General Principles of the Sensory Systems

General Principles of the Sensory Systems

General Principles of the Sensory Systems and Perception by Ken Koenigshofer, Ph.D. Copyright 2004 Imagine that your brain was isolated from the external world. Could you experience the world? The answer is “No.” Could you direct your behavior successfully (adaptively) in the world if your brain was isolated from contact with the external world? Again the answer is “No.” The brain, without sensory systems, is in fact isolated from the world. After all, the brain is inside your skull, hidden away from the external world. So, there must be systems that can get information about the external world into your head. We will consider several major ideas in this lecture. What I want to do is to give you several principles that apply in general to all of our sensory systems, and to the sensory systems of most animals as well (and perhaps life forms elsewhere in the universe if they exist). If you can understand these general principles it will be easier to learn the specific facts about each sensory system. In addition, your understanding of these general principles will also allow you to gain insight into some very interesting issues, some of which border on the philosophical. Have you heard the question posed, perhaps in a philosophy class, “If a tree falls in a forest and there is no one there to hear it, was there a sound?” You will be able to answer this and to explain the rationale for your unexpected answer to others who probably won’t agree with you (you’ll be able to convince them!). Well, let’s get started. Sensory systems are the input systems to the brain. However, interestingly, the brain itself is completely insensitive to the external world in its raw forms. The brain uses neural code. It deals in neuron potentials. It cannot deal with the world in its raw forms. Energies in the external world must be converted into neuron potentials. Here’s what I mean. Imagine that you are a neurosurgeon. Like others of your profession, when you do brain surgery, one of the first steps is to open up the skull of your patient under a local anesthetic, which deadens the scalp and the skull, but leaves your patient conscious and alert. The reason this is possible is because there are no pain receptors in the brain itself, but only in the surrounding scalp, skull, and meninges (a three-layered membrane covering the brain and attached to the skull). Now imagine, that with the skull opened up and the brain exposed, you direct a beam of light from a flashlight in the darkened surgery room at the visual area of the brain, at the rear of it’s exposed surface (this is the primary visual cortex). To make this example even more clear, imagine that your patient is blindfolded. Would your patient “see” the light beam, which is now striking and flooding with illumination the visual cells of his or her brain? I think you can see that, obviously, the patient does not see the beam of light, even though the beam of light is flooding with illumination the brain’s visual cells (located in the Occipital lobe at the very back of the head). Why does the patient experience no visual sensation? After all these cells in the primary visual cortex are the cells in the brain upon which visual experience depends. Why doesn’t stimulation of these cells with light cause your patient to see? The answer is that the brain itself, including even the visual parts of the brain, is insensitive to the world and its energies, in their raw forms. We can illustrate this same principle using other senses. Imagine you plug the nose of your patient and then place a rose or some dirty socks right beside the exposed olfactory cortex (the cortex for smell). Will your patient smell the rose or dirty socks? Again, I think you can see the answer is “no.” Suppose you pour chocolate syrup over your patient’s taste cortex, will the patient taste the chocolate? Again the answer is “No.” Suppose you place a thin slice of lemon on the surface of the taste cortex. Will your patient taste the lemon? Again, no. Why not? Again, the answer is that the brain itself is insensitive to the world and its energies, in their raw forms. What’s necessary then is the conversion of the energies from stimuli in the external world into a form that the brain can deal with. From the lecture and chapter on the brain and nerve cells, recall that the brain and its neurons use electrochemical signals, neuron potentials, to code and process information. Therefore, the first major step in any sensory system is the conversion of environmental energy into neuron potentials. This process is called transduction. The cells that perform transduction of environmental energy into neuron potentials are specialized neurons called sensory receptors. There are different kinds of energy in the external world. Light is a form of electromagnetic energy. Sound and touch depend upon forms of mechanical energy, and taste and smell depend upon chemical energies. Sensory receptors are specialized to convert or transduce only one type of energy. Therefore, each of the sensory systems must have its own specialized sensory receptors. The job of the sensory receptors in each of the sensory systems is to transduce or convert some specific form of environmental energy into the brain’s code, neuron potentials. In the visual system, the visual receptors, located at the back inner surface of the eyeball, the retina, are of two major types– rods and cones. The auditory receptors, called “hair cells” and located in a structure called the cochlea in the inner ear, transduce mechanical energy in the form of vibrations in the air into neuron potentials. The touch receptors are located throughout the skin’s surface all over the body and convert mechanical energy from pressure on the skin into neuron potentials. Temperature receptors in the skin convert heat or cold into neuron potentials. And pain receptors transduce any very intense and potentially injurious stimulus into