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Summary form only given. Recently, photorefractive (PR) spatial optical solitons have been the focus of considerable attention. Of particular interest are the so-called screening solitons that occur in steady state when an external bias voltage is appropriately applied to a PR crystal. These soliton states are known to occur when the drift term dominates in the expression of the space charge field. In a recent study, the effects arising from first-order diffusion terms have also been investigated. It was found that the PR soliton can experience adiabatically self-deflection that varies linearly with the applied electric field. However, recent experiments suggest that this self-deflection can exceed that predicted by theory, especially in the regime of quite high bias fields. To account for this discrepancy, we investigate the effects of other higher-order electric field terms on the evolution of bright steady-state solitons in PR media.