Preface If the analysis of an unknown information processing structure is based on a single model, be it chosen because of its rigour, its simplicity, its biological plausibility, historical renown or current acceptance, or simply because it was the first which came to mind, one generally runs the risk of overlooking basic alternative solutions to the problem under study. To guard against this, Rose and Dobson (1985) advocate to first deduce the logical tree of classes of alternative solutions to a given problem. In a contribution to the 1958 Macy Conference on group processes (M ittelstaedt,

1960; cf. also sequelae in M ittelstaedt, 1961 & 1969), I have tried to develop such a logical tree for possible solutions to the problem of head-centric visual localization. This analysis, though in need of some corrections, may still be useful for the ever livelier discussion on this subject (Howard, 1982; Matin, 1982; M ittelstaedt, 1971; Shebilske, 1984; Bridgeman, 1986; Grusser, 1986; Steinbach, 1987; & W indhorst, 1988), and is here presented in a revised form.