Mémoire (M.Ps)-- Université du Québec à Chicoutimi. 2002. / Document électronique également accessible en format PDF. CaQCU

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Extensive research has supported the existence of a specialized face-processing network that is distinct from the visual processing areas used for general object recognition. The majority of this work has been aimed at characterizing the response properties of the fusiform face area (FFA) and the occipital face area (OFA), which together are thought to constitute the core network of brain areas responsible for facial identification. Although accruing evidence has shown that face-selective patches in the ventral anterior temporal lobes (vATLs), within perirhinal cortex, play a necessary role in facial identification, the relative contribution of these brain areas to the core face-processing network has remained unarticulated. The current study assessed the relative sensitivity of the anterior face patch, the OFA, and the FFA, to different aspects of person information. Participants learned to associate a name and occupation label, or a name only, with different facial identities. The sensitivity of the face processing areas to facial identity, occupation, and the amount of information associated with a face was then assessed. The results of a multivoxel pattern analysis (MVPA) revealed that distributed activity patterns in the anterior face patch contained information about facial identity, occupation, and the amount of information associated with a face, with the sensitivity of the anterior face patch to occupation and amount of information being greater than the more posterior face processing regions. When a similar analysis was conducted that included all voxels in the perirhinal cortex, sensitivity to every aspect of person information increased. These results suggest that the human ventral anterior temporal lobes may be critically involved in representing social, categorical, information about individual identities. / Psychology

It is well established that older adults show a deficit in episodic memory. The associative deficit hypothesis (ADH) (Naveh-Benjamin, 2000) suggests that an age-related reduced ability to create links between units of information is a major contributor to the episodic deficit. It has been a robust finding that older adults show a disproportionate decline in associative memory relative to item memory when compared to young adults. Previous researchers have investigated the role of the frontal lobes (FL) by studying the effect of reduced attentional resources in the associative deficit. However, they have not found that divided attention in young adults produces the disproportionate associative decline seen in aging and it is thought that some cognitive process other than the allocation of attentional resources may contribute to the associative deficit. The present study intended to use a divided attention (DA) task that also engages medial temporal brain regions (MTL) in order to tax additional parts of the network involved in creating associations and provide indirect support for the role of the MTL in the associative deficit. However, the associative memory deficit in older adults was not replicated due to unique poor associative memory performance of some young adults in the study. Analyses excluding these participants show support for the role of the MTL in the associative deficit. However, the young poor performers may provide support for the role of FL function in the associative deficit and show that poor associative memory may not be limited to the older adult cohort.

A word-generation experiment explored the relative contributions of the left temporal neocortex and the left hippocampal region to verbal recall. Patients with large hippocampal excisions (LTH) were impaired in immediate recall of synonyms, whereas those with small hippocampal excisions (LTh) were not. Both groups were impaired in immediate recall of rhymes, and in delayed recall of both synonyms and rhymes. A nonverbal associative-learning task, where the stimulus-items came from a visual continuum, also resulted in a deficit for the LTH (but not the LTh) group. Patients with right temporal lobectomy performed both tasks normally. On an absolute-judgement task involving the accurate numbering of a set of six individually-presented rectangles, only the groups of patients with large hippocampal excisions were impaired, regardless of side of lesion. These findings support the view that the temporal neocortex is important for gaining access to information in semantic memory, whereas the hippocampal region is essential for the temporary storage of new information beyond the span of immediate memory.

This thesis investigated the role of the anterior temporal lobes (ATLs) in conceptual knowledge and name relearning by studying people with semantic dementia (SD). People with SD have atrophy focussed on the ATLs and they exhibit a pan-modal semantic impairment (e.g., Hodges, Patterson, Oxbury, & Funnell, 1992). Recent evidence suggests that modality-invariant concept representations are built up in the ATLs and that these modality-invariant representations are crucial for abstracting away from the surface features of items in order to generalise conceptual information based on their core semantic similarity (e.g., Lambon Ralph & Patterson, 2008). In order to test this, two of the studies described in this thesis (Chapters 2 and 3) assessed semantic generalisation in people with SD. These studies showed that people with SD are less able to generalise conceptual information on the basis of the deeper semantic structure of concepts but instead are increasingly influenced by the superficial similarity of the items. These studies support the hypothesis that the modality-invariant representations formed in the ATLs are crucial for semantic-based generalisation. Previous SD relearning studies have reported relatively good learning but a lack of generalisation to untrained items, tasks, and/or contexts (i.e., under-generalisation). This has been interpreted based on the Complementary Learning Systems (CLS) (McClelland, McNaughton, & O'Reilly, 1995) to suggest that the neocortical semantic system no longer makes a meaningful contribution to relearning but instead relearning is primarily dependent upon the sparse representational medial temporal lobe (MTL) learning system. The studies described in two of the thesis chapters (Chapters 4 and 5) investigated the role of the underlying systems further and found that the neocortical semantic system does still contribute to relearning in SD (although its contribution is disordered and based on the degraded concept representations in the ATL) but there is a shift in the division of labour such that the MTL system takes over more of the work. Finally, in order to clarify the outcomes of relearning in SD, Chapter 6 reviewed all of the previous SD relearning studies and confirmed that people with SD are able to relearn the specific information that they study but that this relearning is rigid. The review and a subsequent re-analysis of the data from Chapters 4 and 5 also showed that relearning in SD can have negative side-effects as well as positive effects.

