The Sixth Sense (And Seventh, Eighth, and Ninth)

Everyone knows about the five “main” senses. But what about those other types sensations that don’t neatly fit into those categories?

Vestibular sense: 

This is the sense involving body position and equilibrium. Movement of the fluid in the inner ear, due to head movement or gravitational pull, stimulates this sense. It can also be referred to as equilibrioception (22). 

Kinesthetic sense: 

This is the sense involving detection of the movement of individual body parts. It allows us to judge our position within a given space. The sense can also be referred to as proprioception (23). 

Sense of temperature: 

This is the ability of the human body to detect changes in external and internal temperatures by using, respectively, cutaneous (skin) thermoreceptors and homeostatic (related to maintaining equilibrium) thermoreceptors within the body. This sense is also known as thermoception (24).

Time perception: 

The sense that allows humans to estimate how much time has passed since a given moment. What allows us to sense time remains unknown. Some theorize that we have an internal stopwatch, so to speak, which allows us to measure time through a series of neural “pulses” (25).

Merely an Ear

Behold, the ear!

(source 21)

The Hills Are Alive With the Sounds of Physics (Or, rather, physics of sound)

Definitions: 

Frequency: The number of sound waves that pass within a certain time, typically a second (18).

Pitch: How high or low a sound is as qualified by its frequency (19).

Amplitude: The length between either consecutive crests or troughs of a wave, defining loudness in sound waves (20).

Which Ear is Here?

The ear, being a complex structure, is divided into multiple sections, including “inner” and “middle”. The middle ear contains the eardrum, the tympanic cavity, and three small bones called the hammer, the anvil, and the stirrup (16). The inner ear contains the cochlea, the semicircular ducts (which are attached to the cochlea), and the auditory tube (this drains excess fluid). It also contains the cochlear and vestibular nerves, which connect the inner ear to the brain (17).

Hear Ye, Hear Ye

How does this all come together to allow us to hear? How do sound waves become auditory signals in the brain?

The pinna (the outermost part of the ear) reflects sound waves into the ear canal, which then reach the ear drum and cause it to vibrate. The hammer, anvil, and stirrup- collectively known as the ossicles- transmit vibrations to the cochlea. The vibrations cause the fluid within the cochlea to move, which causes the cilia to move. The movement of these hairs create neural signals where it is transmitted to the brain via the auditory nerve. The brain then processes these impulses and converts it into recognizable sound (15).

At a Loss for Hearing Loss?

There are two types of hearing loss:

Conductive Hearing Loss- Any damage or obstruction that prevents sound waves from traveling from the outer ear to the inner ear. This can happen as a result of any damage to the outer ear, ear canal, or middle ear. Hearing loss caused by age or exposure to loud sounds is typically conductive hearing loss (14).

Sensorineural Hearing Loss- Any damage to the cilia within the cochlea or the auditory nerve. This nerve damage is often congenital or as a result of disease. It is possible that significant exposure to loud sounds or aging can cause sensorineural hearing loss, but it is less likely (14).

I Spy an Eye

Voila, the human eye:

(source 13)

Processing the Visual Process

Knowing what the eye looks like is all well and good, but how does it work? How do the light waves that enter your eyes turn into neural impulses and signals?

Light enters the eye via the cornea and is slightly bent as it passes through. It travels to the back of the eye by way of the pupil, which is simply a hole in the center of the eye. The amount of light that is allowed into the eye is controlled by the iris, which, though it is best recognized by its color, is actually a muscle. As the muscle expands or contracts, it causes the pupil to dilate or constrict. The light then passes through the lens, which bends and focuses the image onto the retina. The image is actually projected upside down and mirrored at this stage of the visual process. The main point upon which light is focused on the retina is called the fovea. This central area contains a high concentration of visual receptors called cones, which see color and fine detail. In the periphery are rods, which see dim light. The visual signals are then transferred from the retina to the optic nerve, which carries the information to the brain’s visual centers. The image is then fixed to be correctly oriented and sent to other areas of the brain that deal with perception (11, 12). 

All Hue Need To Know About Color

Definitions: 

Hue: A color or shade as defined by its dominant wavelength (8).

Intensity: Either the lightness of a light source or the brightness of the surface upon which light is reflected. A color’s intensity is determined by its amplitude (9).

Saturation: The purity of a color. A highly saturated color will be nearly pure, whereas an unsaturated color will be mixed with neighboring colors (10).

But can you SEE all the colors in the wind?

Photoreceptors are retinal cells used for sensing light that exist in two forms. One, the cone, is used for discerning color and fine details. The other, the rod, can be used to distinguish form and objects in low light (4).

The opponent-process theory is a concept that was developed by Ewald Hering, who is this dashing man right here:

(source 5)

The theory states that photoreceptors are grouped into opposing pairs (red and green, blue and yellow, white and black). The activation of one color in an opposing pair stops the activation of another, so therefore you cannot observe two opposing colors in the same place at the same time. This also explains the afterimage effect. When you look at one color for an extended period of time (without any eye movement) and then look away, you see its opposing color. This happens because the photoreceptor used for an extended period of time becomes tired, so its activity levels decrease. By comparison, the opposing color’s receptor has high activity, so the brain perceives it as you seeing the other color (6).

To test this theory for yourself, click below: