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# Research Project - On the analysis and interpretation of inhomogeneous quadratic forms as receptive fields

Pietro Berkes and Laurenz WiskottRecent research in neuroscience has seen an increasing number of extensions of established linear techniques to their nonlinear equivalent, in both experimental and theoretical studies. This is the case, for example, for spatio-temporal receptive-field estimates in physiological studies and information theoretical models like principal component analysis (PCA) and independent component analysis (ICA). Additionally, new nonlinear algorithms have been introduced, like slow feature analysis (SFA). The study of the resulting nonlinear functions can be a difficult task because of the lack of appropriate tools to characterize them qualitatively and quantitatively.

In this work we introduce some mathematical and numerical methods to analyze and interpret generic inhomogeneous quadratic forms. The resulting characterization is in some aspects similar to that given by physiological studies of the receptive fields of cortical cells, making it particularly suitable for application to second-order approximations and theoretical models of physiological receptive fields. We illustrate the proposed algorithms using the quadratic forms computed in a theoretical model of self-organization of complex-cell receptive field (Berkes and Wiskott, 2002).

We first discuss two standard ways of analyzing a quadratic form by directly visualizing the coefficients of its quadratic and linear terms and by considering the eigenvectors of its quadratic term. This method often allows to gain some intuition of the behavior of the quadratic form, but a direct interpretation is sometimes difficult and the interaction between linear and quadratic term is not considered.

We then present an algorithm to compute the optimal excitatory and inhibitory stimuli, i.e. the stimuli that maximize and minimize the considered quadratic form, respectively, given a fixed energy constraint. This is similar to the physiological practice of characterizing a neuron by the stimulus to which it responds best. The solution to this problem is unique and its mathematical formulation can be reduced to an efficient linear search over one parameter on a bounded range.

Since we are considering non-linear functions, the optimal stimuli do not provide a complete description of their properties. We therefore next consider the invariances of the optimal stimuli, which are the transformations to which the quadratic form is most insensitive. This is similar to the common interpretation of neurons as detectors of a specific feature of the input with an invariance to certain local transformations of that feature. For example, complex cells in the primary visual cortex are thought to respond to oriented bars and to be invariant to a local translation. To do this, we compute the second derivative of the quadratic form at the optimal stimulus but restricted to the sphere of input patterns of constant energy. This can be done by considering special paths on the sphere known as geodetics and computing the second derivative of the function along them. We derive in this way an explicit formula for the quadratic form that describes the second derivative. The invariances are found to be the eigenvectors corresponding to the eigenvalues of largest magnitude. The invariances can then be visualized by moving on the sphere in the direction of the eigenvectors and stopping as soon as the output of the function drops under a given threshold. For each quadratic form with N variables we can find in this way N-1 invariances. We propose a test to determine which of these invariances are statistically significant. This can be done by comparing the second derivatives in the direction of the invariances with those of random quadratic forms that are equally adapted to the statistics of the input data.

We further derive simpler versions of the above results in the special case of a quadratic form without linear term and discuss the analysis of such homogeneous quadratic forms in some theoretical and experimental studies.

Finally, we show that for each quadratic form one can build an equivalent two-layer neural network, formed by a first layer of linear subunits followed by a quadratic nonlinearity and a multiplication with weighting coefficients. The output neuron also receives a direct linear contribution from the input layer. Since the weighting coefficients can be negative, some of the subunits give an inhibitory contribution to the output. Such a network is compatible with (but more general than) related networks used in some recent papers and the energy model of complex cells. We show that the subunits are unique only up to two arbitrary orthogonal transformations of the positive and negative subunits, respectively.