Feature detection using spikes: the greedy approach

A goal of low-level neural processes is to build an efficient code extracting the relevant information from the sensory input. It is believed that this is implemented in cortical areas by elementary inferential computations dynamically extracting the most likely parameters corresponding to the sensory signal. We explore here a neuro-mimetic feed-forward model of the primary visual area (VI) solving this problem in the case where the signal may be described by a robust linear generative model. This model uses an over-complete dictionary of primitives which provides a distributed probabilistic representation of input features. Relying on an efficiency criterion, we derive an algorithm as an approximate solution which uses incremental greedy inference processes. This algorithm is similar to 'Matching Pursuit' and mimics the parallel architecture of neural computations. We propose here a simple implementation using a network of spiking integrate-and-fire neurons which communicate using lateral interactions. Numerical simulations show that this Sparse Spike Coding strategy provides an efficient model for representing visual data from a set of natural images. Even though it is simplistic, this transformation of spatial data into a spatio-temporal pattern of binary events provides an accurate description of some complex neural patterns observed in the spiking activity of biological neural networks.Comment: This work links Matching Pursuit with bayesian inference by providing the underlying hypotheses (linear model, uniform prior, gaussian noise model). A parallel with the parallel and event-based nature of neural computations is explored and we show application to modelling Primary Visual Cortex / image processsing. http://incm.cnrs-mrs.fr/perrinet/dynn/LaurentPerrinet/Publications/Perrinet04tau

The Emergent Logic of Health Law

The American health care system is on a glide path toward ruin. Health spending has become the fiscal equivalent of global warming, and the number of uninsured Americans is approaching fifty million. Can law help to divert our country from this path? There are reasons for deep skepticism. Law governs the provision and financing of medical care in fragmented and incoherent fashion. Commentators from diverse perspectives bemoan this chaos, casting it as an obstacle to change. I contend in this Article that pessimism about health law’s prospects is unjustified, but that a new understanding of health law’s disarray is urgently needed to guide reform. My core proposition is that the law of health care provision is best understood as an emergent system. Its contradictions and dysfunctions cannot be repaired by some master design. No one actor has a grand overview—or the power to impose a unifying vision. Countless market players, public planners, and legal and regulatory decisionmakers interact in oft-chaotic ways, clashing with, reinforcing, and adjusting to each other. Out of these interactions, a larger scheme emerges—one that incorporates the health sphere’s competing interests and values. Change in this system, for worse and for better, arises from the interplay between its myriad actors. By quitting the quest for a single, master design, we can better focus our efforts on possibilities for legal and policy change. We can and should continuously survey the landscape of stakeholders and expectations with an eye toward potential launching points for evolutionary processes—processes that leverage current institutions and incentives. What we cannot do is plan or predict these evolutionary pathways in precise detail; the complexity of interactions among market and government actors precludes fine-grained foresight of this sort. But we can determine the general direction of needed change, identify seemingly intractable obstacles, and envision ways to diminish or finesse them over time. Dysfunctional legal doctrines, interest group expectations, consumers’ anxieties, and embedded institutional and cultural barriers can all be dealt with in this way, in iterative fashion. This Article sets out a strategy for doing so. To illustrate this strategy, I suggest emergent approaches to the most urgent challenges in health care policy and law—the crises of access, value, and cost