Neuronal identity and hybrid coding

Recent studies of the olfactory system report that differentiated sensory neurons have an identity evidenced by surface markers. These markers relate to the specificity of the neuron's odorant receptor. During axonal outgrowth the markers co-determine, with which of the second stage neurons synapses are formed. The markers and the resulting connectivity (or network structure) amount to static, contextual information about the what and where of a firststage neuronal message. This static information, long-sought by Radical Constructivism, complements the rapidly variable information carried by the spike pattern of single fibres. Thus neural coding is hybrid: in first-stage neurons stimulus intensity is encoded by spike pattern and contextual what and where by molecular markers decisive for neuronal connectivity. Based on this connectivity, the answer to the contextual question what odorant? is found by pattern analysis across the population of first-stage neurons.