Impaired spatial performance in rats with retrosplenial lesions: Importance of the spatial problem and the rat strain in identifying lesion effects in a swimming pool

Behavioral, electrophysiological, and anatomical evidence suggests that retrosplenial (RS) cortex (areas RSA and RSG) plays a role in spatial navigation. This conclusion has been questioned in recent work, suggesting that it is damage to the underlying cingulum bundle (CG) (areas CG and IG), and not RS, that disrupts spatial place learning (Aggleton et al., 2000). We revisited this issue by comparing Long-Evans rats, the strain used in studies that report RS deficits, to Dark Agouti rats, the strain in which no RS deficit has been reported. Rat groups with RS, RS + CG, or no lesion were tested on a place task in a swimming pool, a test of nonspatial and spatial learning, and a matching-to-place task, a relatively selective test of spatial learning. Long-Evans rats given RS and RS + CG lesions, either before or after training on the two tasks, were impaired on both tasks, a deficit not attributable to impaired visual acuity. Control Dark Agouti rats and RS Dark Agouti rats, although not different on the place task, were both significantly impaired relative to Long-Evans rats. The RS Dark Agouti group, however, was also impaired on the matching-to-place task. Thus, we show that RS cortex is part of an extended neural circuit involved in spatial behavior in both Long-Evans and Dark Agouti rats, but its role in the place task may be masked by an innate nonspatial deficit in Dark Agouti rats. The results are discussed in relation to the importance of assessing spatial learning with appropriate spatial tests, the problems of interpretation posed by rat strain differences, and the role of retrosplenial cortex in spatial behavior.