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Brain tissue accommodates non-linear deformations and exhibits time-dependent mechanical behaviour. The latter is one of the most pronounced features of brain tissue, manifesting itself primarily through viscoelastic effects such as stress relaxation. To investigate its viscoelastic behaviour, we performed ramp-and-hold relaxation tests in torsion on freshly slaughtered cylindrical ovine brain samples ($25\,\,\text{mm}$ diameter and $\sim 10\,\,\text{mm}$ height). The tests were conducted using a commercial rheometer at varying twist rates of $\{40,240,400\}\,\,\text{rad}\,\,\text{m}^{-1}\,\,\text{s}^{-1}$, with the twist remaining fixed at $\sim 88\,\,\text{rad}\,\,\text{m}^{-1}$, which generated two independent datasets for torque and normal force. The complete set of viscoelastic material parameters was estimated via a simultaneous fit to the analytical expressions for the torque and normal force predicted by the modified quasi-linear viscoelastic model. The model's predictions were further validated through finite element simulations in FEniCS. Our results show that the modified quasi-linear viscoelastic model - recently reappraised and largely unexploited - accurately fits the experimental data. Moreover, the estimated material parameters are in line with those obtained in previous studies on brain samples under torsion. When coupled with bespoke finite element models, these material parameters could enhance our understanding of the forces and deformations involved in traumatic brain injury and contribute to the design of improved headgear for sports such as boxing and motorsports. On the other hand, our novel testing protocol offers new insights into the mechanical behaviour of soft tissues other than the brain., Comment: 19 pages, 11 figures