Summation is a bodily process within the visual process. The following article deals with the clarification of terms as well as the function of summation and pursues the question, what do affected persons perceive, in whom the process of summation is disturbed? What are the clinical pictures within this framework?
What is summation?
Summation is a clearing process in (human) optical perception. It is one of the ways the retina of the eye adapts to changing light conditions.
Function and task
Summation is one of the ways the retina of the eye adapts to changing light conditions. To understand the role that summation plays, first explain the structure of the retina. The human retina is estimated to consist of 120 million rods and 6 million cones. The rods are responsible for twilight, night and motion vision. The cones are only stimulated at higher light intensities and are responsible for color vision. A retinal cross-section shows the ganglion cells in the uppermost layer, which unite in the blind spot to form the optic nerve. Below this is a layer of switch cells that play a role in various retinal clearing processes, the receptive fields, and the process of summation. This layer consists of three different cell types. The bipolar cells connect the rods and cones to the ganglion cells. The horizontal cells connect light sensing cells to each other, while the amacrine cells connect ganglion cells to each other. The switch cell layer is followed by the layer of light sensing cells, rods and cones. Thus, they are not directly exposed to incident light. The parts of the visual sensory cells that are continuously engaged in the visual process are stuck to the outside in the black retinal pigment epithelium – visible through the pupil opening – and are nourished by it. The macula is the most metabolically active area in the human body. The distribution of rods and cones varies and depends on their function in the retina. In the center of the retina, in the optic axis, is the visual pit, also called the fovea centralis. Only cones are found here; rods are not present. In the adjacent area of the macula, the yellow spot, the visual acuity already decreases rapidly. Here, depending on the distance to the center, fewer and fewer cones and more and more rods are interconnected. Outside the macula, the rods are in the overwhelming majority. Since “only” about 1 million ganglion cells are available, these are interconnected in clusters – receptive fields – with the 126 million sensory cells. In the fovea centralis, one cone cell is interconnected with one ganglion cell for the highest visual acuity. In the adjacent area of the macula smaller receptive fields occur, where about 20-100 cones interconnect with 3 -15 bipolar cells and 1 ganglion cell in a receptive field. This is based on the finding that one bipolar cell networks with one ganglion cell: thus, for a receptive field of cones, there is about a 1:6 ratio. In contrast, about 15-30 rods form a receptive field with one bipolar cell. Now summation comes into play. In addition to dark adaptation and light adaptation, summation is another adaptation process of the human retina to regulate the light sensitivity of the rods and cones depending on the illuminance. A distinction is made between spatial and temporal summation. In spatial summation, for the rods, an incoming weak light signal is
incoming weak light signal is amplified by convergence into the receptive field. Many rods must be active at the same time. The electrical impulse must be large enough in the larger receptive fields to trigger a stimulus in the downstream ganglion cell. With increasing luminance, the cones are increasingly stimulated. Here, the smaller receptive fields are addressed. The principle of lateral inhibition applies: Conversely, the signals can also attenuate each other depending on the point of origin – assuming that neighboring sensory cells are stimulated with different light intensities. This principle applies to contrast enhancement: If one observes a grid of black filled squares on a white background, a slightly dark illusion appears in the crossing points of the white lines, only not in the fixation point. The crossing points are surrounded by more white than the white areas adjacent to the black squares.The excitations emanating from the crossing points are ultimately inhibited more strongly than those of the white lines between the black squares. Temporal summation is a process in which the exposure time of the light stimulus to the retina is increased at low light intensities, such as by slowing eye movements or prolonged fixation.
Diseases and disorders
In some diseases, these control processes in the retina can no longer be performed with the intended quality or completely. The affected person is massively blinded, for example, because control processes in the retina no longer function. The contrast processing does not run as usual, as described in the test with the black squares on a white background: The illusions of the black areas appear less intense. The person affected should also have great problems in adjusting when he moves from a bright room to a dark one or vice versa. Or when he crosses an intersection with an avenue of trees on a sunny day. Or he is about to cross the intersection and suddenly finds himself in the cast shadow of a house. Diseases that affect the control process of the retina are those in which the layers of ganglion cells, switch cells, visual sensory cells and retinal pigment epithelium that are set up directionally in the retinal cross-section are no longer present in this form. As a rule, the ophthalmologist should see these irregularities in the retinal structure in the form of hyper- or depigmentation when viewing the fundus of the eye with an ophthalmoscope. These may be localized to the macula, or localized to the retinal periphery. Some retinal dystrophies progress from the periphery to the visual field center or vice versa. Optical coherence tomography, which provides a cross-sectional view of a large portion of the retina, should also be able to provide more detailed information. Fundus autofluorescence (FAF) is able to visualize normally functioning from off-normal retinal areas. Thus, FAF ultimately depicts visual field boundaries or minor deficits called scotomas. This examination detects the accumulation of lipofuscin in the retina, which should normally be discarded. If a disease related to the processing of sensory stimuli in the retina is suspected, the patient is examined in the retinal laboratory. Here are used: Dark adaptation according to Goldmann-Weekers, to check how the rods react to low light intensities. In case of suspicion that processes of switch cells and ganglion cells have been affected, the VEP can be used. In this procedure, the patient views an increasingly rapidly changing black and white honeycomb pattern on a monitor. Multifocal ERG (mfERG) examines the summation response or cellular response in the macula. The ERG is a derivation of the sum response of the rod and cone retina based on scotopic and photopic stimulation of the sensory cells and derivation of the potentials. In some cases of infantile cerebral palsy, the retina behaves as if it has retinitis pigmentosa and mimics the progression.
