Some parts of the visual field are more equal than others February 13, 2008Posted by David Corney in Uncategorized.
Tags: evolution, Fovea, psychophysics, retina, visual cortex
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It’s well-known that visual acuity is far higher in the centre of the visual field than the periphery. Something we see out of the corner of our eyes is blurred, until we turn our eyes to look directly at it, when we can see it much more clearly. This is partly due to the sparse distribution of cones in the periphery, but also due to later neural structures. For example, the visual cortex has more neurons dedicated to the central than the peripheral visual field. That much I knew. However, I just read a paper that describes many other non-uniformities in the visual field, which are rather less intuitive, at least at first glance.
The paper, by Fuller, Rodriguez and Carrasco, starts with a detailed review of what is already known about various asymmetries in perception, including the peripheral drop in acuity I just mentioned. But some other examples of asymmetry were new to me. For example, we have better acuity along the horizontal mid-line of the visual field than we do along the vertical mid-line (known as “horizontal-vertical asymmetry”). So if you fix your eyes on one point, you have (slightly but measurably) higher acuity 5 degrees left or right than you do 5 degrees up or down. Similarly, we have better acuity below the mid-line of the visual field than above (known as “vertical meridian asymmetry”). Again, these effects are due to both the non-uniform distribution of photoreceptors in the retina, and to the characteristics of the visual cortex and the rest of the visual pathway.
In the work presented here, the authors presented subjects with pairs of gratings (alternating dark and light bars) above and below a fixation point. The subject then had to decide which of the pair was of higher contrast, and whether its bars sloped to the left or the right. Over a large number of trials, they found a significant bias towards people choosing the “south” grating (the one below the centre of the visual field) as being the higher contrast one, even when it was physically identical to the “north” grating.
They then varied the experiments by providing an extra cue before the gratings appeared, either above of below the mid-line. The idea was to test whether “exogenous” attention (i.e. automatic pre-conscious attention) effected the visual asymmetries.
They found that this kind of peripheral cue exaggerated the perceived difference in contrast. So a stimulus that grabs your attention appears to have a higher contrast than it would do otherwise, and also that increase in attention is greater if it’s in the bottom half of your field of view.
The authors only briefly touch on why this all happens at the end of the paper. They comment that things on or near the ground in front of us may tend to be more important than things in the air – presumably because they’re much closer, and so require a faster fight-or-flight style response. The authors also question how this effect might vary during childhood: as one grows from being really short to being adult-sized, does the likely location of threats / rewards change?
This fits in with the whole ecological view of perception that I find fascinating, namely that we perceive the world in a way that has led to our (ancestor’s) evolutionary survival, irrespective of whether that perception happens to be “accurate”. I wonder if the area of ground in front of you that is worth paying extra attention to grows over time? Is this effected by your growing motor skills as well? If you’re a child, you’re not going to be able to run very far or very fast, so perhaps it makes sense to pay less attention to things happening, say, 50 feet away, compared to an adult with longer legs. And a similar argument holds for the “horizontal-vertical asymmetry”: things on the horizon would tend to be more significant than things above us if our ancestor’s were used to hunting or running away from land-based animals, like gazelles and lions. It is a bit of “evo-psych” style speculation, but a few computer simulations might shed some light on the issue…
Fuller, S., Rodriguez, R.Z., Carrasco, M. (2008). Apparent contrast differs across the vertical meridian: Visual and attentional factors. Journal of Vision, 8(1), 1-16. DOI: 10.1167/8.1.16