Lesions of the intralaminar thalamic nuclei (ILn), the medial wall (MW) area of prefrontal cortex, and the hippocampus were compared and found to have distinct effects on delayed matching-to-sample (DMS) and delayed non-matching-to-sample (DNMS) tasks based on different types of stimulus cues. Hippocampal lesions impaired DNMS trained in a radial arm maze but had little effect on DMS trained with retractable levers or olfactory DNMS. MW lesions affected the DMS task but had limited effects on olfactory DNMS and radial arm maze DNMS. ILn lesions resulted in a more generalized pattern of impairment for radial maze tasks and (in previous studies) for the DMS and olfactory DNMS tasks. Only the hippocampal lesion was associated with a delay-dependent impairment. It is argued that ILn lesions disrupt remembering through their effects on the recurrent, feedback pathways that link functionally related areas of the basal ganglia and cortex.

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This study compared the effects of lesions damaging hippocampus-related pathways in anterior thalamus (AT) and parahippocampal (PH) cortex on allocentric spatial memory. Rats were trained to perform radial maze delayed nonmatching (DNM) with random selection of arms to prevent egocentric solutions. After experimental treatment (control, excitotoxic AT, radiofrequency PH, or combined AT-PH lesions), rats were retrained for 30 sessions from 2 to 8 weeks after surgery. Results showed comparable impairments for AT and PH lesions that added without interaction in the combined AT-PH group. During chronic recovery, the AT-PH group exhibited delay-dependent deficits comparable to previous results for hippocampal lesions. Thus, AT and PH lesions appear to have separate effects that together disrupt hippocampus-dependent spatial memory.

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