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Receptors and Stimulation

The first step in sensing the world is translating energy from the environment into information that can be used by the brain. This process of converting energy from one form to another is called transduction.

Every one of our senses starts with transduction; that is how energy of the world gives rise to nerve impulses that encode information from the external environment. The first link in the chain, for every sensory system, is a specialized type of cell called a sensory receptor.

A sensory receptor is a living cell that responds to a particular type of energy by generating electrical potentials that influence neurons. Thus receptors are transducers. They translate external energy to internal electrical impulses or gradients.

Receptors are often housed in a sense organ, a specialized structure con­structed to collect one type of energy particularly well. The eye and the ear are examples of sense organs.

The skin may also be considered a sense organ (for the sense of touch). Other senses such as the sense of balance or equilibrium have sense organs that are hidden inside the body.

Four types of receptors are known. Photoreceptors are those sensitive to light. Chemoreceptors respond to particular chemicals or molecular shapes of particles.

Mechanoreceptors respond to physical touch or movement. Finally, magneto­receptors respond to magnetic fields.

What are the four basic types of receptors?

Magnetoreceptors are found in species such as honeybees, mollusks, hornets, salmon, tuna, turtles, salamanders, and homing pigeons. Receptors for magnetoreception contain magnetite, a mineral which is magnetic itself and therefore responds to magnetic fields.

Humans have receptors that contain magnetite. However, researchers have had difficulty finding evidence of human sensitivity to magnetic fields. Reports of human magnetic sensitivity have surfaced several times only to be shot down by failures of replication.

A receptor responds to a stimulus. The word stimulus comes from the Latin stylus for prod or poke and can refer to any measurable input to the nervous system.

Each type of receptor responds with exquisite sensitivity to a particular type of stimulation. The form of energy to which a receptor is most sensitive is called the specific stimulus for that receptor.

What is a stimulus? A specific stimulus?

Although each receptor has a specific stimulus, it can be made to respond to other types of stimulation that are very strong. For example, a poke in the eye (a mechanical stimulus) activates photoreceptors in the eye, producing a flash of light (the proverbial "stars" a person might see after a blow to the eye or head).

Photoreceptors are billions of times more sensitive to light than they are to touch. Therefore light is the specific stimulus for photoreceptors.

The senses of most animals are optimized. In other words, they are as sensitive as they could possibly be without causing problems due to oversensitivity.

What does it mean to say that senses are "optimized"?

Rob de Ruyter illustrated sensory optimization by embedding a fly in orange wax and monitoring the neurons of its visual system. Ruyter found that neurons in the fly's visual system were so sensitive that they were close to the theoretical limit imposed by their optical system (Flam, 1993).

The same is true of human senses. Human hearing, for example, is so sensitive that if it was any more powerful, we would hear the movements of molecules in the air (Brownian movement) hitting our eardrums.

What would we hear, if our ears were more sensitive?

Light can be conceived as waves (light waves) or as a stream of particles (photons). Photons are extremely tiny units of energy.

A 50 watt incandescent bulb, almost too dim for reading, puts out 3,800,000,000,000,000,000 photons per second. Yet a single photoreceptor in the human eye can respond to three or four photons, when the eye is completely adapted to darkness.

How sensitive are human photoreceptors?

Light and the Visual Waveform

Light is a form of electromagnetic radiation. It is the same type of energy as television waves and radio waves. The diagram shows electromagnetic radiation from gamma rays (left) to radio broadcast bands (right). Visible light is a tiny segment of the spectrum.

the electromagnetic spectrum
Visible light as a form of electromagnetic radiation

What type of energy is light? What is the relationship of wavelength to frequency?

Frequency of light refers to the number of cycles in the electro­magnetic radiation per second. Cycles are fluctuations in strength of radiation. Wavelength is the distance from one cycle to the next.

The visible spectrum is the band of electromagnetic radiation we see: the part we call light. The visible spectrum contains all the colors of the rainbow, from red to violet.

