Abstract
Despite being historically one of the first brain regions linked to memory loss, there remains controversy over the core features of diencephalic amnesia as well as the critical site for amnesia to occur. The mammillary bodies and thalamus appear to be the primary locus of pathology in the cases of diencephalic amnesia, but the picture is complicated by the lack of patients with circumscribed damage. Impaired temporal memory is a consistent neuropsychological finding in Korsakoff syndrome patients, but again, it is unclear whether this deficit is attributable to pathology within the diencephalon or concomitant frontal lobe dysfunction. To address these issues, we used an animal model of diencephalic amnesia and examined the effect of mammillothalamic tract lesions on tests of recency memory. The mammillothalamic tract lesions severely disrupted recency judgements involving multiple items but left intact both recency and familiarity judgements for single items. Subsequently, we used disconnection procedures to assess whether this deficit reflects the indirect involvement of the prefrontal cortex. Crossed-lesion rats, with unilateral lesions of the mammillothalamic tract and medial prefrontal cortex in contralateral hemispheres, were unimpaired on the same recency tests. These results provide the first evidence for the selective importance of mammillary body efferents for recency memory. Moreover, this contribution to recency memory is independent of the prefrontal cortex. More broadly, these findings identify how specific diencephalic structures are vital for key elements of event memory.
Highlights
The medial diencephalon was probably the first brain region to be linked to memory loss (Gudden 1896), but there still remains much uncertainty over the core features of diencephalic amnesia and how structures within the medial diencephalon support mnemonic functions
The mammillothalamic tract (MTT) lesions were quantified on the basis of Nisslstained sections and the absence of calbindin staining in the anteroventral thalamic nucleus (Fig. 3a, b)
We examined the impact of selective transection of the rat mammillothalamic tract (MTT) on tests of recency memory analogous to the tasks used in patients (e.g., Hunkin et al 2015)
Summary
The medial diencephalon was probably the first brain region to be linked to memory loss (Gudden 1896), but there still remains much uncertainty over the core features of diencephalic amnesia and how structures within the medial diencephalon support mnemonic functions. The pathology is rarely circumscribed and, in the case of Korsakoff’s syndrome, there are often concomitant changes to both white matter tracts and grey matter structures beyond the medial diencephalon, including frontal lobe dysfunction (Harper and Corbett 1990; Harper 2009; Langlais et al 1996; Torvik et al 1982). It has proved difficult to attribute specific cognitive impairments to particular brain regions (Kopelman 2015). Diencephalic amnesic patients are impaired on tests of recency memory that require individuals to make judgments about the temporal context in which an item was encountered (Hildebrandt et al 2001; Huppert and Piercy 1976; Kopelman et al 1997; Meudell et al 1985; Parkin et al 1990). Temporal or recency memory is classically associated with the frontal cortex; both patients (McAndrews and Milner 1991; Milner et al 1991; Shimamura et al 1990) and animals (Barker et al 2007; Petrides 1991)
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