Involvement of the medial thalamus in multiple attributes of memory
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
It is widely regarded that the amnesic deficits associated with diencephalic amnesia and medial temporal lobe amnesia are similar. However, the neural basis of diencephalic amnesia and the contributions of different medial thalamic nuclei continue to prove a contentious issue. Contrary to the view that specific medial thalamic nuclei are responsible for profound amnesia after diencephalic injury in humans, the neural connections associated with three aggregates of thalamic nuclei suggest that they each contribute to independent memory systems. These three aggregates were identified by a review of the neuroanatomical tracing studies from the research literature. One thalamic aggregate comprised of the anterior thalamic nucleus (AT), while the other two consisted of less conventional groupings of medial thalamic nuclei, namely a lateral thalamic group (LT) and a posteromedial thalamic group (MT). The LT aggregate was identified as the rostral intralaminar nuclei (paracentral, centrolateral, and rostral central medial) and the lateral and paralamellar segments of the mediodorsal thalamic nucleus. The MT aggregate was identified as the central and medial segments of the mediodorsal nucleus as well as the intermediodorsal nucleus. The functional contributions to specific attributes of memory were assessed in rats across a variety of behavioural tasks. Highly localised lesions to the MT produced deficits on a reward magnitude task, which previously has been found to be sensitive to amygdala and lateral prefrontal lesions, which supports the view that the MT contributes to an amygdala-based memory system. Only AT lesions produced deficits in spatial memory tasks, which are also sensitive to hippocampal lesions, which confirms earlier evidence that the AT contributes to an extended hippocampal system responsible for spatial memory processing. Only LT lesions produced deficits in an egocentric response memory task, which is also disrupted by dorsomedial prefrontal cortex and dorsal striatum lesions, and which thus supports the notion that the LT is a functional component of a dorsal striatum memory system. In addition, LT and MT lesions, but not AT lesions, impaired temporal order memory, while no lesion impaired object recognition or sustained attention. These new dissociations indicate that distributed neural circuits incorporating many brain structures including the medial thalamus can be identified as contributing to independent learning and memory processes.