This chapter discusses the methods the brain uses to bring in information from the external environment (in these terms, the "external environment" is anything outside of the nervous system: the world outside the body and the world inside the body (body parts and systems that are not the nervous system).

the stimulation of a sensory receptor.
the conscious experience of a sensation after it has been organized and interpreted by the nervous system.

Neural cells specialized to respond to stimuli that are not neural in nature. They respond to environmental stimuli such as light, vibration, chemicals. We will be discussing these receptors and how they work. In order to understand the receptors we will briefly discuss the stimulus as well.
Specialized neurons that respond to specific features of a stimulus: orientation of a line, direction of movement, upward frequency glide.


Electromagnetic spectrum
The continuum of all frequencies of radiated energy from the shortest wavelengths (gamma, X-ray, and ultraviolet) to the longest wavelengths (infrared, radar, radio & television)
  • Wavelength: the distance between waves as light travels. Since light moves at the same speed always, the wavelength determines the frequency (number of waves that pass a certain point in one second). Wavelength determines the color of the light and whether it is visible or not.
  • Amplitude: the height/strength of the waves. This gives the sensation of brightness to light.

Visible Light
A part of the electromagnetic spectrum ranging in wavelengths from 400 nm (violet & deep blue) to 700 nm (red). {nm = nanometers, 1 inch = 25,400,000 nm}
° The concept of "visible light" is a humano-centric concept, members of some other species are able to see wavelengths outside of our visible spectrum.

The structure of the eye

Click on the image for an animated version
(opens in a new window)

It would be a good idea to test yourself on the anatomy of the eye since it's very likely that it will appear on the exam.


The eye has two basic types of receptors:

The rods and cones comprise the first of five layers of the retina. The majority of the cones are concentrated in The Fovea. It is in this region, directly behind the lens, that clearest vision occurs because of the high number of cones. There are no rods here, which means that faint light (like faint starlight) is not seen very well with the fovea.

In the periphery of the visual field there are few cones (fewer and fewer as one gets farther and farther from the middle of the back of the eye). In this area, there are many rods.

The Visual Pathway

We will begin with the retina and move into the brain.


The definition of this term is essentially "the conversion of a stimulus from a physical stimulus to a neural impulse." In the case of light, this is accomplished by the action of Retinaldehydes: a family of chemicals that respond to light by undergoing a chemical change. It is the reaction of these chemicals in the rods and cones that causes a neural response to light. Rods, and each of the three sorts of cones have slightly different retinaldehydes that respond to different wavelengths and intensities of light.


The ability of the eye to adjust the incoming amount of light and to adjust the focus of the that light. This is done by changing the shape of the lens and the pupil. Accommodating is done to enable the eye to focus on objects both nearby and far away. Of course, when unable to fully adjust, we have some of the simple disorders for which many of us wear contact lenses or glasses to correct.


Rods respond best in dim light. In bright light they get overwhelmed and are essentially useless. When the light gets abruptly dimmer (like when you turn off the lights in a brightly lit room), it takes the rods a while to recover from being overwhelmed and to start functioning properly. Once they recover, one is able to see fairly well, even in low lighting situations. Rods do not provide color information, so seeing at night is essentially seeing in black and white.

Cones respond poorly or not at all to dim light. Remember that there are no rods, only cones, in the fovea (the "center" of your vision). Can you think of phenomena that reflect the relative insensitivity of cones to faint light?


colorful birds

A number of theories were arrived at to explain how color vision works. It turns out that, although they started as rival theories, they were all correct to an extent.


There are several forms of colorblindness:

A couple other interesting comparisons appear below. First we show two images of the SAME color vision testing cards. The one on the left is what the card would look like to a person with normal color vision. The card on the right is what it would look like to someone with deuteranopia (if you have deuteranopia, they probably look virtually identical). To see a selection of these, check out this website.

Below are two copies of the same image of strawberries. The one on the left is normal, the one on the right simulates deuteranopia. Imagine how difficult it could be to find berries, much less ripe berries, if one had deuteranopia. Some researchers believe that this is the way our vision worked prior to the mutation that gave us red-green color.

Here's a bit of fun... the Colorblind Web Filter allows you to put in the address of a web page and have it rendered as it would be seen by someone with various kinds of color blindness. You may not get perfect results with every web page, since many have weird little applets running or what have you. Obviously, sites with lots of colors will appear very different... as will color images... but the more images the filter has to handle, the slower it might be to display the results.