neuron potentials. For the taste system, you can probably guess where the sensory receptors for this sensory system are located. It is the taste buds. The taste buds transduce chemical energies in chemicals dissolved in saliva into neuron potentials. The sensory organs, such as eyes and ears, are really accessory organs severing the sensory receptors. These organs contain the specialized nerve cells, the sensory receptors, whose function it is to convert the raw energies in the external world into a form the brain can handle. The eye for example just focuses light upon the visual receptors, the rods and cones in the retina at the back of the eyeball, wherein the actual transduction, the crucial step, occurs. The external ear just gathers vibrations in the air (“sound waves”). The structures of the inner ear magnify or amplify the mechanical energy in the vibrations in the air before it reaches the hair cells for transduction. To summarize, we have identified three general principles in the organization of sensory systems in animals and humans. The brain is completely insensitive to the external world in its raw forms. The brain can deal with information only if it is in the form of brain code, neuron potentials. Because of this, the first step in any sensory system is transduction, the conversion of some specific form of environmental energy into neuron potentials. Each of the sensory systems has its own sensory receptors “designed” to transduce one specific type of environmental energy (mechanical, chemical, or electromagnetic–light for example) into neuron potentials. But these first three principles only take us to the point where transduction has occurred in sensory receptors in sensory organs. This alone is insufficient for us to have sensory experience of the world. Could eyes not connected to the brain see anything? Or ears not connected to the brain hear anything? No. Seeing, hearing, touch and other skin sensations, tastes and smells occur in the brain. Somehow, after transduction, the resulting neuron potentials go to the brain and cause us to have psychological experiences, internal mental states, conscious sensations, which represent within our minds the external world, in the form of sights, sounds, smells, and skin sensations. How is this done? How do neuron potentials in our brains become mental experiences representing the external world to us? Several additional general principles of sensory systems are involved. After transduction of some specific form of environmental energy into neuron potentials, the resulting neuron potentials go from the specific sensory organ (i.e. eye or ear, for example) along specific sensory nerves (for example, the optic nerve in the case of vision, the auditory nerve in the case of hearing) to the brain, specifically to the Thalamus. In mammals such as us, each of the senses, except the sense of smell, has its own area of Thalamus, which receives neuron potentials from the respective sensory organ. For example, in the visual system, the part of the Thalamus that receives neuron potentials from the optic nerve is called the LGN, lateral geniculate nucleus. LGN is all you need to know. For the auditory system, it is the MGN, medial geniculate nucleus, that receives neuron potentials along the auditory nerve from the inner ear. As stated above, each of the senses, except the sense of smell, has its own specific area of thalamus. (Smell, the olfactory sense, has an anatomical organization different from the other senses and there is no area in the thalamus for the sense of smell.) Information, coded in the form of neuron potentials, from the various senses (except smell) is processed in these sensory-specific regions of the thalamus. After this processing in the thalamus, new neuron potentials generated there are sent on to the cerebral cortex for additional processing. In mammals, each area of thalamus (i.e. LGN, MGN, etc.) “projects”, sends neural impulses (action potentials; see lecture and other materials on neuron potentials) to a specific area of cerebral cortex for more processing. The specific area of cortex that receives neural impulses from a specific region of the thalamus is called the primary sensory cortex for that sense. Each of the senses has its own primary sensory cortex. For example, the area of cerebral cortex that receives projections (nerve pathways carrying action potentials) from the LGN is the primary visual cortex (in the occipital lobe). The area of cortex that receives projections from the MGN of the thalamus is the primary auditory cortex (located in the temporal lobe). Interestingly, there is an orderly mapping of the sensory surface of each sense onto the surface of the respective primary sensory cortex. For example, the retina of the eye is mapped in an orderly way onto the surface of the primary visual cortex. For each point on the retina (the light sensitive surface at the back inner surface of each eyeball, containing the visual receptors–rods and cones), there is a corresponding point on the primary visual cortex in the Occipital lobe. Adjacent points on the retina have adjacent points on the surface of the primary visual cortex. This point-for-point mapping or representation of the retina (the visual receptive surface) onto the primary visual cortex is called a “retinotopic mapping.” There is similar topographical mapping of the other sensory surfaces onto their respective primary sensory cortices. For example, the skin surface of the body is laid out, point-for-point, on the surface of the primary somatosensory cortex (located in the post-central gyrus of the Parietal lobe). However, this “somatotopic” mapping is upside down, but nevertheless orderly. In the auditory system, the auditory receptors (“hair cells”) are distributed over a membrane, called the Basilar membrane, located inside the cochlea in the inner ear. The orderly distribution of these hair cells along the Basilar membrane is mapped in an orderly way onto the surface of primary auditory