What are retinal receptive fields?
In order to answer this question, we need to respond two others beforehand. The first is what’s the retina? A short answer for this question is that the retina is the bowl shaped tissue located in the back of the eye. This light sensitive tissue contains a complex network of neurons that ends in what’s called the optic nerve, which is the ensemble of ganglion cells’ axons plus support cells. The second question is what’s a receptive field? A short answer is that a receptive field is the region of space that is associated to a neuron so when a certain stimulus is present in that region, the neuron’s firing will change. With this knowledge, we may understand that a retinal receptive field is the region of the visual space that makes a retinal ganglion cell change its firing under the presence of a certain visual stimulus. Going a little further, this means that for a given ganglion cell, its receptive field would enclose all the synapsing network of photoreceptors, bipolar, horizontal and amacrine cells that converge to it.
Receptive fields in the retina have been classically treated as elliptical center-surround structures, with the parts being either ON or OFF, and antagonistic between them. For an ON-center cell to fire, it needs to be stimulated by light mainly on its center, while for an OFF-center cell, it fires when the light is mainly on its surround. Why “mainly”? Because when light is in both center and surround, these cells fire in a slow way (low-frequency), whereas when stimulated only in their ON area, they fire rapidly.
Over the years, numerous methods have been developed to estimate these receptive fields. Classically, one would analyze a combination of the responses from ganglion cells to different visual stimuli and the statistics of the stimuli being used. The responses used may be obtained in a lab using patch clamps or multiple electrode arrays; the difference is that a patch clamp allows to record the responses of a single cell, whilst with a multiple electrode array one may also study populations of neurons. The statistics of the stimuli are usually considered when estimating the retinal receptive fields. The classic estimation methods are Spike-Triggered Average and Spike-Triggered Covariance, and another noteworthy technique is Maximally Informative Dimensions.
One important goal for many branches of science is to further understand how sensory systems encode stimuli. In the visual system, the retina is the very first structure that treats (visual) stimuli with a well elaborated encoding mechanism, thus it is crucial for this understanding to successfully identify its receptive fields, since the processings at higher levels will depend on the retinal output.
