Multiunit activity: can it predict population dynamics of specific neuronal ensembles?
Single neuron recordings remain the gold standard for monitoring the activity of neuronal populations. In the absence of isolated neurons, high frequency or ‘multiunit activity’ (MUA) is often used as a surrogate. However, MUA does not monitor specific neuronal subtypes, the knowledge of which may be critical for understanding electrophysiological processes. Here, we asked whether the knowledge of the single unit waveform of specific neuron types is sufficient to allow the use of MUA to monitor and distinguish differential activity of individual neuron types. We compared single unit activity of medium spiny neurons (MSNs) and fast-spiking interneurons (FSIs) in the mouse dorsal striatum with the MUA. We demonstrate that when a well-isolated spike is recorded, the MUA at frequencies greater than 100Hz is primarily correlated with single unit spiking, highly dependent on the waveform of each neuron type, and accurately reflects the timing and spectral signature of each neuron. However, in the absence of well-isolated spikes, the low and high frequency bands of simulated MUA did not typically contain sufficient information to allow for accurate prediction of the respective population activity of MSNs and FSIs. Even when data were optimally simulated using averaged spike waveforms, the accuracy for correctly predicting population firing rates of both subtypes was below 50%. Overall, we demonstrate that under ideal conditions accuracy was higher for predicting MSN activity than FSI activity and was more dependent on the firing rate of neurons than on the proportion of neurons driven by the input. Predictive accuracy was particularly low when MSNs exhibited high firing rates or when FSIs exhibited low firing rates. Together, these results demonstrate that ev
en under ideal conditions for the MUA to reliably predict the moment-to-moment activity of specific local neuronal ensembles, knowledge of the spike waveform of the underlying neuronal populations is necessary, but not sufficient.