What Is Visual Learning?

Vision is a physiological term. Light acts on the visual organs, causing them to sense cell excitement, and the vision is processed after the information is processed by the visual nervous system. Through vision, people and animals perceive the size, brightness, color, and movement of external objects, and obtain various information that is important to the survival of the body. At least 80% of external information is obtained through vision. Vision is the most important for humans and animals. feel.

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Image stimulus

Light

The information processed by the retinal neural network is transmitted to the center by the axons of ganglion cells, the optic nerve fibers. in

The imaging principle of light passing through the eye refractive system is basically the same as that of a camera and

1. Eyesight

Eye and retina

The eyes are spherical and surrounded by the sclera. The sclera follows the transparent cornea in front. Behind the cornea is a lens, which is equivalent to a camera lens and is the main refractive system of the eye. The anterior and posterior chambers between the lens and the cornea contain aqueous humor. The entire eyeball behind the lens is filled with gelatinous vitreous body, which can provide nutrition to various tissues of the eye and also help maintain the shape of the eyeball. On the inner surface of the eyeball, there is a layer of the retina only 0.3 mm thick, which is the peripheral part of the visual nervous system. Between the retina and sclera is a choroid covered with blood vessels, which plays a nutritional role in the retina.

The cornea and the lens make up the refractive system of the eye, so that external objects form an inverted image on the retina. The curvature of the cornea is fixed, but the curvature of the lens can be adjusted by the ciliary muscles through the ligament. When the observation distance changes, the focal length of the entire refractive system is changed by the change of the curvature of the crystal, thereby ensuring that the external object has a clear image on the retina. This function is called visual adjustment. When the visual regulation is abnormal, the object cannot be clearly imaged on the retina, and nearsightedness or farsightedness can occur. At this time, an appropriate lens is required to correct it.

Between the cornea and the lens, a pupil formed by an iris acts as a diaphragm. The pupil shrinks when it is illuminated, and expands in the dark to adjust the amount of light entering the eye. It also helps to improve the imaging quality of the refractive system. The pupil and visual regulation are controlled by the autonomic nervous system.

The movement of the eyeball is realized by six extraocular muscles. The coordinated action of these muscles ensures that the eyeball moves freely in all directions and changes the sight as needed. The movement of the extraocular muscles of the two eyes must be coordinated, otherwise it will cause double retina (double vision) or strabismus.

The retina is a layer of neural tissue containing hundreds of millions of nerve cells. According to the shape and location of these cells, they can be divided into six categories, namely photoreceptors, horizontal cells, bipolar cells, amacrine cells, ganglion cells, and recent years. Newly discovered intercellular cells. Only photoreceptors are sensitive to light, and the initial biophysical and chemical processes triggered by light occur in the photoreceptors. The vertebrate retina is inverted due to embryonic development, that is, after the light enters the eyeball, it first passes through the network of nerve cells and finally reaches the photoreceptor. However, the high transparency of nerve cells does not affect the quality of imaging.

Neural network of the retina and its information processing

The billions of nerve cells in the retina are arranged in three layers, forming a complex network that processes information through synapses. The first layer is photoreceptors, the second layer is intermediate nerve cells, including bipolar cells, horizontal cells and amacrine cells, and the third layer is ganglion cells. The synapses between them form two synaptic layers, namely the outer reticular layer composed of photoreceptors and bipolar cells, horizontal intercellular synapses, and synapses composed of bipolar cells, amacrine cells, and ganglion cells. Inner reticular layer. After the photoreceptors are excited, their signals are mainly transmitted to ganglion cells through bipolar cells, and then to the nerve center through the latter's axons (optical nerve fibers). However, the signals in the outer reticular layer and the inner reticular layer are modulated by horizontal cells and amacrine cells. This signal transmission is mainly achieved through chemical synapses, but there are also electrical synapses (gap connections) between photoreceptors and horizontal cells, linking interactions between each other.

Signals from rod cells and cones are relatively independent in the retina, and the ganglion cells do not converge. There is only one type of bipolar cells that receive signals from rod cells (rod bipolar cells), but bipolar cells that receive signals from cone cells can be divided into two types, trapped and flat, according to their synaptic characteristics. Cells have different functional characteristics. In the outer reticular layer, horizontal cells receive signals from photoreceptors over a wide range and interact with bipolar cells at the synapse. In addition, horizontal cells modulate signals in the form of feedback to photoreceptors. Signals from bipolar cells in the inner reticular layer are transmitted to ganglion cells, while amacrine cells connect neighboring bipolar cells. Convergence of rod and cone signals may also occur in amacrine cells.

The signal of the photoreceptor is mainly transmitted to the intermediate nerve cells by changing the amount of the transmitter released by the chemical synapse. The activity of bipolar cells and horizontal cells still takes the form of graded potentials without nerve impulses. But they are no longer like photoreceptors, they only react when light hits a certain point of the retina, but they spread over a region, and the range of the retina they feel increases significantly. Some horizontal cells even respond to any part of the illuminated retina, which indicates the convergence of photoreceptor signals in different spatial parts. It is particularly important that the receptive fields of bipolar cells exhibit a certain spatial configuration. Some cells are depolarized at the center of the photoreceptive field, and the polarity of the reaction is reversed when they are illuminated at the peripheral area-hyperpolarization; other cells have the opposite reaction pattern; horizontal cells are at this center-peripheral The type of receptive field plays an important role. These two cells are morphologically equivalent to trapped and flat bipolar cells, respectively.

