In this work we report on the pressure dependence of the refractive index of indium selenide (InSe) for polarization perpendicular to the c-axis (E⊥c). The refractive index dispersion n ⊥ (ω) of InSe has been measured between 0.77 and 1.5 eV for hydrostatic pressures up to 8 GPa and interpreted through a single oscillator Phillips-van Vechten model yielding the electronic refractive index and dielectric function. This model also yields the pressure dependence of the Penn gap in InSe, whose interlayer and intralayer deformation potentials have been obtained and compared with those of the lower energy optical gaps. The possible electronic transitions responsible for the Penn gaps for both polarizations (perpendicular and parallel to the c-axis) in InSe and other III-VI layered materials are also discussed on the basis of recent band structure calculations.