Brain lateralization and neural plasticity for musical and cognitive abilities in an epileptic musician

Abstract

The use of intracarotid propofol procedure (IPP) when assessing musical lateralization has not been reported in literature up to now. This procedure (similar to Wada Test) has provided the opportunity to investigate not only lateralization of language and memory functions on epileptic patients but also offers a functional mapping approach with superior spatial and temporal resolution to analyze the lateralization of musical abilities. Findings in literature suggest that musical training modifies functional and structural brain organization. We studied hemispheric lateralization in a professional musician, a 33 years old woman with refractory left medial temporal lobe (MTL) epilepsy (TLE). A longitudinal neuropsychological study was performed over a period of 21 months. Before epilepsy surgery, musical abilities, language and memory were tested during IPP by means of a novel and exhaustive neuropsychological battery focusing on the processing of music. We used a selection of stimuli to analyze listening, score reading, and tempo discrimination. Our results suggested that IPP is an excellent method to determine not only language, semantic, and episodic memory, but also musical dominance in a professional musician who may be candidate for epilepsy surgery. Neuropsychological testing revealed that right hemisphere's patient is involved in semantic and episodic musical memory processes, whereas her score reading and tempo processing require contribution from both hemispheres. At one-year follow-up, outcome was excellent with respect to seizures and professional skills, meanwhile cognitive abilities improved. These findings indicate that IPP helps to predict who might be at risk for postoperative musical, language, and memory deficits after epilepsy surgery. Our research suggests that musical expertise and epilepsy critically modifies long-term memory processes and induces brain structural and functional plasticity.

Keywords: Hippocampus and memory; hemispheric lateralization; memory; neural plasticity; neuropsychology; temporal lobe epilepsy; wada test.

Figure 1

Pre- and post-surgical MRI study. (A) Pre-surgical MRI; IR coronal image shows reduction of left hippocampal size, with hypointensity, and loss of internal differentiation (white arrow). (B) Pre-surgical MRI; Flair coronal image reveals hyperintensity and decreased hippocampal size (white arrow). (C) Post-surgical MRI; T1 coronal image shows the absence of the hippocampal gyrus, the subiculum, and the parahippocampal gyrus, medial to collateral sulcus (white arrow). (D) Post-surgical MRI; Flair coronal image hyperintensity is observed, surrounding the surgical cavity, secondary to gliosis (white arrow).

Figure 2

Score-reading task. (A) Musical score excerpt presented during encoding condition (left hemisphere inactivated). (B) Musical scores excerpts presented during retrieval condition (in order to assess left hemisphere functions).

Figure 3

EEG activity recorded from 6 s. after propofol injection in the left hemisphere (A) and from 5 s. in the right hemisphere (B). Both EEG recordings show bilateral theta-delta slowing and theta band activity peaks post injection, which gradually decrease. No significant inter-hemisphere asymmetry is revealed. EEG activity returns to pre-injection baseline status approximately 10 min. after propofol injection. Abbreviations: LH, left hemisphere; RH, right hemisphere.