In amacrine cells, some impulsive responses begin, but still dominated by graded potentials. To the ganglion cells, the response to light is completely pulsed, and the center-peripheral ridge-type receptive field develops more completely. The receptive field of ganglion cells in higher animals is usually concentric and consists of two parts, the center and the surrounding area. Some cells have a series of pulses when they illuminate the central area of their receptive fields. The stronger the light, the higher the pulse frequency. When the peripheral area is illuminated, the cell's spontaneous pulses are inhibited. Such cells are often called light- Central cells. Other so-called light-removal-center cells not only do not appear pulses when they illuminate the central area of their receptive fields, but suppress spontaneous pulses, but suddenly appear a series of pulses after the light stops. When the light is moved to the peripheral area, the reaction pattern is the opposite. For example, if the light irradiates the entire receptive field, the ganglion cells often have no response or only a weak response. On a dark background, a light spot (for the light-center cell) filled with the central area of the receptive field or a bright background light is filled with the receptive field. The dark spot in the central area (for light-removing-central cells) causes the strongest response of the cells.

The emergence of the central-peripheral receptive field marks an important stage of visual information processing. The most important function of vision is to distinguish images, and any image is ultimately a combination of different light and dark parts. After the photoreceptor detects the presence of light, it needs a neural mechanism to specifically process the light and dark contrast information. The central-peripheral receptive field is an important manifestation of this neural mechanism.

Color vision is another important aspect of vision. Although color information is encoded at the photoreceptor level with three different signals: red, green, and blue, these three signals are not transmitted to the brain by dedicated lines, as assumed by the three-color theory. In horizontal cells, signals of different colors converge in a specific way. For example, some cells are depolarized when irradiated with red light, and the reaction polarity is changed to hyperpolarized when irradiated with green light. Other cells respond in opposite patterns. Similarly, there are cells that respond to green-blue colors. Although the other nerve cells of the retina respond to different types (either hierarchical potentials or nerve impulses), they respond to the color signals in a trance-like manner. In ganglion cells, this form of reaction is more complete, and many of them are also in spatial response. For example, there is a so-called double-type cell, which responds to light when red light illuminates the central area of its receptive field and withdraws light when irradiated around its receptive field; the reaction pattern to green light is the opposite. This type of -type coding ensures that different photoreceptor signals will not be confused during the transmission process. This way is exactly the assumption of another theory of color vision-ochre theory. Therefore, with the deepening of the understanding of objective laws, the three-color theory and the black-color theory have been dialectically unified at a new level.

The cell body of the reticulum cells is arranged at the same level as the amacrine cells, and its protrusions are widely extended in the two synaptic layers. They receive signals from amacrine cells and feed them back to horizontal cells. This centrifugal feedback pathway is combined with the main pathway of information transmission from photoreceptors bipolar cells ganglion cells to the heart, making the retina a complete Neural Networks.

Photoreceptive transduction of the retina

As mentioned above, there are photoreceptor cell layers in the retina. Human and most vertebrate photoreceptor cells include rod cells and cone cells. Photoreceptor cells can synapse with bipolar cells through terminal feet. Bipolar cells are then connected to ganglion cells. The ganglion-like processes converge into bundles on the surface of the retina, then pass through the choroid and sclera to form the optic nerve. The eye passes through the optic canal into the cranial cavity and is connected to the mesencephalon through the optic cross.

At present, it is believed that the falling of the object image on the retina first causes a photochemical reaction, and a photosensitive substance has been extracted from the retina. These substances are purple-red in the dark, and quickly fade to white when exposed to light. If you place a frog or rabbit in a dark room, orient the animal towards a bright window for a certain period of time, then take out the eyeballs immediately after shading, remove the retina, and treat the retina with a suitable chemical such as alum, you can find the image of the window on the retina of the animal. The light transmitting part of the window is white, and the window frame part is dark red. All these indicate the photochemical reaction of the light-sensitive substances on the retina under the action of light. In a large number of studies of photoreceptor cells, the study of rod cells has been relatively clear. The light-sensitive substance of rod cells is called rhodopsin, which is made by combining opsin and retinal. Retinal is converted from Vitamin A. Rhodopsin quickly decomposes into opsin and retinal when illuminated. At the same time, rod receptor cells can be seen to develop sensory potentials, which then cause other retinal cells to move.

Rhodopsin decomposes in the light and can be re-synthesized in the dark. When people look at things in the dark, they actually have both the decomposition of rhodopsin and its synthesis. The darker the light, the more the synthesis process exceeds the decomposition process, which is the basis for people to constantly see matter in the dark. On the contrary, under the action of strong light, rhodopsin decomposition is enhanced and synthesis is reduced, and rhodopsin in the retina is greatly reduced, so the sensitivity to low light is reduced. Therefore, rod cells are sensitive to low light, which is related to dusk dark vision. During the decomposition and resynthesis of rhodopsin, a part of retinal will be consumed, which is mainly supplemented by vitamin A in the blood. Such as vitamin A deficiency, will affect people's vision in the dark is called night blindness.

Cones also contain special light-sensitive pigments. It is called rhodopsin. According to a variety of studies on the photoreceptor pigments of cone cells in animals, they are also considered to be a combination of retinal and opsin.

There are three types of cone cells or corresponding light-sensitive pigments that are particularly sensitive to red, green, and blue light, respectively. Due to the proper mixing of the three colors of red, green and blue, it can cause the feeling of any color in the spectrum. Therefore, cone cells are considered to be related to color vision. Color blindness may be caused by the lack of corresponding cone cells. The difference in the sensitivity of the three cone cells is related to the difference in the light-sensitive substances. The three photosensitive pigments are composed of retinal and opsin. Among them, retinal is basically the same, but there are slight differences in the optoproteins of the three. This difference may be the reason for their different sensitivity characteristics.