cortex in the Temporal lobe. These mappings of the sensory surfaces onto their respective primary sensory cortex probably is important in the coding of various features of sensory stimuli such as the location of objects and their parts in visual space, the locations of stimuli on the skin, and the frequencies of “sound” waves. After information processing in primary sensory cortex, additional information processing occurs in additional areas of cerebral cortex. These areas, in turn, are called secondary sensory cortex, third level (or tertiary) sensory cortex, fourth level sensory cortex, etc. For example, in the visual system, the primary visual cortex (also known as striate cortex or V1) is located in the central area of the Occipital lobe at the back of your head. Surrounding this area of cortex is secondary visual cortex (V2). In addition, there are visual areas 3, 4, 5 and 6 (V5, for example, processes information that allows you to see motion; in people with damage here, they can’t see motion, but only a series of still views in successively different positions). In fact, it is estimated that in us, and in other primates, there may be over thirty different areas of cortex involved in the later stages of processing of visual information. One of these is the Inferotemporal (IT) Cortex, involved in our ability to recognize objects by sight alone. Damage there allows us to still see, but we can’t recognize what it is we are seeing (this disorder is called visual agnosia). After these steps in information processing, somehow the resulting patterns of electrical activity occurring in large populations of neurons (which make up complex circuits in the brain) produce mental experiences of the external world. (Mental experiences may be so-called “emergent properties” of the structure and functioning of extremely complex circuits in the brain–entirely material in structure and function). These conscious, psychological experiences we have from the operation of our sensory systems are called “sensory qualia.” For example, luminosity of light or colors of objects, both produced by neural activity in visual areas of the brain, are examples of visual qualia. Sounds, such as the sound of a cricket chirping, are auditory qualia. Tastes such as the taste of sugar or the taste of a lemon are taste qualia. There are also somatosensory qualia (skin sensations) and olfactory qualia (smells). Notice that all of the sensory qualia are produced when patterns of neural impulses reach and activate the neurons in a particular sensory cortex. Neural impulses are action potentials and they are the same everywhere. The thing that determines the nature of the sensory qualia, the type of sensory experience that one has from a particular sensory input, is where in the brain (which sensory cortex) the neural impulses, from the sensory organs, end up. So, for example, if we could somehow surgically redirect the optic nerves and connect them to taste cortex, then sensations of taste, taste qualia, would result when light was transduced by rods and cones in the eyes. In other words, under these conditions, you would taste light, not see it. If our nervous systems were in fact actually wired this way, you would grow up thinking that light was tasty (just like you think light is luminous and colored) and that different wavelengths of light had different tastes. And you would be right to say that light tasted, as right as you are when you say, now, that light is luminous and colored. Which is to say, you would be right, not at all. Light is neither tasty, nor luminous and colored. These different properties which we would ascribe to light are really properties of the activity of the neurons that get activated in the presence of light. These 8 general principles apply to all of our sensory systems and to the sensory systems of all the mammals. Furthermore, the first four also apply to the sensory systems of all animals in general. However, some non-mammal species don’t have a thalamus, and no species, except mammal species, have cerebral cortex. In species without cortex or thalamus, other brain structures characteristic of those species carry out additional processing of sensory information. Nevertheless, transduction of environmental energies by sensory receptors into neuron potentials which are then processed by additional neural structures in the brain of the species is universal in all animals, even invertebrates such as jellyfish and insects. Forms of life which we might discover elsewhere in the universe someday (alien life forms) may be expected to follow a similar organization. An extremely interesting and important thing to understand is that: Sensory qualia are entirely in your head (more accurately, in your brain). Although we grow up thinking that light is luminous and colored, in fact it is not. Light in fact is no more luminous or colored than X-rays or radio waves (both of which, like light, are forms of electromagnetic energy). Luminosity (the glowing quality that we attribute to light) and color are really properties of the brain’s response to light, not properties of light itself. Light, in the external world, is really just as dark as other forms of electromagnetic energy (the forms of electromagnetic energy are gamma rays, X-rays, ultraviolet, visible light, infrared, T.V. , radio, in order of increasing wavelength). The luminosity we associate with light is not in the light, but results from brain activity in the visual system of our brains. Luminosity and color are not in the world at all, but are creations of our brains’ visual cortical neurons. Here is a simple demonstration of this surprising fact that you can do at home (but be careful not to hurt yourself). Here’s what to do. At night, go into your room, shut off the lights, then go into your closet, close the door behind you, making sure the closet light (if any) is off. Now, when you are sure that there is absolutely no light at all reaching your eyes, whack yourself hard on the back of