Abstracts

The function and connectivity of the subcortical heterotopic band in double cortex syndrome (DCS) is only partially understood. Tracers studies that examined the thalamo-cortical connections in the somatosensory system of the tish mutant rat model of double cortex have shown that both neurons in the normotopic cortex and those in the heterotopic band establish topographically-organized bidirectional connections with the thalamus (Schottler et al., 1998), suggesting that primary sensorimotor information is represented in a parallel manner in both cortices. However, it is unclear whether sensorimotor organization of the normotopic cortex in DCS involves only primary areas or a more widespread network. The aim of this study was to investigate the motor and somatosensory function in patients with DCS using functional magnetic resonance imaging (fMRI). We studied six DCS patients (five females and one male; mean age 23 years) with intractable epilepsy, except for one patient whose seizures were controlled with medication. High-resolution anatomical MRI showed bilateral diffuse subcortical band heterotopia with varying degrees of asymmetry in all patients. Blood oxygen level dependent (BOLD) fMRI images were obtained on a 1.5 T scanner using a T2*-weighted gradient EPI sequence. The motor task consisted of acoustically cued fingers-to-thumb opposition. The sensory stimulus consisted of light manual brushing using a soft brush over the skin of the palm and fingers. Two runs of 100 volumes each were obtained sequentially for right and left motor task and sensory stimulus using a block design. Image processing and statistical analysis was done using fMRISTAT-MULTISTAT software (Worsley et al., 2002). Simple motor task elicited a contralateral activation of both the primary motor cortex and the underlying heterotopic band in all patients. Ipsilateral motor activation was seen in 4/6 DCS patients. Furthermore, there were additional areas of activation of non-primary normotopic cortical areas (supplementary motor area and premotor area). Sensory stimuli resulted in activation of the contralateral primary sensory cortex in all patients and secondary somatosensory areas in 5/6. The underlying band beneath the primary sensory area became activated in 3/6 patients. Activations were also seen in subcortical structures for both paradigms. The activation patterns found in our patients suggest that in DCS the heterotopic band may participate to the processing of simple motor and sensory tasks. To perform these tasks, patients with DCS recruit a widespread network involving bilateral primary areas and higher order association normotopic cortices, suggesting an increased attentional demand and motor programming. (Supported by Canadian Institutes for Health Research )