Come To Your Senses

COME TO YOUR SENSES

Sensation and Perception

Sensation is what our senses do to make us aware of a stimulus, such as a tree or someone calling our name or cupcakes baking in the oven. Perception is what our brain does with the information that allows us to form a concept of the stimulus: the sight of the tree, the sound of a friend’s voice calling or name, or the sweet smell of the baking cupcakes. However, the sights, sounds, smells, tastes, and touches we experience are not a property of the stimulus itself, but a reaction of neurons in your brain to these particular stimuli.

Sensation and Perception

Each of the receptors for the various senses in our brain (for vision, hearing, taste, smell, and touch) is specialized to use one kind of energy and convert that energy into an electrochemical pattern in our brain. The activity of your brain in response to the stimuli does not duplicate the object that your senses respond to, but the representation of that object that the brain has built because of our experiences using our various senses in our world. A certain pattern of activity in our neurons allows us to experience “green tree” or “familiar voice” or “baking cupcakes.”

Vision

Light comes into our eye through an opening in the center of the iris called the pupil. The lens of the eye is adjustable and focuses the light and the cornea (which is not adjustable) also helps focus the light and it is then projected onto the retina at the rear surface of the eye. The rear surface of the eye is lined with visual receptors. The highest concentration of receptors specialized for particularly detailed vision is known as the fovea. The receptors send action potentials to the brain along the optic nerve. The point at which the optic nerve leaves the eye has no receptor cells and is called the blind spot. We cannot see anything with the part of the eye where the blind spot is. The message about vision is carried to the primary visual cortex in the occipital lobe of the brain.

The human eye with major parts labeled

Vision

The receptors that are located in the back of the eye in the retina include the bipolar cells which send their messages to ganglion cells, located closer to the center of the eye than the bipolar cells. The ganglion cells bind together and form the optic nerve. Another set of receptors are amacrine cells, and these send their messages to bipolar cells, other amacrine cells and to ganglion cells. Horizontal cells are also receptor cells, but they are cells that inhibit rather than excite.

Vision

In addition to the bipolar, ganglion, amacrine, and horizontal cells, the retina also is made up receptor cells called rods and cones. There are more rods at the edges of the retina, and they respond to faint light and seeing in the dark. There are more rods than cones in the brain, but they are smaller. Cones are clustered in and around the fovea and are more useful in bright light, and they are critical for color vision. Cones have a more direct route to the brain than rods do.

Both rods and cones contain a chemical that releases energy when hit by light called photopigments. Photopigments have a sensitivity to different wavelengths of light. The different lengths of light waves are what allow us to perceive distinct colors.

Rod (left) and cone (right)

For humans, the shortest light waves are seen as violet and light waves that get longer and longer are seen as blue, then green, then yellow, then orange, and the longest are seen as red.

Color Vision

Color Vision

There are three major theories about how it is that we see color and also shades of color because a neuron in the visual system can only vary the frequency of action potentials. A single action potential can’t respond to both brightness and color, for example. And we don’t have separate receptors for different colors.

Color Vision

The first theory is called the trichomatic theory of color vision or the Young-Helmholtz theory (after the people who developed the theory). This theory suggests that we have three kinds of cones: cones that respond best to the short wavelength blue part of the visual color spectrum, cones that respond best to the medium wavelength green part of the visual color spectrum, and cones that respond best to the long wavelength red part of the visual color spectrum. We see different shades of color depending on how active cones sensitive to each particular color are because they are firing at the same time.

Color Vision

The second theory is called the opponent-process theory. This theory is based on the fact that if you stare at an image of one of the colors a specific cone is sensitive to for a minute or so and then look at a white surface or white piece of paper, the color should be replaced by its opposite. For example, in the picture below, if you stare at the red square for a time and look away, you should see a blue afterimage. This theory proposes that we see colors based on paired opposites: red versus green, yellow versus blue, and white versus black. This theory proposes that shades of color occur both because of the active firing of some cones but also a decrease in firing of cones sensitive to a different color.

Paired Opposites

Color Vision

The third theory is called the retinex theory. This theory explains the idea of color constancy, that is the ability to recognize the color of an object even when light changes. For example, if someone wearing a bright shirt walks into a tunnel where it is pretty dark, the shirt will look more gray than yellow, but we know the color of the shirt has not actually changed. Also, we see how bright an object is by comparing it with other objects. This theory suggests that the visual cortex compares information from many parts of the retina to decide the brightness and color of objects. This theory suggests that vision requires thinking about what we are seeing and having some experience with vision to be able to make judgments about objects, including what the object is, what color it is, and how bright it is.

Color Blindness

Some people don’t see all of the colors in the light spectrum because they have a condition known as color vision deficiency or color blindness. Color vision deficiency is caused by differences in our genes. Some people are missing one or two kinds of cones, and some have all three kinds of cones, but one kind of cone has some abnormalities.