Gestalt Laws of organization
Proximity Perception of items occurring close together as being related or connected
Similarity Perception of items similar to each other as being related or connected
Continuity Perception that an item is continuous and whole when another item is interposed upon the first item
Closure Similar to continuation, if part of an item is interrupted we still perceive the item as whole. This is the rule used by the visual system to handle you Blind Spot
Good form Perception of items following rules that make sense, such as symmetry.

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Perception of movement

Perception of movement is done using sensory cues from vision, the vestibular sense, and the orientation and movement of the eyes.

Illusory movement

Induced movement: If the background is moving and the foreground is stationary, we often perceive the foreground object as moving and the background object as stationary because experience tells us that the background either moves very slowly or not at all relative to nearby objects: cartoons make use of this phenomenon.

Stroboscopic movement: If a series of stationary images that are similar but slightly different are presented in rapid succession, the result is the perception of a continuous, moving image. Movies, television, etc.

Depth and Distance Perception:

The ability to determine what is closer than something else. There are many many clues used to determine this:

Binocular cues: Cues to depth requiring both eyes to be functioning

Monocular cues: Cues to depth needing only one functioning eye

Optical Illusions

Here is a site (opens in a new window) with a number of interesting visual illusions and some descriptions of what they are all about and why they happen. You may find these quite interesting, although some are more dramatic than others. You should definitely try out the "Experience your blind spot".

Here is another site (opens in a new window). Some of these are a bit cheesy, but some are pretty good.

Here is another site (opens in a new window) with more examples of visual illusions. The eyes that follow the mouse are just annoying...

I just added this site (opens in a new window), it has some very nice examples of perceptual illusions, both visual and auditory.

These are caused by tricking the above cited clues to size, distance, and depth.


The visual system makes use of both top-down and bottom-up processing.


The structure of the ear

click on the image or title for an interactive animation
(opens in a new window)

Types of hearing deficit

Sound localization

One of the tasks of the auditory system is to determine where a sound is in relation to the person. to do this it uses a number of strategies.




The sense of smell is a complex sense, having well more than 100 distinct receptors located in the top of the nasal passage, extending down from the Olfactory Bulb. They are stimulated by different chemical substances. Our understanding of olfaction is not as complete as our understanding of the other senses because of the great variety of chemoreceptors but advances in technology are causing the field to open up.

These are chemicals that organisms release into the environment. They are meant to signal other members of their species: warning them off, signaling sexual readiness/receptiveness, etc.
In many cultures people prefer to mask many of these scents by washing, using cologne and perfume, etc. However, they still appear to influence behavior, some research findings seem to indicate.


Taste, in psychological terms, is the collection of sensory responses one gets only from the tongue and mouth. What we think of as the "taste" of most foods is a combination of the sense of taste and the sense of smell.

There are five types of taste receptors in the human, most located on the tongue, some on the walls of the mouth near the entrance to the throat. These receptor sites tend to be clustered in groups called "taste buds".

Phenothiocarbamide (PTC) & genetics; nontasters, and supertasters.


We tend to refer to the "sense of touch". But it's a complex system of several senses, in reality. Collectively we call them the cutaneous senses.

One area of focus with regard to the sense of touch is pain: how it works, how it can be reduced.

Phantom limb sensations


(listed in the book as a Somasthetic sense, but more closely related to hearing than to touch and so I give it a separate listing.)

This sense gives the brain information on the tilt of the head, it's acceleration (or lack thereof) and the head's orientation with respect to gravity. This allows your eyes and brain to compensate for motion of the head and orientation The sensory apparatus is found in the inner ear. It two distinctive components

Do we have other senses? Many animals seem to have a magnetic sense which allows them to orient to various directions. There is not yet strong evidence of this in humans, though preliminary research seems to indicate that it may be present, but weak.

Extrasensory perception (ESP). The idea that we might be able to transmit thoughts or emotions or perform other feats purely by "will of mind." While this has been researched a great deal throughout the years, there is very little solid evidence of its existence. Research has been difficult or impossible to replicate, and there is no understandable way in which these transmissions could occur (although we may simply not have discovered it yet.

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