- touch is a short-range sense; have to be in contact w/something to have
- smell has intermediate range system, you smell better when you are closer
dogs have a history of smell and use their past to tell them what happened
before coming into a room (for example)
- vision and audition are long range system and are favored (avoiding a person to
stay out of trouble or hunting can be used to prepare attack)
- visual system must provide the stability in the face of rapid shifts in its input
- sound is not so variable and intensity changes as we move toward or away from
its origins; changes are more gradual than those face by visual system
sounds carry around obstacles in ways that light does not and auditory sense
has more time to process signals
- b/c they are attuned to different aspects of our world, sense contribute to the
richness of experience
Sensation vs. Perception – linked but description is a little false
– raw sensory input that is striking the retina and it stimulates the retina and
brought to the brain, which is then analyzed in perception
– the detection of simple stimulus properties such as brightness, color, sound
– seeing color red
Perception – categorizing all things
– the detection of objects, their location, their movements, their background.
– Seeing a red apple
– seeing purple and blue in the picture is sensation, whereas seeing beautiful
fireworks is perception
+ seeing movement is sensation but seeing a soccer ball coming toward us and
realizing that we will have to move the left to block it is perception
How do these messages from the world get transferred into our body?
- our senses are input from the world
- through transduction
Monday, October 23, 2006
Transduction – is the process by which sense organs convert energy from
environmental events into neural activity, eventually ending up in the brain.
- Sense organs detect stimuli provided by light, sound odor, taste or mechanical
contact w/ environment – info. about these stimuli transmitted to the brain
through neural impulses – action potentials carried by axons in sensory nerves
- 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.
- Energy we’re able to pick up and turn into something else that makes sense
inside the brain.
Light is out there in the real world and that is what our eyes can pick up, so it
has to deal w/ radiant energy which goes to the neural system Vestibular system is the tilt and rotation of head
Tongue is all about chemical analysis, recognizes various shapes of molecules
and that’s what produces taste
Human is sensitive to a very narrow range of it (ie. We’re sensitive to light but
we’re not sensitive to all light – infrared light)
Location of Sense Environmental Energy Transduce
Eye Light Radiant energy
Ear Sound Mechanical energy
Vestibular System Title and rotation of Mechanical energy
Tongue Taste Recognition of molecular shape
Nose Odor Recognition of molecular shape
Skin, Internal Organs Touch Mechanical energy
Temperature Thermal Energy
Vibration Mechanical Energy
Muscle Pain Chemical Reaction
Stretch Mechanical Energy
- in most senses, specialized neurons called receptor cells release chemical
neurotransmitter substances that stimulate other neurons, thus altering rate of
firing of their axons
Wednesday, October 25, 2006
- How we take the rich source of information we see in front of us, reduce it to a
bunch of firing patterns and somehow get it back a to a rich image.
- Nerve cells can only send a message or not; they tell other neurons to fire 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?
- In Morse Code complex, semantic messages were transmitted using simple
- In brain, through specific firing of nerve cells, able to capture the info. the
richness of what’s out there and relay the information to the cortical areas, where
it is somehow unpacked and turned into what we perceive.
- A code is a system of symbols or signals representing info; as long as we know
the rules of a code, we can convert a message from one medium to another w/o
losing any information
How does the brain code information? not absolutely sure yet; puzzle needs more
Anatomical Coding – what is imp. is where the message originates and where it ends up.
- 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
Example, rubbing your eyes & phantom limbs. Tap your eye and you will see stars; eye is not getting light but by squeezing
the fluid in your eyes, you are stimulating the sensory cells in your retina and
the message is sent back to the visual cortex, which becomes the light pattern.
Phantom limb is where that part of the brain becomes randomly stimulated and
you feel like you need to itch but you have no arm.
- any information that ends up in the visual cortex, will produce a visual message
- people on psychedelic drugs usually say things like the number 13 is a bluish
color, or that light has a nice smell; signals get mixed up
Temporal Coding – Information can be coded according to time
- The easiest way to do this is with respect to rate of neural firing
- By firing at a faster or slower rate according to intensity of a stimulus, an axon
can communicate quantitative info. to the brain (ie. Light touch to skin can be
encoded by low rate of firing and more forceful touch by a high rate)
- Thus, firing of particular set of neurons (anatomical code) tells where the body is
being touched; the rate at which these neurons fire (temporal code) tell us how
intense the touch is
- May be the main way to code the intensity of stimulation
Imagine grandmother in your mind; there are certain visual/auditory/smell
patterns associated with her and all those things come to mind when you think
of her; and all the nerve cells fire in synchrony and they are somehow glued
together and maybe that is what underlies our conscious experience.
Psychophysics – physics of the mind
The systematic study of the relation between the physical characteristics of stimuli, and
the sensations they produce.
Just Noticeable Differences
- Ernst Weber was the first to measure JNDs, the smallest change in the
magnitude of a stimulus that can be detected.
- found that the JND is directly related to the magnitude of the original stimulus
- 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.
- [Used in modern times, to measure eye visions to see what prescription is
- Weber examined all the senses in this respect (example, to detect a diff. in
brightness, the differ. must exceed 1/60 of the average brightness of the stimuli)
- These ratios are now called Weber Fractions (b/w jnd and magnitude of a
- Construct graph indicating strength of sensation of brightness (in jnds) in relation
to the intensity of a stimulus – graph relates strength of sensory experience to
- Each dot marks another jnd – amount pg physical energy necessary to produce a
jnd increases w/ magnitude of stimulus; shape is characteristic of the
mathematical function of a logarithm
- This was a big deal in psychology because ability to experience difference in
brightness, sound, heaviness follows a mathematical rule and can be studied.
Measuring Sensitivity via Thresholds
- The JND is a threshold aka difference threshold. - It reflects how big a difference has to be before it is detected.
- There are 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?
- Simple threshold – just being able to detect the presence of sound
- Absolute threshold – minimum value of stimulus that can be detected –
discriminated from no stimulus at all.
- Threshold was not a fixed value; it is the point at which a participant detects the
stimulus 50% of the time
- We may have a conscious and unconscious system where unconscious system
has a lower threshold and it takes less to detect presence of energy (ie. A
message for an ad in lighter color, in background can be detected by our
unconscious system to do something – such as travel on an airplane because of
- 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
How do we know if a message is subliminal?
Subliminal - so subtle a sensory stimulus that one is not aware of its existence but may
in some manner respond to it.
- if you want to make, real lasting changes in any area of your life, you must reach
the subconscious mind where the changes begin
simple threshold approach – establish the ‘limon’, the point where it is noticed
half the time you notice something, and half the time you can’t
subliminal – point (more than half the time) where you can’t notice the stimulus
Advertising expert for a theater put ‘eat popcorn’ 1/60 of a second, and the
popcorn sales increased
You can somehow stimulate the unconscious mind w/o the conscious mind
being aware and it causes hunger
It got everyone interested in subliminal perception, they wanted to not be
controlled and went to go see movies to see if they got hungry
The statement of ‘eat popcorn’ was made up; but his sales increased
The Problem with Simple Thresholds
- Jastrow’s (1897) Subliminal Perception Experiment (first empirical study in N.A)
- You sit there and he started walking backwards and you have to tell him when
you can’t see the word
- At that point, he would show the cards and you had to guess what they said –
saw that the results were 60% correct, which is above chance
- He concluded that this is strong evidence for some existence of some entity other
than our primary waking self – somebody that is good at telling us letters from
digits, when you cant see them
- This is a very controversial area because of method; the way the experimenter
presents the whole stimuli to people - subject may get tired and may not be
telling the truth; experimenter has no way in verifying the truth of the subject
- Demand characteristics and response bias can prevent accurate measurements
of a threshold – subject has to make up what the experimenter wants (ie. When
can you not see the words on the paper // not knowing what the experimenter
- Introspection – relying on subject to give you data is scientifically dangerous Freud – one of his ideas; in conscious minds we like to think of ourselves as pure
- but there are perceptual defense mechanisms; try to keep dirty stuff out of minds
- experiment was done where words were presented and the person had to blurt
out word when he/she saw it – showed that it took longer to say the swear word;
Freud may have been right
- or it could have been that the person saw the swear words around the same time
as the other word but the willingness to say it took longer
- there could have been a response bias – willing to say it may have been reduced
or taken longer because it was uncomfortable to say it out loud (emotionally
- response bias is a personal thing
Friday, October 27, 2006
Separating Bias from Sensitivity
- 1974 Green & Swets came up w/ a way of measuring sensitivity free of resp. bias
- approach called signal detection theory
- theory involves presenting stimulus on some trials, and not on others; then
asking subjects on each trial to state (guess) whether or not a stimulus was
- every stimulus event requires discrimination b/w signal (stimulus) and noise
(consisting of both background stimuli and random activity of nervous system)
- It is a mathematical theory of the detection of stimuli, which involves
discriminating a signal from the noise in which it is embedded and which takes
into account participants’ willingness to report detecting the signal.
