Effect of cortical feedback on nerve-cell response in lateral geniculate nucleus (LGN) to visual stimuli

Abstract

A feedback connection from the visual cortex has been shown to affect characteristics of cells in the lateral geniculate nucleus (LGN) in the brain. Research of cortical feedback have been performed with experiments and modelling, but its functional role is still not resolved. Studies of cortical feedback on LGN cells found in literature mainly focus on the feedback's spatial effect on the LGN cell. The aim for this thesis is to study the feedback's temporal effects on LGN cells with the use of a newly developed tool: A neural circuit simulation software, pylgn. This simulation tool had not been tested prior to this thesis for scientific purposes by others than the developer himself. Thus, this study is the first application of pylgn.

Results of simulations of the relay-cell response with the use of pylgn featured effects in accordance with experimental research results found in literature, for example, suppression of LGN cell response when stimulus sizes are magnified over a certain diameter value. This thesis' study of effects from cortical feedback's temporal features was performed by simulations of a neural network with separated excitatory and inhibitory feedback components. Impact of delay and prolonging of feedback was studied for each feedback component. The results revealed dependence of the temporal oscillation of the neural response and the size of the stimulus for an oscillating spot and a patch grating of lower spatial frequency (k=1,47/deg), but no corresponding dependence for stimulus of a patch grating with higher spatial frequency (k=1,47/deg). The results showed that the dependence of the temporal oscillation of neural responses and the size of the stimulus occurred for delayed or prolonged excitatory feedback. A temporal shift in the response was also observed for delayed or prolonged excitatory feedback. The results also revealed that the oscillations in neural response for delayed or prolonged inhibitory feedback were less dependent of the patch size. This indicates that the effect of inhibitory feedback is simply to reduce response of LGN relay cells, while the excitatory feedback potentially determines the behaviour of the LGN relay cell response.

Det er bevist at en kobling som gjennomfører tilbakesending av signaler fra den visuelle hjernebarken (korteks) har e_ekt på egenskapene til nerveceller i lateral geniculate nucleus (LGN) i hjernen. Forskning på kortikal tilbakesending av signaler har blitt gjennomført med eksperimenter og modellering, men dens funksjonelle rolle er fremdeles ikke avklart. Studer av kortikal tilbakesending funnet i litteraturen fokuserer i hovedsak på tilbakesendingens romlige e_ekter på celler i LGN. Målet med denne avhandlingen er å utforske kortikal tilbakesendings tidsmessige e_ekter på celler i LGN ved å benytte et nylig utviklet verktøy: En simuleringsprogramvare for nevrale kretser, pylgn. Før denne masteroppgaven hadde dette simuleringsverktøyet ikke blitt testet til vitenskapelige formål av andre enn utvikleren selv. Dermed er denne studien den første til å benytte seg av pylgn.