Spike Train Statistics from Empirical Facts to Theory: The Case of the Retina - Modeling in Computational Biology and Biomedicine - page 254

Fig. 8.2 Visual pathway in the human brain. The principal projection of the eye, which is formed
by the optic nerve is carried to a first synapses in the lateral geniculate nucleus ( LGN )inthe
thalamus and then for a second synapses to the main cortical visual area V1, from where many
other projections target secondary cortical areas ( V2 , etc) (Reproduced from [ 32 ])

8.2.1.2 Retina Circuits and Receptive Fields

As a result of its stratified, horizontal and vertical structure, and of the various
type of synaptic connections (electrical fast synapses

∼ 0.1 ms for short distance;

chemical slow synapses 10 ms for long distances) between the different type of

neurons (P,H,B,A,G) a large number of “circuits” are present in the retina. The main
connectivity structure of the retina is shown in Fig. 8.1 b. This circuitry results in the
capacity of specific G cells to respond to specific stimuli in the visual field.
The receptive field (RF) of a sensory neuron is a region of space where the
presence of a stimulus modifies the activity of that neuron. In the retina this change
of activity is precisely the result of the transduction chain, from photoreceptor to G
cells, converting photons into spike trains. As a consequence, one also defines the
RF of a G cell as the input from all of the photoreceptors which synapse with it (via
B, H, A) cells.
The RF of a cell can have different forms, depending on the network of neurons
connected to this cell. A prominent example, is the antagonist ON-OFF center-
surround arrangement . An ON-center cell is stimulated when the center of its
receptive field is exposed to light, and is inhibited when the surround is exposed
to light. OFF-center cells behaves exactly the opposite way.
The physiological properties of G cells are determined at the center and the
surround of their RF by the input of ON or OFF B cell. First, photoreceptors make
synapses with ON (excitatory) B cells and OFF (inhibitory) B cells according to
their response to light. Figure 8.3 a explains in a schematic way how this property
results from the connectivity between P, B, and H cells. In the example, the
illumination of the photoreceptors in the center of the RF results in a depolarization
of ON B cells so in an increase of spikes rate in the respective connected G cells.

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