This leads to four possibilities – stimulus was either present or it was not
1. Hit – saying a stimulus is present when it is.
2. Miss – saying a stimulus is not present when it is.
3. False Alarm – Saying a stimulus is present when it is not.
4. Correct Rejection – Saying a stimulus is not present when it is not.
So far bias can still have an effect, but it affects both hits and false alarms.
Conservative – less likely to say yes; even when stimulus is present
Liberal – more likely to say yes when it is present, but it also increases the false alarm.
Graph on pg. 170, is a receiver operating characteristic curve (ROC) – graph of hits and
false alarms of participants under different motivational conditions; indicates people’s
ability to detect a particular stimulus.
- difference b/w two curves demonstrates that louder tone is easier to detect –
detectability is measured by relative distances of the curves from 45-degree line
- signal detection method is best way to determine person’s sensitivity to
occurrence of particular stimulus (dimmest light eye can pick up; lightest sound
that ear can pick up – best sensitivity to avid bias)
- signal detection theory emphasizes that sensory experience involves factors
other than the activity of the sensory systems, such as motivation and prior
In the signal detection theory, which bias is the null sensitivity line constructed
under? Is it the 50 cents for hits and -50 cents for false alarms? The null sensitivity line is really just a theoretical thing. It is a line depicting all points on
the graph where hits equal false alarms. So it goes from .00 hits and .00 false alarms to
1.00 hits and 1.00 false alarms. Again, the idea is that if one is saying yes just as often
when something was presented (a hit) as when nothing was presented (a false alarm)
then they are apparently insensitive to whether a stimulus has or has not been
presented. Thus, whenever the proportion of hits equals the proportion of false alarms
this implies null sensitivity, and the null sensitivity line merely connects all of these pairs
How do we get rid of bias?
Trick here is to use some manipulation that will
vary response bias & test given subject under a
number of levels of this mani. (Payoffs)
- Hits are greater than false alarms
- When below the line – more likely to say
yes when its not there, no when there is
- Maximum sensitivity when bias is at a
level that allows us to show the best
sensitivity you can show; you always
- Want to know what’s the maximum sensitivity of the system, under the best
possible systems – you can find the best sensitivity of every sense receptor this
- Graph shows results from different levels of response bias (5) and you can
compute a curve and see the sensitivity from the null sensitivity
- Two sources of information – one is any information you get from sense organs;
two is the general willingness to say yes or no
- Sum up: the whole issue is what if you want to know how good a system is at
picking up information – point of signal detection theory
Vision – Near and Far
- Some of our senses are primarily concerned with providing information about
stimuli and events in our immediate environment (touch, perhaps smell).
- Others provide info. about stimuli/events that are further away (vision, audition).
- Frontal lobe – we got good at planning things because our hands became free
and were able to get into mischief but we needed advance information.
- Vision gives us a lot of information in advance on what is happening and allows
us to plan our actions better.
- knowledge about things not in our immediate environment can be critical as
actions can than be taken that either bring the stimulus close if it is desirable, or
make sure it stays far away if it is undesirable.
- Of our “early warning” senses, vision is primary.
Vision – what you see, and what you don’t
- First of all, we often believe that we see all there is to see in the world. That is
- Light is the stimulus and consists of radiant energy similar to radio waves, ad
oscillates as it is transmitted from its source
- the light spectrum ranges from short wavelength signals like Gamma Rays, up to
long wavelength like AC television and radio waves - We see only a small part of this called the visible spectrum. It ranges from light
with wavelengths of between 380 nanometers (violet) up to 760 nanometers
- This is really a narrow, tiny part of the light spectrum, other beasties are sensitive
to what we don’t pick up and can see more.
