References
Tao, Cai, McLaughlin, Shelley, Shapley; PNAS 103-34, 12911–12916, 206.
Figure 1 : The Fraser spiral illusion, the false spiral, or the twisted cord illusion. The overlapping black arc segments appear to form a spiral but are actually a series of concentric circles.
Within a large-scale neuronal network model of macaque primary
visual cortex, we examined how intrinsic dynamic fluctuations in
synaptic currents modify the effect of strong recurrent excitation
on orientation selectivity. Previously, we showed that, using a
strong network inhibition countered by feedforward and recurrent
excitation, the cortical model reproduced many observed properties
of simple and complex cells. However, that network’s complex
cells were poorly selective for orientation, and increasing cortical
self-excitation led to network instabilities and unrealistically high
firing rates. Here, we show that a sparsity of connections in the
network produces large, intrinsic fluctuations in the cortico-cortical
conductances that can stabilize the network and that there is a
critical level of fluctuations (controllable by sparsity) that allows
strong cortical gain and the emergence of orientation-selective
complex cells. The resultant sparse network also shows near
contrast invariance in its selectivity and, in agreement with recent
experiments, has extracellular tuning properties that are similar in
pinwheel center and iso-orientation regions, whereas intracellular
conductances show positional dependencies. Varying the strength
of synaptic fluctuations by adjusting the sparsity of network
connectivity, we identified a transition between the dynamics of
bistability and without bistability. In a network with strong recurrent
excitation, this transition is characterized by a near hysteretic
behavior and a rapid rise of network firing rates as the synaptic
drive or stimulus input is increased. We discuss the connection
between this transition and orientation selectivity in our model of
primary visual cortex.
Web Resource: www.pnas.org
Keywords: inhibition, visual cortex, recurrent excitation, feedback, cortical circuitry
