Sensation and Perception Part 1

Sensation

the detection of physical stimuli and transmission to the brain

Perception

the brain’s further processing, organization, and interpretation of the sensory info

Sensation Examples

Loud sound and flashing light

Perception Examples

Police siren and pull over!

Bottom-up processing

perception based on the physical features of the stimulus

Top-down processing

how knowledge, expectations, or past experiences shape the interpretation of sensory information

Transduction

turning physical stimuli into neural impulses

Sensory receptors

receive sensory stimuli

• pass info to brain via neural impulses

• all info sent to/sorted in thalamus except smell!

Quality vs Quantity

the brain needs qualitative and quantitative info about a stimulus

Qualitative Information

consists of the most basic qualities of a stimulus

Quantitative Information

consists of the degree, or magnitude of those qualities

Qualitative Information- Ex. traffic light

sensory receptors respond to differences by firing in different combinations

• a green light is coded by different receptors than a red light

Quantitative Information- Ex. traffic light

Sensory receptors respond to differences by firing at different rates

• a bright light causes receptors to fire more rapidly (at a higher frequency) than a dim light

Psychophysics

• scientific study of psychological experience of physical stimuli

• researchers present subtle changes in stimuli (observe how participants respond)

Absolute threshold

the minimum intensity of stimulation that must occur before you experience a sensation

• Ex: starting to use a plugin

Difference threshold

smallest noticeable difference between two stimuli

• also called the “Just Noticeable Difference” or JND

• Ex: increasing the plugin’s intensity

Weber’s Law

Amount of change for JND is proportional to the original stimulus

• the smaller a stimulus is at the start, the less it has to change for you to notice

• the larger the stimulus at the start, the more it has to change

Weber’s Law Example

• lift a 5 lb. weight vs a 10 lb. weight (noticeable- 5 lb. gain is a huge % increase!)

• lift a 200 lb. weight vs a 205 lb. weight (not noticeable- 5 lb. gain is a ting % increase!)

Signal detection theory (SDT)

• detection of a stimulus requires a judgement

• it is not an all-or-nothing process

Response bias

a participant’s tendency to report or not report detecting the signal in an ambiguous trial

Sensory adaptation

a decrease in sensitivity to a constant level of stimulation

• information stops being meaningful

McGurk Effect (McGurk and MacDonald, 1976)

• our perceptions often rely on multiple senses

• input from one sense can confuse another sense

• “ba” sound appears to be different sound “va”

Cornea

where light first passes through- focuses light

Lens

bends light further inward toward the retina- accommodation

Retina

contains sensory receptors that transduce light into neural signals

Pupil

determines how much light enters the eye

Iris

color and determines the size of the pupil

Accommodation

behind the iris, muscles change the shape of the lens

• flatten lens to focus on distance objects

• thicken lens to focus on closer objects

Rods

(120 million)- shape/contrast

• responsive even in dark (scotopic)

Cones

(6 million) - color

• responsive in daylight (photopic)

Fovea

center of retina, high density of photoreceptors

• greater visual acuity/resolution

• high number of cones, low number of rods

Peripheral

sides, low density of photoreceptors

• poor visual acuity

• high number of rods, low number of cones

Optic Nerve

(no sensory receptors)

Optic Chiasm

½ of the axons of the optic nerve cross to contralateral side

Optic Tract Projects to multiple areas

Superior Colliculus and Lateral geniculate nucleus

Superior Colliculus

processes movement

Lateral geniculate nucleus (LGN) of thalamus

relays info bound for primary visual cortex

Blind spot

where the optic nerve leaves the eye

• no sensory receptors

Why don’t we notice our Blind Spot?

the brain fills in the missing info

• both eyes open = uses info from other eye

• one eye open = pattern completion

Secondary visual areas (the what system)

temporal lobe

• identification of visual objects

• occipital- temporal pathway

• damage: can lead to visual agnosia

Secondary visual areas (the where system)

parietal lobe

• location of visual objects and guiding of actions

• occipital- parietal pathway

• damage: can lead to problems with reaching for seen objects

Color Perception

• See a small portion of the spectrum (400 -700 nm)

Lower (shorter) wavelengths

perceived as blue/purple

Middle wavelengths

perceived as green

Higher (longer) wavelengths

perceived as red and orange

Trichromatic Theory

Three types of cones

• Each responsible for different

wave lengths

• Activation leads to color

perception

• Some colors can mix

– EX) Yellow

• Doesn’t explain unimaginable

colors

Color Blindness

• There are two main types of color blindness, determined by the relative activity among the three types of cone receptors.

• red–green color blindness

• blue–yellow color blindness

• These genetic disorders occur in about 8 percent of males but less than 1 percent of females.

Opponent-process theory

• receptor cells are linked in pairs working in opposition

• opposite colors inhibit signals

• Red and green

• Yellow and blue

• Black and white

How do you perceive the car?

“unified whole”

• innate principles to view sensory information as a whole

Figure and Ground

In identifying what is “figure,” the brain assigns the rest of the scene to the background.

Similarity

We tend to group figures according to how closely they resemble each other, whether in shape, color, or orientation

Proximity

The closer two figures are to each other, the more likely we are to group them and see them as the same object

Good Continuation

We tend to see the green as one rectangle rather than two rectangles

Closure

We tend to complete figures that have gaps

Common Fate

Grouping elements together if they move in the same direction, travel at the same speed, or change in a synchronized way