membrane voltage is variable, the spikes along the axon are all-or-none. To accomplish this, there is a conversion of an analog signal (the gradual changes in membrane voltage) to a digital signal (the pulses or spikes along the axon) that occurs within the neuron. This conversion is very much like the action of an analog-to-digital converter seen in electronics as, for example, when a visual image is converted to a series of digital bits by the CCD of a digital camera. This pulsatile or digital signal is then reconverted back to an excursion in membrane potential (or analog signal) at the synapse.

The synapse is a specialized region where two neurons come very close to each other. At the synapse, the first cell (the presynaptic neuron) releases a chemical called a neurotransmitter in amounts proportional to the number of action potentials. The postsynaptic neuron has receptor molecules in its membrane that bind to this neurotransmitter and open gates for ion channels, resulting in a change in the membrane voltage of the postsynaptic cell. Most neurons have many synapses, so all the changes in membrane voltage sum to create the new membrane voltage of the postsynaptic neuron, which determines its action potential rate and amount of neurotransmitter release onto a third neuron. This process continues, with the synapse essentially performing a mathematical function on the information transmitted. The result is that some synapses will simply add signals, some will determine their rate, some will integrate, and so on. In this way, our synapses act as individual processors, with their processing determined both by the chemical used as the neurotransmitter at that synapse and the molecule used as its receptor. These two combine to determine the type of processing that occurs at any one synapse. Given that our brains have about 100 billion neurons, and each neuron may have a thousand synapses, this makes our brains equivalent to a massively parallel computer comprised of 100 billion processors [1].

So, what does all this have to do with photoreceptors? The rods and cones are the cells of the visual system that create the first change in membrane potential, and they then communicate this information to the next cells via their synapse. The RPE regenerates the visual pigment, allowing the photoreceptors to continue functioning over and over.

Blind Spot

There is a unique spot in each eye where there are no photoreceptors of any type. This spot is where the optic nerve exits the eye and where the blood vessels that supply the retina enter and leave. The region is called the optic disk, or the blind spot, because we cannot see anything in that area. It is located nasal of the fovea by several millimeters and thus is in the opposite side of the visual field for each eye.

The optic disk is centered about 4–5mm from the fovea and is typically about 1.5–2mm in diameter. It is not a perfect circle and varies somewhat in shape from person to person. It is necessary for our high-acuity vision as this is the port through which blood flows to nourish the metabolically active retina and information leaves from the eye. But, in that region we see no light. How is it that we do not notice this absence of vision [1, 2]?

First, we have binocular vision, meaning that each eye is conveying information about the same visual scene, but from a slightly different vantage point. This, of course, allows us to determine the distance of various objects, but it also allows us to fill in the full information of a scene when a part of it is missing from one eye.

Second, even when we are observing a scene with just one eye, the visual center of our brain fills in when there is information lacking. So, when we do not have information