SENSATION AND PERCEPTION
Course overview:
Tutorial meetings--> 2/03, 9/03, 23/03
- Preparing 4 questions
- Presentation based on other groups questions
Exam --> 28/03 Re-sit-->2/06 open ended questions
Sensations
• Sensations are pieces of meaningless information
• Sensation is input about the physical world obtained by our sensory receptors
(physical world represented in the inner world). Often identified as involving simple
“elementary” processes that occur right at the beginning of a sensory system, such as
when light reaches the eye, sound waves enter the ear, or your food touches your
tongue.
• SENSES are the PHYSIOLOGICAL BASIS OF PERCEPTION
• Perceptions are pieces of meaningful information
• Perception is the process by which the brain selects, organizes and interprets
these sensations information in order to represent and understand the presented
information or environment. Often identified with complex processes that involve higher-
order mechanisms such as interpretation and memory that involve activity in the brain—
for instance, identifying the food you’re eating and remembering the last time you had it.
Sensation involves detecting elementary properties of a stimulus, and perception
involves the higher brain functions involved in interpreting events and objects.
Deciding what is sensation and what is perception is not always obvious, or even that useful.
Non-truthful experiences such as illusions and hallucinations show that perceptual
phenomenology is not enough for perceiving. Just objects are not enough to
perceive.
Philosophy
- Descartes 1596-1650 > I think therefore I am. Physical reality exists.
- Kant 1724-1804 > 2 kinds of worlds: Phenomenon X Noumenon
- Helmholtz 1867-1910 > unconscious inferences
Philosophers had been thinking about this a lot, and then the psychologists came. Their question
was: do you add the tiny parts together to get a whole, or do you perceive the whole and focus
on the details after? We have two different points of view:
1. STRUCTURALISTS: believed that we had hundreds of basic pieces that were being
used to create the perception of a complex whole.
2. GESTALT PSYCHOLOGISTS: believed that our mind is bound by a set of rules that
organize the separate parts as a whole. In the same way we understand sentences based
on a set of rules, and not just by putting together their individua parts. The whole is
automatically perceived and then there’s the detail. THEY WON. The whole is not the
same as the parts that make up a whole.
,Perceiving as information processing
How do we construct a representation of the environment for the demands of thought and
action, from minimal stimulation?
– we try to develop a theory of perceiving
– we try to discover the mechanisms responsible for our capacity to extract useful information
from sensory stimulation
– we try to present it in a form that suits the needs of other cognitive functions
When perception fails
Patient H.J.A: he can see details and describe them but fails in recognizing the full. He can see
properly; he has no color vision but that alone does not explain his problem. Difficulties in
identifying anything. There are no abnormalities apparent in the CT scans. He suffers from visual
agnosia: inability to recognize objects even though the patient is not blind.
Munk 1881 called the disease mind-blindness because the patient can see but cannot
understand.
PERCEPTUAL PROCESS: involves 7 steps,
beginning with a stimulus in the environment, and
ending with perceiving the stimulus, recognizing it and
acting in relation to it. The steps in the perceptual
process do not always unfold in a one-follows-the-other
order; ex. research has shown that perception (“I see
something”) and recognition (“That’s a tree”) may not
always happen one after another, but could happen at
the same time, or even in reverse order. -
Distal stimulus: stimulus in the environment; -
Proximal stimulus: stimulus in proximity to the
receptors. The
light and pressure waves that stimulate the receptors
introduce one of the central principles of perception, the
principle of transformation, which states that stimuli
and responses created by stimuli are transformed, or
changed, between the distal stimulus and perception.
Sensory receptors-->TRANSDUCTION: transformation of environmental energy into
electrical energy. Ex. retinal receptors transduce light energy into electrical energy.
Knowledge influences what we perceive, our expectations and biases, culture all influence the
way in which we perceive (top-down process). We study the perceptual process in chunks>
1. behavior (perception- recognition- action),
2. stimulus (proximal- distal)
3. physiology (studied mainly on animals, processing- receptors)
STIMULUS-BEHAVIOR RELATIONSHIP: relates stimuli to behavioral responses. One way
to study the stimulus–behavior relationship is using an approach called psychophysics, which
measures the relationships between the physical (the stimulus) and the psychological (the
behavioral response).
STIMULUS-PHYSIOLOGY RELATIONSHIP: relationship between stimuli and physiological
responses, like neurons firing.
,PHYSIOLOGY-BEHAVIOR RELATIONSHIP: relationship between physiological responses and
behavioral responses.