your head. What happens? Well, you should “see stars.” These “stars” have a technical name; they are called “phosphemes,” sensations of luminosity and color. But look what has happened. You are experiencing visual qualia (luminosity and colors) in the complete absence of light. These visual qualia are produced by the whack to the back of your head, which activates neurons in the primary visual cortex. It is the activation of those cells that produces luminosity and color, even in the total absence of light. Somehow activation of those visual cortical neurons by any means will produce in the mind the visual qualia, luminosity and color. With this simple demonstration, you have verified an astounding fact–luminosity and color are not properties of light, but are properties of the activation of visual cortical neurons. Under normal conditions, light (remember, just a form of electromagnetic energy) from the external world strikes the eyes and is transduced by rods and cones into neuron potentials. Those neuron potentials are transmitted along the optic nerve to the LGN of the thalamus, and from there on to the visual cortex. The activation of neurons in the visual cortex by neuron potentials from the LGN causes the visual qualia, luminosity and color. Luminosity and color don’t exist in the external world at all, and are not really properties of light at all, but instead are properties of the activation of visual cortical neurons in the brain. Thus, luminosity and color exist only inside brains, not in the external world, not in light itself. I know this may be hard to accept, but there is other more scientific evidence, primarily from studies on the effects of brain damage and brain stimulation in conscious patients during brain surgery. Electrical stimulation of the cortex in conscious human patients during brain surgery causes different sensory qualia, different sensory experiences, depending upon the area of sensory cortex stimulated. Electrical stimulation of the visual cortex by the neurosurgeon causes the patient to report “seeing” visual qualia such as flashes of luminosity and color. Electrical stimulation of the auditory cortex produces auditory qualia, sounds. Electrical stimulation of the taste cortex produces mental experiences of taste, taste qualia. Electrical stimulation of the olfactory cortex produces olfactory sensations or qualia, smells. All of these realistic sensory qualia can be produced in the total absence of any of the corresponding external stimuli. That is, in the absence of any light, or sound sources, or smelly or tasty objects in the external world, realistic sensory experiences can be produced by stimulation of sensory cortex. So, if the sensory qualia can be produced in someone’s mind just by stimulating neurons in one sensory cortex or another, even in the absence of any sensory stimuli in the external world, then those qualia must be properties of brain activity, not properties of the external world. Similar conclusions can be drawn from observing the effects of injury to sensory cortex. For example, damage to the primary visual cortex causes blindness (called “cortical blindness”). Even though the eyes are still working normally, and there may be plenty of light to illuminate objects in the person’s field of view, someone with total destruction of primary visual cortex is completely blind–there is only darkness for the person with total destruction of primary visual cortex; there are no longer any visual qualia at all. Total destruction of the primary sensory cortex of other senses produces similar loss of particular sensory qualia. With regard to color sensations similar arguments can be made. The experience of different colors is really a brain code for different wavelengths of light. (Light travels in waves through space; the distance between adjacent wave peaks is the wavelength of the light; the wavelength of light reflected from an object depends upon the chemical composition and other physical properties of the material out of which an object is composed). Within our eyes we have rods and cones (the visual receptors). There are three different types of cones (but just one type of rod). Each type of cone is maximally sensitive (maximally able to transduce) light waves within its own particular range of wavelengths. “Color vision” begins when a particular wavelength of reflected light gets transduced by a particular set of cones. These, in turn, send a particular pattern of neural impulses to specific neurons in the visual cortex (via the LGN of the thalamus), which when stimulated produce the mental experience of a particular color. None of this occurs in the brain of a dog or a cow or many other species which lack cones in their eyes (they have rods only). Is our perception of reality more complete or more accurate than that of the dog which lacks color vision? In one sense, the answer is yes–we have the capacity to code the wavelength of reflected light in the external world, the dog does not. But, nevertheless, the “color” we experience in our minds only exists there, not in the external world. So, in a way, we are somewhat misled by our color perception. There are different wavelengths of light, and it is adaptive for us to possess a mental code for these different wavelengths. (It makes it very easy to see ripened fruit against the leafy background of a tree, a real advantage to our tree-living primate ancestors who also had good “color” vision). But, the color in our minds is an illusion. The light in the external world is not really colored, in fact, it is not even luminous (it is as dark and non-luminous as X-rays or radio waves). The luminosity of light, as we experience it, is dependent upon the fact that light (a particular range of wavelengths within the electromagnetic spectrum) gets transduced by rods and cones leading to activation of visual cortical neurons in our heads. If rods and cones were constructed differently, so that they transduced a different range of wavelengths within