This post is the second in a series about vision and visual perception. It will explore the anatomy of the eye as well as how rays of light are transformed into electrical impulses that can be transmitted along neural pathways to facilitate visual perception.
We look at a cup. Make it a mug. Of steaming hot tea. Perhaps with a biscuit or scone with jam and cream beside it. In order to understand the visual process, it is important to break this picture down into two important aspects. First, the physical properties of the scene: brown substance held in a container with a semi-circular protrusion on its side, and a golden short cylindrical object with a layer of shiny red matter, topped with a white substance. Doesn’t sound too appealing.
This information is what is presented to our eyes in the form of rays of light. It is only when this information is transmitted throughout the brain that we can make sense of what we are looking at. Suddenly, the golden scone with a layer of sticky red jam and soft, fresh cream has me thinking of my grandmother.
In order to understand the diseases and injuries that affect the visual processing system, we must understand what happens at each point within that process. And the first point of that process is the cornea of the eye. This transparent layer covering the eye acts as a camera lens, bringing the light into focus. The iris then constricts in brighter conditions and dilates in dim conditions, to allow the optimal level of light to enter the eye.
Behind the pupil is a lens that further focusses the light before it is reflected onto the retina at the back of the eye. The retina is 0.5mm thick and contains ten layers of neurons. Within these layers are the rods and cones which are photoreceptors containing pigments. These split apart when exposed to light, triggering action potentials in downstream neurons; the translation of light into signals that will travel through the neural pathways.
Rods contain rhodopsin, which is destabilised at low levels of light. This pigment is quickly depleted and takes time to replenish. They are therefore more effective in dim conditions. Cones contain photopsin which destabilises at high levels of light and can replenish quickly, therefore making cones more useful for vision in well-lit environments.
While cones are located primarily at the fovea which is the centre point of the retina, rods are spread throughout the retina. Cones and rods are connected to bipolar neurons. Here we see another difference between cones and rods. A number of rods are connected to one bipolar neuron.
The sum of the information received by the bipolar neuron is transmitted to the ganglion neuron. The axons of the ganglion neuron form the optic nerve, which transmits visual information along the neural pathway.
While the number of rods connected to one bipolar neuron means that there is more chance for the ganglion to receive input above the threshold required to initiate an action potential through the optic nerve, we have also learnt that rods are positioned in a diffuse pattern across the retina, and that information from these diverse locations send messages to the same bipolar neuron. It should be no surprise therefore, that rods help us to identify low levels of light, but that they are not able to provide significant detail.
Bipolar cells are innovated by a small number of cones. The ability of a few cones to meet the threshold requirements to initiate an action potential is reduced, but there is a higher degree of detail given the specific location.
So we have learnt that the eye is able to focus and control the amount of light the retina is exposed to. It has neurons that facilitate both clear, detailed vision in well-lit conditions and basic, generalised sight in dim lighting. The next post in this series will explore the path of the electrical impulses once they enter the optic nerve.