Research ReportI love my grandkid! An NIRS study of grandmaternal love in Japan

•We investigated the involvement of the APFC in grandmaternal love.•We measured cerebral hemoglobin concentration changes in the APFC of grandmothers.•The sight of one's own grandchild activated the APFC irrespective of their expression.•The sight of the smiling grandchild increased the activation in the medial APFC.•The sight of the smiling grandchild activated the superior APFC additionally.

Grandmaternal love is essential for the grandmother-grandchild attachment relationship and thus aids an infant's development and mental health, but the underlying neural mechanism is unknown. Recent studies have shed light on involvement of the prefrontal cortex (PFC) in maternal and romantic love. Here, we investigated the involvement of the PFC in grandmaternal love by examining cerebral hemoglobin concentration changes using near-infrared spectroscopy (NIRS). Seventeen grandmothers viewed video clips which included their own or other's (unknown) grandchild smiling or showing a neutral expression while the oxy-hemoglobin (oxy-Hb) concentration was measured from the anterior prefrontal cortex (APFC). The sight of one's own grandchild activated the inferior and medial APFC irrespective of their expression. In addition, the sight of the smiling grandchild induced an increased activation in the medial APFC involved in reward monitoring and mentalizing and an additional activation in the superior APFC involved in cognitive and attentional control. Both medial and superior activations significantly correlated with emotional mood rating. These findings indicate that the different regions of the APFC are involved in grandmaternal love.