Pii: S0304-3940(01)02133-4

The main purpose of the present paper was: (1) to study the phase synchronization pattern in the g-band while performing the classical Shepard±Metzler task of mental rotation; (2) to investigate the role of musical training; and (3) to study hemispheric differences in the degree of synchronization during mental rotation. Multivariate electroencephalograph signals from 20 male subjects (ten musicians and ten non-musicians) were recorded while performing the mental rotation task and also at resting condition. Phase synchronization was measured by a recent index, mean phase coherence. It was found that synchronization between frontal cortex and right parietal cortex was signi®cantly increased during mental rotation with respect to rest, whereby musicians showed signi®cantly higher degrees of synchronization than non-musicians. Left hemispheric dominance in the degree of phase synchronization, stronger in the posterior right parietal and occipital regions, was observed in musicians. Right hemispheric dominance was generally observed in nonmusicians. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Electroencephalograph; Mental rotation; Gamma band; Phase synchronization; Musical training; Cerebral asymmetry Mental rotation is a widely discussed concept that suggests an analogous mode of visual information processing in certain visuospatial cognitive tasks. This task, originally introduced by Shepard and Metzler [16], demands discrimination between the image and mirror-image of rotated 3D objects, for which human subjects need an increasing reaction time depending on the angular disparity between the rotated objects. Other than behavioral results, one great interest in the study of mental rotation is the neural mechanism underlying the task. Imaging studies indicated the primary activation in parietal cortex with additional coactivations in premotor and supplementary motor areas while performing mental rotation [3,12,18,19]. An electroencephalograph (EEG) study found activity over the left premotor regions and other areas of the frontal cortex [20] and ERP based results indicated a marked negative component over the right frontocentral region [7]. Further, the question of cerebral asymmetry during mental rotation demands a lot of attention; an early imaging study reported right hemispheric dominance [6], whereas no clear differences were found by some later studies [4,18]. These different results suggest that mental rotation like any other complex cognitive function is performed by a host of subprocesses working synchronously while sub-processes are being carried out in different cortical areas [4]. In this study, we addressed the problem of ®nding the synchronization pattern in the g-band while performing the classical mental rotation task as compared with resting state. The g-band was chosen because there are numerous evidences that neuronal oscillations and synchronization in the high frequency grange (. 30 Hz) provide a general framework of largescale cognitive integration [13,17]. A recent index was used to detect phase synchronization (or synchrony, used interchangeably), which was found to be robust for noisy and non-stationary time series [9]. Beside the topic of gender differences [11,19], another interesting but unresolved debate in this context is the possible correlation between musical training and spatiotemporal reasoning [10]. Based on a model incorporating the columnar organization of cortex, it was predicted that musical training would enhance performance in some spatial tasks [8]. Therefore, we compared the degrees of synchronization between musicians and non-musicians while performing mental rotation and at resting condition. The four ®gures used in the task of mental rotation were Neuroscience Letters 311 (2001) 29±32 0304-3940/01/$ see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(01)02133-4 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 143-1-515-81-325; fax: 143-1512-8901. E-mail address: [email protected] (J. Bhattacharya). similar to the original ®gures [16]. Subjects were requested to answer by `yes' or `no', whether or not the two ®gures were identical. In the control state, subjects were looking at a white wall. In other studies, identical or mirror-rotated ®gures were used as a control condition but there was a chance of induced carry-over processing effects from the experimental to the control stimulus condition because the subject might try to mentally rotate also the control stimuli due to similarity in shape and outline as the experimental stimuli. Thus in the present study, the overall neuronal dynamics during mental rotation was maximally emphasized by choosing a control condition requiring minimal cognitive demand. Spontaneous EEG signals were recorded from 20 right-handed male subjects (ten musicians, mean age 25.7 years, each with at least 5 years of musical training, and ten non-musicians, mean age 25.4 years with no musical training) by 19 electrodes (Fig. 1) with a sampling frequency of 128 Hz and A/D precision of 12 bit. Average of signals from the two ear-lobes was used as the reference. The general condition [14] for phase synchronization between two coupled non-linear oscillators is de®ned as: wn;m ˆ nf1…t†2 mf2…t† j j , a where n and m are positive integers, f 1,2 are the phases of two oscillators, and a is an arbitrary constant. The instantaneous phase of any signal {x(t)}is: f…t† ˆ tan xH…t† x…t† where {xH(t)} is the Hilbert transform of {x(t)}. After ®nding the instantaneous phases (f 1,2) of individual signals and their phase differences (c ˆ f 1±f 2, mˆ nˆ 1) between two signals of length L, the index to characterize the strength of phase synchrony is de®ned as: