Impairments in the acquisition of new object-name associations after unilateral temporal lobectomy despite fast-mapping encoding

Learning new object-name associations (i.e., word learning) is an ability crucial to normal development starting in early childhood and continuing through the lifespan. To learn a new word, an object must be associated with an arbitrary phonological (or orthographic) string representing a word. The declarative memory system formulates and encodes associations between two arbitrary stimuli and has been well established as playing a critical role for adult word learning. Research investigating the neural substrates of the declarative memory system and word learning has implicated the hippocampus and the surrounding medial temporal lobe (MTL) as crucial structures. A substantial literature on populations with damage to these particular structures (e.g., hippocampal amnesia, temporal lobectomy) has supports the view that without these structures, declarative learning, and word learning by extension, is grossly impaired. However, a recent study Sharon and colleagues (2011) suggested that non-MTL structures may be sufficient to support word learning under special study conditions ("fast mapping") (Sharon, Moscovitch, & Gilboa, 2011). Fast mapping is a word-learning phenomenon described as the ability to acquire the name for a new word in a single exposure to an unknown word and unfamiliar referent alongside a known word with its referent (e.g., Carey & Bartlett, 1978; Carey, 2010).

This study evaluated the ability of patients with unilateral temporal lobectomy (TL) following early-onset temporal lobe epilepsy to learn new object-name associations in two different word learning conditions: fast mapping (FM) and explicit encoding (EE). The word learning performance was evaluated relative to a group of healthy normal comparison participants (NC). The goal of this study was to examine the role of the hippocampus in word learning to answer the question: does a FM condition promote word learning in participants with temporal lobe epilepsy who have had a left temporal lobectomy?

NC participants were able to acquire a rich representation of novel items (as evidenced by improved familiarity ratings and generalization of items) while TL participants had severely impaired performance on free recall, recognition testing, and generalization tasks. TL participants did not learn novel object-name associations despite a FM paradigm while the NC group performed significantly above chance on recognition testing. These findings in conjunction with broadly similar results obtained from hippocampal amnesic patients tested using the same paradigm (Warren & Duff, 2012), support the necessity of the hippocampus for rapid and flexible associations to be obtained via the declarative memory system.