[psychology-visionlist] External Talk - Thurs 6th Oct

[Craig Scott]

Hi all,

Dr Mark Schira<http://socialsciences.uow.edu.au/psychology/contacts/UOW174512.html>, from the University of Wollongong (Australia), will be giving a talk at the Sir Peter Mansfield Centre at 1PM next Thursday the 6th Oct.

He will be presenting his work on:

Natural images, fractal distributions and modeling fMRI in early visual cortex

In the last couple of years the basis for modeling fMRI measurements in early visual cortex have come together.  Accurate algebraic models of the retinocotical projection and sophisticated spatiotemporal BOLD modeling have provided the corner stones to generate concrete predictions of BOLD responses in time and cortical space. However, the effects of cortical visual processing on BOLD amplitude is still rather crudely understood, barely beyond a crude contrast to BOLD amplitude function. We have investigated the effect that frequency and fractal distributions have on cortical BOLD responses. It is well known that natural images share a specific frequency distribution often simplified referred to as 1/f distribution. This describes the observation that in natural images, spatial frequencies are not equally frequent, as they are for example in white noise. Instead, there is high energy at low spatial frequencies and less energy at higher spatial frequency, where the distribution roughly falls on a line,  with typical slope between -0.75 and -1.25. What is less known is if and how responses in visual cortex are adapted to this property of our environment. It is also less known that the fractal properties of natural scenes are even more stable. We tested the responses in early visual cortex using filtered and thresholded natural noise images, manipulating the slope of the 1/f distribution and the fractal dimension D (Mandelbrot 1982). We found that early visual cortex responds maximally to naturalistic noise images specifically to it's fractal dimension. We found that frequency distribution and fractal dimension determine the BOLD response as strongly as contrast.

Mark will also be visiting the School of Psychology on Wednesday the 5th Oct, and would be happy to meet people within our group perhaps over lunch. Should you wish to contact him his email is mark.schira at gmail.com<mailto:mark.schira at gmail.com>.


Best Wishes

Craig

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