A cohomological invariant of degree 3 for symplectic involutions on central simple algebras has been defined in a joint work with Garibaldi and Parimala. A new approach to its definition for algebras of degree 8 has been developed jointly with Becher and Grenier-Boley. This talk will describe this new approach and the relation with an invariant of Barry, which gives a criterion for the quadratic descent of biquaternion algebras.