Abstract
Multi-electrode array recordings of spike and local field potential (LFP) activity were made from primary auditory cortex of 12 normal hearing, ketamine-anesthetized cats. We evaluated 259 spectro-temporal receptive fields (STRFs) and 492 frequency-tuning curves (FTCs) based on LFPs and spikes simultaneously recorded on the same electrode. We compared their characteristic frequency (CF) gradients and their cross-correlation distances. The CF gradient for spike-based FTCs was about twice that for 2–40 Hz-filtered LFP-based FTCs, indicating greatly reduced frequency selectivity for LFPs. We also present comparisons for LFPs band-pass filtered between 4–8 Hz, 8–16 Hz and 16–40 Hz, with spike-based STRFs, on the basis of their marginal frequency distributions. We find on average a significantly larger correlation between the spike based marginal frequency distributions and those based on the 16–40 Hz filtered LFP, compared to those based on the 4–8 Hz, 8–16 Hz and 2–40 Hz filtered LFP. This suggests greater frequency specificity for the 16–40 Hz LFPs compared to those of lower frequency content. For spontaneous LFP and spike activity we evaluated 1373 pair correlations for pairs with >200 spikes in 900 s per electrode. Peak correlation-coefficient space constants were similar for the 2–40 Hz filtered LFP (5.5 mm) and the 16–40 Hz LFP (7.4 mm), whereas for spike-pair correlations it was about half that, at 3.2 mm. Comparing spike-pairs with 2–40 Hz (and 16–40 Hz) LFP-pair correlations showed that about 16% (9%) of the variance in the spike-pair correlations could be explained from LFP-pair correlations recorded on the same electrodes within the same electrode array. This larger correlation distance combined with the reduced CF gradient and much broader frequency selectivity suggests that LFPs are not a substitute for spike activity in primary auditory cortex.
Highlights
Comparison of the receptive fields at the input and output of a cortical cell can reveal information of what the cell, under the influence of all its excitatory, inhibitory, and modulatory inputs, does
We show that the space constant of the cross-correlogram peaks is about a factor 2 larger for local field potential (LFP)-pair correlations compared to spike-pair correlations
For multi-frequency stimulus ensembles presented at 65 dB SPL, we evaluated 259 spectro-temporal receptive fields (STRFs) based on the 2–40 Hz LFP and multiple sorted unit (MSU) recorded on the same electrode
Summary
Comparison of the receptive fields at the input and output of a cortical cell can reveal information of what the cell, under the influence of all its excitatory, inhibitory, and modulatory inputs, does. An extracellular reflection of many synchronous PSPs in a limited volume around the recording electrode is found in the short latency parts of the stimulus evoked local field potential (LFP). For synchronous activation of many cells in the recording volume of an extracellular electrode, one expects the LFP to be a weighted sum of the time derivative of these PSPs [1]. The LFP constitutes a collective property of a neuronal ensemble, i.e., is a measure of synchronous post-synaptic activity of a population of neurons. This ensemble activity may, besides excitatory PSPs (EPSPs), include inhibitory PSPs, subthreshold membrane potential oscillations, and afterpotentials of somatodendritic action potentials [2]. It is important to know how LFPs compare to multi-unit activity, especially with respect to their topographic specificity
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