Hemispheric differences in the control of finger tapping

Abstract

The human brain is composed of two hemispheres. Even though most functions are represented in both, they differ in processing abilities, enabling the left hemisphere to speak and control learned motor sequences. One current hypothesis how the hemispheres differ is in their processing of relative frequencies of sensory stimuli (Ivry and Robertson, 1998; Flevaris et al., 2010). The Double-filtering-by- frequency (DFF) theory proposes that the left hemisphere has a pref- erence to process relative high frequencies and the right hemisphere relative low frequencies. The authors hypothesize that, hemispheric differences in sensory processing should transfer to the speech and motor domain. The goal of this thesis was to investigate frequency dependent hemispheric preferences for hand motor control. An fMRI and an MEG experiment were performed to answer the following questions: Is there a hemispheric preference for relative movement frequencies visible in behavioral measures? What are the cortical areas and neural mechanisms involved explaining possible prefer- ences? In a first auditory paced finger tapping study (Pflug et al., 2017) we investigated whether the left and the right hand differ in their temporal precision to tap relative slow and fast rhythms. While the fast tapping rate was defined in tapping to every beat of a sequence of auditory beats, the slow tapping rate differed between two experimental groups. While the first group tapped a standard quadruple meter (tap on beat position one), the second group had to tap a syncopated quadruple meter (tap on beat position four). Participants were asked to tap either bimanual monofrequent (fast or slow) or multifrequent (one hand fast, the other slow). As pre- dicted by the DFF-theory, the right hand was more precise when tapping the fast rate, while the left hand performed better during slow tapping. This effect was found only in the group tapping the syncopated rhythm, suggesting that frequency-dependent lateraliza- tion can be masked in case the two rhythms can be interpreted in a hierarchical Gestalt as in the standard quadruple meter. To investigate frequency-dependent lateralization further, synco- pated slow tapping was compared with fast tapping during fMRI and MEG. The internal generation of a slow rhythm increased BOLD ac- tivity in the SMA and in the right auditory association cortex (A2). MEG revealed that the right A2 represented the slow tapping more strongly compared to the left A2 in an amplitude modulation of low beta power. In contrast, the left A2 represented the fast auditory beat rate more strongly than the right A2, although, both cortices received the same auditory input. These results identify the sensory cortices as the source of lateralization of hand motor control and validate the DFF-hypothesis. The motor cortices represented only the rhythm of the motor output in an amplitude modulation of low beta oscillations with a non-linear relationship between beta power and BOLD activity. Finally, effective connectivity analysis of auditory-motor interac- tions revealed that stronger bidirectional auditory-motor coupling in the left compared to the right hemisphere could privilege the left cerebral hemisphere for rhythm integration in a hierarchical Gestalt. This could represent a mechanism explaining at least in part left lat- eralized speech processing.