'Dynamics of neuronal interactions' cannot be explained by 'neuronal transients'

In a recent paper, Vaadia et al. demonstrated that patterns of firing correlation between single neurons in the cortex of behaving monkeys can be modified within a fraction of a second. These changes occur in relation to sensory stimuli and behavioral events, and even without modulations of the neurons' firing rates. These findings call for a revision of prevailing models of neural coding that solely rely on single neuron firing rates. In a defense of these models, Friston put forward an alternative explanation, proposing that the observed correlation dynamics emerge solely from co-modulations of the firing rates of each of the neurons, while the strength of their interaction remains constant. To test this possibility we re-examined the data, adopting Friston's 'neuronal transients' model, and the associated equations and procedures. We found that, to explain the dynamic correlation between a pair of neurons, the alternative interpretation requires that each neuron's response to a single stimulus is composed of a relatively large number of independent components, which co-vary with their counterparts in the companion neuron. This large number of components and their shapes lead us to conclude that, although in principle possible, the neuronal transients model: (i) does not provide a simpler explanation of the experimental results; and (ii) cannot explain these results without itself deviating significantly from most rate code models.