Rats depend on habit memory for discrimination learning and retention

Abstract

We explored the circumstances in which rats engage either declarative memory (and the hippocampus) or habit memory (and the dorsal striatum). Rats with damage to the hippocampus or dorsal striatum were given three different two-choice discrimination tasks (odor, object, and pattern). These tasks differed in the number of trials required for learning (approximately 10, 60, and 220 trials). Dorsal striatum lesions impaired discrimination performance to a greater extent than hippocampal lesions. Strikingly, performance on the task learned most rapidly (the odor discrimination) was severely impaired by dorsal striatum lesions and entirely spared by hippocampal lesions. These findings suggest that discrimination learning in the rat is primarily supported by the dorsal striatum (and habit memory) and that rats engage a habit-based memory system even for a task that takes only a few trials to acquire. Considered together with related studies of humans and nonhuman primates, the findings suggest that different species will approach the same task in different ways.

Figure 1.

(A) Reconstructions of coronal sections through the hippocampus showing the smallest (black) and largest (stippled) lesion for the hippocampal lesion group. The average size of the hippocampal lesions was 85%. Numbers (right) represent the distance (in millimeters) posterior to bregma. (B) Reconstructions of coronal sections through the dorsal striatum showing the smallest (black) and largest (stippled) lesion for the dorsal striatum lesion group. Numbers (right) represent the distance (in millimeters) relative to bregma. The average size of the dorsal striatum lesion was 34.6%.

Figure 2.

Odor discrimination. (A) Preoperative acquisition of a two-choice odor discrimination by controls (CON, n = 6) and rats in the dorsal striatum (DS, n = 6) or hippocampal (H, n = 8) groups. (B) Postoperative retention of the odor discrimination 17–19 d after learning. (C) Postoperative acquisition of a new two-choice odor discrimination. (D) Retention 13–16 d after acquiring the new odor discrimination. Brackets show the standard error of the mean. (**) Impairment relative to both the control and hippocampal groups (p < 0.05).

Figure 3.

Object discrimination. (A) Preoperative acquisition of a two-choice object discrimination by controls (CON, n = 9) and rats in the dorsal striatum (DS, n = 8) or hippocampal groups (H, n = 10). (B) Postoperative retention of the object discrimination 17–21 d after learning. (C) Postoperative acquisition of a new object discrimination. (D) Retention 2 wk after acquiring the new object discrimination. Brackets show the standard error of the mean. (*) Impairment relative to the control group (p < 0.05).

Figure 4.

Pattern discrimination. (A) The number of trials needed to learn a pattern discrimination by controls (CON, n = 8) or rats with dorsal striatum (DS, n = 9) or hippocampal (H, n = 9) lesions. (B) Retention 2 wk after learning by each group (CON, n = 8; DS, n = 8; H, n = 6). Brackets show the standard error of the mean. (*) Impairment relative to the control group (p < 0.05). (**) Impairment relative to both the control and hippocampal groups (p < 0.05).

Figure 5.

Z scores for 2-wk retention of postoperative learning by the hippocampal and dorsal striatum lesion groups across the three discrimination tasks (odor, object, and pattern discrimination). The z scores are based on Figures 2D, 3D, and 4B. Z score calculations were based on control scores, so that a mean of zero represents control performance. Brackets show the standard error of the mean. (*) Impairment relative to the control group (p < 0.05). (**) Impairment relative to both the control and hippocampal groups (p < 0.05).