Connectionists: Two papers available

[Ben.Vincent@bristol.ac.uk]

Two papers are available for download from
http://ben.psy.bris.ac.uk/#publications


Vincent B. T, Baddeley R. J, Troscianko T, Gilchrist I. D, 2005, Is the
early visual system optimised to be energy efficient?, Network:
Computation in Neural Systems, special issue on Sensory Coding and the
Natural Environment, 16(2/3): 175-190

Abstract
This paper demonstrates that a representation which balances natural
image encoding with metabolic energy efficiency shows many similarities
to the neural organisation observed in the early visual system. A simple
linear model was constructed that learned receptive fields by optimally
balancing information coding with metabolic expense for an entire visual
field in a 2-stage visual system. The input to the model consists of a
space variant retinal array of photoreceptors. Natural images were then
encoded through a bottleneck such as the retinal ganglion cells that
form the optic nerve. The natural images represented by the activity of
retinal ganglion cells was then encoded by many more cortical cells in a
divergent representation. Qualitatively, the system learnt by optimising
information coding and energy expenditure and matched 1) The centre
surround organisation of retinal ganglion cells; 2) The gabor-like
organisation of cortical simple cells; 3) Higher densities of receptive
fields in the fovea decreasing in the periphery; 4) Smaller receptive
fields in the fovea increasing in size in the periphery; 5) Spacing
ratios of retinal cells and 6) Aspect ratios of cortical receptive
fields. Quantitatively however there are small but significant
discrepancies between density slopes which may be accounted for by
taking optic blur and fixation induced image statistics into account. In
addition, the model cortical receptive fields are more broadly tuned
than biological cortical neurons, this may be accounted for by the
computational limitation of modelling a relatively low number of
neurons. This paper shows that retinal receptive field properties can be
understood in terms of balancing coding with synaptic energy expenditure
and cortical receptive fields with firing rate energy expenditure and
provides a sound biological explanation of why sparse distributions are
beneficial.



Vincent B. T, Baddeley R. J, 2003, Synaptic energy efficiency in retinal
processing, Vision Research 43, 12831290

Abstract
Recent work suggests that the visual system may represent early visual
information in an energy efficient manner [Nature 381 (1996);Nature, 381
(1996) 607;Neural Comput. 3 (2001) 799;Curr. Opin. Neurobiol. 11 (2001)
475]. This paper applies the idea of energy efficient representations to
understand retinal processing, and provides evidence that centre
surround processing observed is efficient in terms of minimizing
synaptic activity. In particular, it is shown that receptive fields at
different retinal eccentricities and at different levels of noise, can
be understood in terms of maximizing the transmission of visual
information given a constraint on total synaptic strengths and hence
energy consumption.



Ben Vincent, DPhil
Department of Experimental Psychology, University of Bristol, 8 Woodland
Road, Bristol, BS8 1TN, UK
http://ben.psy.bris.ac.uk/