Spatio-Temporal Credit Assignment in Neuronal Population Learning

n learning from trial and error, animals need to relate behavioral decisions to environmental reinforcement even though it may be difficult to assign credit to a particular decision when outcomes are uncertain or subject to delays. When considering the biophysical basis of learning, the credit-assignment problem is compounded because the behavioral decisions themselves result from the spatio-temporal aggregation of many synaptic releases. We present a model of plasticity induction for reinforcement learning in a population of leaky integrate and fire neurons which is based on a cascade of synaptic memory traces. Each synaptic cascade correlates presynaptic input first with postsynaptic events, next with the behavioral decisions and finally with external reinforcement. For operant conditioning, learning succeeds even when reinforcement is delivered with a delay so large that temporal contiguity between decision and pertinent reward is lost due to intervening decisions which are themselves subject to delayed reinforcement. This shows that the model provides a viable mechanism for temporal credit assignment. Further, learning speeds up with increasing population size, so the plasticity cascade simultaneously addresses the spatial problem of assigning credit to synapses in different population neurons. Simulations on other tasks, such as sequential decision making, serve to contrast the performance of the proposed scheme to that of temporal difference-based learning. We argue that, due to their comparative robustness, synaptic plasticity cascades are attractive basic models of reinforcement learning in the brain.

The Penetration of Financial Instability in Agricultural Credit and Leveraging. Factor Markets Working Paper No. 2, September 2011

This paper describes the aggregate rural capital markets of the EU and the main differences between the markets of its member countries. The results of our study suggest that the agricultural credit markets are still quite segmented and the segments are country- rather than currency- or region specific. Financial instability in Europe is also penetrating the agricultural sector and the variation of interest rates for agricultural credit is increasing across countries. Perhaps the most dramatic signal of growing financial instability is that the financial leverage (gearing rate) of European farms rose in 2008 by almost 4 percentage points, from 14 to 18%. The 4 percentage-point annual rise was twice the 2 percentage-point rise observed during the economic recession in the late 1980s and early 1990s. The distribution of the financial leverage of agriculture across countries does not, however, reflect the distribution of country-specific risk premiums in the manner that they are observed in government bond yields. Therefore, in those countries that have the weakest financial situation in the public sector and in which the bond markets are encumbered with high country-specific risk premiums, the agricultural sector is not directly exposed to a very large risk of increasing interest rates, since it is not so highly leveraged. For example in Greek and Spanish agriculture, the financial leverage (gearing) rate is only 0.6% and 2.2% respectively, while the highest gearing rates are found elsewhere (in Denmark), reaching 50%.

Central America aid : status of the Trade Credit Insurance Program : fact sheet for the Honorable Patrick Leahy, U.S. Senate /

"B-221422"--1st. prelim. p.

Small farmer credit in Mexico and Central America.

PB 224 025

Credit for marginal groups; a study of the principal private sector credit institutions providing loans to marginal groups in the countries of the Dominican Republic, Chile, Ecuador, Colombia, and Guatemala.

"Sponsored by: Pan American Development Foundation, in cooperation with: CUNA International, Inc. [and others]"

Small farmer credit in Colombia.

PB 224 029

Small farmer credit in Costa Rica.

Cover title.

Small farmer credit in Ecuador.

PB 224 028.

The Cash discount act : the Fair credit practices act : hearing before the Subcommittee on Consumer Affairs of the Committee on Banking, Finance, and Urban Affairs, House of Representatives, Ninety-sixth Congress, second session, on H.R. 6928 ... H.R. 7038 ... April 23, 1980.

Mode of access: Internet.

Meeting the credit needs of minority communities : hearing before the Subcommittee on Domestic Monetary Policy of the Committee on Banking, Finance, and Urban Affairs, House of Representatives, Ninety-sixth Congress, second session, May 29, 1980.

"Serial no. 96-65."

Amending the Export-import bank act of 1945 with respect to credit to Communist countries : hearings before the Subcommittee on International Trade, Investment, and Monetary Policy of the Committee on Banking, Finance, and Urban Affairs, House of Representatives, Ninety-sixth Congress, first session, on H.R. 1908 ... July 23 and 24, 1979.

"Serial no. 96-24."

The federal farm loan system, new method of farm mortgage finance, under national supervision; a practial manual upon organizing and conducting national farm loan associations, also joint stock land banks. Showing how farmers, investors, bankers and the public may obtain the fullest benefit of the system.

Mode of access: Internet.

The Farm credit act of 1937.

Mode of access: Internet.

Effects of lender decisions on farm financial planning : some solutions to problems arising from credit limitations set by commercial lenders /

Cover title.

Credit practices : final report to the Federal Trade Commission and proposed trade regulation rule (16 CFR part 444) /

"Staff report and recommendation on proposed trade regulation rule. 16 CFR part 444, public record 215-42."