RECUPERACIÓN DE ALUMNOS CON LA MATERIA PENDIENTE DE CURSOS

2.4.1 The traditional model of memory processes.

The famous case of the patient HM (Henry Molaison) who suffered from the pharmaco-resistant temporal lobe epilepsy and was surgically treated with a bilat-eral hippocampal resection has driven research on memory for many years [Corkin, 2002, Scoville and Milner, 1957]. After removal of both hippocampi he developed an anterograde amnesia and lost the ability to register new episodic memories. In contrast, his working and procedural memory were intact. This clear dissociation between the long-term declarative memory and short-term working memory was so striking that it made a foundation for the classical taxonomy of memory processes [Gazzaniga et al.,2002,Squire and Zola,1996]. The long-term declarative memory has been suggested to depend critically on the medial temporal lobe (MTL) and the hippocampus in particular but not on the PFC. Working memory in contrast has been exclusively associated with the prefrontal cortex but not with the MTL or hippocampus. Assuming a double dissociation between these two cognitive functions and the brain structures this model has been very productive in guiding research for decades. However, recent experiments both in human subjects and rodents have questioned the main assumptions of this model in two ways.

2.4.2 Aporetic findings.

First, there are other non-declarative forms of episodic memory [i.e. statistical learning: Schapiro et al., 2014, 2012] and more general cognitive processes like object representation [Schapiro et al., 2014] or decision making [Wimmer and Shohamy, 2012] which have been identified to relay on the hippocampus [for re-view see Shohamy and Turk-Browne, 2013]. These results are difficult to explain within the classical model which expects the hippocampus to exclusively contribute to the episodic memory [Squire and Zola, 1996]. Second, and for the purpose of this thesis more important, working memory has been also found to depend on the

hippocampus. In particular studies using neuroimaging [Nichols et al.,2006, Ran-ganath and D’Esposito, 2001], intracranial EEG [Axmacher et al., 2010b,c, 2007, Leszczy´nski et al.,2015,van Vugt et al., 2010] have shown that the hippocampus activates during WM maintenance of multiple items. Furthermore, together with my collaborators I have found that the relevant parameters of EEG signal recorded from the hippocampus predict individual WM capacity limits [Chaieb et al.,2015, Leszczy´nski et al.,2015]. Studies have also shown that rats [Aggleton et al.,1992]

and human participants [Nichols et al., 2006, Olson et al., 2006] with lesioned hippocampus have deficits during maintenance of multiple items or associations of multiple features. For example, Olson and colleagues compared the performance of patients with the medial temporal lobe amnesia and a control group of healthy participants in a task that required maintaining either three objects, three loca-tions or object-location conjuncloca-tions in WM. The authors observed that patients and controls performed equally well in memorizing objects and locations separately but were impaired while maintaining conjunctions of both features. The authors suggest that the hippocampus is critical for relating or binding separate features into conjunctions at both short and long time intervals. Similarly, Nichols et al.

[2006] observed that performance of amnesiac patients with variable lesions in the medial temporal lobe was impaired in WM task that included maintenance of a face. Although face was a singular stimulus but remembering it requires an inte-gration of multiple features (i.e. its shape, relations between eyes and eyebrows, etc.). These studies suggest that the hippocampus is not only activated during WM tasks, but it is also necessary under certain conditions which include the maintenance of multiple items or a number of features. Along the lines of these findings Ranganath and Blumenfeld have doubted the double dissociation between short-term and long-term memory and questioned if theories of memory need to propose a distinct storage mechanisms for both [Ranganath and Blumenfeld,2005].

In particular, the authors reviewed studies showing that working memory indeed do rely on the medial temporal lobe when complex or novel visual objects need to be maintained.

2.4.3 A hypothetical role of the hippocampus in WM.

The emerging doubts on the double dissociation and increasing amount of re-search showing hippocampal contribution to WM begs the question as to what is being represented in the hippocampus during WM maintenance? What is the role which hippocampus plays during maintenance of multiple items? One may imagine a twofold role for the hippocampus during working memory maintenance.

First, a domain general involvement of the hippocampus would assume that it does not represent any particular feature maintained in working memory but is rather driven to encode information to episodic memory or plays some form of an executive function [for a similar idea see Shohamy and Turk-Browne, 2013]. Al-ternatively, a feature specific account would suggest that the hippocampus codes a particular feature during a working memory task. This feature would likely be an abstract and complex association across items (for example temporal or spa-tial relations). Importantly, the feature specific account would assume that the hippocampus is involved only in a subset of working memory tasks. These would be the type of task which requires such a relational representation. This in turn explains why the performance in some WM tasks is spared after the hippocampal lesion.

In sum, the double dissociation between the brain structures (hippocampus, PFC) and cognitive functions (episodic memory, working memory) with the the hip-pocampus being essential for the episodic memory but not for the working mem-ory becomes questionable. There is accumulating evidence pointing towards hip-pocampal involvement beyond episodic memory in a broad range of cognitive func-tions spanning working memory [Axmacher et al., 2010b, Chaieb et al., 2015, Leszczy´nski et al., 2015] but also perception, decision making [for review see Shohamy and Turk-Browne, 2013] or conflict processing [Oehrn et al.,2015].