| Object naming is essentially a mnemonic task of accessing stored associations between visual and auditory features. Therefore, understanding the neural basis of naming requires a better understanding of the means by which these and similar associations are acquired, stored, and retrieved from long-term memory. Presented here are a series of three experiments designed to address these questions using a novel paired associate memory paradigm in which the same image sound pairings can be recalled nonlinguistically or linguistically, depending on experience. Twelve participants were trained to pair images of animals, degraded using low-pass filers, with sine-wave speech analogues of the spoken animal name; though unaware of the potential linguistic content. The neuroanatomic basis of memory recall for these pairs, as well as additional nonlinguistic controls, is examined over time using event-related functional magnetic resonance imaging. The first experiment identifies the neural network related to accurate pair recall immediately after successful pair learning, and how this substrate evolves during the task time-course. Regions within the hippocampal complex were identified and their functional connectivity to other neo- and subcortical regions was mapped. The second experiment examines how the neural substrate of associate memory recall is altered by time by comparing regions involved in successful immediate recall to those involved in recall one month later. Seven predications of the standard model of systems level consolidation, including decreased activity in some medial temporal regions, were confirmed though the full pattern of results is potentially irreconcilable with the theory. The final experiment examines the patterns of memory storage by identifying how the locus of visual memory changes when participants become aware of the linguistic nature of the paired stimuli. Utilizing a lexical strategy resulted in magnitude but not location changes in the fusiform and inferior temporal gyri, providing strong evidence against theories which propose that memories are stored in anatomically distinct regions according to semantic categories or category features. The full pattern of consolidation and lexical status related effects argues for a modified process specific theory of memory where inter-regional connectivity, varying dynamically with task demands, plays a key role in lexical memory storage and recall. |
