Chapter 4: Lecture 11- Sensation & Perception: Subliminal Perception
Just Noticeable Differences
- Earnst weber: the real pioneer of psychophysics
- His goal was to study how the external world is represented into the human mind
- Wanted to find the link between the external world and our perception of it
Difference vs. Absolute Thresholds
- Weber’s technique was all about finding difference thresholds … the point where participants
can notice a difference
- But what we’d really like to know is how sensitive our sensory systems are … that is, what is the
weakest stimulus they are sensitive to … what is their absolute threshold
- When you cross the threshold, something changes
- Threshold: the point where you don’t see, feel, taste, smell, then suddenly you do.
- How do we measure this? That is the challenge we want to figure out
- Maybe if we talk to subconscious, we can find an efficient way to modify behaviour Subliminal Seduction!
Signal Detection Theory. Typical Experimental Context
- Something is presented and you are asked if you saw anything, yes or no
- Third no: a miss. Stimulus was presented but the person didn’t see anything
- Last yes: nothing was presented by person said they saw something… called a false alarm Signal Detection Theory Terminology
- In an trial a stimulus could be present or absent
- A person could respond yes or no
- The 4 types of trials we end up with
- When there is a trial and a person says yes: HIT
- Says no: MISS
- Present: Hit 7/10= .70 = miss 3/10= 0.3
- Absent: False alarm 2/10= .2 = Correct Negative = 8/10=0.8
SDT: The Logic, Step 1 SDT: The Logic, Step 2
SDT: The Logic, Step 2
So is Subliminal Perception Real? Lecture 12: Vision Sensation
Perception vs. Sensation
- THC messes with associated cortex
The Limits of Sensation
- Humans do not sense all there is to sense;
- bats have echo-location, sharks sense minute electrical currents, rattlesnakes are sensitive to
low frequency vibrations, dogs can smell cancer
- Dogs know who was in the room before
- Vision is about transducing light into perception
Iris - The colored part of your eye … it is actually a muscle that controls
the size of the pupil.
Pupil - The black part in the middle of the eye … the opening that
allows light into the eyeball … lights out!
Sclera - The white part of the eye … a tough membrane that serves as
Cornea - The fluid filled outer coating of the eye … provides moisture
Lens – Focuses the 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).
Aqueous Humor - This fluid nourishes the front of the eye
Vitreous Humor - This fluid nourishes and supports the inner part of
Retina – The surface that the image lands on. The inner coating is the
retina is the part that transmits the light signal (focused or not) into a
neural signal. Transduction
- Sensory Neurons play the critical role of translating the physical properties of the outside world
into neural signals … a process termed transduction
- goes from physical stimulus to sensory(turns into action potentials that go and travel through
- perception can induce behaviour
Photoreceptors – Rods and
Cones (Transduction proper –
Red, Green, Blue)
Bipolar Cells – Image
sharpening, edges and
contours made crisper
Ganglion Cells – Colour
sharpening, and introduction
of yellow) Photoreceptors
- Light strikes the back of the eye stimulating photoreceptor cells which can be either rods or
cones (show overhead).
- Rods are not responsive to colour, but they are very responsive to dim light … great for low light
- Cones are sensitive to colour (red, green and blue) and provide a much more detailed image …
great for high light, detailed imaging.
- The transduction is done via a bleaching process in which the photo-pigments are split, causing
an action potential.
- Where transduction happens
- -Eye works backwards from how a computer works
Red, blue and green mix to produce a certain colour that you see (computer)
- The signal from the photoreceptors is then passed on to the bipolar cells which reprocess the
signal in a way that tends to emphasize edges and contours.
- Essentially, when the photoreceptors associated with spatially close parts of the retina are
sending very different signals, the bipolar cells accentuate these spots aiding us in our ability to
Ganglion Cells - Help sharpen colours, use an interesting cell called an opponent process cell
- When ganglion cells are resting, it fires at a middle rate.
- They come in two types, red/green and blue/yellow
- The rate increases when red is present, and decreases when green is present.
- When there is none there is a mid-level rate, and if both colours as well.
- Equal amount of red and green stimulates yellow
- Yellow kind of becomes for a while red and green, if blue is present it slows down Lecture 13: Vision: Perception
Perception vs. Sensation
- Top-down perception, what is this?
- When people say bottom up they mean the raw input from the world coming in through the
senses, top down our brain right at the moment input is coming in, our brain is expecting certain
stuff, so it is exerting an influence right from the beginning that will determine how we percieve
what we perceive, brain (top)
- Brain influence is top down, raw input is bottom up
12 13 14, A B C (this is the top down, it is perceived different)
- Flipping the cube, psychologically a hard thing to describe Takes effort to hold it in that flipped
- Interesting aspect is its not really context, its different from the previous, its just your mind,