Retinal Processing of Visual Stimuli

Light stimuli hyperpolarize the sensor potential of photosensors A, left) from ca. - 40 mV to ca. - 70 mV (maximum) due to a decrease in conductance of the membrane of the outer sensor segment to Na+ and Ca2+ (^ p.348ff.). The potential rises and falls much more sharply in the cones than in the rods. As in other sensory cells, the magnitude of the sensor potential is proportional to the logarithm of stimulus intensity divided by threshold-intensity (Fechner's law). Hyperpolarization decreases glutamate release from the receptor. When this signal is relayed within the retina, a distinction is made between "direct" signal flow for photopic vision and "lateral" signal flow for scotopic vision (see below). Action potentials (APs) can only be generated in ganglion cells (^ A, right), but stimulus-dependent amplitude changes of the potentials occur in the other retinal neurons (^ A, center). These are conducted electrotonically across the short spaces in the retina (^ p.48ff.).

Direct signal flow from cones to bipolar cells is conducted via ON or OFF bipolar cells. Photostimulation leads to depolarization of ON bipolar cells (signal inversion) and activation of their respective ON ganglion cells (^ A). OFF bipolar cells, on the other hand, are hyperpolarized by photostimulation, which has an inhibitory effect on their OFF ganglion cells. "Lateral" signal flow can occur via the following pathway: rod ! rod-bipolar cell ! rod-amacrine cell ! ON or OFF bipolar cell ! ON or OFF ganglion cell. Both rod-bipolar cells and rod-amacrine cells are depolarized in response to light. Rod-amacrine cells inhibit OFF bipolar cells via a chemical synapse and stimulate ON bipolar cells via an electrical synapse (^ p. 50).

A light stimulus triggers the firing of an AP in ON ganglion cells (^ A, right). The AP frequency increases with the sensor potential amplitude. The APs ofON ganglion cells can be measured using microelectrodes. This data can be used to identify the retinal region in which the stimulatory and inhibitory effects on AP frequency originate. This region is called the receptive field (RF) of the ganglion cell. Retinal ganglion cells have concentric RFs comprising a central zone and a ringlike peripheral zone distinguishable during light adaptation (^ B). Photic stimulation of the center increases the

AP frequency of ON ganglion cells (^ B1). Stimulation of the periphery, on the other hand, leads to a decrease in AP frequency, but excitation occurs when the light source is switched off (^ B2). This type of RF is referred to as an ON field (central field ON). The RF of OFF ganglion cells exhibits the reverse response and is referred to as an OFF field (central field OFF). Horizontal cells are responsible for the functional organization of the RFs (^ p. 344). They invert the impulses from photosensors in the periphery of the RF and transmit them to the sensors of the center. The opposing central and peripheral responses lead to a stimulus contrast. At a light-dark interface, for example, the dark side appears darker and the light side brighter. If the entire RF is exposed to light, the impulses from the center usually predominate.

Simultaneous contrast. A solid gray circle appears darker in light surroundings than in dark surroundings (^ C, left). When a subject focuses on a black-and-white grid (^ C, right), the white grid lines appear to be darker at the cross-sections, black grid lines appear lighter because of reduced contrast in these areas. This effect can be attributed to a variable sum of stimuli within the RFs (^ C, center).

During dark adaptation, the center of the RFs increases in size at the expense of the periphery, which ultimately disappears. This leads to an increase in spatial summation (^ p.353 C3), but to a simultaneous decrease in stimulus contrast and thus to a lower visual acuity (^ p. 349 B2).

Color opponency. Red and green light (or blue and yellow light) have opposing effects in the RFs of p ganglion cells (^ p. 358) and more centrally located cells of the optic tract (^ p.357E). These effects are explained by Hering's opponent colors theory and ensure contrast (increase color saturation; ^ p.356) in color vision. When a subject focuses on a color test pattern (^ p. 359 C) for about 30 min and then shifts the gaze to a neutral background, the complementary colors will be seen (color successive contrast).

RFs of higher centers of the optic tract (V1, V2; ^ p.358) can also be identified, but their characteristics change. Shape (striate or angular), length, axial direction and direction of movement of the photic stimuli play important roles.

i— A. Potentials of photosensor, ON-bipolar and ON-ganglion cells



Weak stimulus

Sensor potentials (s)

Weak stimulus

Strong stimulus mV

ON-bipolar cell

Sensor potentials (s)

Potential in ON-bipolar cell

ON-ganglion cell Optic nerve fiber

Weak stimulus

Strong stimulus

Action potentials

I— B. Receptive fields of "on" ganglion cells (1, 2) and "off" ganglion cells (3, 4)

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