Why can some people’s brains see better than others’?

On Friday I had a new paper published in the open access journal PLoS ONE. It addresses the question of why some people have better sensitivity to contrast (variations in light levels across an image) than others, sometimes by quite substantial amounts. Unlike differences in eyesight (acuity) that can usually be optically corrected, contrast sensitivity differences occur even for large (low frequency) stimuli that aren’t affected much by optical blur. Presumably then, the sensitivity differences are neural in origin. I was surprised that nobody had really tried to answer this question before, so thought I should give it a go.

The paper is divided into two parts. The first section uses an equivalent noise technique to assess whether sensitivity differences are due to different amounts of noise, or a difference in the efficiency with which stimuli are processed. Although I rule out the latter explanation, the noise masking method cannot tease apart a difference in internal noise from a difference in contrast gain. So, the second part of the study looks at a large corpus of contrast discrimination data, collated from 18 studies in the literature. By looking at the between-subject differences in discrimination performance, I conclude that individual differences at threshold are primarily a consequence of differences in contrast gain. Whether this is due to differences in structure, anatomy, neurotransmitter levels or developmental factors is unclear at the moment.

Since I spent quite a long time putting together all of the dipper function data, I thought I should make it available online. Most of the data were extracted from the published figures using the excellent GraphClick program. The data can be downloaded here in Matlab format. They are organised into a cell array, with each of the 22 cells containing data from one experiment. Each cell is further divided into separate cells for each individual observer, with the ‘data’ array containing the x- and y-values used to produce these plots. I hope these data become a useful resource for other researchers interested in basic visual processes.

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