Involvement of the human thalamus in relational and non-relational memory

Representation of viewed and recalled film clips in patterns of brain activity in a person with developmental amnesia

As clear memories transport us back into the past, the brain also revives prior patterns of neural activity, a phenomenon known as neural reactivation. While growing evidence indicates a link between neural reactivation and typical variations in memory performance in healthy individuals, it is unclear how and to what extent reactivation is disrupted by a memory disorder. The current study characterizes neural reactivation in a case of amnesia using Multivoxel Pattern Analysis (MVPA). We tested NC, an individual with developmental amnesia linked to a diencephalic stroke, and 19 young adult controls on a functional magnetic resonance imaging (fMRI) task during which participants viewed and recalled short videos multiple times. An encoding classifier trained and tested to identify videos based on brain activity patterns elicited at perception revealed superior classification in NC. The enhanced consistency in stimulus representation we observed in NC at encoding was accompanied by an absence of multivariate repetition suppression, which occurred over repeated viewing in the controls. Another recall classifier trained and tested to identify videos during mental replay indicated normal levels of classification in NC, despite his poor memory for stimulus content. However, a cross-condition classifier trained on perception trials and tested on mental replay trials-a strict test of reactivation-revealed significantly poorer classification in NC. Thus, while NC's brain activity was consistent and stimulus-specific during mental replay, this specificity did not reflect the reactivation of patterns elicited at perception to the same extent as controls. Fittingly, we identified brain regions for which activity supported stimulus representation during mental replay to a greater extent in NC than in controls. This activity was not modeled on perception, suggesting that compensatory patterns of representation based on generic knowledge can support consistent mental constructs when memory is faulty. Our results reveal several ways in which amnesia impacts distributed patterns of stimulus representation during encoding and retrieval.

Thalamic-Medial Temporal Lobe Connectivity Underpins Familiarity Memory

The neural basis of memory is highly distributed, but the thalamus is known to play a particularly critical role. However, exactly how the different thalamic nuclei contribute to different kinds of memory is unclear. Moreover, whether thalamic connectivity with the medial temporal lobe (MTL), arguably the most fundamental memory structure, is critical for memory remains unknown. We explore these questions using an fMRI recognition memory paradigm that taps familiarity and recollection (i.e., the two types of memory that support recognition) for objects, faces, and scenes. We show that the mediodorsal thalamus (MDt) plays a material-general role in familiarity, while the anterior thalamus plays a material-general role in recollection. Material-specific regions were found for scene familiarity (ventral posteromedial and pulvinar thalamic nuclei) and face familiarity (left ventrolateral thalamus). Critically, increased functional connectivity between the MDt and the parahippocampal (PHC) and perirhinal cortices (PRC) of the MTL underpinned increases in reported familiarity confidence. These findings suggest that familiarity signals are generated through the dynamic interaction of functionally connected MTL-thalamic structures.

The Regulatory Role of the Human Mediodorsal Thalamus

The function of the human mediodorsal thalamic nucleus (MD) has so far eluded a clear definition in terms of specific cognitive processes and tasks. Although it was at first proposed to play a role in long-term memory, a set of recent studies in animals and humans has revealed a more complex, and broader, role in several cognitive functions. The MD seems to play a multifaceted role in higher cognitive functions together with the prefrontal cortex and other cortical and subcortical brain areas. Specifically, we propose that the MD is involved in the regulation of cortical networks especially when the maintenance and temporal extension of persistent activity patterns in the frontal lobe areas are required.

Medial thalamic stroke and its impact on familiarity and recollection

Models of recognition memory have postulated that the mammillo-thalamic tract (MTT)/anterior thalamic nucleus (AN) complex would be critical for recollection while the Mediodorsal nucleus (MD) of the thalamus would support familiarity and indirectly also be involved in recollection (Aggleton et al., 2011). 12 patients with left thalamic stroke underwent a neuropsychological assessment, three verbal recognition memory tasks assessing familiarity and recollection each using different procedures and a high-resolution structural MRI. Patients showed poor recollection on all three tasks. In contrast, familiarity was spared in each task. No patient had significant AN lesions. Critically, a subset of 5 patients had lesions of the MD without lesions of the MTT. They also showed impaired recollection but preserved familiarity. Recollection is therefore impaired following MD damage, but familiarity is not. This suggests that models of familiarity, which assign a critical role to the MD, should be reappraised.

