Let us first discuss, briefly, how electrophysiological recordings are made. An electrode (which can be made in a number of ways) but can be thought of as thin wire with a super tiny point, is placed in (intra-cellular) or near (extra-cellular) a neuron from which one wishes to record the electrical activity. Yes, information is passed along neurons by means of changes in electrical charges. The electrical signal that the electrode senses is delivered to an amplifier (the electrophysiological signal is extremely small and needs to be amplified) and then to an oscilloscope. An oscilloscope can be thought of as a specialized TV screen on which one can see the shape of the electrical pulse traveling along our nerves.
Let us assume that the electrode is placed very close to one of the optic nerves. Next the retina will be stimulated with light. Since the electrode will be adjusted to optimally pick up nerve impulses from only one optic nerve, stimuli delivered to only a portion of the retina will be observed on the oscilloscope.
Before we continue, one additional piece of information is needed. The electrode we placed near an optic nerve will show signs of activity even when there is absolutely no light applied to the receptors. This neural activity in darkness is call the resting level or potential of the neuron.
Now let us assume that a small spot of light shines on the receptors and that action potentials (also called spikes) are observed on the scope. Actually what you will see is an increase or decrease in the number of spikes above or below the resting potential. So if the light shines on a receptor from say a center excitatory receptor (now would be a good time to refresh your memory of the receptive field diagram) you would see an increase in the number of spikes. If the light fell on those receptors from the surround inhibitory part of the retina you would see a decrease in the number of spikes. Now, here is what visual scientists do. They draw a map which shows the area of the receptors that respond in an excitatory (spike increase) fashion and those areas that respond in an inhibitory (spike decrease) fashion. This map is called the receptive field of the neuron. Retinal receptive fields frequently have the center surround configuration seen in the receptive field diagram.
Receptive fields are also found in the visual cortex. If you are interested in obtaining more complete information about cortical receptive fields and, in general, how the brain processes visual information I would recommend a very readable text by Semir Zeki entitled A Vision of the Brain, published by Blackwell Scientific Publications in 1993.
Hubel and Wiesel illustrated the connections between the LGN and the visual cortex utilizing a cortical cube model. This model also shows some of the cortical receptive fields they have mapped, especially the blob areas which are reported to be important color structures, the columns responsible for orientation detection as well as the columns to which the right and left eyes feed. For more information see the model and also further explanations about the cortex.
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