MCAT Behavioral Sciences - Sensation and Perception

sensation

a raw signal, which is unfiltered and unprocessed until it enters the central nervous system; performed by receptors in the peripheral nervous system, which forward the stimuli to the central nervous system in the form of action potentials and neurotransmitters

transduction

taking the physical, electromagnetic, auditory, and other information from our internal and external environment and converting this information into electrical signals in the nervous system

Perception

processing this information within the central nervous system in order to make sense of the information’s significance; include both the external sensory experience and the internal activities of the brain and spinal cord

Sensory receptors

are neurons that respond to stimuli by triggering electrical signals that carry information to the central nervous system; may encode multiple aspects of a stimulus

distal stimuli

Physical objects outside of the body that can be sensed

proximal stimuli

Photons, sound waves, heat, pressure, etc. are frequently produced by objects and directly interact with sensory receptors

psychophysics

relationship between the physical nature of stimuli and the sensations and perceptions these stimuli evoke

threshold

the minimum amount of a stimulus that renders a difference in perception

absolute threshold

minimum of stimulus energy that is needed to activate a sensory system

e.g. sweetness (a teaspoon of sucrose dissolved in two gallons of water); light (naked eye may see one candle burning thirty miles away)

threshold of conscious perception

level of intensity that a stimulus must pass in order to be consciously perceived by the brain or reach the higher-order brain regions that control attention and consciousness

subliminal perception

information that is received by the central nervous system but that does not cross this threshold

just-noticeable difference (jnd) threshold

to the minimum change in magnitude required for an observer to perceive that two different stimuli are, in fact, different

e.g. pitch (~3Hz)

discrimination testing

A participant is presented with a stimulus. The stimulus is then varied slightly, and researchers ask the participant to report whether they perceive a change. The difference continues to be increased until the participant reports they notice the change, and this interval is recorded as the just noticeable difference.

Weber’s law

observation that difference thresholds are proportional and must be computed as percentages; discovered by Ernst Heinrich Weber (1795–1878)

Signal detection theory

studies how internal (psychological) and external (environmental) factors influence thresholds of sensation and perception.

noise trials

trials in which the stimulus is presented

catch trials

trials in which the stimulus is not presented

hit (signal detection)

a trial in which the signal is presented and the subject correctly perceives the signal

miss (signal detection)

a trial in which the subject fails to perceive the presented signal

false alarm (signal detection)

a trial in which the subject indicates perceiving the signal, even though the signal was not presented

correct negative (signal detection)

a trial in which the subject correctly identifies that no signal was presented

adaptation

ability to detect a stimulus can change over time; physiological and psychological

eye

specialized organ used to detect light in the form of photons

sclera

white of the eye, thick structural layer that covers much of the exposed eye excpet the cornea

choroidal vessels

complex intermingling of blood vessels between the sclera and the retina

retinal vessels

blood vessels that go to and from the retina

retina

innermost layer of the eye; contains the actual photoreceptors that transduce light into electrical information

cornea

a clear, domelike window in the front of the eye, which gathers and focuses the incoming light

anterior chamber

lies in front of iris

posterior chamber

between iris and lens

iris

colored part of the eye; composed of muscle

dilator pupillae

muscle in iris that opens the pupil under sympathetic stimulation

constrictor pupillae

muscle in iris that contracts the pupil under parasympathetic stimulation

choroid

vascular layer of connective tissue that surrounds and provides nourishment to the retina

ciliary body

produces the aqueous humor that bathes the front part of the eye

aqueous humour

transparent water-like fluid similar to blood plasma, but containing low protein concentrations

canal of Schlemm

where aqueous humor flows into

lens

behind the iris; helps control the refraction of the incoming light

ciliary muscle

parasympathetic control; pulls on suspensory ligaments to change the shape of the lens to focus on image as distance varies

accomodation

focus on an image as the distance varies

vitreous humour

a transparent gel behind the lens that supports the retina

ganglia

collections of neuron cell bodies found outside the central nervous system

projection areas

areas in the brain that further analyse sensory input

photoreceptors

respond to electromagnetic waves in the visible spectrum (sight)

mechanoreceptors

respond to pressure or movement. Hair cells, for example, respond to movement of fluid in the inner ear structure (movement, vibration, hearing, rotational and linear acceleration)

nociceptors

respond to painful or noxious stimuli (somatosensation)

thermoreceptors

respond to changes in temperature (thermosensation)

osmoreceptors

respond to the osmolarity of the blood (water homeostasis)

olfactory receptors

respond to volatile compounds (smell)

taste receptors

respond to dissolved compounds (taste)

retina

a screen in the back of the eye consisting of neural elements and blood vessels; converts incoming photons of light to electrical signals