Topological Schemas of Cognitive Maps and Spatial Learning

Spatial navigation in mammals is based on building a mental representation of their environment-a cognitive map. However, both the nature of this cognitive map and its underpinning in neural structures and activity remains vague. A key difficulty is that these maps are collective, emergent phenomena that cannot be reduced to a simple combination of inputs provided by individual neurons. In this paper we suggest computational frameworks for integrating the spiking signals of individual cells into a spatial map, which we call schemas. We provide examples of four schemas defined by different types of topological relations that may be neurophysiologically encoded in the brain and demonstrate that each schema provides its own large-scale characteristics of the environment-the schema integrals. Moreover, we find that, in all cases, these integrals are learned at a rate which is faster than the rate of complete training of neural networks. Thus, the proposed schema framework differentiates between the cognitive aspect of spatial learning and the physiological aspect at the neural network level.

Thalamo-Cortical Disruption Contributes to Short-Term Memory Deficits in Patients with Medial Temporal Lobe Damage

Short-term (STM) and long-term memory (LTM) have largely been considered as separate brain systems reflecting fronto-parietal and medial temporal lobe (MTL) functions, respectively. This functional dichotomy has been called into question by evidence of deficits on aspects of working memory in patients with MTL damage, suggesting a potentially direct hippocampal contribution to STM. As the hippocampus has direct anatomical connections with the thalamus, we tested the hypothesis that damage to thalamic nuclei regulating cortico-cortical interactions may contribute to STM deficits in patients with hippocampal dysfunction. We used diffusion-weighted magnetic resonance imaging-based tractography to identify anatomical subdivisions in patients with MTL epilepsy. From these, we measured resting-state functional connectivity with detailed cortical divisions of the frontal, temporal, and parietal lobes. Whereas thalamo-temporal functional connectivity reflected LTM performance, thalamo-prefrontal functional connectivity specifically predicted STM performance. Notably, patients with hippocampal volume loss showed thalamic volume loss, most prominent in the pulvinar region, not detected in patients with normal hippocampal volumes. Aberrant thalamo-cortical connectivity in the epileptic hemisphere was mirrored in a loss of behavioral association with STM performance specifically in patients with hippocampal atrophy. These findings identify thalamo-cortical disruption as a potential mechanism contributing to STM deficits in the context of MTL damage.

Accelerated forgetting of contextual details due to focal medio-dorsal thalamic lesion

Effects of thalamic nuclei damage and related white matter tracts on memory performance are still debated. This is particularly evident for the medio-dorsal thalamus which has been less clear in predicting amnesia than anterior thalamus changes. The current study addresses this issue by assessing 7 thalamic stroke patients with consistent unilateral lesions focal to the left medio-dorsal nuclei for immediate and delayed memory performance on standard visual and verbal tests of anterograde memory, and over the long-term (>24 h) on an object-location associative memory task. Thalamic patients showed selective impairment to delayed recall, but intact recognition memory. Patients also showed accelerated forgetting of contextual details after a 24 h delay, compared to controls. Importantly, the mammillothalamic tract was intact in all patients, which suggests a role for the medio-dorsal nuclei in recall and early consolidation memory processes.

Associative learning beyond the medial temporal lobe: many actors on the memory stage

Decades of research have established a model that includes the medial temporal lobe, and particularly the hippocampus, as a critical node for episodic memory. Neuroimaging and clinical studies have shown the involvement of additional cortical and subcortical regions. Among these areas, the thalamus, the retrosplenial cortex, and the prefrontal cortices have been consistently related to episodic memory performance. This article provides evidences that these areas are in different forms and degrees critical for human memory function rather than playing only an ancillary role. First we briefly summarize the functional architecture of the medial temporal lobe with respect to recognition memory and recall. We then focus on the clinical and neuroimaging evidence available on thalamo-prefrontal and thalamo-retrosplenial networks. The role of these networks in episodic memory has been considered secondary, partly because disruption of these areas does not always lead to severe impairments; to account for this evidence, we discuss methodological issues related to the investigation of these regions. We propose that these networks contribute differently to recognition memory and recall, and also that the memory stage of their contribution shows specificity to encoding or retrieval in recall tasks. We note that the same mechanisms may be in force when humans perform non-episodic tasks, e.g., semantic retrieval and mental time travel. Functional disturbance of these networks is related to cognitive impairments not only in neurological disorders, but also in psychiatric medical conditions, such as schizophrenia. Finally we discuss possible mechanisms for the contribution of these areas to memory, including regulation of oscillatory rhythms and long-term potentiation. We conclude that integrity of the thalamo-frontal and the thalamo-retrosplenial networks is necessary for the manifold features of episodic memory.

