Mammalian vision is the process of mammals perceiving light, analyzing it and forming subjective sensations, on the basis of which the animal's idea of the spatial structure of the external world is formed. Responsible for this process in mammals is the visual sensory system, the foundations of which were formed at an early stage in the evolution of chordates. Its peripheral part is formed by the eyes, the intermediate (by the transmission of nerve impulses) - the optic nerves, and the central - the visual centers in the cerebral cortex.
The recognition of visual stimuli in mammals is the result of the joint work of the eyes and the brain. At the same time, a significant part of the visual information is processed already at the receptor level, which allows to significantly reduce the amount of such information received by the brain. Elimination of redundancy in the amount of information is inevitable: if the amount of information delivered to the receptors of the visual system is measured in millions of bits per second (in humans - about 1×107 bits/s), the capabilities of the nervous system to process it are limited to tens of bits per second.
The organs of vision in mammals are, as a rule, well developed, although in their life they are of less importance than for birds: usually mammals pay little attention to immovable objects, so even cautious animals such as a fox or a hare may come close to a human who stands still without movement. The size of the eyes in mammals is relatively small; in humans, eye weight is 1% of the mass of the head, while in a starling it reaches 15%. Nocturnal animals (for example, tarsiers) and animals that live in open landscapes have larger eyes. The vision of forest animals is not so sharp, and in burrowing underground species (moles, gophers, zokors), eyes are reduced to a greater extent, in some cases (marsupial moles, mole rats, blind mole), they are even covered by a skin membrane.
Like other vertebrates, the mammalian eye develops from the anterior brain vesicle and has a rounded shape (eyeball).
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A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons. These neurons typically communicate with one another by means of long fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.
The optic chiasm, or optic chiasma, is the part of the brain where the optic nerves cross. It is located at the bottom of the brain immediately inferior to the hypothalamus. The optic chiasm is found in all vertebrates, although in cyclostomes, it is located within the brain.
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.
Depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object. Depth sensation is the corresponding term for non-human animals, since although it is known that they can sense the distance of an object, it is not known whether they perceive it in the same subjective way that humans do.
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different wavelengths independently of light intensity. Color perception is a part of the larger visual system and is mediated by a complex process between neurons that begins with differential stimulation of different types of photoreceptors by light entering the eye. Those photoreceptors then emit outputs that are propagated through many layers of neurons and then ultimately to the brain. Color vision is found in many animals and is mediated by similar underlying mechanisms with common types of biological molecules and a complex history of evolution in different animal taxa. In primates, color vision may have evolved under selective pressure for a variety of visual tasks including the foraging for nutritious young leaves, ripe fruit, and flowers, as well as detecting predator camouflage and emotional states in other primates.
The visual system comprises the sensory organ and parts of the central nervous system which gives organisms the sense of sight as well as enabling the formation of several non-image photo response functions. It detects and interprets information from the optical spectrum perceptible to that species to "build a representation" of the surrounding environment. The visual system carries out a number of complex tasks, including the reception of light and the formation of monocular neural representations, colour vision, the neural mechanisms underlying stereopsis and assessment of distances to and between objects, the identification of particular object of interest, motion perception, the analysis and integration of visual information, pattern recognition, accurate motor coordination under visual guidance, and more. The neuropsychological side of visual information processing is known as visual perception, an abnormality of which is called visual impairment, and a complete absence of which is called blindness. Non-image forming visual functions, independent of visual perception, include the pupillary light reflex (PLR) and circadian photoentrainment.
The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, and balance. Senses are transducers from the physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.
A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.
In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision as cone cells are only able to function at higher illumination levels. Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds.
Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.
The superior colliculus is a structure lying on the roof of the mammalian midbrain. In non-mammalian vertebrates, the homologous structure is known as the optic tectum, or optic lobe. The adjective form tectal is commonly used for both structures.
The star-nosed mole is a small mole found in moist, low areas in the northern parts of North America. It is the only member of the tribe having a touch organ with more than 25,000 minute sensory receptors, known as Eimer's organs, with which this hamster-sized mole feels its way around. With the help of its Eimer's organs, it may be perfectly poised to detect seismic wave vibrations.
In medicine and anatomy, the special senses are the senses that have specialized organs devoted to them:
Many researchers have found the evolution of the eye attractive to study because the eye distinctively exemplifies an analogous organ found in many animal forms. Simple light detection is found in bacteria, single-celled organisms, plants and animals. Complex, image-forming eyes have evolved independently several times.
Cat senses are adaptations that allow cats to be highly efficient predators. Cats are good at detecting movement in low light, have an acute sense of hearing and smell, and their sense of touch is enhanced by long whiskers that protrude from their heads and bodies. These senses evolved to allow cats to hunt effectively at dawn and dusk.
Vision is the most important sense for birds, since good eyesight is essential for safe flight. Birds have a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". Birds are theropod dinosaurs, and the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.
Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.
Visual perception in animals plays an important role in the animal kingdom, most importantly for the identification of food sources and avoidance of predators. For this reason, blindness in animals is a unique topic of study.
A sense is a biological system used by an organism for sensation, the process of gathering information about the world and responding to stimuli. Although traditionally around five human senses were known, it is now recognized that there are many more. Senses used by non-human organisms are even greater in variety and number. During sensation, sense organs collect various stimuli for transduction, meaning transformation into a form that can be understood by the brain. Sensation and perception are fundamental to nearly every aspect of an organism's cognition, behavior and thought.
Odor molecules are detected by the olfactory receptors in the olfactory epithelium of the nasal cavity. Each receptor type is expressed within a subset of neurons, from which they directly connect to the olfactory bulb in the brain. Olfaction is essential for survival in most vertebrates; however, the degree to which an animal depends on smell is highly varied. Great variation exists in the number of OR genes among vertebrate species, as shown through bioinformatic analyses. This diversity exists by virtue of the wide-ranging environments that they inhabit. For instance, dolphins that are secondarily adapted to an aquatic niche possess a considerably smaller subset of genes than most mammals. OR gene repertoires have also evolved in relation to other senses, as higher primates with well-developed vision systems tend to have a smaller number of OR genes. As such, investigating the evolutionary changes of OR genes can provide useful information on how genomes respond to environmental changes. Differences in smell sensitivity are also dependent on the anatomy of the olfactory apparatus, such as the size of the olfactory bulb and epithelium.
