Functional Specialization in the Human Medial Temporal Lobe

Abstract

Investigations of memory in rats and nonhuman primates have demonstrated functional specialization within the medial temporal lobe (MTL), a set of heavily interconnected structures including the hippocampal formation and underlying entorhinal, perirhinal, and parahippocampal cortices. Most studies in humans, however, especially in patients with brain damage, suggest that the human MTL is a unitary memory system supporting all types of declarative memory, our conscious memory for facts and events. To resolve this discrepancy, amnesic patients with either selective hippocampal damage or more extensive MTL damage were tested on variations of an object discrimination task adapted from the nonhuman primate literature. Although both groups were equally impaired on standard recallbased memory tasks, they exhibited different profiles of performance on the object discrimination test, arguing against a unitary view of MTL function. Cases with selective hippocampal damage performed normally, whereas individuals with broader MTL lesions were impaired. Furthermore, deficits in this latter group were related not to the number of discriminations to be learned and remembered, but to the degree of "feature ambiguity," a property of visual discriminations that can emerge when features are part of both rewarded and unrewarded stimuli. These findings resolve contradictions between published studies in humans and animals and introduce a new way of characterizing the impairments that arise after damage to the MTL.

Regional activation of the human medial temporal lobe during intentional encoding of objects and positions

“…This supports our hypothesis that the perirhinal cortex is involved in object-specific encoding; and the finding corroborates similar results from both the human and non-human primate literature (Aggleton and Brown, 2005;Wan et al, 1999;Bussey et al, 2002Bussey et al, , 2005. Converging evidence from lesion studies of monkeys (Baxter and Murray, 2001), rats (Prusky et al, 2004) and humans (Barense et al, 2005), as well as from neuroimaging studies (Pihlajamäki et al, 2005) have implicated the perirhinal cortex in object processing. Furthermore, studies using immediate early gene imaging in rats indicate that the perirhinal cortex, but not the hippocampus, is involved in processing novel, as opposed to familiar, visual objects.…”

“…It has been suggested that, during encoding, visual object features are processed in the perirhinal and lateral entorhinal cortex, while spatial context is processed in the adjacent medial entorhinal cortex and parahippocampal cortex (Eichenbaum et al, 2007). Indeed, lesions to the perirhinal cortex alone, or together with the entorhinal cortex, have been shown to cause significant deficits in the ability of humans (Barense et al, 2005), monkeys (Baxter and Murray, 2001) and rats (Prusky et al, 2004) to perform visual categorization and memory tasks, deficits not found when lesions are restricted to the hippocampus. It has also been suggested that the memory functions of MTL structures are dissociated according to stimulus content, where the perirhinal cortex contributes more heavily to processing of visual object information (Bussey et al, 2006), while the hippocampus and parahippocampal cortex contribute more in the processing of spatial information (Epstein et al, 1999;Burgess et al, 2002).…”

The perirhinal cortex modulates V2 activity in response to the agreement between part familiarity and configuration familiarity

“…Interestingly, the performance of MTL-damaged patients deviated from control performance in two ways: first, they reported seeing the Familiar Configurations as figure somewhat less often than controls, and second, they reported seeing the Part-Rearranged Novel Configurations as figure more often than controls. Taken alone, the first finding is consistent with the theoretical view that the PRC of the MTL contains representations of complex configurations Lee et al, 2005;Barense et al, 2005Barense et al, , 2007Barense et al, , 2010a; Bartko et al, 2007;Lee and Rudebeck, 2010;Burke et al, 2011), and damage to these configural representations removes effects of configuration familiarity on figure assignment. Taken together, however, the two findings suggest that either output from the PRC is privileged over that from lower-level visual regions during figure-ground assignment (a feedforward explanation for the data) or that the intact PRC plays a role in modulating processing in lower-level visual areas (a feedback explanation).…”

Dynamic updating of hippocampal object representations reflects new conceptual knowledge

“…The finding that anterior HPC forms concept-specific representations speaks to the debate on HPC's role in representing complex visual objects (44) and classification learning (4). Although findings from seminal rodent and patient studies suggest perirhinal cortex rather than HPC is critical for processing objects composed of multiple features (45)(46)(47), the current findings are consistent with the account that HPC is important for organizing complex object representations according to changing contexts (39,41,48). Specifically, our results suggest that HPC plays an important role in forming new concepts; these HPC-based concepts may then be consolidated into long-term cortical representations of conceptual Stimulus-specific neural representations from the HPC region in A were used to estimate attention weights to the three feature dimensions.…”