Neural hybrid model of semantic object memory: implications from event-related timing using fMRI.

Previous studies by our group have demonstrated fMRI signal changes and synchronized gamma rhythm EEG oscillations between thalamus and cortical regions as subjects recall objects from visually presented features. Here, we extend this work by estimating the time course of fMRI signal changes in the cortical and subcortical regions found to exhibit evidence for task-related activation. Our results indicate that there are separate loci of signal changes in the thalamus (dorsomedial and pulvinar) that exhibit notable differences in times of onset, peak and return to baseline of signal changes. The signal changes in the pulvinar demonstrate the slowest transients of all the cortical and subcortical regions we examined. Evaluation of cortical regions demonstrated salient differences as well, with the signal changes in Brodmann area 6 (BA6) rising, peaking, and returning to baseline earlier than those detected in other regions. We conclude that BA6 mediates early designation or refinement of search criteria, and that the pulvinar may be involved in the binding of feature stimuli for an integrated object memory.