Spontaneous events outline the realm of possible sensory responses in neocortical populations

Abstract

Neocortical assemblies produce complex activity patterns both in response to sensory stimuli and spontaneously without sensory input. To investigate the structure of these patterns, we recorded from populations of 40-100 neurons in auditory and somatosensory cortices of anesthetized and awake rats using silicon microelectrodes. Population spike time patterns were broadly conserved across multiple sensory stimuli and spontaneous events. Although individual neurons showed timing variations between stimuli, these were not sufficient to disturb a generally conserved sequential organization observed at the population level, lasting for approximately 100 ms with spiking reliability decaying progressively after event onset. Preserved constraints were also seen in population firing rate vectors, with vectors evoked by individual stimuli occupying subspaces of a larger but still constrained space outlined by the set of spontaneous events. These results suggest that population spike patterns are drawn from a limited "vocabulary," sampled widely by spontaneous events but more narrowly by sensory responses.

Figure 1

Individual neurons respond to different tones with stereotyped temporal profiles, but varying firing rates. (A–D) Raster plots showing responses of representative neurons to presentations of 5 pure tones (100 trials for each tone). Red lines represent peri-stimulus time histograms. (E) Scatter plot showing each neuron’s mean spike latency (MSL) to its preferred tone frequency vs. to all other tones. The red line corresponds to equal latencies. Blue dots indicate putative interneurons as defined by spike width. While neurons typically show earlier firing to their preferred tone, this difference is an order of magnitude smaller than the differences between cells.

Figure 2

Similar temporal activity patterns initiated by presentation of tones and natural sounds. (A) Raster plots showing spike times for two representative neurons to repeated presentations of a pure tone stimulus. (B) Average activity of 90 simultaneously recorded neurons to tone stimuli. Grey bars show pseudocolor representations of each neuron’s perievent time histogram (PETH), red dots denote each neuron’s mean spike latency in the 100ms after tone onset. Neurons are ordered vertically by the mean latency over all stimuli, to illustrate sequential spread of activity. (C) Response of the same two neurons as in (A) to a natural sound (insect vocalization; sound spectrogram shown below rasters), illustrating similar temporal response profiles as to the tone. (D) Response of the same population as (B), displayed in the same vertical order, indicating that the sequential order of firing is preserved. The dots on the right indicate at which shank neurons were recorded. (E) Scatter plot showing each neuron’s mean spike latency for tones and natural sounds with putative interneurons marked in blue. The distribution of points along the diagonal indicates preservation of sequential structure across conditions. (F) Histogram of rank correlations between mean spike times for individual tone presentations and mean response profile across all tones (see Supplementary Figure 4A). (G) Histogram of rank correlations between mean spike times for single natural sound presentation and average across all tones. The prevalence of positive correlations indicates that for the majority of trials, the sequence of neuronal activation was preserved.

Figure 3

Spontaneous upstates initiate sequential patterns homologous to evoked responses. (A) Representative raw data plot showing a tone response and spontaneous firing event. The green trace is a synchronization pulse indicating the duration of a tone stimulus; blue traces show local field potentials (LFP) from four separate recording shanks; underneath is a raster plot showing the spike trains of simultaneously recorded neurons. At bottom is the multiunit firing rate (MUA) computed by averaging all neurons. Neurons are sorted by average spontaneous mean spike latency to facilitate visual examination of temporal patterns. (B) Raster plots showing spike times for the same neurons as in Figures 2A and 2C, triggered by upstate onsets. Note the similar temporal pattern to Figure 2. (C) Average upstate-triggered activity of all neurons, sorted in the same order as in Figures 2B and D. (D) Cross-correlograms of one neuron’s spike times with the summed activity of all other cells, during different experimental conditions. Vertical arrows indicate the center of mass (mean spike time) of correlograms (μ_cc). Cross-correlograms are normalized between 0 and 1 to facilitate comparison. (E) Conservation of μ_cc across different stimuli and spontaneous events, indicating preservation of sequential order. Each point represents the values of μ_cc for a given cell in the conditions indicated on the axes. (F) Histogram of rank correlations between mean spike times for single-trial tone presentations and average mean spike times for spontaneous events.

Figure 4

Preservation of sequential structure between sensory-evoked and spontaneous events in unanesthetized animals. (A, B) Representative raw data plots from an unanesthetized, head-fixed subject in a passive listening paradigm. Again, global fluctuations in activity are seen, although downstates are typically shorter than under anesthesia. (C,D) Similar analysis as in Figure 3B&C, showing preservation of individual neurons’ PETH, and conservation of sequential structure. (E) Conservation of μ_cc across tones and spontaneous events in unanesthetized animals (similar analysis to Figure 3E). (F,G) Histograms of rank correlations between mean spike times for single tone presentation and average across all tones and spontaneous events respectively.