The most common type of color blindness occurs because a person has the same kind of photopigments in their long wavelength and medium wavelength cones instead of different ones. This causes them to have trouble telling the difference between red and green.

Vision and the Brain

The optic nerve leaves the eye and forms the optic tract to the brain. At a place called the optic chiasm, the optic nerve sends part of its signal to the same side of the brain and part of its signal to the opposite side of the brain. Before the signal from the optic nerve gets to the brain, the thalamus and some other brain areas below the cerebral cortex help route the information to the primary visual cortex in the occipital lobe of the brain.


The primary visual cortex manages the first stage of visual processing and is responsible for most of the visual information of which we are consciously aware. Some other areas of the cortex process visual information about shape, other areas about movement, and still other areas about brightness and color. Areas in the temporal cortex of the brain process information about what something is and where something is that we are looking at. All of these areas of the brain work together so that we see a complete object when we are looking at it, even if different information about the object is processed in different places in our brains.

Optic chiasm and route of visual information to the brain


Hearing

When we hear, we are processing sound waves that are made up of compressions of air or water. We hear vibrations in the air as they strike a part of the ear called the eardrum and make it vibrate. These vibrations are sent through other parts of the ear and finally sent as action potentials to the brain.

Sound waves have both amplitude and frequency. Amplitude is a sound’s intensity, and loudness is the perception of that intensity. Frequency of a sound is the number of compressions per second. Pitch is closely related to frequency.

Hearing – Outer Ear

The part of the hearing system that we see on the outside of the head is called the pinna (the ear). It is designed to capture sound. When a sound reaches the ear, it passes through the tube called the external auditory canal until it reaches the tympanic membrane or eardrum.

Hearing – Middle Ear

The eardrum vibrates at the same frequency as the sound waves that hit it. Attached to the eardrum are three very small bones (the smallest bones in the body!) that also vibrate to the frequency of the sound. These bones are known as the hammer, anvil, and stirrup because of their shapes. Together, they are known as the ossicles. The three bones are attached to the oval window. The oval window is the beginning of the inner ear.

Hearing – Inner Ear

The inner ear has the cochlea, a snail-shaped fluid-filled structure. Vibrations from sound in the fluid in the cochlea displace hair cells that are the neuron receptors for sound at the bottom of the cochlea in the basilar membrane. The tectorial membrane covers the hair cells and protects them. The hair cells send signals to the auditory nerve, which sends a signal about sound to the temporal lobe of the brain.

Visualization of the Ear

Theories about Hearing

There are three theories about hearing. The first theory is known as the frequency

theory. This theory says that the basilar membrane that holds the hair cells vibrates at the same frequency as sound. This causes the auditory nerve axons to produce action potentials at the same frequency. However, the maximum firing rate of a neuron is short of the highest frequencies we can hear.


The second theory is known as the place theory. This theory suggests that the basilar membrane is similar to the strings of a piano and that each area along the membrane is tuned to a specific frequency and vibrates to that frequency. The nervous system would have to decide among the frequencies based on which neurons are active. However, the problem with this theory is that some parts of the basilar membrane are bound together too tightly for any part to vibrate like a piano string.


The final theory is known as the volley principle. This theory suggests we use methods that combine aspects of the frequency theory and the place theory. The basilar membrane is stiff at its base where the stirrup connects with the cochlea and floppy at the other end of the cochlea. Hair cells along the basilar membrane would act differently depending on their location. When we hear sounds at a very high frequency, we use something like the place theory. When we hear lower pitched sounds, we use something like the frequency theory. So combining parts of the first two theories explains how we hear better than using either one of the first two theories separately.

Hearing and the Brain

Information about hearing, just like information about vision, is first routed through the thalamus and other brain areas below the cortex before reaching the primary auditory cortex, located in the temporal lobe of the cerebral cortex. Different areas of the auditory cortex, just like is true in the visual cortex, process information in different ways, including about the location of a sound and the motion of sound. And just like vision, hearing requires a certain amount of experience with sounds for our hearing to be fully developed.

Areas of the brain associated with vision and hearing

Vestibular Sensation

When you move your head, the organs of the vestibular system adjust the movement and allow your eyes to adjust as well. For example, when your head moves left, your eyes move right and when your head moves right your eyes move left. This allows you to keep your eyes focused on what you want to see.


The vestibular organ looks like it is part of the structures used for hearing and is attached to the hearing structures. It is composed of three semicircular canals. Calcium carbonate particles called otoliths lie next to the hair cells (similar to the hair cells used in hearing) that are the receptor cells for vestibular sensations. When the head tilts in different directions, the otoliths push against sets of hair cells and cause them to be excited and produce action potentials.


The semicircular canals are on three different planes, and they are filled with a jellylike substance and lined with the hair cells. During head movement, the jellylike substance also pushes against the hair cells. This causes action potentials to be produced, and they travel to the brainstem and cerebellum. Two baglike structures, known as saccule and utricle, are also part of the vestibular system.