Lesions of the mammillothalamic tract impair the acquisition of spatial but not nonspatial contextual conditional discriminations.

This study examined the influence of selective mammillothalamic tract lesions in rats on the acquisition of two kinds of contextual conditional discrimination: one involving two contexts (A and B) that differed in their visuo-spatial properties and another involving two contexts (C and D) that differed in temperature. In contexts A (and C) presentations of a tone were paired with food whereas presentations of a clicker were not; and in contexts B (and D) presentations of the clicker were paired with food whereas those of the tone were not. Mammillothalamic tract lesions disrupted initial acquisition of the conditional discrimination involving visual contexts (A and B), but not the formally equivalent discrimination involving thermal contexts (C and D). These results provide support for the suggestion that mammillothalamic tract lesions disrupt visuo-spatial encoding.