Dynamical regime exhibiting correlated background activity. The top of (
a) shows raster-plots of the activity during an 8 second stretch of time. The clusters are organized such that the first eight clusters correspond to
θ
1, the second eight to
θ
2, and the third eight
θ
3. Thus, the clusters in the same hypercolumn are not plotted adjacent to one another. Each row represents the total number of firing-events of either the E-(
top) or I-(
bottom) population within each cluster binned over intervals of 1 ms. This total number of firing-events is represented logarithmically using the colorscale to the
left (i.e., the value 3 corresponds to 2
3 = 8 total events within the
N
Q
= 128 neuron population over a 1 ms interval, or an instantaneous population firing-rate of 64 Hz). In this panel it can clearly be seen that a typical epoch within this background regime is dominated by the activity of E- and I-cells associated with one of the 3 orientations. Below the rasters in panel-(
a) we display the summed E-(
red) and I-(
blue) activity for each orientation. The
vertical scale-bar represents a magnitude of 256 total events per 1 ms bin across all 8 clusters corresponding to a given
θ—i.e., a single-cluster event-count of 16 spikes per ms. Note that when any one
θ
j
is active, the other two orientations are not typically active. Nevertheless, the activity is not always restricted to a single orientation—it is not rare for multiple orientations to activate simultaneously. Note also that, within this regime, there are multiple events which involve brief heightened activity across both the E- and I-populations in many iso-orientation clusters (see
arrowheads). On the left of panel-(
b) we plot the crosscovariance
C (
E,
Q,
A, 0,
τ) in spiking activity between cells in different hypercolumns. See Section
5 for a definition of crosscovariance. The crosscovariance
C is plotted for
Q =
E (
red) and
Q =
I (
blue), as well as for
A = 1 (
upper curves) and
A = 0 (
lower curves). Note that iso-orientation activity is correlated on a timescale of ∼ 300 ms, whereas non-iso-orientation activity is anticorrelated over a similar timescale. Also note that the crosscovariance is nearly symmetric and positive for both
Q =
E and
Q =
I. In the right of panel-(
b) we show experimental crosscovariance measurements from two paired recordings presented in (Kohn and Smith
2005) (
gray, reproduced with permission), superimposed with the crosscovariance
C (
E,
E, 1, 0,
τ measured in our simulation (
light red, taken from top of panel-(
b)). Although the vertical scale is not directly comparable, the persistence time-scale observed in experiment is similar to that seen in simulation. In panel (
c) we plot the spike-triggered average voltage-distribution (see Section
5)
SVD(
E,
Q,
A, 0,
τ,
V) for
Q =
E and
Q =
I and
A = 1 (Same-
θ) and
A = 0 (Diff-
θ). The
solid white lines indicate the mean and mean±stdev for the voltage distribution