Nir Y, Dinstein I, Malach R, Heeger DJ. BOLD and spiking activity - a comment on Viswanathan and Freeman. Nature Neuroscience, 11:523, 2008.
Summary: Viswanathan and Freeman (Nat Neurosci, 2007) claim that oxygen concentration and by inference blood oxygen level-dependent (BOLD) fMRI reflect synaptic activity more than spiking activity. This is a fundamental and controversial issue in fMRI research, so this claim, if incorrect, may erroneously bias the interpretation of a large body of data. The authors simultaneously recorded multi-unit activity (MUA), local field potentials (LFP), and tissue oxygen concentration in primary visual cortex of anesthetized cats stimulated with moving gratings. During high temporal frequency stimulation, when thalamic inputs are active but few cortical neurons respond, oxygen signals were observed without MUA. Hence, they concluded that oxygen responses reflect synaptic inputs more than spiking. However, careful inspection of their results leads to the opposite conclusion and supports a tight coupling between oxygen signals and local cortical spiking.