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I just wasn’t smiling at you*. January 31, 2008

Posted by Emma Byrne in Uncategorized.
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ResearchBlogging.org(Or the effect of gaze aversion vs direct gaze on reward and desire).

This isn’t specifically about vision, but Strick et al [1] (Cognition) report on a study of the effect of stimuli paired with pictures of attractive or unattractive faces with gaze averted from or directed towards the viewer.

In the first trial the images of faces were paired with unknown peppermint brands and in the second the images were paired with positive and negative adjectives. It was ensured that volunteers attended to the faces by the inclusion of images of a third condition (faces with eyes closed). When one of these was detected, the volunteer had to press a button.

To determine the effect of attractiveness and gaze on desire, the volunteers were asked to rate the peppermint brands for desirability at the end of the test. Mean self-reported desire for brands associated with the attractive/direct gaze condition was 4.46 out of a possible 7 and for the attractive/averted condition was 4.05. Direct gaze resulted in significantly higher desirability where the face was attractive (p= .04). For the unattractive faces, averted gaze led to fractionally but not significantly higher ratings than direct gaze.

The adjective experiment is an affective priming test. In affective priming, respondents must indicate whether an adjective they read is positive (‘exciting’, ‘happy’) or negative (‘boring’, ‘angry’) by pressing one of two keys. Responses to positive adjectives are generally faster than responses to negative adjectives, but priming can exaggerate or eradicate this difference. The experiments showed that attractive faces lead to a greater difference in response time than unattractive faces (a mean difference between positive and negative judgements of 55ms for attractive faces versus 36ms for unattractive faces, p =0.4). Attactive faces with direct gaze also made the response time difference significantly longer than unattractive faces with direct or averted gaze (both p < .02). However for unattractive faces there was no significant effect on the difference in in response times.

This doesn’t seem to have made the same splash as last November’s press coverage of the Conway et a [2] (Royal Soc) study that showed the same face is more attractive when gazing directly at the viewer than when not. It seems that for some of us, the power of our gaze may not be all we’d hoped it might be. Especially if we’re trying to sell mints.

[1]Strick, M., Holland, R.W., van Knippenberg, A. (2008). Seductive eyes: Attractiveness and direct gaze increase desire for associated objects. Cognition, 106(3), 1487-1496. DOI: doi:10.1016/j.cognition.2007.05.008

[2]C.A. Conway, B.C. Jones, L.M. DeBruine, and A.C.Little. “Evidence for Adaptive Design in Human Gaze Preference.” Proc. R. Soc. B 275, no. 1630 (January 2008): 63-39.

* You Could Have it So Much Better, Franz Ferdinand.

“The last message you sent said I looked really down,
and that I ought to come over and talk about it. Well,
I wasn’t down;
I just wasn’t smiling at you.”


Kiwi and night vision January 29, 2008

Posted by David Corney in Uncategorized.

ResearchBlogging.orgA (fairly) new paper by Graham Martin et al. in the wonderful PLoS ONE discusses kiwi and their eyes. Coming from (nearly) the opposite side of the world, I am (was) embarrassingly ignorant about kiwi. I knew they were large and flightless birds from New Zealand, but that was about it. (I even thought the plural was “kiwis” [1].) I now know that they’re nocturnal, like quite a few birds, but they’ve evolved surprising eyes.

To see in the dark, many nocturnal animals have evolved relatively large eyes, such as owls, lemurs and some monkeys, to gather what little light there is. Against this however, eyes are heavy, being balls of mostly-water, and weight is always a concern if you’re flying. So at first glance, you might expect (as the authors mention) that a bird that stops flying might evolve to have larger and larger eyes, as weight becomes less of an issue. Especially if it’s nocturnal. However, kiwi have small eyes for their bodies, and what’s more, they have small optic nerves and small visual cortices. They’re not blind like cave fish, although given a few million years more, who knows?

Moving forward a few inches, all birds have nostrils, usually at the base of the bill or even inside the mouth. Kiwi, uniquely, have their nostrils at the tip of their bills, coupled with fine touch sensors all over the bill tips. They feed by pecking at surface-living insects or by probing the soil with their long bill and sensing underground insects, suggesting a convergent evolution to the same ecological niche filled by mammals in many parts of the world. And if you’re finding grubs underground, you don’t need vision, of course.