Unraveling the contributions of the diencephalon to recognition memory: a review

Both clinical investigations and studies with animals reveal nuclei within the diencephalon that are vital for recognition memory (the judgment of prior occurrence). This review seeks to identify these nuclei and to consider why they might be important for recognition memory. Despite the lack of clinical cases with circumscribed pathology within the diencephalon and apparent species differences, convergent evidence from a variety of sources implicates a subgroup of medial diencephalic nuclei. It is supposed that the key functional interactions of this subgroup of diencephalic nuclei are with the medial temporal lobe, the prefrontal cortex, and with cingulate regions. In addition, some of the clinical evidence most readily supports dual-process models of recognition, which assume two independent cognitive processes (recollective-based and familiarity-based) that combine to direct recognition judgments. From this array of information a "multi-effect multi-nuclei" model is proposed, in which the mammillary bodies and the anterior thalamic nuclei are of preeminent importance for recollective-based recognition. The medial dorsal thalamic nucleus is thought to contribute to familiarity-based recognition, but this nucleus, along with various midline and intralaminar thalamic nuclei, is also assumed to have broader, indirect effects upon both recollective-based and familiarity-based recognition.

Differential impairment of remembering the past and imagining novel events after thalamic lesions

Vividly remembering the past and imagining the future (mental time travel) seem to rely on common neural substrates and mental time travel impairments in patients with brain lesions seem to encompass both temporal domains. However, because future thinking-or more generally imagining novel events-involves the recombination of stored elements into a new event, it requires additional resources that are not shared by episodic memory. We aimed to demonstrate this asymmetry in an event generation task administered to two patients with lesions in the medial dorsal thalamus. Because of the dense connection with pFC, this nucleus of the thalamus is implicated in executive aspects of memory (strategic retrieval), which are presumably more important for future thinking than for episodic memory. Compared with groups of healthy matched control participants, both patients could only produce novel events with extensive help of the experimenter (prompting) in the absence of episodic memory problems. Impairments were most pronounced for imagining personal fictitious and impersonal events. More precisely, the patients' descriptions of novel events lacked content and spatio-temporal relations. The observed impairment is unlikely to trace back to disturbances in self-projection, scene construction, or time concept and could be explained by a recombination deficit. Thus, although memory and the imagination of novel events are tightly linked, they also partly rely on different processes.

Vascular thalamic amnesia: a reappraisal

In humans lacunar infarcts in the mesial and anterior regions of the thalami are frequently associated with amnesic syndromes. In this review paper, we scrutinized 41 papers published between 1983 and 2009 that provided data on a total of 83 patients with the critical ischemic lesions (i.e. 17 patients with right-sided lesions, 25 with left-sided lesions and 41 with bilateral lesions). We aimed to find answers to the following questions concerning the vascular thalamic amnesia syndrome: (i) Which qualitative pattern of memory impairment (and associated cognitive and behavioral deficits) do these patients present? (ii) Which lesioned intrathalamic structures are primarily responsible for the amnesic syndrome? (iii) Are the recollection and familiarity components of declarative memory underlain by the same or by different thalamic structures? Results of the review indicate that, similar to patients with amnesic syndromes due to mesio-temporal lobe damage, patients with vascular thalamic amnesia display a prevalent deficit of declarative anterograde long-term memory, a less consistent deficit of declarative retrograde long-term memory and substantially spared short-term and implicit memory. Unlike mesio-temporal lobe patients, however, vascular thalamic amnesics often present dysexecutive and behavioral deficits similar to those observed in patients with frontal damage. The presence of an amnesic syndrome in patients with thalamic lacunar infarcts is strongly predicted by involvement of the mammillo-thalamic tract, which connects the anterior nuclei complex to the hippocampus proper via the fornix and the mammillary bodies. Finally, data reported in a few single cases provide support for the hypothesis that thalamic regions connected to distinct areas of the mesio-temporal lobe play differential roles in recollection and familiarity processes. The mammillo-thalamic tract/anterior nuclei axis seems primarily implicated in recollective processes, whereas the ventroamygdalofugal pathway/medio-dorsal axis primarily underlies familiarity processes.