Verbal and visual memory binding in patients with unilateral temporal lobectomy

The role of the medial temporal lobes in long-term declarative memory is well established, but their role in short-term relational memory and learning, however, is still under investigation. Research suggests there are dissociations between items that form unified representations (e.g. shapes with colours) and inter-item bound representations that do not form single entities (e.g. word pairs). Evidence from brain damaged patients also suggests a functional dichotomy between left and right hemisphere for language and visuo-spatial memory respectively. This study investigates the role of the left and right medial temporal lobes in verbal and visual memory in patients after unilateral anterior temporal lobectomy for temporal lobe epilepsy (left = 9, right = 12, controls = 53). Subjects underwent a binding paradigm to assess short-term memory (STM), learning, and long-term memory (LTM) for unitized visual representations (shapes and patterns versus shape-pattern combinations) and inter-item verbal associations (single words versus word-pairs), as well as a battery of neuropsychological tests to assess verbal and visual memory, executive function, attention, and visual perception. Binding in STM reduced performance for all groups. Both patient groups performed worse than controls in all conditions of STM, learning, and LTM. Patient groups displayed an additional binding deficit in the verbal STM condition. Only left-sided patients had LTM verbal binding deficit. There were no binding deficits in the visual tasks. Findings suggest that binding in STM is resource demanding but does not additionally impact learning. MTL damage in either hemisphere affects both verbal and visual memory and learning. While MTL damage does not affect intra-item visual binding it does affect STM binding of within-domain verbal material. The LTM deficit in bound verbal material seen in left patients is explained via a laterality effect. Results are discussed in relation to working memory theory.