Claudia Clopath
Wednesday 30th October 2013
Time: 4pm
Basement Seminar Room
Alexandra House, 17 Queen Square, London, WC1N 3AR
Receptive field formation by interacting excitatory and inhibitory plasticity.
Cortical neurons receive a balance of excitatory and inhibitory currents. This E/I balance is thought to be essential for the proper functioning of cortical networks, because it ensures their stability and provides an explanation for the irregular spiking activity observed in vivo. Although the balanced state is a relatively robust dynamical regime of recurrent neural networks, it is not clear how it is maintained in the presence of synaptic plasticity on virtually all synaptic connections in the mammalian brain. We recently suggested that activity-dependent Hebbian plasticity of inhibitory synapses could be a self-organization mechanism by which inhibitory currents can be adjusted to balance their excitatory counterpart (Vogels et al. 2011). The E/I balance not only generates irregular activity, it also changes neural response properties to sensory stimulation. In particular, it can lead to a sharp stimulus tuning in spiking activity although subthreshold inputs are broadly tuned, it can change the neuronal input-output relation and cause pronounced onset activity due to the delay of inhibition with respect to excitation. This control of neuronal output by the interplay of excitation and inhibition suggests that activity-dependent excitatory synaptic plasticity should be sensitive to the E/I balance and should in turn be indirectly controlled by inhibitory plasticity. Because we expected that excitatoryplasticity is modulated by inhibitory plasticity, the question under which conditions excitatory Hebbian learning rules can establish receptive fields needs to be reevaluated in the presence of inhibitory plasticity. In particular, it is of interest under which conditions neurons can simultaneously develop a stimulus selectivity and a cotuning of excitatory and inhibitory inputs.
To address these questions, we analyze the dynamical interaction of excitatory and inhibitory Hebbian plasticity. We show analytically that the relative degree of plasticity of the excitatory and inhibitory synapses is an important factor for the learning dynamics. When excitatory plasticity rate is increased with respect to the inhibitory one, the system ungoes a Hopf bifurcation, losing stability. We also find that the stimulus tuning of the inhibitory input neurons also has a strong impact on receptive field formation. When stimulus tuning of the inhibitory input neurons has the same width than the one of the excitatory input neurons, stimulus selectivity is prevented, but if the inhibitory input neurons are not tuned, selectivity emerge. This latter scenario, together with our analysis, suggests that the sliding threshold of BCM rules may not be implemented on a cellular level but rather by plastic inhibition arising from interneurons without stimulus tuning. If the stimulus tuning of the inhibitory input neurons is broader than that of the excitatory inputs, constant with experimental findings, we observe a local BCM behavior that leads to a stimulus selectivity on the spatial scale of the inhibitory tuning width. This work is done in collaboration with Tim Vogels and Henning Sprekeler.