It has been shown that the gut and the brain are linked through a multimodal, bidirectional pathway called the gut-brain axis. In the gut-to-brain way, the gut microbiota has been shown to be the main regulator. In clinical practice, evidence of microbiota and brain interactions comes from the association of gut microbiota alterations with neurological and psychiatric conditions. However, until now, it remains unknown how the gut microbiota influences brain activity. In this paper, we show that different microbiota profiles from healthy older people are associated with different spontaneous activity in medial posterior cortical areas. These areas are associated with memory, language, and emotion processing abilities. Therefore, the results obtained provide evidence that non-pathological gut microbiota profiles are correlated to spontaneous cortical activity associated with cognitive functions that typically deteriorate with age. This implies that early nutritional interventions that modify microbiota composition could help delay or ameliorate natural age-related cognitive decline.