One of the few areas in the brain that still exhibits experience-dependent neuroplasticity in adulthood is found in the medial temporal lobe (MTL) system. Within the MTL, this plasticity has been observed in the hippocampus in both humans and animal models. Rodent model studies focusing on the effect of aerobic exercise have shown a positive increase of neuroplasticity in the dentate gyrus subregion of the hippocampus. Another area in the MTL, the entorhinal cortex (EC), serves as a primary input to the hippocampus, and studies on environmental enrichment have reported greater EC volume in rodents supplied with toys and running wheels. Previous work in our lab working with healthy young adults showed a positive correlation between right EC volume, and aerobic fitness (VO2 max). In this thesis, I examined two aims, first whether aerobic fitness predicts changes in thickness or volume of the MTL as well as performance in an MTL dependent task in healthy young adults. Additionally, whether the brain morphology measures of the MTL can predict performance on the memory task. The second aim looks at the longitudinal effect a 12-week exercise intervention has on thickness or volume in the MTL and performance on an MTL dependent task in the same population. Results indicate that there is a positive baseline correlation between aerobic fitness and thickness of the EC on the left hemisphere but there are no longitudinal changes in morphology after the exercise intervention. These data extend previous work on the effects aerobic exercise has on MTL structure and offer interesting venues to combat neurodegenerative diseases that affect the MTL memory system like Alzheimer’s disease.

The question of how we form memories has fascinated scientists for decades. The hippocampus and surrounding medial-temporal-lobe (MTL) structures are critical for both memory and spatial navigation, yet we do not fully understand the neuronal representations used to support these behaviors. Much research has examined how the MTL neurally represents spatial information, such as with “place cells” that represent an animal’s current location or “head-direction cells” that code for an animal’s current heading. In addition to attending to current spatial locations, navigating to remote destinations is a common part of daily life. In this dissertation I investigate how the human MTL represents the relevant information in a goal-directed spatial-memory task. Specifically, I analyze single-neuron and local field potential (LFP) data from neurosurgical patients with respect to their spatial navigation and memory behavior, with a focus on probing the link between neuronal firing, oscillations, and memory. In Chapter 2, I find that the firing rates of many MTL neurons during navigation significantly change depending on the position of the current spatial target. In addition, I observe neurons whose firing rates during navigation are tuned to specific heading directions in the environment, and others whose activity changes depending on the timing within the trial. By showing that neurons in our task represent remote locations rather than the subject’s own position, my results suggest that the human MTL can represent remote spatial information according to task demands. In Chapter 3, I find that during encoding the left hippocampus exhibits greater low theta power for subsequently recalled items compared to unrecalled items. I also find that high frequency activity and neuronal firing in the hippocampus distinguish between item-filled compared to empty chests. Finally, I find that MTL cells’ firing rates and the differential timing of spikes relative to low frequency oscillations in the LFP distinguish between subsequent recall conditions. These results provide evidence for a distinct processing state during the encoding of successful spatial memory in the human MTL. Overall, in this thesis I show new aspects of the neural code for spatial memories, and how the human MTL supports these representations.

