Inferring Nonlinear Neuronal Computation Based on Physiologically Plausible Inputs

The computation represented by a sensory neuron's response to stimuli is constructed from an array of physiological processes both belonging to that neuron and inherited from its inputs. Although many of these physiological processes are known to be nonlinear, linear approximations are commonly used to describe the stimulus selectivity of sensory neurons (i.e., linear receptive fields). Here we present an approach for modeling sensory processing, termed the Nonlinear Input Model (NIM), which is based on the hypothesis that the dominant nonlinearities imposed by physiological mechanisms arise from rectification of a neuron's inputs. Incorporating such ‘upstream nonlinearities’ within the standard linear-nonlinear (LN) cascade modeling structure implicitly allows for the identification of multiple stimulus features driving a neuron's response, which become directly interpretable as either excitatory or inhibitory. Because its form is analogous to an integrate-and-fire neuron receiving excitatory and inhibitory inputs, model fitting can be guided by prior knowledge about the inputs to a given neuron, and elements of the resulting model can often result in specific physiological predictions. Furthermore, by providing an explicit probabilistic model with a relatively simple nonlinear structure, its parameters can be efficiently optimized and appropriately regularized. Parameter estimation is robust and efficient even with large numbers of model components and in the context of high-dimensional stimuli with complex statistical structure (e.g. natural stimuli). We describe detailed methods for estimating the model parameters, and illustrate the advantages of the NIM using a range of example sensory neurons in the visual and auditory systems. We thus present a modeling framework that can capture a broad range of nonlinear response functions while providing physiologically interpretable descriptions of neural computation.Author Summary: Sensory neurons are capable of representing a wide array of computations on sensory stimuli. Such complex computations are thought to arise in large part from the accumulation of relatively simple nonlinear operations across the sensory processing hierarchies. However, models of sensory processing typically rely on mathematical approximations of the overall relationship between stimulus and response, such as linear or quadratic expansions, which can overlook critical elements of sensory computation and miss opportunities to reveal how the underlying inputs contribute to a neuron's response. Here we present a physiologically inspired nonlinear modeling framework, the ‘Nonlinear Input Model’ (NIM), which instead assumes that neuronal computation can be approximated as a sum of excitatory and suppressive ‘neuronal inputs’. We show that this structure is successful at explaining neuronal responses in a variety of sensory areas. Furthermore, model fitting can be guided by prior knowledge about the inputs to a given neuron, and its results can often suggest specific physiological predictions. We illustrate the advantages of the proposed model and demonstrate specific parameter estimation procedures using a range of example sensory neurons in both the visual and auditory systems.