Figure 1.

Left. In red the nonlinear transfer function relating membrane potentials and rates in the neural model (1). The typical activity rates enforced by the intrinsic plasticity rule (y_t = 0.2) result in the neuron operating at the foot of the non-linearity, where it is practically indistinguishable (m.s.e. = 0.027 for x ∈ [−5:0]) from a threshold-powerlaw with exponent n = 2.5 (in blue), typically considered a suitable model for experimental findings in cortical neurons^33–35. Right. The effective synaptic strength multiplier φ(t)u(t) of the Tsodyks-Markram model (4). Here β = α = 0.01 and U_max = 4 was used. The red/violet curves correspond to the values as measured respectively for excitatory synapses in the medial prefrontal cortex of ferrets⁴³ and for inhibitory layer 2–4 neurons of the somatosensory cortex layer of Wistar rats⁴⁴. The presynaptic neuron is active for t ∈ [100, 300] (ms), and inactive otherwise.