  Functional Magnetic Resonance Imaging (fMRI) has transformed our
understanding of brain function in-vivo. However, the neurovascular coupling
mechanisms underlying fMRI are somewhat "distant" from neural activity.
Interestingly, evidence from Intrinsic Optical Signals (IOSs) indicates that
neural activity is also coupled to (sub)cellular morphological modulations.
Diffusion-weighted functional MRI (dfMRI) experiments have been previously
proposed to probe such neuromorphological couplings, but the underlying
mechanisms have remained highly contested. Here, we provide the first direct
link between in vivo ultrafast dfMRI signals upon rat forepaw stimulation and
IOSs in acute slices stimulated optogenetically. We reveal a hitherto
unreported rapid onset (<100 ms) dfMRI signal which (i) agrees with fast-rising
IOSs dynamics; (ii) evidences a punctate quantitative correspondence to the
stimulation period; (iii) and is rather insensitive to a vascular challenge.
Our findings suggest that neuromorphological coupling can be detected via dfMRI
signals, auguring well for future mapping of neural activity more directly.
