Constrained spherical deconvolution determines the orientation of white matter fibres from the diffusion MRI signal. To do so, the diffusion profile of a single fibre is estimated and assumed constant across the sample. However, the diffusion signal is dependent on microstructural properties such as axonal diameter, packing and myelination, which questions the assumption of a single-fibre response function. This multimodal study combines diffusion MRI with histology from the same tissue sample, to test the validity of a ‘brain-wide’ fibre response function. Preliminary results indicate that in practice the fibre response function is indeed dependent on local anatomy.