Hypofrontal activity during word retrieval in older adults: a near-infrared spectroscopy study

The supplementary motor area (SMA) has been regarded as a third speech area. The SMA is anatomically classified into two regions, pre-SMA and SMA proper, but the functional specialization of speech production between the two regions remains unknown. Although word retrieval difficulties were often observed in older adults, there was no report as to whether the SMA would be involved in the retrieval difficulties. We focused on the SMA as a function of word production and then used near-infrared spectroscopy (NIRS) with the verbal fluency task (VFT) to explore the possible mechanism underlying the retrieval difficulties related to aging. Based on the anatomical differences within the SMA, we relied on region-of-interest (ROI) analysis to compare the brain activation patterns in the SMA during VFT between 11 healthy elder and 11 younger subjects in the situation where both groups show comparable task performance. Notably, the anterior VFT-related SMA response was more robust in the younger than in the elder group. Furthermore, anterior SMA responses in the elder group may only have a positive correlation with the VFT performance. The findings imply that anterior SMA hypoactivity in elders may cause word retrieval difficulties, while bilateral prefrontal cortices, having close connection with the pre-SMA, may contribute to the compensatory process that enables equivalent performance of the elder group with the younger one.