Chapter 5 - Sensation
In this chapter we will be focusing on our “input systems”, the senses we use to perceive the world
We will focus primarily on vision, but will also discuss audition, touch and smell.
Much of the information will attempt to explain how these systems work … though we will also discuss
illusions that show us how our sensory systems can be tricked.
Throughout, we will again emphasize the distinction between sensation, the information arriving
through the sense organs,and perception, the subjective impression of the world that we end up with.
Sensation versus Perception:
The textbook actually defines sensation and perception slightly differently from how I have, though the
flavor is the same. According to the text:
Sensation is the detection of simple stimulus properties such as brightness, colour, sound frequency,
Perception is the detection of objects, their location, their movements, their background.
Thus, seeing purple and blue in the picture is sensation, whereas seeing beautiful fireworks is perception
Transduction is the process by which sense organs convert energy from environmental events into
neural activity … eventually ending up in the brain.
Sense organs differ in terms of the kinds of environmental energies they are sensitive to, and in the
manner they transduce that energy.
Usually the transduction is accomplished via specialized receptor cells that release specialized
neurotransmitters that stimulate other neurons.
Steve, show table 6.1 here to show some of the variety across the sense organs
As we discussed earlier, nerve cells can only send a message or not. The message does not vary in terms
of what it says, or how it says it.
So how then can the same nerve cells transmit the fact that bananas are yellow, but carrots are orange?
The answer lies in the use of code … think of Morris Code for example. In Morris Code complex
semantic messages were transmitted using simple clicks
How does the Brain Code Information?:
The first answer to this question is: We’re not absolutely sure yet … the puzzle still needs more
But, we do think the following two codes are involved. Anatomical Coding - Sensory organs located in different parts of the body send their signals to different
locations in the brain … the brain uses this to interpret the signals correctly. Example, rubbing your eyes
& phantom limbs
Temporal Coding - Information can be coded according to time. The easiest way to do this is with
respect to rate of neural firing. May be the main way to code the intensity of stimulation
Psychophysics - Physics of the Mind:
The systematic study of the relation between the physical characteristics of stimuli, and the sensations
they produce. Example – JNDs
Just Noticeable Differences - JNDs:
Ernst Weber was the first to measure JNDs, the smallest change in the magnitude of a stimulus that can
He found that the JND is directly related to the magnitude of the stimulus.
For example, when examining people’s ability to detect differences in weights, he found that if the
weights were within 1/40th of each other, no difference was detected.
Weber examined all the senses in this respect. For example, to detect a difference in brightness, the
difference must exceed 1/60 of the average brightness of the stimuli.
These results are now called Weber Fractions.
Measuring Sensitivity via Thresholds:
The JND is a threshold is a difference threshold. It reflects how big a difference has to be before it is
There is a more simple thresholds as well, how intense does a stimulus have to be before the subject
claims to see, hear, smell, taste, or feel it?
These sorts of thresholds have been used to assess the sensitivity of our sensory apparatus … and have
lead to areas of research such as subliminal perception.
How do we know if a message is subliminal?
> simple threshold approach - establish the “limon”, the point where it is noticed half the time, and go
below The Problem With Simple Thresholds:
Jastrow’s (1897) Subliminal Perception Experiment
The notion (and evidence) for perceptual defense
Demand characteristics and response
bias can prevent accurate measurements
of a threshold
Separating Bias from Sensitivity:
In 1974 (1974!) Green & Swets came up with a way of measuring sensitivity free of response bias … it is
an approach called signal detection theory.
Signal detection theory involves presenting a stimulus on some trials, and not on others … then asking
subjects on each trial to state (guess) whether or not a stimulus was presented.
This leads to four possibilities:
Hit - saying a stimulus is present when it is.
Miss - saying a stimulus is not present when it is.
False Alarm - Saying a stimulus is present when it is not.
Correct Rejection - Saying a stimulus is not present when it is not.
Is there a word?:
In Reality, stimulus was:
So far bias can still have an effect, but it effects both hits and false alarms.
How do we get rid of bias?:
The trick here is to use some manipulation that will vary response bias, and test a given subject under a
number of levels of this manipulation .. E.g., payoffs Vision - Near vs. Far:
Some of our senses are primarily concerned with provide information about
stimuli and events in our immediate environment (e.g. touch, perhaps smell). Others provide
information about stimuli and events that are further away (e.g., vision,