In this paper, ab initio CASSCF computations are used to investigate the photoisomerization path of the protonated Schiff base (PSB) 4-cis-gamma-methylnona-2,4,6,8-tetraeniminium cation: a five conjugated double bond model of the retinal chromophore of rhodopsin (the human retina visual pigment). We show that, after initial skeletal relaxation from the Franck-Condon region (which involves a large increase in the central C=C bond length), the system is "trapped" in an energy plateau on the S-1 energy surface which may be the origin of the "slow" cis --> trans isomerization dynamics observed in retinal PSBs in solution. The energy plateau is absent in shorter retinal chromophore models which have a steeper S-1 isomerization path. The rhodopsin cavity (where the native chromophore is embedded) may have the effect of removing the energy plateau from the S-1 potential thus dramatically increasing the photoisomerization rate from picoseconds to femtoseconds.