Lesions affecting the right hippocampal formation differentially impair short-term memory of spatial and nonspatial associations.

Converging evidence from behavioral and imaging studies suggests that within the human medial temporal lobe (MTL) the hippocampal formation may be particularly involved in recognition memory of associative information. However, it is unclear whether the hippocampal formation processes all types of associations or whether there is a specialization for processing of associations involving spatial information. Here, we investigated this issue in six patients with postsurgical lesions of the right MTL affecting the hippocampal formation and in ten healthy controls. Subjects performed a battery of delayed match-to-sample tasks with two delays (900/5,000 ms) and three set sizes. Subjects were requested to remember either single features (colors, locations, shapes, letters) or feature associations (color-location, color-shape, color-letter). In the single-feature conditions, performance of patients did not differ from controls. In the association conditions, a significant delay-dependent deficit in memory of color-location associations was found. This deficit was largely independent of set size. By contrast, performance in the color-shape and color-letter conditions was normal. These findings support the hypothesis that a region within the right MTL, presumably the hippocampal formation, does not equally support all kinds of visual memory but rather has a bias for processing of associations involving spatial information. Recruitment of this region during memory tasks appears to depend both on processing type (associative/nonassociative) and to-be-remembered material (spatial/nonspatial).