The colors of the rainbow can be memorized in order by remembering the name Roy G. Biv. This stands for red, orange, yellow, green, blue, indigo and violet.

Indigo is hard to see as a distinct color. Indigo probably would not be in the list except for the fact that Isaac Newton, who proposed this classification, liked the number seven.

What does Roy G. Biv stand for?

The colors of the spectrum are called hues. Going from red to violet through Roy G. Biv, each color represents a slightly higher frequency, therefore a shorter wavelength.

With shorter wavelength, more cycles fit into a second. Blue has the highest frequency and the shortest wavelength in the visual spectrum.

How is frequency related to wavelength?

graph shows wavelength and amplitude
Wavelength and amplitude

How is amplitude related to luminance and brightness?

On a graph, the amount of energy in light is shown by the height of the wave. This is called its amplitude or strength. In the visible spectrum, greater amplitude means greater amounts of light.

Reflected or transmitted light is called luminance, the amount of illumination. Increased luminance (a physical quantity) results in increased brightness (a psych­ological sensation).

Light is hardly ever pure. It usually contains a mixture of different frequencies.

When you see the color red, it is typically a complex mixture of different wavelengths with red frequencies dominating. Light containing only one frequency is said to be completely saturated.

Completely saturated colors do not occur in nature, although the colors of a rainbow come close. Highly saturated colors look intense, while unsaturated colors look dull and gray.

To summarize, there are three main properties of the visual waveform that correlate with psychological experiences. The frequency of the wave influences our perception of hue. The amplitude of a wave influences luminance and our experience of brightness. The saturation of light corresponds to purity or complexity of a color.

Sound and the Auditory Waveform

The experience we call sound results from our detection of pressure changes in a medium such as air. In a diagram of a pressure wave, peaks represent moments of relatively high pressure or compression of the air molecules.

Valleys (dips in the waveform) represent low pressure or partial vacuum, which is called rarefaction. A vibrating string on a musical instrument produces alternating compression and rarefaction in the air, resulting in sound.

graph shows auditory waveform
Properties of a sound wave

The distance from one peak to the next is a single cycle. A single cycle per second is called one Hertz (abbreviated Hz) after the German physicist Heinrich Hertz (1857-1894).

What produces sound? What does Hz mean?

Amplitude of a waveform, its height on a graph, showing the amount of pressure change, correlates with loudness of a sound, although there are numerous other factors that affect perceived loudness of sounds.

The wavelength or frequency of the wave in cycles per second affects the pitch of a sound: how high or low it sounds. As the frequency goes up, the sound becomes higher; as the frequency goes down, the sound becomes lower.

Other factors besides frequency affect perception of pitch. Many professional piano tuners prefer to tune a piano by ear instead of by using an electronic instrument. An electronic device would be more precise, but it might not produce a piano that sounded "in tune" to the human ear.

How do changes in amplitude or frequency affect sound? Why do piano tuners tune "by ear"?

Recall that a third characteristic of light (after brightness and hue) was saturation. Saturation corresponded to purity or complexity of the waveform.

Similarly, in analyzing sounds, the word that labels the distinctive, complex mix of a sound wave. This is timbre, pronounced TAM-ber.

Timbre refers to quality of a sound, as distinct from loudness or pitch. Timbre is shown on a graph as the shape of a waveform.

For example, a flute produces a very smooth sound that appears on an oscilloscope (an instrument that displays wave form) as a perfect sine wave: a smoothly rolling waveform. A piano, by contrast, produces a highly complex mix of different frequencies that looks very noisy on an oscilloscope. This happens because the timbre of a piano is very complex.

Because the timbre of a piano is complex, it is difficult to reproduce faithfully. One way to test the fidelity of reproduction in a good music system is to play a high quality audio track of a solo piano performance.

What is timbre? Why is a good recording of a piano useful for testing a music system?

Lean back, close your eyes, and see if the piano sounds like it is there in the room. If it does, you have a good music system, capable of reproducing highly complex timbres.

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Reference:

Flam, F. (1993) Physicists take a hard look at vision. Science, 261, 982-984.


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