Figure 5

Preservation of temporal structure between sensory-evoked and spontaneous events in somatosensory cortex. (A) Representative raw data plots of spontaneously occurring upstates and air-puff evoked activity. (B) Conservation of μ_cc across different stimuli and spontaneous events, indicating preservation of sequential order. (C) Histogram of rank correlations between mean spike times for single air puff and average mean spike times for spontaneous events.

Figure 6

Combinatorial constraints on population firing rate vectors. (A) Spike counts of two neurons (recorded from separate tetrodes) during the first 100ms of spontaneous upstates (black), responses to a tone (green), and natural sound (magenta). Data were jittered to show overlapping points. Note that regions occupied by responses to the sensory stimuli differ, but are both contained in the realm outlined by spontaneous patterns. (B) Contour plot showing regions occupied by points from (A). The blue outline is computed from spike counts shuffled between upstates, indicating the region that would be occupied in the absence of spike count correlations. (C) Firing rate vectors of entire population, visualized using multidimensional scaling; each dot represents the activity of 45 neurons, nonlinearly projected into two-dimensional space. (D) Contour plot derived from multidimensional scaling data, with responses to individual stimuli marked separately. Sensory-evoked responses again lie within the realm outlined by spontaneous events. (E) Scatter plot showing the Euclidean distances from each evoked event to its closest neighbor in the spontaneous events (E_spont), and in the shuffled spontaneous events (E_shuf). Dashed red line shows equality. (F, G) Histogram showing the difference between distances to shuffled and spontaneous events (E_shuf − E_spont). Top and bottom: data from all anesthetized and unanesthetized experiments, respectively. Almost every evoked event was closer to a true spontaneous vector than to a shuffled vector.

Figure 7

Analysis of pair-wise correlations during evoked and spontaneous conditions. (A) Correlation matrix between spike counts of 45 neurons calculated during upstates. For ease of visualization, neurons were ordered so that the highest correlations are close to the diagonal. (B) Spike count correlation matrix for responses to all sensory stimuli (5 tones and 5 natural sounds), with neurons ordered the same as in A. The dots on the right indicate at which shank neurons were recorded. (C) Correlation matrices for repeated presentations of a single tone and a single natural sound (“noise correlations”), with neurons again ordered the same as in A; note the similar appearance of all matrices. (D) Boxplots showing distribution across experiments of elementwise correlation coefficients between correlation matrices (diagonal excluded). (E) The volume of response space occupied by spontaneous events (black), all evoked activity (red), and the responses to a single stimulus (green), was estimated as a fraction of the volume occupied by shuffled spontaneous events as the square root ratio of covariance matrix determinants (see Supplementary Figure 11). Note the monotonic decrease with population size. (F) Box plots showing distribution of slopes of log-volume fraction as a function of population size for anesthetized (left) and unanesthetized (right) experiments.

Figure 8

Preserved constraints on firing rate vectors allow prediction of a neuron’s sensory tuning based on spontaneous activity, and a geometrical interpretation of these constraints. (A) Prediction of a neuron’s receptive field from correlations with simultaneously recorded neurons during spontaneous activity, and from the receptive fields of these other neurons. Here, w denotes a vector of weights, optimized to maximize prediction of the target neuron’s activity during spontaneous activity, and r denotes the receptive field vectors of N other neurons. (B,C) Actual and predicted receptive fields for a representative neuron. On average, predicted and actual receptive fields showed a correlation of 0.62 (see text for details). (D,E) General relationship between correlation matrix and cluster orientation. (D) shows a set of simulated spike count vectors with the correlation matrix shown in (E); the values of the correlation matrix provide information about the orientation of the cluster relative to the coordinate axes. (F) Cartoon illustrating the geometrical interpretation of our findings. The gray area illustrates the space of all rate vectors theoretically possible in the absence of relationships between neurons. The black outline represents the space of spontaneous events; this is shown elongated and of small volume to illustrate strong constraints at the population level. Responses to individual stimuli occupy smaller subsets within this (colored blobs; the irregular shape illustrates possible non-Gaussianity of these clusters). The orientations of the spaces for individual stimuli (corresponding to noise correlation matrices) are approximately aligned with the space of spontaneous events. The mean response to each stimulus also lies within the space of spontaneous events (see Supplementary Figure 10).