= Solution
\b[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.<The medial temporal lobe memory system by Larry Squire and others>[1991, anatomical components, declarative learning, cortical binding and consolidation]{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.<The hippocampus and mechanisms of declarative memory by Howard Eichenbaum>[1999, systematic organization and flexible expression of memories]{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.<Perirhinal cortex resolves feature ambiguity in complex visual discriminations by Timothy J. Bussey and others>[2002, fixed number of pairs and manipulated feature ambiguity]{ref}<Selective perceptual impairments after perirhinal cortex ablation by Mark J. Buckley and others>[2001, object oddity and difficult simple-feature controls]{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.<Multiple routes to memory by Lila Davachi and others>[2003, differentiated subsequent item and source memory effects]{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.
\b[A coherent account can combine a differentiated medial temporal system, relational binding and hierarchical representations. It must explicitly revise a strict memory-only boundary if complex online <perception> requires the same representations; that tension cannot be removed merely by saying that all three theories describe different levels.]
Back to article page