duplexity/duplicity theory of vision

the retina contains two kinds of photoreceptors: those specialized for light-and-dark detection and those specialized for color detection

cones

eye cells that sense color and fine detail; most effective in bright light, named for the wavelengths at which they have highest light absorption: short (S, blue), medium (M, green), and long (L, red)

rods

eye cells that sense light and dark; most effective in low light; highly sensitive to photons and is somewhat easier to stimulate than a cone cell

rhodopsin

pigment in rods

macula

central section of the retina; contains many cones

fovea

centermost region of the macula, only cones

blind spot/optic disk

region of the retina where the optic nerve leaves the eye, devoid of photoreceptors

bipolar cells

highlight gradients between adjacent rods or cones; synapsed with rods and cones; located in front of rods and cones

ganglion cells

axons group together to form optic nerve; synapsed with bipolar cells; located in front of rods and cones; output from each ganglion cell represents the combined activity of many rods and cones

amacrine cells

receive input from multiple retinal cells in the same area before the information is passed on to ganglion cells (a); accentuate slight differences between the visual information in each bipolar cell (edge detection)

horizontal cells

receive input from multiple retinal cells in the same area before the information is passed on to ganglion cells (h); accentuate slight differences between the visual information in each bipolar cell (edge detection)

visual pathways

both the anatomical connections between the eyes and brain and to the flow of visual information along these connections

temporal/nasal retinal fibers

where the photons from an object travel to the opposite side to reach your eyes

left side stimulus → right temporal & left nasal → right side brain

right side stimulus → left temporal & right nasal → left side brain

temporal/nasal visual fields

the placement of a stimulus in space relative to each eye is opposite of the retinal path they take

optic chiasm

nasal fibers from the left and right eyes cross paths to route apporpriate information to each side of the brain

optic tracts

reorganized pathways going to each side of the brain

lateral geniculate nucleus (LGN)

in thalamus, synapse with nerves that then pass through radiations in the temporal and parietal lobes to the visual cortex in the occipital lobe

visual cortex

Region of the brain that processes visual information

occipital lobe

Part of the brain at the back of the head, contains visual cortex

thalamus

a well-known connecting and routing center of the forebrain

superior colliculi

control some reflexive responses to visual stimuli and reflexive eye movements; midbrain

parallel processing

the brain’s ability to analyze information regarding color, form, motion, and depth simultaneously, using independent pathways in the brain

form

the shape of an object, but also our ability to discriminate an object of interest from the background by detecting its boundaries

parvocellular (P) cells

in the lateral geniculate nucleus; high spatial resolution but low temporal resolution; colour and low contrast

magnocellular (M) cells

in the lateral geniculate nucleus; high temporal resolution but low spatial resolution; monochrome and high contrast

spatial resolution

ability to detect very fine detail when thoroughly examining an object

temporal resolution

ability to detect a fast-moving object

Depth perception

our ability to discriminate the three-dimensional shape of our environment and judge the distance of objects within it

binocular neurons

specialised cells repsonsibles for comparing the inputs to each hemisphere

feature detectors

detects a very particular, individual feature of an object in the visual field

vestibular sense

ability to both detect rotational and linear acceleration and to use this information to inform our sense of balance and spatial orientation

pinna/auricle

cartilaginous outside part of the ear; channel sound waves into external auditory canal

external auditory canal

directs the sound waves to the tympanic membrane

tympanic membrane/eardrum

vibrates in phase with the incoming sound waves

intensity

corresponds to an increased amplitude

ossicles

three smallest bones in body; in middle ear; help transmit and amplify the vibrations from the tympanic membrane to the inner ear

malleus (hammer)

ossicle 1; attached to tympanic membrane

incus (anvil)

ossicle 2

stapes (stirrup)

ossicle 3; rests on the entrance to cochlea

Eustachian tube

connects auditory canal to nasal cavity; helps equalize pressure

between the middle ear and the environment

bony labyrinth

hollow region of the temporal bone containing the cochlea, vestibule, and semicircular canals

membranous labyrinth

a continuous collection of tubes and chambers inside the bony labyrinth; filled with endolymph

endolymph

a potassium-rich fluid that fills mebranous labyrinth

perilympth

suspends membranous labyrinth in bony labyrinth; transmits vibrations from the outside world and cushions the inner ear structures in scalae

cochlea

a spiral-shaped organ that contains the receptors for hearing, divided in three scalae

organ of Corti

hearing apparatus on basilar membrane in middle scala; composed of thousands of hair cells, which are bathed in endolymph; transduce the physical stimulus into an electrical signal

basilar membrane

stiff structural element within the cochlea of the inner ear that separates two scalae

tectorial membrane

relatively immobile membrane over the organ of Corti

round window

a membrane-covered hole in the cochlea, permits the perilymph to

actually move within the cochlea