Dr. Gerald S. Hecht
Associate Professor of Psychology
College of Sciences
webmaster@psiwebsubr.org
PSYC 381 - Sensation & Perception Exam 2 Study Guide
Vision
Vision is the most complex of the senses, culminating in the high resolution, color images of the higher primates. The pathway must handle such demands as transduction of light into coherent and synchronous neural impulses, binocular and more distant depth perception, motion in the visual field or of the observer or both--in as near to real time as possible...all in a wide range of focal lengths and light levels. The receptor is constrained by the physical laws of optics. Motion not only must be perceived but also tracked by coordinated eye, head, and body movements as well as lens-iris accomodation. These paths ultimately lead into the realms of memory and mentation, where we recognize and understand what is being seen...the so-called "mind's eye" which can be tapped by dreams and hallucinations as well.
Structure of the Eye
Proper size and shape are extremely important for eye function.

The visual axis is along a line which passes straight through the apex of the cornea, exactly through the center of the pupil, through the center of the lens, and finally strikes the retina. Since such a line is perpendicular to all refractory surfaces in that pathway, there is no deflection of the "beam." Notice the only frames of reference are in the eye structure itself, so it doesn't matter which direction the eye is pointing. As you can see, any pathway of light not on the axis is refracted as it passes through the eye, and at the focal point, such a beam crosses both the visual axis and other beams (along the same circumference). The result is that it reaches the retina on the side opposite the axis from where it entered. This doesn't cause visual confusion, but it does cause thought chaos until you grow accustomed to thinking in these terms.

 Thus, there are visual fields and retinal fields of vision.
Retinal Paths
We can teach a glass lens and plastic box to focus an image of the correct brilliance. We call those "cameras" and some film companies almost give them away so you will buy their film. The hard part we will talk about here is a more advanced technology--developing that film into (what turns out to be) a negative. This is the job of the retina, the innermost layer of the eyeball. In the final part of this study guide, we will send that negative along to the brain where the picture will be printed and appreciated.

The initial processing of the image that occurs in the layers of the retina is some of the most complex that makes up the sense of vision. Histologists recognize 10 layers to the retina, but our approach will be directed more to the component cells and their interrelationship.
Rod and Cone Function
THESE PHOTORECEPTOR cells form a dense outer layer of the retina (remember, the outer layer is farthest from the center of the eye sphere).The outer segment, which is the cell region where transduction of light actually occurs, is the outermost part of the visual retina.

Visible Spectrum Sensitivity

(1) The further away from the modal (maximum) sensitivity for each color, the brighter the light must be before retinal receptors will respond.
(2) Red and green overlap extensively, as do green and rod, but red has little overlap with rod wavelengths.
(3) Blue is virtually isolated from the other two colors.
Light Transduction
Both rods and cones transduce light along similar pathways, although the pigment compounds are different in the two cell types. It is not important at this time to go into minute chemical details of this transduction-- rodfunctionimagethe generalities are quite sufficient. Let's limit this discussion to rod transduction. Follow along in the illustration below.

Convergence: Bipolar Cells and Ganglion Cells
As we continue to examine the retina, it would be a good idea now for you to get an appreciation of physical scale. The rods and cones are smaller than most of the cell bodies of spinal or brain stem cells. They are not myelinated. A proper dendrite or axon cannot be distinguished. At their inner surface, however, they do synapse into a column of interneurons which will process the light signal into a sensable image. These interneurons are also quite small. The output of their interaction with the receptor cells and with each other converges onto ganglion cells, which are the output layer from the retina to the brain proper. The direct throughput path is receptor to bipolar cell(s) to ganglion. Follow the drawing below as these three cell interrelationships are described.

As their name implies, bipolar cells have a dendritic process, a cell body, and an axon. However, the cells are so small that these distinctions are minor. The cells are not myelinated, and their excitation produces an inhibitory generator potential. Synaptic input to bipolars is from receptor cells and from a second type of interneuron, the horizontal cells. We'll consider horizontals later.
Some of these cell interactions will seem counter-intuitive--just grit your teeth and hang on. Rods and cones have leaky cell membranes with a continuing inflow of sodium ion. This qualifies them as pacemaker cells with a standing generator potential. As a result:
1. they are also leaky to calcium ions.
2. they are continuously releasing their neurotransmitter (glutmate) onto the post-synaptic membrane of the bipolars.
Bipolars are also leaky, but reception of the cone transmitter closes those leaks--the dark adapted bipolar is inhibited. If the light receptor is illuminated, the cone stops leaking, stops pacing...stops releasing transmitter. The bipolar is freed from its ongoing inhibition and depolarizes (generator), causing release of its transmitter (glutamate) onto its ganglion cell. There are details you need to know right now, but first you need to understand the ganglion cell.
The Ganglion cell layer is the innermost layer of the retina. Ganglion cells have relatively large cell bodies, and from these arise long myelinated axons that will exit the eye and make up the optic nerve/tract synapsing in the lateral geniculate or optic tectum of midbrain. They are leaky cells, pacemakers, but the result of this sodium inflow is paced action potentials. Ganglion cells are inhibited by bipolars cells which are inhibited by rods/cones which are inhibited by light. Let's try that again:
GANGLION cells spontaneously generate action potentials that are transmitted along the visual nerve pathway. Because each ganglion has its own schedule independent of all the others, this input is disorganized. Your subjective appreciation is--nothingness. The ganglion cells can behave in this fashion because the bipolar cells that can inhibit them are not inhibiting them because they are inhibited by dark rod/cones.
If the light receptor cell is stimulated, it stops inhibiting the BIPOLAR, which now begins to inhibit the ganglion cell. This imposes order onto the discharge of the ganglion. An object has been perceived, and as a result the ganglion communication with higher centers is silent. You see what you don't see. Unfortunately, it's not quite that simple. Before we go on down this maze, let's clean up three details.
Having just read about the pigment layer and its function, you should understand that if polarity of the retina was reversed--if receptor cells were the innermost layer of the retina--much incident light could stimulate the outer segments from a variety of incoherent directions, and image quality would be seriously fuzzy.