Why pigeons?

 

This morning, I took this photo of a flock of lovely wild pigeons foraging on grain. This reminded me of a question someone asked me recently.

Question: "Can we expect the visual processing being studied on birds standing at rest to be the same as for those who are flying or “walking” where birds move their heads in jerks in contrast to humans?"

This is an excellent question, and I'd like to address it here since you, dear reader and supporter, are interested in pigeon research.

There are multiple ways to respond to this question. First, to compare pigeon vision to human vision experimentally typically involves comparing how pigeons perform in psychophysical tasks (a.k.a., "computer video games") similar to those used to study human vision. In human psychophysics experiments, the subject typically sits in a chair facing a computer screen. The subject is presented with visual displays and is instructed to respond to the displays in certain ways, such as with a button press or touching the screen directly. In this way, testing the pigeon on a psychophysical task where it pecks at visual stimuli presented on a screen allows comparisons across species.

However, as the question implies, vision is not only used when we are stationary. It must function well while we move around the world and as the world moves around us! There is a growing body of human psychophysical research using virtual reality setups to study human vision in these dynamic conditions. There is also some work on vision in pigeons and other nonhumans (especially primates) tested under dynamic conditions (see http://pigeonrat.psych.ucla.edu/resources/6/Cook%2... for an example from my own research). This can be achieved either by using movies on a 2D computer screen, virtual reality (easier with primates than pigeons!), or even having the subject navigate a small space--like in an enclosed arena. This work is more challenging and costly given the type of equipment and procedures involved, but can illuminate many aspects of vision not captured by stationary psychophysics tasks.

As an aside, another thing we must be aware of are anatomical differences in the sensory systems between species. For example, (most) humans are trichromats, dividing the visible light spectrum into three distinct primary colors, from which the rest of the color spectrum is constructed. Pigeons and (as far as I know) all other birds are pentachromats, dividing their visible light spectrum into 5 separate primary color categories! We can only guess as to what the color space of a pigeon looks like! This actually can create a confound when using color stimuli rendered on a color monitor because the color blending system uses the RGB system suitable for trichromats. We can't be sure that our pentachromatic subjects see the displays in a similar way. For my research this tends to be inconsequential because we only require the birds to be able to discriminate between a set of colors, which we have show in prior research with our stimuli that they do.

Also, humans have a single fovea--where spatial resolution is highest--in each retina. Pigeons, however, have two fovea in each retina. One is lower down and centrally located in the retina and is likely used for myopic view of objects (like seeds on the ground) right in front of their face (beak?). The other is higher up and more lateral on the retina, and as far as we know is more likely used for long-range vision, to detect predators, and possibly to coordinate flight in a flock. Have you ever seen how gracefully every bird in a flock can suddenly and smoothly veer in different directions as if a single mind is controlling them? This may require input from these laterally-placed fovea, but I don't know if there's research substantiating this.

Finally, as mentioned in the question, you may have noticed that when a pigeon is walking it jerks its head back and forth in a front to back motion. Why is that? It turns out that while walking, the pigeon cannot smoothly follow objects while its head (and therefore eyes) are moving. (They must be able to do this while flying, however, as they can't jog their head back and forth in flight, yet they are able to use their vision to navigate.) To solve this problem, what the pigeon does is to thrust its head forward (what scientists call a saccade, or ballistic motion), then hold its head still in place while the body catches up underneath. Once the body is repositioned under the head, the next saccade is executed. In this manner the head moves forward, then while it is still it takes a picture of the world before the next saccade. This is quite similar to how our eyes scan the environment in a series of short jumps (saccades) from one location to the next and the next, etc. All visual species must solve the problem of viewing a world full of objects in 3D. While there are many common solutions, there are also many that are species unique.

You see, the real world situation is much more complex and complicated. Comparative psychology is not easy. But by carving out a well-defined research question, and systematically testing hypotheses concerning that question, we can incrementally build our knowledge of how rich is the mind of another species, and how similar and different it may be to that of a human! Thanks for the question, and thanks for your support!