Visual Field Map

It is school holiday time here in NZ, so I thought for something a little different I'd share with you a fun activity - one I enjoyed from biology class in my own school days. Equally, curious adults might enjoy participating just as much!

You may remember a couple of previous posts that talked a little about some of the amazing visual capabilities of creatures like the mantis shrimp. Well, today we are going to explore a little about our own vision and create our own map of our very own visual field.

You will need:

• an A3 piece of paper (or two A4 pieces with their long edges taped together, but only tape it on one side as you'll need to write on the paper)
• a selection of coloured pencils (or felt tip pens)
• a HB pencil 
• a ruler 
• a helper
• a piece of wall without pictures on it, furniture in front of it etc.
• blue tack, or some other means of affixing the paper to the wall
• a protractor (optional)

Instructions:

• Draw a dot in the centre of your piece of A3 paper with the HB pencil (if using two A4 pieces, make sure the dot is on the side without cellotape)

• Rule a bunch of lines radiating out to the edge of the paper from the centre dot. We made our lines at approximately 15 degree intervals, and I would suggest this as a minimum (it doesn't matter if they aren't exactly 15 degrees, so if you don't have a protractor its fine to just estimate). Lines that are closer together are fine, and will give a more accurate map of your visual field.

Your piece of paper should now look like this...

• Next attach the piece of paper to the wall, landscape orientation, so that when you stand against the wall, facing it, the tip of your nose is on the central dot. 

• Give your helper the coloured pencils, and looking straight ahead with your nose touching the central dot, ask them to pick one pencil and starting at the edge of the paper, slide the pencil along one of the lines (you should not know which colour pencil they have or which line it is coming down).

• As soon as you can see the pencil, tell your helper to stop, and make a mark on the paper where the end of the pencil is. Repeat until each line has been used. For this first part of the map, the same pencil may be used on all the lines if you wish. Once there is a mark for each line, use a ruler to join them together. This will map the edge of your peripheral vision. 

The map of your peripheral visual field should look something like this...

The human retina consists of two different types of photoreceptor cells - rods and cones. There are some 120 million rods in the human retina that detect changes in light intensity, shape and movement. Rods are distributed across the surface of the retina and it is these cells we use in our peripheral vision. Rods allow us to see in low light conditions but because they are not sensitive to colour, we have a hard time seeing colour in such situations. The same is true with the coloured pencil. Because it is detected first by the rods in our peripheral vision, we see it without knowing what colour it is...

• Repeat the exercise in the bullet point above, but this time make sure your helper mixes up which colour pencil is being used (Your helper may reuse a pencil if they need to, but make sure you don't know what colour they are about to use). This time don't mark where you can first see the pencil, but where you are first able to tell what colour it is. This will map your colour visual field. 

Your map should now look something like this...

The area of your colour visual field should be smaller than the area of your peripheral vision. This is because the second type of photoreceptor - the cones - are sensitive to colour, but they are concentrated near the centre of the retina, so the pencil has to pass further into your visual field before you are able to detect its colour. Each of the 6 million cone cells in your retina is sensitive to one of three pigments (unless you are colour blind, in which case one or more pigments is missing) - blue, green or red, and it is the combination of those pigments being stimulated within the eye that allows us to see the whole array of colours we see. They allow us to see the scene in front of us in detail, while the rods in our peripheral vision allow us to detect to if something is going on there, e.g. movement, so we can then turn our head to focus on what was there in more detail.

• Finally, don't forget to swap places with your helper so that they can have a turn! 

My New Room Mate

WARNING: This post contains graphic images of spiders. People with spider phobias may not wish to view the rest of this post.


Have you seen a spider with its prey, and been compelled to watch with fascination and intrigue, perhaps with a degree of disgust or a measure of fear thrown in? Or perhaps you've watched with curiosity, a spider weaving its delicate web and wondered how such a little creature could engineer such a beautiful death trap?

A couple of days ago, I noticed I had a little friend living by my window. He (or she) is much smaller than previous room mates I've had, such as this lovely fellow who kept me company a few years ago when I was in Nigeria: 

I was sitting on my bed, with my back leaning against the wall, reading a book one evening. This guy had been sitting quietly at the top of the wall, next to the ceiling, and I hadn't given much thought to his presence. All off a sudden I see movement out the corner of my eye, and turned to see him almost fly down the wall and stop, next to my head. Recovering from the surprise, I noticed he'd just caught a moth. I sat and watched him with fascination and a sense of awe, that I'd been privileged to be a witness to his hunt and subsequent feed.

While I do not know what sort of spider my Nigerian room mate was, my new little friend is a jumping spider. Jumping spiders belong to the family Salticideae, and are incrediably bright for being such little creatures with miniscule brains.

There are two major categories of spiders. There are sit and wait spiders. These are those that build webs to catch their prey, and they typically just sit out of sight near their web until some unsuspecting insect flies into it and gets caught up in the threads. Other spiders are active hunters. Both my Nigerian room mate and jumping spiders are active hunters. They still produce silk, which they might use as a saftely line (as with the jumping spiders) or to make their nests or egg sacs etc, but they do not construct webs to catch prey. Instead they are like the lions of the invertebrate world and actively stalk their prey.

As jumping spiders are active hunters, they have great vision with their eight eyes and spatial mapping abilities. Web building spiders have special tapping signals that the males tap out on the female's web, to tell her that he isn't prey but a potential mate. Some jumping spiders even mimic these tapping signals, pretending to be a potential mate, enticing the female out, where instead of finding a mate, she gets eaten! Spiders can be smart! They are incredible creatures. 

Anyway, I'd like you to meet my new little friend :)

Don't you wonder what those dark eyes are seeing and what is going on in his little brain? If you are in Christchurch, you might enjoy this seminar by Ximena Nelson, a scientist who studies jumping spiders.

I Spy With My Little Eye

Sometimes I think about how beautiful the world is. I feel blessed to live on such a beautiful planet and to have eyes that allow me to witness that beauty. The human eye contains four different photoreceptors: rods that detect the presence of light (i.e allow us to see in black and white and detect movement) and three types of cones that allow us to see in colour. Each cone type detects a different colour - red, green or blue. All the colours we see are the result of different combinations of those cones being stimulated simultaneously by the light entering the eyes. As humans, our eyesight is one of our more developed senses - one that we rely heavily on each day, or at least I know I do. Yet, Louie Schwartzburg compares our visual range to a single octave of the musical scale.

Many animals have the capacity to see light frequencies we cannnot. For example, honey bees can see ultraviolet, and many flowers have patterns on their petals that are not visible to the naked human eye that become visible under ultraviolet light, that aid bees in locating nectar and pollen. We already admire flowers for their beauty, but imagine being able to see the additional beauty visible to the bees. But there is one creature that makes me marvel even more at its visual capabilities...

The Mantis Shrimp 

http://en.wikipedia.org/wiki/File:OdontodactylusScyllarus.jpg

This shrimp-like creature is not actually a shrimp, but a group of crustaceans belonging to the Order Stromatopoda. They are predatory creatures found in shallow tropical and sub-tropical seas around the world. The mantis shrimp is noteworthy for several reasons, including its viciousness and strength, its complex signaling behaviours and its extraordinary eyesight.

The eye of the mantis shrimp has at least 16 different photoreceptors, of which 12-13 are cones for detecting colour. Moreover, they are capable of seeing polarised light. If the world looks as amazing as it does through our human eyes, imagine how amazing it would look through the eyes of the mantis shrimp...