Vision allows us to make sense of our world and to acquire knowledge about what is around us. It is commonly thought that the eye is like a camera, sending a high-resolution image of the visual world to the brain. In fact, what actually happens is that light enters the opening in the front of the eye, the pupil, and the lens focuses the image on the retina which is a sheet of layers of neural tissue on the back of the eyeball. These layers are comprised of ganglion, bipolar, and horizontal cells. The photoreceptor cells are, which follow in this layering, are the cells that respond directly to light. They contain light-absorbing chemicals, pigments that generate a neural signal when they absorb light. The light activates the rod and cone photoreceptors, which then convert the light into an electrical signal that is sent to the brain via retinal neurons.
When light passes into the photoreceptor layer, some of it is absorbed by the photoreceptor and any leftover light is absorbed by a layer of pigment cells at the back of the eye. This pigment connects directly to the fact that we are diurnal animals, active when there is plenty of light. Our eyes are designed for acuity rather than for light sensitivity.
We have two kinds of photoreceptors, rods and cones, which generate signals in response to light. Rods and cones are not distributed evenly across the retina – the cone density is higher at the center of the gaze than in the periphery, where the rods do the work.
We have three different types of cones, each kind containing a different kind of pigment and each responding best over a different range of visible wavelengths, long-, middle-, and short-wavelength cones. They are sometimes called red, green, and blue cones. These three cone types allow us the benefit of color discrimination – being better able to distinguish differences between light reflected off various surfaces.
It is helpful to think of visual processing in terms of two systems, the what system and the where system. Each are different in the ways in which they process light signals. The where system is the more primitive of the two visual systems. It deals with information about depth, spatial organization, position, figure/ground and motion. In other words, it deals with all of the aspects that a human would need to navigate his or her environment. Most animals have only the where system although there are a few animals that have both, such as certain squirrels and primates. Humans, as well as most mammals, navigate their environment using this system. The what system uses both color and luminance to define the shape of objects and the color of surfaces.
There are four fundamental ways in which these systems process light signals: color selectivity, contrast sensitivity, speed and acuity (resolution). The "where" system is colorblind, has a high sensitivity to differences in brightness (contrast), is fast and transient, and has a slightly lower acuity than the "what" system. The "what" system uses and carries information about color, requires larger differences in brightness, is slower and higher in sharpness (acuity) than the where system.
Throughout this unit, students will be engaged in a variety of experiments with light. We are fortunate to be in a school with many windows, which I see as an avenue of research that will include shadows, darker or brighter days, color through prisms and acetate panel. How does light appear on different materials and does the nature of the material matter? What are the physical behaviors of the light throughout our school day in our classroom? Documentation and measurement will help students track their findings. Students will learn about the eye itself and create models that show their learning. With some background knowledge now of light and vision, students will, in the classroom and on our travels to the museum, begin to apply their knowledge as they analyze what they see.
