A BOLD Assumption

intrinsic signals and fMRI, and more recently with optogenetic methods (Lee et al., 2010) have permitted remarkable insights into the organization of brain activity and the underlying neuronal mechanisms, suggesting that, within reasonable approximation, this assumption is correct. However, recent reports seem to challenge this assumption. How much? Schummers et al. (2008) recently reported that hemodynamic responses depend on astrocytic rather than on neu-ronal activity. The tuning properties of hemodynamic responses would thus reflect those of astrocytic populations rather than of neuronal ones. How different are those? Schummers et al. find that the tuning curves of nearby astrocytes and neurons are centered on the same maximum. Yet, astrocytes are somewhat more narrowly tuned, which would introduce some non-linearity in the translation of the neuronal into the hemo-dynamic response. More data and extensive simulation work are needed to determine the amplitude of this effect – in particular under anesthesia, which the authors report to affect neurons and astrocytes differently. However, hemodynamic responses integrate over comparatively large neuronal – and thus astrocytic – populations (at least the size of a cortical column). Moreover, both neuronal and astrocytic tuning curves are in most cases symmetric around their maximum. Therefore, at least the spatial bias introduced by the astrocytic filter is likely to be small. In other words, assuming that the hemodynamic responses reflect a linear integration over the neuronal tuning curves of a local population of neurons is probably not such a bad approximation – at least when it comes down to determine what kind of stimulus with respect to another a given patch of cortex is preferably responsive to. In more specific visual stimulus designs, involving perceptual suppression of a physically present visual stimulus (Maier et al., 2008), BOLD responses and neuronal ones (spikes and LFP, in particular the high frequency range) have been reported to not always go " hand in hand " , but to co-vary differently with visual perception. Although a caveat exists here with respect to the lack of detailed knowledge about the local network activity (Nir et al., 2007), these results suggest that hemodynamic responses might be coupled to the different neuro-nal processes in a stimulus-dependent way. Put differently: the relationship between hemodynamic and neuronal responses is not rigid, but can depend on the specific circumstances and the task. Finally, in a recent study performed by Sirotin and Das (2009) in awake monkey V1 it has been …