Commonality of processes underlying visual and verbal recognition memory

Memory for visual and verbal material engages widely distributed systems, to a large degree focussed in different hemispheres. It might thus be expected that these disparate neuronal populations should display significantly different characteristics in regard to mnemonic performance. Visual memory, fundamental to all human beings, and whose characteristics are largely shared with macaques, was assayed using unique non-objective, colored images lacking ready verbal description and was contrasted with memory for four-letter non-offensive English words. The effects of memory loading, stimulus duration and long-term test intervals (1-2 weeks) were studied in regard to accuracy and reaction times for recognizing initial versus re-exposure to these two types of items. No effects of memory loading were apparent despite the incrementing memory load in the 240-item, running recognition sessions. Words were better remembered than images, both in the long and short term, but the detailed characteristics of reaction times and accuracy in relation to number of intervening items, and in long-term memory were strikingly similar. Given the wide and well-established disparity in cerebral loci participating in linguistic versus image analysis, these multiple similarities in the pattern of mnemonic performance indicate that the underlying neuronal processes must be comparable for remembering either images or words. Furthermore, the strong link manifested between individual items across a varying number of intervening intervals and added items suggests that a phenomenon highly similar to the "stimulus specific adaptation" (SSA), displayed by units in macaque inferotemporal cortex, occurs for each item to be recognized. Finally, the significant augmentation in accuracy both in short- and long-term memory for images when viewing time permits saccades is explained if each saccade and fixational pause recruits additional neurons into the pool displaying SSA, or its equivalent, for the item being viewed.