The Receptive Field
Consider an arbitrary receptor in the retina. The optics of the eye focus the light from some subscribed area in the visual field onto the receptor. That is, in some area of the visual field, subtending some visual angle, the receptor will detect changes in light energy. We can consider this area out in the visual field the receptive field for this receptor.
Now consider that this receptors signals are sent via other neurons in the retina to a ganglion cell. The ganglion cell may also receive inputs from other receptors. So there is an area out in the visual field, which includes the receptive field of the original receptor, that is the receptive field for the ganglion cell. That is changes in the light stimulus in the receptive field will modulate the activity of the ganglion cell.
For any cell in the visual system, from the retina to the brain, there is an area out in the visual field that will produce a change in the response in that cell. This is the definition of the receptive field. Sometimes the receptive field is defined by an area of the retina that elicits the response but I prefer to think of the receptive field as existing out in the visual field.
Physiologists can take electrodes and place them in or close to neurons in the visual system and measure the response of the cell to light stimuli. The area of the visual field that causes responses in the cell define the receptive field. By trying different stimulus configurations (size, shape, spatial frequency, temporal frequency, etc.) researchers can define the receptive field in terms of optimal stimuli. A particular stimulus configuration may increase the cells response (excitation) while another may decrease it (inhibition). In this way a map of the receptive field can be constructed.
This diagram shows microelectrodes recording responses from from retinal ganglion cells from the cell bodies in the retina and from the axons in the optic nerve.
The little squiggles near the electrodes show the action potentials recorded from the ganglion cells. The electrode is hooked up to an amplifier and oscilloscope or a computer which can record responses. The time scale is on the horizontal axis and the potential is shown on the vertical axis. The time scale is compressed so each action potential looks like a spike.
Visual stimuli can be presented to the eye in either case. (The electrode is small and unobtrusive.)
Here the recordings are being made in a cat's brain. (This figure is modeled after the early experiments by Hubel & Wiesel.) The caption of the above figure tells the story pretty well. Often the signal from the electrode is also passed to a speaker that makes a clicking sound for each action potential. The experimenter can then judge the response by listening to the change in the rate of the clicks. He can then in turn adjust the location or configuration of the stimulus to try to figure out the receptive field for the neuron.
In reality, the animals vitals (temperature, blood pressure, etc.) are monitored, the animals head is in a stereotaxic device to keep it still and to put the electrode in the correct place and anesthetic is continuously administered. Typically, a lesion would be produced in the animals brain after recording so that upon histological study of the animals brain after sacrifice, the location of the electrode recordings can be determined. Today, studies of visual response are done with awake, alert, and behaving animals using implanted stereotaxic devices and head and eye-tracking devices. Consider how physiology done on an unconscious animal might compare to an awake animal.
Think about this: Let's say you put an electrode in a motor neuron in the ear of an awake, alert dog. If you flash a light in the dog's peripheral visual field, the dog might turn to look at the light and its ears might perk up to listen for something. So you would get a response in the neuron. Is this neuron a visually responsive cell?