https://tdlc.ucsd.edu/SV2013/Feldman_%202012_Spike_Timing.pdf

The spike-timing dependence of plasticity.

The descending projections from motor cortex share many features with top-down or backward connections in visual cortex; for example, corticospinal projections originate in infragranular layers, are highly divergent and (along with descending cortico-cortical projections) target cells expressing NMDA receptors. This is somewhat paradoxical because backward modulatory characteristics would not be expected of driving motor command signals. We resolve this apparent paradox using a functional characterisation of the motor system based on Helmholtz’s ideas about perception; namely, that perception is inference on the causes of visual sensations. We explain behaviour in terms of inference on the causes of proprioceptive sensations. This explanation appeals to active inference, in which higher cortical levels send descending proprioceptive predictions, rather than motor commands. This process mirrors perceptual inference in sensory cortex, where descending connections convey predictions, while ascending connections convey prediction errors. The anatomical substrate of this recurrent message passing is a hierarchical system consisting of functionally asymmetric driving (ascending) and modulatory (descending) connections: an arrangement that we show is almost exactly recapitulated in the motor system, in terms of its laminar, topographic and physiological characteristics. This perspective casts classical motor reflexes as minimising prediction errors and may provide a principled explanation for why motor cortex is agranular.

https://link.springer.com/content/pdf/10.1007%2Fs00429-012-0475-5.pdf

Predictions not commands: active inference in the motor system

Endocannabinoid mediated spike timing-dependent depression (t-LTD) is crucially involved in the development of the sensory neocortex. t-LTD at excitatory synapses in the developing rat barrel cortex requires cannabinoid CB(1) receptor (CB(1)R) activation, as well as activation of NMDA receptors located on the presynaptic terminal, but the exact signaling cascade leading to t-LTD remains unclear. We found that astrocytes are critically involved in t-LTD. Astrocytes gradually increased their Ca(2+) signaling specifically during the induction of t-LTD in a CB(1)R-dependent manner. In this way, astrocytes might act as a memory buffer for previous coincident neuronal activity. Following activation, astrocytes released glutamate, which activated presynaptic NMDA receptors to induce t-LTD. Astrocyte stimulation coincident with afferent activity resulted in long-term depression, indicating that astrocyte activation is sufficient for the induction of synaptic depression. Taken together, our findings describe the retrograde signaling cascade underlying neocortical t-LTD. The critical involvement of astrocytes in this process highlights their importance for experience-dependent sensory remodeling.

Astrocyte signaling controls spike timing–dependent depression at neocortical synapses

It has been suggested that excitatory and inhibitory inputs to cortical cells are balanced, and that this balance is important for the highly irregular firing observed in the cortex. There are two hypotheses as to the origin of this balance. One assumes that it results from a stable solution of the recurrent neuronal dynamics. This model can account for a balance of steady state excitation and inhibition without fine tuning of parameters, but not for transient inputs. The second hypothesis suggests that the feed forward excitatory and inhibitory inputs to a postsynaptic cell are already balanced. This latter hypothesis thus does account for the balance of transient inputs. However, it remains unclear what mechanism underlies the fine tuning required for balancing feed forward excitatory and inhibitory inputs. Here we investigated whether inhibitory synaptic plasticity is responsible for the balance of transient feed forward excitation and inhibition. We address this issue in the framework of a model characterizing the stochastic dynamics of temporally anti-symmetric Hebbian spike timing dependent plasticity of feed forward excitatory and inhibitory synaptic inputs to a single post-synaptic cell. Our analysis shows that inhibitory Hebbian plasticity generates ‘negative feedback’ that balances excitation and inhibition, which contrasts with the ‘positive feedback’ of excitatory Hebbian synaptic plasticity. As a result, this balance may increase the sensitivity of the learning dynamics to the correlation structure of the excitatory inputs.

/pdf/balancing-feed-forward-excitation-and-inhibition-via-hebbian-1lp58uwckm.pdf

Balancing Feed-Forward Excitation and Inhibition via Hebbian Inhibitory Synaptic Plasticity

Regular Spiking and Intrinsic Bursting Pyramidal Cells Show Orthogonal Forms of Experience-Dependent Plasticity in Layer V of Barrel Cortex

Layer 2/3 (L2/3) pyramidal cells receive excitatory afferent input both from neighbouring pyramidal cells and from cortical and subcortical regions. The efficacy of these excitatory synaptic inputs is modulated by spike timing-dependent plasticity (STDP). Here we report that synaptic connections between L2/3 pyramidal cell pairs are located proximal to the soma, at sites overlapping those of excitatory inputs from other cortical layers. Nevertheless, STDP at L2/3 pyramidal to pyramidal cell connections showed fundamental differences from known STDP rules at these neighbouring contacts. Coincident low-frequency pre- and postsynaptic activation evoked only LTD, independent of the order of the pre- and postsynaptic cell firing. This symmetric anti-Hebbian STDP switched to a typical Hebbian learning rule if a postsynaptic action potential train occurred prior to the presynaptic stimulation. Receptor dependence of LTD and LTP induction and their pre- or postsynaptic loci also differed from those at other L2/3 pyramidal cell excitatory inputs. Overall, we demonstrate a novel means to switch between STDP rules dependent on the history of postsynaptic activity. We also highlight differences in STDP at excitatory synapses onto L2/3 pyramidal cells which allow for input specific modulation of synaptic gain.

Input Specificity and Dependence of Spike Timing--Dependent Plasticity on Preceding Postsynaptic Activity at Unitary Connections between Neocortical Layer

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Input Specificity and Dependence of Spike Timing--Dependent Plasticity on Preceding Postsynaptic Activity at Unitary Connections between Neocortical Layer