2. Senses and Perception Flashcards
Vision involves __% of the cerebral cortex
30%, more than any other sense
Animals used to study vision
Knowledge of how light energy is converted to electrical signals comes from studies of fruit flies and mice. Higher level visual processing studied in monkeys and cats
Process of light entering eye and being processed
Light passes through cornea and enters eye through pupil. Iris regulates amount of light entering by changing pupil size. Lens bends light to focus it on retina. Cornea does initial focusing, lens can thicken or flatten to adjust to distance. Signals then travel via optic nerves to brain
3 types of neurons in retina
Photoreceptors, interneurons, ganglion cells
Location of photoreceptors
Most peripheral layer. Light reaches other cells before photoreceptors
Role of ganglion cells and interneurons
Don’t directly respond to light, but process and relay information from photoreceptors
What is the optic nerve made of?
Axons of ganglion cells
Number of photoreceptors in each eye
125 million
Transduction
Process of converting one form of energy to another, occurs in most sensory systems
Rods
95% of photoreceptors. Extremely sensitive, enable sight in dim light
Cones
Pick up fine detail and colour. 3 types, each sensitive to different colours (red, green, blue). Sensitivities overlap, different combinations.
Area of retina with most cones
Centre, so vision there is sharper than in periphery
Fovea
Very centre of retina. Where cones are most densely packed. Only red and green cones
Macula
Area around fovea, critical for reading and driving.
Macular degeneration
Death of photoreceptors in macula, leading cause of blindness in people over 55
How does the relationship between ganglion cell and photoreceptors affect vision
Each ganglion cell in macula receives input from very few cones, allowing for resolution of fine details. Near margins of retina, each ganglion cell receives inputs from several photoreceptors. Less detailed
Receptive field
Portion of visual space providing input to a ganglion cell
How is light processed?
Comparing amounts of light hitting small, adjacent areas on retina. Receptive fields tile the retina, providing a representation of the visual scene. Receptive field is activated when light hits its centre, inhibited when light hits donut-shaped area around centre. If light strikes both donut and hole, ganglion cell responds weakly.. Maximizes perception of contrast, key to object detection
How is the optic nerve blind spot filled in?
Using information from other eye
Where do nerve fibres from eyes first cross?
Optic chiasm. Fibres carrying information from left side of both retinas continue together on left side of brain and vice versa.
Passage of optic information starting from eyes
Optic nerve - lateral geniculate nucleus (part of thalamus) - primary visual cortex
Middle layer of visual cortex
Receives messages from thalamus, has receptive fields similar to those in retina, preserve retina’s visual map
Cells above and below visual cortex
More complex receptive fields, register stimuli shaped like bars or edges or with particular orientations
Two processing streams into which visual signals are fed in the visual cortex
Dorsal stream, which heads to parietal lobe. Ventral stream, which heads to temporal lobe. Carry out separate processing of unconscious vision. Ventral = what, dorsal = where
Binocular vision
Seeing with two eyes, allows for perception of depth because each eye sees things from slightly different angles. Only works if both eyes are equally active and properly aligned, visual fields must overlap. Allows for larger visual field
Strabismus
Crossed eyes, results in decreased depth perception. Children initially have good vision in both eyes, but because they can’t fuse images from both eyes, start to favour one and lose vision in other. Vision can be restored only if treated before 8.
Passage of sound through ears
Sound reaches pinnae of ears, funneled into each ear canal to reach eardrum, which vibrates and sends vibrations to three bones in middle ear: malleus (hammer), incus (anvil), and stapes (stirrup). Stapes acts like a tiny piston, pushing on oval window, a membrane that separates air-filled middle ear from fluid-filled cochlea of inner ear. Oval window converts mechanical vibrations of stapes to pressure waves in fluid of cochlea, where they’re transduced into electrical signals by hair cells
Basilar membrane and role in hearing
Runs along inside of cochlea. Tuned to different frequencies. When fluid ripples, membrane moves, vibrating to higher-pitched sounds near oval window and to lower-pitched sounds in centre. Hair cells located on top of membrane. When membrane vibrates, stereocilia extending from hair cells bend against tectorial membrane, opening channels that allow ions in fluid to rush in, converting physical movement to electrochemical signal. Excites auditory nerve.
Passage of information starting from auditory nerve
Auditory nerve - brainstem - thalamus - auditory part of cerebral cortex
Each hair cell responds to a narrow range of frequencies determined by…
Its location on the cochlea
Side of brain responsible for understanding speech
Sound processed on both sides of brain, but left side is typically responsible for speech
Damage to Wernicke’s area causes…
Inability to understand speech, although hearing is unimpaired
Most common cause of hearing loss
Loss of hair cells. Don’t regrow
Regeneration of taste receptor cells and olfactory neurons
Taste receptor cells regularly regenerate, olfactory neurons are only sensory neurons continually replaced throughout life
How many taste buds do humans have?
Between 5000 to 10,000 taste buds
When do we begin to lose taste buds?
Around 50
How many sensory cells does each taste bud contain?
50-100
What are the five basic tastes?
Sweet, sour, salty, bitter, umami (savoury)
Specific tastes are limited to specific regions of the tongue. True or false?
False. All tastes are detected all across the tongue
Taste buds send signals through…
3 cranial nerves: facial, glossopharyngeal, and vagus nerves
Passage of taste information starting from cranial nerves
Brainstem - thalamus - gustatory cortex in frontal lobe and insula where tastes are identified
Process of smelling
Odors bind to olfactory cells on mucus membrane inside nasal cavity. Axons of these cells enter olfactory bulbs (2, one for each nostril) after crossing through small holes in skull. Information then travels to olfactory cortex in temporal lobe. Only sensory system that sends information directly to cerebral cortex without passing thalamus first.
How many different types of olfactory cells do we have, and how many different smells can we identify?
1000 different types of olfactory cells, can identify 20x as many smells
How do olfactory cells respond to odors?
Have cilia that are receptive to different odors
When smelling, what exactly is being transmitted to the brain?
Specific smell will stimulate a unique combination of cells, creating a distinct pattern which is transmitted to brain
System responsible for touch sensations
Somatosensory system
Passage of touch sensations to somatosensory cortex
Receptors - spinal cord - thalamus - somatosensory cortex
Types of fibres along which touch sensations travel
A-beta fibres: myelinated fibres with thick axons. Faster, evokes sharp pain
C fibres: thin, unmyelinated axons. Nerve endings spread over larger area. Slower, evokes dull ache
Sensitive areas stimulate _____ regions of cortex
Larger
How is sensitivity measured?
Two-point discrimination: minimum distance between two points on skin that a person can identify as distinct stimuli.
What characteristic do sensitive areas have?
Acuity is greatest in most densely nerve-packed areas
Pain is what kind of experience?
Sensory and emotional
Nociceptors
Respond to strong stimuli, helps detect true danger. Different types are sensitive to different types of pain (thermal, mechanical, chemical). Also respond to spicy food. Some respond to itch (e.g., histamine receptors activated when skin irritation, bug bites, and allergies trigger release of histamine)
Prostaglandins
Enhance sensitivity of receptors to tissue damage, make you feel pain more intensely. Contribute to allodynia, condition where even soft touch can produce pain
Neuropathic pain
Hypersensitivity to pain. Due to nervous system malfunction, not injury
Diabetic neuropathy
Nerves in hands/feet damaged by prolonged exposure to high blood sugar, send signals of numbness, tingling, burning, pain
Periaqueductal grey matter
In brain stem. Cortex sends it messages. Activates pathways that modulate pain, triggers release of endorphins.
