Two papers available on 1) neural coding, and 2) psychophysics

[Roland Baddeley]

The following two papers are available. The first is on the nature of
the 
neuronal code (rate coding, information theory and all that), and the
second
is on using neural nets to analyse psychophysical experiments.  

They can now both be found on my web page (hope they are of interest):

http://www.mrc-bbc.ox.ac.uk/~rjb/

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Title: Responses of neurons in primary and inferior temporal visual
cortices to natural scenes

Baddeley, Abbott, Booth, Sengpiel, Freeman, Wakeman, and Rolls (in
press)

To appear in Proceedings of the Royal Society B

Abstract: The primary visual cortex (V1) is the first cortical area to
receive visual input, and inferior temporal (IT) areas are among the
last along the ventral visual pathway.  We recorded, in area V1 of
anaesthetised cat and area IT of awake macaque monkey, responses of
neurons to videos of natural scenes.  Responses were analysed to test
various hypotheses concerning the nature of neural coding in these two
regions.  A variety of spike-train statistics were measured including
spike-count distributions, interspike interval distributions,
coefficients of variation, power spectra, Fano factors and different
sparseness measures.  All statistics showed non-Poisson
characteristics and several revealed self-similarity of the spike
trains.  Spike-count distributions were approximately exponential in
both visual areas for eight different videos and for counting windows
ranging from 50 ms to 5 seconds.  The results suggest that the neurons
maximise their information carrying capacity while maintaining a fixed
long-term-average firing rate\cite{Baddeley96,Levy96}, or
equivalently, minimise their average firing rate for a fixed
information carrying capacity.

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Insights into motion perception by observer modelling

Roland Baddeley and Srimant P. Tripathy

Journal of the Optical Society of America (In press)

The statistical efficiency of human observers performing a simplified
version of the motion detection task used by Newsome et al. is
high but not perfect. This reduced efficiency may be because of noise
internal to the observers, or it may be because the observers are
using strategies that are different from that used by an ideal
machine. We therefore investigated which of three simple models best
accounts for the observers' performance. The models compared were: 1)
a motion detector that uses the proportion of dots in the first
frame that move coherently (as would an ideal machine), 2) a model
that bases its decision on the number of dots that move, 3) a
model that differentially weights motions occurring at different
locations in the visual field (for instance differentially weighting
the point of fixation and the periphery). We compared these models by
explicitly modelling the human observers performance. We recorded the
exact stimulus configuration on each trial together with the
observer's response, and, for the different models we found the
parameters that best predicted the observer's performance in a least
squares sense. We then used N fold cross-validation to compare the
models and hence the associated hypotheses.  Our results show that the
performance of observers is based on the proportion of dots moving,
not the absolute number, and that there was no evidence for any
differential spatial weighting. Whilst this method of modelling the
observers' response is only demonstrated for one simple psychophysical
paradigm, it is general and can be applied to any psychophysical
framework where the entire stimulus can be recorded.

-- 
Dr Roland Baddeley                   WWW:  
http://www.mrc-bbc.ox.ac.uk/~rjb/
Mail: 	Psychology Dept, Oxford      email: rjb at psy.ox.ac.uk
	University, South Parks      phone: +44-1865-271914
        Road, Oxford, OX1 3UD, UK    fax:   +44-1865-272488