The observations discussed in Sections 2 and 4 strongly suggest that the presence of a bar increases both the probability that a nucleus of an early-type spiral galaxy experiences star formation and the rate at which the stars form. By contrast, bars seem to have little influence on the global star formation properties of spirals (Kennicutt 1994). Insofar as numerical studies predict that the large-scale interstellar medium should be channeled to the central region of a barred galaxy, the observational evidence gives reassuring agreement. It should be stressed that star formation is a wide-spread phenomenon in the nuclei of both barred and unbarred galaxies (e.g., Ho et al. 1996b); the presence of a bar is neither a necessary nor a sufficient condition for nuclear star formation to take place. Undoubtedly, other factors such as the availability of gas must play a crucial role.
Interestingly, the enhancement of star-formation activity seems to take place preferentially in early-type spirals, confirming similar findings reported by Devereux (1987) and Dressel (1988) As noted by Devereux (1987), the dichotomy between the response of the gas to a bar in early and late-type spirals probably reflects the influence of the bulge-to-disk ratio on the rotation curve and on the relative positions of the primary resonances. If an ILR is present in an early-type disk, it is expected to be located interior to the bar and close to the nucleus; if a late-type disk contains an ILR, it will likely be found near the ends of the bar, whose extent is smaller than that of early-type systems (Elmegreen & Elmegreen 1985; Combes & Elmegreen 1993). Since the accretion rate depends on the length of the bar (Athanassoula 1992; Friedli & Benz 1993), it is natural to expect the most pronounced star-formation activity to occur in early-type systems.
Although the survey of Ho et al. (1995) lacks detailed two-dimensional information, narrow-band imaging studies (e.g., Phillips, these proceedings) indeed find that circumnuclear star formation appears to exist only in early-type spirals (Sb and earlier). In the few cases where high-resolution CO maps are available (e.g., Kenney et al. 1992), the ring-like distributions of the sites of recent star formation are roughly co-spatial with peaks in the molecular gas distributions, located near the expected position of the ILRs.