance, n.s.).
CONCLUSIONS: The similar dynamics of cortical activation patterns across movement sequences during MEM and NOV 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.