Attending to a stimulus enhances the neuronal responses to it, while responses to non-attended stimuli are not enhanced and may even be suppressed. Although the neural mechanisms of response enhancement for attended stimuli have been intensely studied, the neural mechanisms underlying attentional suppression remain largely unknown. It is uncertain whether attention acts to suppress the processing in sensory cortical areas that would otherwise process the non-attended stimulus or the subcortical input to these cortical areas. Moreover, the neurochemical mechanisms inducing a reduction or suppression of neuronal responses to non-attended stimuli are as yet unknown. Here, we investigated how attention directed towards visual processing cross-modally acts to suppress vestibular responses in the human brain. By using functional magnetic resonance spectroscopy (fMRS) in a group of female and male subjects we find that attention to visual motion downregulates in a load-dependent manner the concentration of excita...