Introduction fMRI signal changes have been found to increase at high diffusion weightings (b ≥ 600 s/mm) in both human and cat visual stimulation [1,2]. Because the high diffusion sensitizing gradients presumably crush the signal from the blood, this finding suggest a change of water diffusion characteristics in the extravascular tissue and has been hypothetically attributed to the activation-induced neuronal cell swelling . In contrast, a recent human hypercapnia study suggested that vascular contribution might dominate the observed change in the diffusion weighted fMRI . We have recently studied the response of the apparent diffusion coefficient (ADC) for a cat visual stimulation model using a gradient echo sequence . When the blood signal was eliminated by the injection of an intravascular contrast agent, stimulation-induced ADC changes could not be observed with b <1050 s/mm in the brain parenchyma, whereas ADC decreased at the surface of the visual cortex suggesting an activation-induced change of the partial volume of cerebrospinal fluid (CSF). In this work, we investigated the diffusion weighted fMRI response for higher b-values using a double spin-echo sequence [1,3]. Experiments were performed with and without the suppression of intravascular signals by a contrast agent, monocrystalline iron oxide nanoparticles (MION).