According to the “wiki-kiwi” page, in areas where people are absent, kiwi are active during the day. Which makes me wonder if they have ended up with reduced visual processing simply because they can’t see what they’re eating anyway, whether it’s day or night, so why waste the effort? It seems to me that there are two evolutionary stories that fit this data:

  1. In version one, kiwi evolved to find food in the topsoil with their beaks and so didn’t need good vision; in turn, they spent less energy growing and using eyes; and then finally they tended towards nocturnal behaviour because there was no extra cost to them.
  2. In version two, kiwi became nocturnal to avoid predators (not that there were any mammals to compete with until recently) or to find nocturnal insects perhaps; and then they developed poorer eyesight because good eyesight was no longer required.

It could of course be some mixture of the two, as evolutionary histories needn’t have a nice clear narrative. In either case, I guess they still need at least rudimentary vision for mate selection, not walking into trees, that kind of thing. Anyway, I’ve learned a lot about kiwi, for which I am grateful!

Martin, G.R., Wilson, K., Wild, J.M., Parsons, S., Kubke, M.F., Corfield, J., Iwaniuk, A. (2007). Kiwi Forego Vision in the Guidance of Their Nocturnal Activities. PLoS ONE, 2(2), e198. DOI: 10.1371/journal.pone.0000198

See also: “The allometry and scaling of the size of vertebrate eyes” doi:10.1016/j.visres.2004.03.023
“Some nocturnal animals rely on senses other than vision, which is reflected in their small eye size. Others take the strategy of increasing eye size as much as possible to compensate for the low light conditions.”

[1] I checked the OED to see what it said about the word “kiwi”. It gives the etymology as “Maori” which isn’t terribly informative, but does have a quote from a Walter Lawry Buller and his 1873 text, A history of the birds of New Zealand: “Last Sunday I dined on stewed Kiwi, at the hut of a lonely gold-digger.” So I’ve learned something already…

Classic paper: The evolution of the human eye January 24, 2008

Posted by David Corney in Uncategorized.

Blogging on Peer-Reviewed ResearchSuppose that one type of eye is better at detecting information about the world than another. Then it will allow its owner to respond more usefully to the visual world, whether it’s finding food, avoiding predators and harsh conditions, monitoring its own movement, or whatever. As always, natural selection works at a local level, local both in space and time. So it doesn’t matter whether this eye is good, only whether it is better than the competition around it. Half an eye is better than no eye. In fact half an eye is better than 49% of an eye…

Darwin admitted that the evolution of something as intricate as the human eye, by nothing more than natural selection, seems “absurd”. I love science when it’s so counter-intuitive! I know that everything in the universe is made out of atoms and quarks and things, but it doesn’t look like it, intuitively. Stars at night don’t look like they’re billions of miles away. The Earth doesn’t look round. Anyway, back to the eye…

Back in 1994, Nilsson and Pelger presented a model of the possible evolution of eyes, consisting of a sequence of steps where each step is a) small enough to happen in one generation; and b) leads to an improvement in spatial acuity. They start with a simple, flat light-sensitive circle and see where it leads. Basically, the sequence starts with an increasing central depression, which means that light from either side gets cut off by the surrounding “ridge”. Thus the light falling on any one part of the patch is coming from a smaller and smaller region of the world, as the depression gets deeper. This “specialisation” defines an increase in spatial acuity. Then later, the “ridge” surrounding this central depression constricts, ultimately forming a “pupil”, and further limiting the amount of “world” whose light reaches each part of the light-sensitive “retina”.

Unfortunately, this process inevitable cuts down the total amount of light reaching the retina, so you end up with an eye that has really high spatial acuity, but that only works in really bright conditions… But if you add a clear protective covering over the initial light-sensitive patch, and if this covering gradually thickens just so, then it will start bending the light in such a way that acuity improves further without having to make the pupil any smaller. This “lens” allows a trade-off between acuity and sensitivity, so you can see clearly now, even in lower light levels.