the electromagnetic spectrum, say, for example, that range known as radio waves, then we would experience radio waves as being luminous, and different wavelengths of radio waves as different colors. Regarding color specifically, here is something you can demonstrate to yourself that will show that the visual qualia, color, exists only inside brains. Imagine being outside around sunset. You will notice that objects in the environment still appear to have color. Trees still appear green. Their trunks still appear some shade of brown. You may see a red car parked nearby, and someone walks past you in blue jeans. But as the sun sets, and the daylight fades more and more, there comes a point at which the trees no longer are green, other objects “lose” their colors, and all becomes blacks and shades of gray, if there are no artificial sources of light, as would be the case in the desert or mountains away from city lights. It is about 9 p.m. as I write this. A moment ago, I stepped outside. I saw a large tree. The shape of the tree including some of its leaves was clearly visible, but the tree was completely black and gray. There was no color at all. Now, think about this. Where did the color go? The answer is, it really wasn’t there to begin with. What has happened, as the intensity of light drops off, is that color-generating systems in the brain and nervous system shut down. When the color-generating systems in the brain and nervous system shut down, the color disappears because the color was only in your head (your brain) in the first place, never really in the external world at all. It is known that the visual receptors that are involved in color perception, the cones, can only transduce the higher levels of light typical of daylight. At night, there is insufficient light to cause transduction in the cones, but still enough for transduction by the rods. Cones activate parts of the visual system, at the level of the LGN and visual cortex, which generate color qualia, sensations of color. Without the activation of these brain systems, there is no color. You experience this absence of color firsthand at night (if there are no artificial light sources to raise levels of light to the threshold necessary to activate the cones and their associated color generating circuits in the brain). Color, like luminosity, or sound, or tastes and smells or sensations on the skin, exist only inside living, functioning brains. During brain surgery, patients who have their primary somatosensory cortex stimulated report feeling things at various places on their skin. In one case which I witnessed (via a film of the surgery), the patient said with astonishment, when a point on his somatosensory cortex was stimulated, “I feel something on my teeth.” When a nearby point on the same cortex was stimulated a moment later, he said with equal astonishment, “Now, it’s on my tongue. I feel something on my tongue!” There was nothing on his teeth or on his tongue. The realistic feeling of something on the teeth or tongue was due to electrical stimulation of different, but nearby, regions of primary somatosensory cortex. The lesson of these examples is that the world that we know and experience is really in our heads (more accurately, in our brain function). The color, the luminosity, the tastes, the smells and other sensory qualia are not properties of the external world or the things in it, but properties produced by the activity of nerve cells organized into complex circuits in the brain. By these arguments, I’m not denying that there is an external world out there, outside our heads, I’m just saying that it doesn’t really look, sound, smell, feel, or taste the way we think it does. In the objective world outside our heads, none of these subjective properties actually exist. So what does the world outside our heads really look like? It doesn’t really look like anything, independent of the properties of the “looker”, the brain and nervous system that is doing the looking. What the world or anything else looks like is dependent as much upon the properties of the nervous system doing the looking as it is upon properties of the things being looked at. The world looks one way to a bee, another way to a snake, another way to a dog, another way to a bat, another way still to us. In each case, the nervous system creates a representation, a model, of the external world, the function of which is guide adaptive behavior of the organism. Natural selection “designed” nervous systems for that adaptive “purpose”, not to give the organism an objective model of absolute Reality with a capital R. Reality is species-specific. Each species’ nervous system represents the aspects of the world important for the organization of successful behavioral adaptation for that species, within the environmental niche occupied by that species. However, in spite of species differences in sensory systems and resulting differences in the representation of reality for each species, there are also likely to be aspects of the external world that are represented in similar ways across a broad range of species. These aspects of reality would be those which are more or less universal to the environmental niches of a broad range of species. For example, representation by the brain of gravitational forces and their direction is something that is probably universally found in a very broad range of species of animal life on earth. This is pretty abstract and somewhat speculative stuff. Let’s make it a bit more concrete. Let’s consider the classic philosophical question: “If a tree falls in the forest, and there is no one [no brain] to hear it, is there a sound?” Go to the main topic in this conference with the appropriate title to respond to this question. YOUR TASK: Here, just tell me and your classmates that you have read this lecture, and then briefly describe two ideas or facts that you found most interesting or important and in addition, post any questions or comments about the lecture that you may have.