Figure 1.
Spike sequences that have identical sets of inter-spike intervals.
Intervals are aligned (A) in a regular order, (B) randomly, and (C) alternating between short and long.
Table 1.
List of the cortical areas, experimental attributes and references for neuronal spike data (in order of ascending mean LvR).
Figure 2.
Performance of LvR and other firing irregularity metrics.
(A) Dependence of LvR on the refractoriness constant, R. Ordinate, performance of LvR estimated as the F-value of ANOVA for the entire 1,307 neurons. (B) Peristimulus spike rasters and histograms for two MT neurons during a stimulus with textured image motion from 500–700 ms (thick horizontal bar) [47]. The spike rasters were aligned to the onset of a fixation target (the origin of the abscissa) upon which the monkey was required to fixate. The perievent metrics Cv, Lv and LvR were determined for spike rasters sampled in time windows of ±50 ms around the time of each bin. Error bars indicate the confidence level (p<0.05, t-distribution). (C) Scatter plots of the six metrics plotted against fluctuation in the firing rate across 20 ISI segments from the 1,307 neurons. The ordinate and abscissa represent the deviations of the metric and the firing rate from the means, normalized to SD, respectively. Colored lines represent average slopes.
Figure 3.
LvR distributions for the entire population and subpopulations of neurons.
(A) The distribution of LvR determined across 2,000 ISI sequences for all 1,307 cortical neurons. (B) Top: Original and rescaled specimen spike sequences of representative neurons with an LvR of 0.5, 1.0, and 1.5 taken from data sets #2 (SMA), #10 (TE) and #19 (MT). Triangle and horizontal bars for the three original spike sequences indicate the onset of wrist movement and periods of visual stimulation, respectively. Dashed lines indicate the correspondence between the original and the rescaled fractions (10 ISIs) for which the time-scale is normalized to the average firing rate of that fraction. Bottom: Metric distributions for fractional sequences derived from neurons whose representative (mean) LvR values are within the range of ±0.05 around 0.5, 1.0, and 1.5 (blue, green and red bars in A, n = 92, 91 and 60). (C) LvR distributions for the 19 neuronal data sets (Table 1), shown in order of ascending mean LvR. The primary motor, higher-order motor, visual, and prefrontal areas are indicated as hexagons, pentagons, triangles, and squares, respectively.
Figure 4.
A MDS map of cortical neuronal firing based on LvR.
The map plots the first and second components of Kruskal's nonmetric multidimensional scaling (MDS) analysis for the Hellinger distances of the metric distributions for all combinations of 19 neuronal data sets. The primary motor, higher-order motor, visual, and prefrontal areas are indicated by black hexagons, blue pentagons, red triangles, and green squares, respectively. The number in brackets next to the notation for each cortical area indicates data sets 1–19 shown in Figure 3C and Table 1.
Figure 5.
Correlation of various metrics.
Mean values of Lv, Cv, and firing rate from the 19 data sets are plotted with reference to the mean LvR. Correlations are r = 0.95, 0.54, and 0.05, respectively. The black hexagons, blue pentagons, red triangles, and green squares represent the primary motor, higher-order motor, visual, and prefrontal areas, respectively, as described in Figure 4.
Figure 6.
ISI distributions and sample firing patterns.
(A), (B) and (C): Left: Distributions of 2,000 interspike intervals (ISIs) from neurons that exhibited LvR = 0.5, 1.0, and 1.5, respectively. For LvR values, three neurons are sampled that exhibited different Cv values close to 1.0, 1.5, and 2.0. Right: Sample firing patterns consisting of 100 consecutive ISIs for each.