Abstract
BackgroundAn essential component of cognition and language involves the formation of new conditional relations between stimuli based upon prior experiences. Results of investigations on transitive inference (TI) highlight a prominent role for the medial temporal lobe in maintaining associative relations among sequentially arranged stimuli (A > B > C > D > E). In this investigation, medial temporal lobe activity was assessed while subjects completed "Stimulus Equivalence" (SE) tests that required deriving conditional relations among stimuli within a class (A ≡ B ≡ C).MethodsStimuli consisted of six consonant-vowel-consonant triads divided into two classes (A1, B1, C1; A2, B2, C2). A simultaneous matching-to-sample task and differential reinforcement were employed during pretraining to establish the conditional relations A1:B1 and B1:C1 in class 1 and A2:B2 and B2:C2 in class 2. During functional neuroimaging, recombined stimulus pairs were presented and subjects judged (yes/no) whether stimuli were related. SE tests involved presenting three different types of within-class pairs: Symmetrical (B1 A1; C1 B1; B2 A2; C2 B2), and Transitive (A1 C1; A2 C2) and Equivalence (C1 A1; C2 A2) relations separated by a nodal stimulus. Cross-class 'Foils' consisting of unrelated stimuli (e.g., A1 C2) were also presented.ResultsRelative to cross-class Foils, Transitive and Equivalence relations requiring inferential judgments elicited bilateral activation in the anterior hippocampus while Symmetrical relations elicited activation in the parahippocampus. Relative to each derived relation, Foils generally elicited bilateral activation in the parahippocampus, as well as in frontal and parietal lobe regions.ConclusionActivation observed in the hippocampus to nodal-dependent derived conditional relations (Transitive and Equivalence relations) highlights its involvement in maintaining relational structure and flexible memory expression among stimuli within a class (A ≡ B ≡ C).
Highlights
An essential component of cognition and language involves the formation of new conditional relations between stimuli based upon prior experiences
A sample stimulus was presented on the left side of a computer screen and two comparison stimuli presented on the right
Behavioral For each subject, response accuracy exceeded 90% correct for each derived relation and Foils during neuroimaging
Summary
An essential component of cognition and language involves the formation of new conditional relations between stimuli based upon prior experiences. Considerable evidence highlights a role for the hippocampus in mediating our ability to derive relations among stimuli [1,2,3,4,5,6] and maintaining representational flexibility [7] These two skills underlie many types of complex performances and successful functioning of humans, and have previously been studied with serial transitive inference (TI) paradigms. During SE training an individual may learn that when presented the spoken word 'cat' (sample stimulus A1), selection of the printed word "CAT" (comparison B1), but not the printed word "DOG" (comparison B2) produces reward This differential reinforcement procedure establishes the auditory-visual conditional relation A1:B1. The resulting stimulus class (A1 ≡ B1 ≡ C1) contains elements that are conditionally related, but not hierarchically or sequentially related, which markedly differs from serially ordered stimuli employed in TI paradigms (e.g., A > B > C > D > E)
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