Psychophysics
• Absolute threshold: smallest amount of energy that we need to detect a stimulus.
The second the stimulus goes from nothing to something it means it has crossed the
absolute threshold. Es linear scale diminishing lights and the person has to say when he
sees the light and not. Es2 turning up the volume and hear from when the sound is heard.
Thresholds measure the limits of sensory systems (measures of minimums).
• Fechner-->Classical psychophysical methods:
- Method of limits: Stimuli of different intensities are presented in ascending and
descending order; observer responds yes/no when perceived. The threshold is then
determined by calculating the average of all the crossover points.
- Method of adjustment: stimulus intensity is adjusted by the observer until he detects it;
average of crossover points is the threshold.
- Method of constant stimuli: like methods of limits, but instead of presenting stimuli in
ascending/descending order they are presented in random order. After presenting each
intensity many times, threshold is defined as the intensity that results in detection on 50
% of trials. (Most accurate one, but also most time-consuming).
Sensitivity= 1/threshold
• Difference threshold: minimum difference that must exist between two stimuli
before we can tell the difference between them. Calculated with the Weber’s contrast.
DI = just noticeable difference / I =intensity
• Magnitude estimation: psychophysical method in which the subject judges the
intensity of a stimulus against a baseline stimulus. The observer is given a standard
stimulus above threshold and a value for its intensity, then the observer compares the
standard stimulus to test stimuli by assigning numbers relative to standard.
Patient H.J.A > limited sensitivity on the top half of left and right visual fields. He has Superior
Altitudinal Hemianopia. Anopia: can arise from damage to optic track or brain damage.
The eye and retina
Electromagnetic spectrum: continuum of electromagnetic energy that is produced by electric
charges and is radiated as waves. The energy in this spectrum can be described by its
wavelength (distance between waves’ peaks). Spectrum ranges from short wavelength
gamma rays to long wavelength radio waves. The visible spectrum for humans ranges from 400
to 700 nanometers. Most perceived light is reflected light. For humans and some other animals,
the wavelength of visible light is associated with the different colors of the spectrum, with short
wavelengths appearing blue, middle wavelengths green, and long wavelengths yellow, orange,
and red.
The eye 👁🗨
Light reflected from objects in the environment enters the eye through the pupil and is focused by
the cornea and lens to form sharp images of the objects on the retina, the network of neurons
, that covers the back of the eye and that contains the receptors for vision, also known as
photoreceptors:
1. RODS: night vision
2. CONES: day vision
Signals from the receptors flow
through the network of neurons
that make up the retina and
emerge in the optic nerve,
which contains a million optic
nerve fibers that conduct
signals toward the brain.
Accommodation
Is the change in the lens’s shape that occurs when the ciliary muscles at the front of the
eye tighten and increase the curvature of the lens so that it gets thicker. Accommodation
occurs unconsciously, so you are usually unaware that the lens is constantly changing its
focusing power to let you see clearly at different distances.
Refractive errors: errors that can affect the ability of the cornea and/or lens to focus
incoming light onto the retina.
- NEAR POINT ISSUES: distance at which your lens can no longer accommodate to bring
close objects into focus. Age-related loss of the ability to accommodate = presbyopia;
- FAR POINT ISSUES: max distance at which light becomes focused on the retina. Inability
to see distant objects clearly = myopia.
- See distant objects clearly but have trouble seeing nearby objects = hyperopia or
farsightedness.
The retina
BLIND SPOT: small area where the optic nerve leaves the back of the eye. There are no
visual receptors in this area, so small images falling directly on the blind spot cannot be seen.
The reasons why we don’t perceive this blind spot are that it is located at the side of our visual
field where objects are not in sharp focus and because the brain is able to fill in the spot in a
coherent way thanks to binocular vision.
Rods and cones differ in:
• Shape:
- Rods--> large and cylindrical;
- Cones--> small and tapered;
• Pigments:
- Rods--> Rhodopsine = Opsine + Retinal;
- Cones--> Iodopsine = Photopsine + 3 kind of Retinal;
• Distribution across the retina:
- FOVEA: only cones
- Peripheral retina: both rods and cones
- More rods than cones in periphery
• Spectral sensitivity:
- Rods-->more sensitive at 500 nm;
- Cones--> most sensitive at 560 nm (419 nm, 531 nm, and 558 nm)
• Dark adaptation (the process of increased sensitivity in the dark)