Through various plausible, even pessimistic, assumptions, Nilsson and Pelger argue that you could get from the initial flat patch of light-sensitive cells to a fully-functioning vertebrate eye in just 1829 steps! Then with some more (plausible) assumptions about evolution and inheritance, they estimate this could occur in a little over 350,000 generations, something like 1500 times faster than the evolution actually managed.

The upshot is a nice straightforward explanation of the development of many different sorts of eyes through natural selection. Whether the historical evolution of humans involved this sequence is not clear, but that hardly matters: here is one way it could have happened, an existence-proof of the possible evolution of the eye.

In the paper, they also point out it could, theoretically, have happened a lot faster, if for example different changes happened in parallel rather than sequentially. Given ideal conditions, a truly benevolent ecology, I wonder just how quickly such an eye could evolve? It’s a big universe out there: what’s the record fastest evolution of an eye, from a standing start, I wonder…?

Nilsson, D., Pelger, S. (1994). A Pessimistic Estimate of the Time Required for an Eye to Evolve. Proceedings of the Royal Society B: Biological Sciences, 256(1345), 53-58. DOI: 10.1098/rspb.1994.0048

Blind as a fish January 18, 2008

Posted by David Corney in Uncategorized.
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Having just written a few days ago about nocturnal vision and moths that can see by starlight, I was intrigued to read about some blind cave fish via Living the Scientific Life. The theory is that over millions of years of living in perpetual darkness, these fish mutated until they no longer grew eyes, either to save the effort and energy, or because eyes are delicate and prone to infection. The news is that by taking fish from different populations and breeding hybrids, fully-functioning eyes reappeared in a single generation! It seems that each population mutated in a different way, so at least some hybrid offspring had all the “original” seeing genes still there. Populations matter in genetics.

One thought: if moths can see (in colour, no less) by something as faint as starlight, and these different populations of fish independently gave up having eyes altogether, those caves must be very dark. Very dark indeed…

I liked the final quote from the paper, too:

This observation underscores the power of a well defined environment to repeatedly direct the evolution of the same end phenotype, regardless of initial genotype.

Full paper: Richard Borowsky. Restoring sight in blind cavefish (2008). Current Biology 8 R23-R24 doi:10.1016/j.cub.2007.11.023

Problem solving? Active vision actively helps. January 15, 2008

Posted by Emma Byrne in Uncategorized.
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Steve Higgins over at Omni-Brain takes a recent paper by Thomas and Lleras as the subject of a great post. According to Higgins’ review, the study showed that subjects solved a problem much more quickly when directed to make one pattern of eye movements over others.

The comments thread is shaping up to be interesting too – is the effect due to embodied cognition (the eyes “acting out” the movements that the correct solution consists of) or is it to do with inter-hemispheric communication?

Definitely worth a look for Higgins’ great summary. The full paper is available here – no Athens password required.

Night and day January 11, 2008

Posted by David Corney in Uncategorized.
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Blogging on Peer-Reviewed ResearchAlthough I’ve been researching vision and creating synthetic images for a while now, I’d never really thought about night-time illumination. Well, not beyond thinking, “It’s dark at night!” I suppose. But then I read a paper [1] by Javier Hernandez-Andres and his group in Granada about crepuscular and nocturnal vision, and learned that natural light at night is a lot more complex (and interesting) than I thought. During the day, all light come from the sun or from skylight, which is just scattered sunlight. But at twilight and through the night, there are many and varied light sources.

After the sun passes below the horizon, it still lights up the sky for a while so that’s one source of light. Then there’s moonlight, which is a direct reflection of the sun and has a very similar spectrum to daylight, at least for a high and full moon. And there’s starlight, which has a spectrum roughly like daylight but fainter and with four distinct spikes around the yellow / red region. Then illumination starts getting really exotic. There’s “airglow”, which was first (officially) noticed by Anders Ångström (he of the unit) in the mid-19th century. It consists of various light-emitting molecular processes in the upper atmosphere, which produces a faint blue-ish glow across the sky. Then there’s “zodiacal light”, which was noted by Cassini (he of the Saturn orbiter) in the 17th century. It consists of sunlight bouncing off scattered cosmic dust between the planets of our solar system, so again it has the same spectrum as sunlight, albeit fainter. And apparently, the very dark blue sky seen during late (“nautical”) twilight is that colour because of ozone absorption, and not (just) due to sunlight scattering effects. In other words, it’s not just “blue sky but a bit darker”, but is blue for a different reason.

