Is Eichenbaum et al.'s proposal testable and how extensive is the hippocampal memory system?

The hippocampal system: Dissociating its functional components and recombining them in the service of declarative memory

Continuing commentary raised several issues concerning our proposal that the hippocampus, parahippocampal region, and cortical association areas mediate different aspects of memory function. Recent relevant findings strengthen our argument that neocortical areas and the parahippocampal region maintain persistent encodings of specific single items and that the hippocampus mediates representations of the relations among these items. The reciprocally and closely interconnected structures that compose the hippocampal memory system work interactively to support flexible memory expression that is relevant to the natural behavior of animals and to conscious recollection in humans.

The hunting of the hippocampal function

Eichenbaum et al.'s (1994a) theory suffers from a lack of ecological validation. It is not at all clear why the hypothesized faculties would have evolved and what their adaptive value would be. I argue that hippocampal function can only be understood if the animal is seen in its natural context.

Sequential processing of “items” and “relations”

Eichenbaum et al. (1994a) hypothesized that perceptually distinct items and the relations among them are processed sequentially by the parahippocampal region and the hippocampal formation, respectively. Predictions based solely on their model's sequential-processing feature might prove easier to disconfirm than those based on its representational features. Two such predictions are discussed: (1) double dissociations should be impossible following hippocampal vs. parahippocampal lesions, and (2) hippocampal lesions should not exacerbate impairments that follow complete parahippocampal lesions.

Recovered consciousness: A proposal for making consciousness integral to neuropsychological theories of memory in humans and nonhumans

Why is consciousness associated with recovery of memories that are initially dependent on the hippocampal system? A hypothesis is proposed that the medial temporal lobe/hippocampal complex (MTL/H) receives as its input only information that is consciously apprehended. By a process termed “cohesion,” the MTL/H binds into a memory trace those neural elements that mediated the conscious experience so that effectively, “consciousness” is an integral part of the memory trace. It is the phenomenological records of events (Conway 1992), integrated consciousness-content packets, that are recovered when memory traces are retrieved.

Hippocampus, delay neurons, and sensory heterogeneity

We raise three issues concerning the Eichenbaum, Otto & Cohen (1994) model. (1) We argue against the strict division of labor that Eichenbaum et al. attribute to neocortical and limbic regions. (2) We raise the possibility that the anterior and posterior portions of the hippocampus may be important for different types of information processing. (3) We argue that, rather than reflecting relational processing, different neural responses to “match” and “nonmatch” trials may relate to different required spatial responses.

The hippocampus seen in the context of declarative and procedural control

Various apparently incompatible theories of hippocampal function have been proposed but integration is now needed. It is argued that the involvement of the hippocampus is most clearly seen when the animal needs to extrapolate beyond current sensory information. Such control can involve both the initiation of behaviour in the absence of appropriate sensory input and the inhibition of behaviour that might otherwise be triggered by current sensory input.