The effect of synaptic plasticity on orientation selectivity in a balanced model of primary visual cortex.

G. MATO; D. HANSEL; C VAN VREESWIJK

Chicago

Conferencia; 45th Meeting Society for Neuroscience; 2015

Society for Neuroscience

Neurons in primary visual cortex (V1) exhibit orientation selectivity (OS). This is true for animals like cats or primates in which V1 has an orientation map (OM) as well as for those without OM (salt-and-pepper organization), e.g. rodents. Whether OS is primarily due to feedforward (FF) connectivity or to recurrent interactions has not been settled. In the former case the presence or absence of an OM hardly matters, but in the latter the spatial organization of preferred orientations (PO) could affect the mechanism. The connectivity in rodent V1 is hotly debated. Unclear is the extent to which connections in adult mice are specific. With specificity, the distribution of the POs of cells projecting to a neuron would be similar to that in cat V1 and the same mechanism could operate in both cases. In contrast, with random connectivity this distribution is flat. Importantly, it has been demonstrated that in mice V1 neural responses are highly selective at eye opening although the connections are not yet specific (Ko etal, 2013). How can OS arise in this case? In a recent theoretical paper (Hansel & van Vreeswijk, 2012) we showed that strong OS emerges naturally in layer 2/3 in V1 if it operates in the balanced excitation/inhibition regime even if the recurrent connectivity is random and FF inputs from Layer 4 (L4) result from selective neurons with random preferred orientations. A question then arises  : But how do L4 neurons exhibit OS in rodents ? To address this issue we consider a model comprising two networks, one for L4 and one for the LGN. We assume first circular LGN receptive fields and purely random FF connections. We show that despite the lack of OS in LGN the FF connectivity carries a weak information on the stimulus orientation which can be extracted and amplified by the L4 network if it operates in the balanced regime. We show that the orientation selectivity index (OSI) depends only weakly on the model parameter with typical mean OSI=0.1-0.2 and 0.15-0.3 for the F0 and F1 components of the response. We also find that for realistic parameter values, the F0 component is substantially less tuned for inhibitory neurons than for excitatory neurons but for F1 components tuning is similar in both populations. Additional contributions to the OS in L4 with salt and paper organization can come from LGN neurons if they have elongated center-surround receptive fields, as recently described in experimental studies or in specificity in the LGN projections as in the Hubel & Wiesel mechanism. We show that these three contributions can be unambiguously disentangled experimentally by characterizing how the PO of the neurons varies with the spatial frequency of a drifting grating stimulus.