Bilateral dorsolateral thalamic lesions disrupts conscious recollection

In an earlier study we disputed the claim that the mediodorsal thalamic nucleus is critical for familiarity. We reported patient (QX) who showed a severe deficit in conscious recollection, and behavioural problems (disinhibition, emotional lability) with relative sparing of familiarity-aware memory following a left mediodorsal thalamic lesion. More recent MR imaging has revealed that QX's lesions are more extensive than previously reported and involve both dorsolateral thalamic nuclei, and whilst there is evidence of left mediodorsal thalamic damage, it is not the main focus of damage. This paper reports a full analysis of QX's thalamic pathology alongside a more detailed investigation of his recognition memory, using yes/no and forced-choice procedures, and executive function. The results revealed impairments in yes/no recognition and conscious recollection rates of famous, artist and unknown names. In addition to the previously noted behavioural disinhibition and emotional lability, a deficit in spontaneous planning ability was evident on the Zoo Map Test (subtest of the Bahavioural Assessment of the Dysexecutive Syndrome). Forced-choice recognition, familiarity estimates and remote memory showed higher levels of preservation. The findings indicate that the dorsolateral thalamus is part of the extended hippocampal circuit which is causally critical only for recall and conscious recollection of complex associations rather than for the more automatic processes linked with novelty detection.