PSY100H1 Chapter Notes - Chapter 5: Gustav Fechner, Superior Colliculus, Amygdala
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Chapter 5 The Perceiving Mind: Sensation and Perception
5.1 HOW DOES SENSATION LEAD TO PERCEPTION?
- The process of
sensation
brings info to the brain that arises in the reality outside our bodies
- Sensation→ the process of detecting environmental stimuli or stimuli arising from the body
- The process of
sensation
brings info to the brain that arises in the reality outside our bodies
- Sensory systems have been shaped by natural selection to provide info that enhances survival w/in a
particular niche
- Sensation differs person to person
- Perception→ the process of interpreting sensory info
5.1a SENSORY INFO TRAVELS TO THE BRAIN
- Sensation begins w/ the interaction b/w a physical stimulus & our biological sensory systems
- Stimulus→ anything that elicits a reaction from our sensory systems
- Before you can use info from your senses, it must be translated into a form the NS can understand
- Process of translation from stimulus to neural signal is known as
transduction
- Transduction→ the translation of incoming sensory info into neural signals
5.1b THE BRAIN CONSTRUCTS PERCEPTIONS FROM SENSORY INFO
- Once the info from the sensory systems has been transduced & sent to the brain, the process of perception
(interpretation) of the sensory info begins
- Perception allows us to organize, recognize, & use the info provided by the senses
--ATTENTION--
- Important gateway to perception is the
process of attention
- Attention→ a narrow focus of consciousness
- Often determines which feat. of the environment influence our subsequent thoughts & behaviours
- Unfamiliar, changing, or high-intensity stimuli often affect our survival & have a high priority for our
attention
- Sensory systems particularly sensitive to change in the environment
- Reduced response to an unchanging stimulus is known as
sensory adaptation
- Sensory adaptation→ the tendency to pay less attention to a nonchanging source of stimulation
- High-intensity stimuli (eg. bright lights & loud noises) draw our attention b/w the situations that produce
these stimuli (eg. nearby explosion) can have obv. consequences for our safety
- In most cases, people experience divided attention (attempt to process multiple sources of sensory info)
- Cannot process all the info converging simultaneously on our sensory systems
- People prioritize input by using selective attention or the ability to focus on a subset of available info &
exclude the rest
- These abilities may be disrupted in cases of ADHD
- We refer to the brain’s use of incoming signals to construct perceptions as
bottom-up processing
- Bottom-up processing→ Perception based on building simple input into more complex perceptions
- Brain also imposes a structure on the incoming info, a type of process known as
top-down
.
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- Top-down processing→ a perceptual process in which memory & other cognitive processes are required for
interpreting incoming sensory info (we use knowledge gained from prior experience w/ stimuli to perceive
them)
- Require bottom-up processing to bring sensations of the letter shapes to our brain
- Then use knowledge & experience to recognize individual words
- If the brain expects to see a particular word, you are likely to see that word, even if it misspelled
- We always use bottom-up processing or else the info won’t be perceived
- Possible that bottom-up processing alone allows us to respond appropriately to simple stimuli
- As stimuli become more complicated, more likely to engage in top-down processing in addition to bottom-
up
--MEASURING PERCEPTION--
- Psychophysics→ the study of relationships b/w the physical qualities of stimuli & the subjective responses
they produce (studied by Gustav Fechner)
- Methods of psychophysics allow us to establish the limits of awareness (thresholds) for each of our sensory
systems
- Smallest possible stimulus that be detected @ least 50% of the time is known as
abs. threshold
- Absolute threshold→ the smallest amount of stimulus that can be detected
- Under ideal circumstances, our senses are surprisingly sensitive
-
Difference threshold
is the smallest diff b/w two stimuli that can be detected @ least 50% of the time
- Difference threshold→ the smallest detectable diff b/w two stimuli
- Amount of diff that can be detected depends on size of stimuli being compared
- As stimuli get larger, diff must become larger to be detected by an observer
--SIGNAL DETECTION--
- Adds the cognitive process of decision making to the process of sensation
-
Signal detection
is a two-step process:
1. The actual intensity of the stimulus, which influences the observer’s belief that the stimulus did occur
2. The individual observer’s criteria for detecting whether the stimulus occurred
- Signal detection→ the analysis of sensory & decision-making processes in the detection of faint, uncertain
stimuli
- Possible outcomes in signal detection
Participant response
Stimulus present
Stimulus absent
Yes
Hit
False alarm
No
Miss
Correct rejection
SUMMARY: Assessing Perception
Concept
Definition
Example
Absolute threshold
The smallest amount of stimulation
that is detectable
Seeing light from a candle flame 30
mi. away on a dark night
Difference threshold
The smallest diff b/w two stimuli
that can be detected
Being able to detect diff b/w two diff
weights
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Signal detection
Correctly identifying when a faint
stimulus is or is not present
A radiologist correctly detecting
cancer from a mammogram
5.2 HOW DO WE SEE?
- Vision→ The sense that allows us to process reflected light
- One of the most important sensory systems in humans
- ~50% of our cerebral cortex processes visual info
- Comparison: 3% for hearing & 11% for touch & pain
5.2 THE VISUAL STIMULUS
- Visible light is the energy w/in the electromagnetic spectrum to which our visual systems respond
- Light energy moves in waves
- Wavelength (λ) is the distance b/w successive peaks of waves
- Λ is decoded by visual system as colour or shades of gray
- Amplitude of waves is translated by visual system into brightness
- LARGE amplitude = appears bright
- SMALL amplitude = appears dim
- Gamma, x-rays, ultraviolet, infrared, mico, & radio waves lie outside visual light spectrum
5.2b THE BIOLOGY VERSION
- Human vision begins w/ the eye
- Hard outer covering helps w/ the fluid-filled eyeball retain its shape
- Toward the front of the eye, the outer covering becomes clear & forms the
cornea
- Cornea→ the clear surface @ the strong of the eye that begins the
process of directing light to the retina
- The cornea begins the process of bending light to form an image on
the back of the eye
- Traveling light enters the
pupil
, which is actually an opening formed
by the muscles of the
iris
- Pupil→ an opening formed by the iris
- Iris→ the brightly coloured circular muscle surrounding the pupil of
the eye
- The iris adjusts the opening of the pupil in response to the amount of
light present in the environment & to signals from the autonomic NS
- Arousal is detected w/ dilated pupils, while relaxation is associated w/
more constricted pupils
- Directly behind the pupil & iris is the main optical instrument of the
eye, the
lens
- Lens→ the clear structure behind the pupil that bends light toward the retina
- Muscles attached to the lens can change its shape which allows us to accommodate or adjust our focus to see
near or distant objects
- Behind the lens in the main chamber of the eye, & located on the rear surface of this chamber is the
retina
(a
thin but complex network of neurons specialized for the processing of light)
- Retina→ layers of visual processing cells in the back of the eye
- In the deepest layer of the retina are specialized receptors, the rods & cones that transduce the light info
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