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A neuropsychologist whose work includes declarative memory and amnesia.^[ref]

An account in which the medial temporal lobe rapidly establishes and initially binds long-term declarative memory, with a changing dependence on the region during systems consolidation. The original strong restriction of the region to long-term memory must be distinguished from the broader claim that it is important for declarative learning.^[ref]

The theories can be reconciled at several explanatory levels, but a strong restriction of medial temporal regions to declarative memory conflicts with evidence for their contribution to demanding online perception. Compatibility should be assessed between explicit claims, rather than achieved by renaming every deficit relational or mnemonic.

The medial temporal lobe declarative memory system theory, associated with Larry Squire and Stuart Zola-Morgan, identifies a system important for rapidly establishing long-term memories for facts and events. Their 1991 account emphasizes binding distributed cortical representations and a time-limited dependence during consolidation. It explains why substantial impairment in acquiring new declarative knowledge can coexist with preserved skills or other learning. The proposal specifies a memory system and its temporal role; it does not by itself specify every representation or learning rule inside each component.^[ref]

Relational memory theory, associated with Howard Eichenbaum and supported by work including Norbert J. Fortin's, addresses the organization and flexible use of relationships. Items may be connected by location, order or another shared element. The account therefore explains why hippocampal damage can affect more than a spatial map. Its proposal that related experiences support flexible queries supplies a candidate computation within a wider declarative system, rather than necessarily competing with the existence of that system.^[ref]

The representational hierarchy theory associated with Timothy J. Bussey and Mark J. Buckley asks what conjunctions a region represents. A simple feature can occur in several objects and be insufficient for distinguishing them. Perirhinal object-level representations can reduce such ambiguity. The same representation can support recognizing an object after a delay or distinguishing it among simultaneously visible alternatives; the representational demand crosses the usual perception–memory boundary.

Two experiments sharpen this claim. Timothy J. Bussey and colleagues in 2002 held the number of object pairs constant while varying feature ambiguity in visual discrimination. Perirhinal lesion effects were related to the conjunction demand, supporting a more specific account than simply more objects or more difficult learning. Mark J. Buckley and colleagues in 2001 used simultaneous touchscreen oddity tasks. Lesioned macaques were impaired when object-level distinctions were required, with spared performance on difficult discriminations based on simple color, shape or size differences. Difficulty alone therefore does not explain the pattern.^[ref][ref]

A constructive reconciliation follows. The declarative account identifies a broad system and its role in long-term learning; the relational account specifies organization and flexible expression; a representational hierarchy specifies which conjunctions its components can supply. Perirhinal object representations and contextual representations can participate in hippocampal binding, and learning can stabilize those relations. Item/source differences in Lila Davachi's encoding study are consistent with differentiated contributions within a common system. Consistency is weaker than a unique confirmation of this particular synthesis.^[ref]

A real disagreement remains. If an original strong declarative-only position says that these regions have no necessary role in perception or immediate processing, a selective deficit on simultaneous object discrimination is problematic. Calling every across-fixation comparison memory would protect that position only by changing what it excludes. Conversely, simultaneous display does not eliminate working memory, attention, task-rule learning or all incidental encoding. The strongest adjudication manipulates representational complexity separately from retention demand and matches sensory and motor requirements. A selective conjunction deficit at negligible imposed delay supports the representational claim more directly than a generic difficult-task impairment.

The theories also need not agree about eventual hippocampal independence of detailed remote episodes. Systems-level consolidation, relational organization and perceptual conjunctions are different issues; adopting one does not logically settle the others. Each retained commitment should generate a prediction that could differ from an alternative, including the kind of error expected after a selective lesion.