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Ausgehend von den primärsensorischen Arealen verlaufen Verarbeitungswege nach anterior durch die Temporallappen, die der Objekterkennung dienen. Besonders die vorderste Spitze der Temporallappen, der anteriore Temporalkortex, wird mit Funktionen der Objektidentifizierung assoziiert. Es existieren jedoch mehrere Vermutungen, welcher Art die Objekte sind, die in dieser Region verarbeitet werden. Es gibt Annahmen über die Verarbeitung von Sprache, von menschlichen Stimmen, semantischen Informationen oder individuellen Konzepten. Um zwischen diesen Theorien zu differenzieren, wurden vier ereigniskorrelierte fMRT-Messungen an jungen gesunden Erwachsenen durchgeführt. Die Probanden hörten in drei Experimenten die Stimmen berühmter und unbekannter Personen und in einem der Experimente zusätzlich Geräusche von Tieren und Musikinstrumenten. Im vierten Experiment wurden Zeichnungen von Comicfiguren gezeigt sowie von Tieren und Obst- und Gemüsesorten. Die neuronale Aktivität bei der Verarbeitung dieser Reize im Vergleich zu Zeiten ohne Stimulation wurde mit Hilfe von Interesseregionen untersucht, die nahezu die gesamten Temporallappen abdeckten und diese in jeweils zwölf Areale untergliederten. In den anterioren Temporallappen waren sowohl mit auditiven als auch mit visuellen Stimuli deutliche Aktivierungsunterschiede in Abhängigkeit von der semantischen Kategorie festzustellen. Individuelle Konzepte (menschliche Stimmen und Zeichentrickfiguren) riefen eine signifikant stärkere Aktivierung hervor als kategoriale Konzepte (Tiere, Musikinstrumente, Obst- und Gemüse). Außerdem war das Signal, dass durch die Stimmen der bekannten Personen ausgelöst wurde, deutlich stärker als das Signal der unbekannten Stimmen. Damit sind die Daten am ehesten kompatibel mit der Annahme, dass die anterioren Temporallappen, bekannte individuelle Konzepte verarbeiten. Da die beschriebenen Signalunterschiede zwischen den verschiedenen Bedingungen ausgehend von den transversalen Temporalgyri nach anterior zum Temporalpol zunahmen, unterstützen die Ergebnisse zudem die Theorie von einem ventralen Verarbeitungsweg, der die Temporallappen nach anterior durchquert und zur Objekterkennung beiträgt. In Übereinstimmung mit den Annahmen der Konvergenzzonentheorie von A. R. Damasio scheint die spezifische Funktion dieses rostral gerichteten Verarbeitungsweges aus der sukzessiven Kombination immer mehr sensomotorischer Merkmale von Objekten zu bestehen. Da bekannte individuelle Konzepte eine besonders hohe Anzahl von Merkmalen aufweisen, ist eine weiter nach anterior verlaufende Verarbeitung zu beobachten als bei unbekannten oder kategorialen Konzepten. / It is assumed that neural pathways run from the primary sensory cortices through the temporal lobes towards their poles crossing areas necessary for object recognition. Especially the most anterior temporal parts were associated with processes contributing to the identification of objects. Yet, there is little agreement on the kinds of objects that are interpreted by the anterior temporal lobes. For example, there are assumptions regarding linguistic processing, voice recognition, the processing of general semantic information or the identification of unique entities. In order to differentiate between those theories, four event-related fMRI experiments were performed in healthy young adults. In three experiments, the subjects heard the voices of famous and unknown persons. In addition, characteristic sounds of animals and musical instruments were presented in one of these experiments. During the fourth experiment, drawings of famous cartoon characters were shown together with animals and fruit & vegetables. The neural activity in response to these stimuli compared to rest was analyzed using a regions-of-interest approach. 12 regions-of-interest that covered the majority of the temporal lobes were defined in each hemisphere. Both with auditory and visual stimuli, there were clear activation differences between the semantic categories in the anterior temporal lobes. Unique entities (human voices and cartoon characters) evoked a significantly stronger signal than categorical concepts (animals, musical instruments, fruit & vegetables). Furthermore, the signal in response to voices of familiar persons was significantly higher than to unfamiliar voices. Thus, the results are most compatible with the assumption that the anterior temporal lobes process supramodal features of familiar unique entities. As the before-mentioned signal differences between unique and categorical concepts and between familiar and unfamiliar voices increased from the transversal temporal gyri towards the temporal poles, the results support the notion of a ventral processing pathway running rostrally through the temporal lobes. In accordance with the convergence zone theory described by A.R. Damasio, the precise function of that pathway seems to consist in the incremental combination of sensorimotor concept features. Since familiar unique entities possess an especially high number of features, their processing was found to be directed into more anterior portions of the temporal lobe than the perception of unfamiliar or categorical concepts.