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What colour chimeras tell us about vision. July 28, 2008

Posted by Emma Byrne in Uncategorized.
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I managed two impossible things before lunch today[1]. I crossed the road this morning, confident I could avoid the big red bus that I saw in my peripheral vision. I read the links in small-type in the Word Press sidebar, despite the fact that they are in blue text. However, if my retina is to be believed, this shouldn’t be possible.

Colour photo-receptors (cones) are not distributed evenly across the retina. In the fovea, “red” and “green” cones are densely packed, and there are no “blue” cones and no rods. So detailed vision takes place with no (direct) knowledge of short wave light.

However, only 5° of visual angle from the fovea the density of the cones drops dramatically (from around 150,000/mm² at the central fovea to <10,000 per mm²) and the density of the rods rises from none at the fovea to >150,000/mm° at around 15° from the centre of the fovea.

5° of visual angle is about two thumbs’ width when held at arms’ length. Hold up your hands with your thumbs side by side(at arms’ length). Keep your hands forward, so they look something like the “bird” hand shadow. Now fixate on your thumbs. Everything around your thumbs is seen by this area of the retina that is much richer in rods than in cones, whereas the thumbs themselves are seen with the part of the retina that is pretty much all cones. Yet the skin on the back of your hand looks just as colourful as the skin on your thumbs.

So why are colours in the periphery and colours in the centre of the visual field perceived so similarly? The short answer is “because it’s useful”. If the things you saw kept changing colour as they moved across your visual field that would make object identification very difficult indeed. But this doesn’t tell us how the visual system “reconstructs” colour. Is the colour “spread” through the visual field by taking the statistics of the centre and applying them to the surround? Or is there are “top-down” effect, such that knowledge of what is in the visual field tells us how it should look.

Balas and Sinha devised a neat experiment to discriminate between these hypotheses. They decided to address the following questions:

  1. “Do observers complete the colour content of natural scenes when larger regions of the image have had colour artificially removed?”
  2. “If colour completion occurs, does it do so more readily from the centre of an image outwards as opposed from the periphery inwards?”
  3. “If colour completion occurs, does it depend on natural scene statistics?”

To answer the first and second questions they created “Colour Chimeras” – images that were desaturated (“greyed out”) either in the centre or at the edges. Volunteers were presented with images that were entirely grey, entirely coloured (“pan-field” coloured), colour centre chimeras or grey centre chimeras. They found that subjects were much more likely to mistake chimeric images for “pan field” colour images than they were to mistake them for grey images. Importantly, if didn’t matter whether the chimera was greyed out at the centre or the edge: the volunteers still saw a significant proportion of the images as pan-field coloured.

To answer the third question the researchers altered the textural and the colour information in the images. In the first experiment the chimeric and non-chimeric images were all natural scenes (beaches, trees etc.) In the second experiment, some volunteers were presented with natural scenes. Some volunteers were presented with scenes in which the colour had been altered to consist of a single hue, so that the colours were not natural, but shapes within the image were still recognisable. Others were presented with scenes in which the textures were changed, so that the structure of objects was no longer recognisable, but the distribution of colours was the same as the original image. Some subjects were presented with images that had both colout and texture manipulations, in which the original objects and colours could no longer be recognised. How would this affect the volunteers’ ability to spot chimeras?

Volunteers were less likely to “fill in” colour when they were presented with the manipulated chimeras instead of the natural scenes. Textural changes reduced the ability to “spread” colour to the rest of the scene, and colour manipulations reduced this ability even more. However when images were manipulated both for colour and texture, subjects were very good at spotting chimeras (or very bad at filling in colour).

The authors conclude that colour spreading is a common perceptual phenomenon (much more common than the occurrence of “grey spreading” – the mis-identification of chimeras as grey images). Furthermore, they conclude that scene statistics provide important perceptual cues that support this colour spreading. So the next time you see a bus in your peripheral fieldand you know that it’s red, it’s probably because you’ve seen red buses before, and not because your retina tells you so.

[1] I know it should be six before breakfast, but it’s very hot today. Vaughan at Mind Hacks is obviously made of sterner stuff.

Balas, B., Sinha, P. (2007). “Filling-in” colour in natural scenes. Visual Cognition, 15 (7), 765-778. DOI: 10.1080/13506280701295453

Some parts of the visual field are more equal than others February 13, 2008

Posted by David Corney in Uncategorized.
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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