A hemodynamic correlate of lateralized visual short-term memories

Neuroimaging studies attempting to isolate the neural substrate of visual short-term memory in humans have concentrated on the behavior of neurons populating the posterior part of the parietal cortex as a possible source of visual short-term memory capacity limits. Using a standard change-detection task, fMRI studies have shown that maintenance of bilaterally encoded objects elicited bilateral increases of hemodynamic activation in the intra-parietal and intra-occipital sulci (IPS-IOS) proportional to the number of objects retained in visual short-term memory. We used a spatially cued variant of the change-detection task to record hemodynamic responses to unilaterally encoded objects using functional near-infrared spectroscopy (fNIRS). Electrophysiological studies that employed this task have shown that maintenance of unilaterally encoded objects elicited posterior unilateral (contralateral) increase in event-related negativity proportional to the number of objects retained in visual short-term memory. We therefore examined whether contralateral increases in oxy-hemoglobin concentration correlated with the number of retained objects. Contrary to the idea that bilateral increases in BOLD responses and unilateral increases in event-related negativity may be different reflections of the same underlying neural/functional processing, memory-related increases in oxy-hemoglobin concentration were found bilaterally even when objects had to be encoded unilaterally. The present findings suggest that EEG and fMRI/fNIRS techniques reveal distinct neural signatures of the mechanisms supporting visual short-term memory.