The final nocturnal light source Hernandez-Andres et al. mention is anthropogenic light – light pollution. This varies enormously across space and time of course, with a strong yellow/red shifted spectrum suddenly appearing whenever a million streetlights click on at dusk, along with car headlights, office lights, advertising hoardings etc. etc. Scientists are now realising that many nocturnal animals, including some moths, rely on very subtle colour cues for foraging and mating, just as diurnal animals do. What effect light pollution is having on these creatures seems to be unknown, but presumably it forms a strong selection pressure, at least near built-up areas. Sounds like a ripe area of future study…

Johnsen, S. (2006). Crepuscular and nocturnal illumination and its effects on color perception by the nocturnal hawkmoth Deilephila elpenor. Journal of Experimental Biology, 209(5), 789-800. DOI: 10.1242/jeb.02053

Other links:




Seeing things similarly January 3, 2008

Posted by Emma Byrne in Uncategorized.
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[Originally posted here in September 2007.]

Blogging on Peer-Reviewed Research Take a look at the figure on the left (click on it to make it bigger). It’s taken from a paper by Corney and Lotto. The left hand column shows some optical illusions that you’ve probably seen before. In Box A for example, the corner lines look lighter than they actually are, and in Box D, there is a tendency to see light spots appearing in the intersections of the darker grey lines.

In the right hand column, the blue line on the graph shows the actual lightness (reflectance) of the stimuli – that is, how much light is really being returned to the eye by the points along the blue line (assuming the blue line wasn’t there!). The red curve shows the perceived reflectance – that is, how light the stimulus appears to be, once the brain is done processing the image in the light of prior experience.

Graph A shows the brain overestimating lightness throughout, but particularly at the points of maximum lightness – the “bands” in the corner that we see as being lighter than they really are. Graph D – my favourite – shows an oscillation: an intersection between two dark grey lines is perceived as lighter than it is (causing the appearing and disappearing “blobs” that we see). At the points furthest from these intersections, the line actually appears to be darker than it is.

Now for the sting: the brain in question was an artificial neural network (ANN) that only ever existed inside a computer. It was trained to successfully perform on a lightness constancy task. Most excitingly, when trained to discern between overlapping layers, the ANN sees White’s illusion (Box E). White’s illusion has been problematic to model as the lightness perception goes “the other way” from the stimuli shown here. Thus, the by-product of learning to see lightness and depth is a susceptibility to these illuaions. This also tells us something about how animal brains, including our own, work.

For the full article see Plos, and for a writeup see New Scientist.

Cool Eye of the Day (Part 1): the Kangaroo January 3, 2008

Posted by Emma Byrne in Uncategorized.
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Many animals have areas of high acuity in their eyes. In humans and many other primates and in some birds, fish and reptiles, this takes the form of a fovea or “pit” that contains a high density area of photoreceptors. Other animals have evolved different strategies, but the idea remains the same: for many visual animals, there is an area of the receptive field of the eye from which the greatest amount of information can be gained.

So let me tell you about kangaroos. What I didn’t know, until recently, is that different types of kangaroos have very different habitats: there are plains dwelling kangaroos and arboreal kangaroos (which I’m guessing from context means ‘roos who live among trees, rather than ‘roos that live up trees). In both of these types of kangroo, instread of a fovea, there is an area of the retina that is more densely connected to ganglion cells (retinal nerve cells). But here’s where it gets interesting: arboreal kangaroos, which tend to have a limited horizon, have a roughly circular area of high acuity (in the form of densely packed ganglia) in the centre of their visual field. In contrast, plains dwelling kangaroos, whose environment tends to be characterised by a large horizon, have a horizontal area of high acuity that stretches across the visual field [1]. What a brilliant example of form following function!

[1] Steinert, R. F. (2005). ASCRS Binkhorst Lecture 2004: The Search for Perfect Vision: Ophthalmology’s Holy Grail? Journal of Cataract & Refractive Surgery, 31(12), 2412.e1-2412.e4.