Basic Anatomy of Outer Eye
a) Cornea – fluid filled outer coating of eye; admits light into eye; can fix far-sighted if
b) Sclera – white part of the eye, tough membrane (structure) serves as protection for
c) Iris – colored part of your eye, a muscle that controls the size of the pupil; when
there’s little light, iris opens pupil; when a lot of light – iris reacts by closing down the
d) Aqueous Humor – now focused light passes through eyeball which is filled with a
liquid called aqueous humor (watery fluid). Fluid nourishes front of eye in a way that
blood vessels normally would.
- if it is produced too quickly, or if passage that return it to the blood becomes
blocked, pressure w/in eye can increase and cause damage to vision – disorder
known as glaucoma, nutrients not delivered properly.
e) Pupil – black part in the middle of the eye; opening in iris that allows light into the
f) Lens – transparent organ situated behind the pupil of the eye; helps focus incoming
light onto the retina. This lens is flexible and slight alterations in it can alter the focus
of it, a process called accommodation (as an example, squint).
- Becomes brittle/stiff as we age – no surgery, still need glasses.
- b/c it must remain transparent, lens contains no blood vessels and is therefore
functionally dead tissue
g) Retina – If eye is properly shaped, nicely focused image lands on inner coating of the
back of the eye. This inner coating is the retina, and it is the part that transmits the
light signal (focused or not) into a neural signal. – performs sensory functions of the
- has three principal layers 1) ganglion cell layer (front) 2) bipolar cell layer
(middle) and 3) photoreceptor layer (back)
OK, so now the light is in the eye, what happens next?
Visual problems can be fixed by reshaping the cornea, at young age.
People whose eyes are too long (front to back) are nearsighted; need a concave lens to
correct the focus.
People whose eyes are too short are farsighted; need convex lens.
As people get older, the lenses of their eyes become less flexible and it becomes difficult
for them to focus on objects close to them. These people need reading glasses w/
Wednesday, November 1, 2006
- Info. from photoreceptors is transmitted to neurons that send axons towards one
point at back of the eye – optic disc. All axons leave eye at this point and join
optic nerve, which travels to the brain.
- Photoreceptors respond to light and pass info. on by means of a transmitter
substance to the bipolar cells, the neurons with which they form synapses
- Bipolar cells transmit this info. to ganglion cells, neurons whose axons travel
across the retina and through optic nerves (back of eye) - Thus, visual info. passes through a 3-cell chain to the brain: photoreceptor
bipolar cell ganglion cell brain.
Vision – Transduction
The process of transducing a light signal into a neural impulse comes about through
several interim steps:
Step 1 - Photoreceptors
- Light strikes the back of the eye stimulating photoreceptor cells which can be
either rods or cones – energy is translated to nerve impulses here
- Rods are not responsive to color, but they are very responsive to dim light, great
for low light situations, allow you to see mostly shades of gray – in periphery area
- Cones are sensitive to color and provide a much more detailed image, great for
high light, detailed imaging; need a lot of light – primarily in centre of retina
- The Fovea which is a small pit in the back of the retina, approx. 1 mm in
diameter, contains only cones and most are connected to only one ganglion cell
- Fovea is responsible for our finest, most detailed vision – when we look at a point
in our visual field, we move our eyes so that the image of that point falls directly
on the cone-packed fovea.
- Further away from fovea, number of cone decreases and number of rods
increase. Up to 100 rods may converge on a single ganglion cell. A ganglion cell
that receives info. from so many rods is sensitive to very low levels of light.
- transduction is done via a bleaching process in which the photopigments are
split, causing an action potential
There are actually three kinds of photoreceptors in the retina.
- Rods have one type, a type that is only sensitive to brightness (shades of grey)
- Cones come in three varieties, one roughly sensitive to the color red, another to
green, and a third to blue (approximately)
- photoreceptors of cones are sensitive to color in the inverse way that a TV (or
computer monitor) displays color.
Step 2 - Bipolar Cells
- signal from photoreceptors is then passed on to the bipolar cells which reprocess