Physiological Psychology Quiz 6

Cochlear implants can help treat some forms of sensorineural deafness

Left: Image of a person with a cochlear implant, showing the external components (transmitter and microphone)

Right: Cochlear implant X-ray; each of the 29 surgically implanted beads is a stimulating electrode that excites the auditory nerve at positions along the organ of corti that correspond to successively lower frequencies from the base to the apex (tonotopy)

• While hair cells are lost, the auditory nerve fibers are still there but there is nothing to stimulate them, sensorineural hearing loss

• Transmitter → transmits electrical signals to cochlea

• Microelectrodes implanted into the cochlea, they are stimulating electrodes, causes stimulation in response to sound based on the frequency

• Electrodes are along the band, and 

• Tonotopic arrangement

• Can help distinguish sounds that cannot be distinguished through lip reading alone

• The earlier in life this is done, the better the outcome

  • Adults don't usually do this, under 1 year is optimal

    • Because of plasticity in the auditory cortex

    • Synaptic plasticity

    • Dendritic growth

    • If you can apply the right amount of stimulation early in life, it works best

    • As you get beyond this critical period, the gains become less

Cochlear implants do not fully replicate normal human hearing, they treat some forms of Sensorineural Deafness by essentially replacing inner hair cells

• But over time the brain can adapt to the auditory signal

• Takes training

• People with cochlear implants struggle with music perception

  • Requires the ability to discriminate really small changes in pitch

  • They perceive music at flat and non-melody rich

  • With training + experience, people get better with differentiating sounds

• A microphone detects sounds and directs the cochlear implant circuitry to stimulate the auditory nerve at frequency-specific locations along the cochlea corresponding to the frequency/pitch of the sound

• This provides a somewhat crude simulation of hearing, but the brain can use this information to decipher speech and other sounds, and it gets better with training

Auditory System Summary/Review

The human ear consists of the outer, middle and inner ear, with each contributing to the sensation of sound

• The inner ear includes the cochlea, which transduces sound waves into a tonotopic map of sound frequencies based on the changing biophysical properties of the basilar membrane

  • The cochlear apex is tuned to low frequencies (~20 Hz), while the base is tuned to high frequencies (~20 kHz)

• Outer hair cells adjust the sensitivity of the basilar membrane, while inner hair cells are the auditory sensory receptors, responsible for transducing sound information to the brain

  • Hair cells have stereocilia which contain K+/Ca2+ channels that are mechanically opened when bent in the direction of the tallest stereocilium

• Tonotopy is preserved from the level of the cochlea all the way to primary auditory cortex—this is thought to be important for the perception of “pitch” mainly through “place coding”

• Brainstem auditory nuclei use different methods to localize sound depending on the frequency/pitch of the sound (low frequency = interaural time differences; high frequency = interaural intensity differences)

• There are three main categories of hearing loss depending on the location of the auditory issue

• Some forms of sensorineural deafness can be treated with cochlear implants which directly stimulate the auditory nerve—it’s a bit crude, but gets better with training!

VISION

Eye has evolved like 50 times

• Eye pupil can take different shapes

Do we see the world the same way a traditional camera does?

• Some things are the same

• But there’s differences

• To focus, camera lens moves

Lens

Refracts/bends light

• Ours doesn’t move back and forth tho

High focuses in on a spot in the back of the eye

Sensory transduction in the visual system

• Light rays converge on a single point

• Retinal cells:

  • Cones

  • Rods

  • (Photoreceptors)

  • These two are responsible for sensation

  • Also taking light rays and turning them into action potentials

  • There are more than just rods and cones in retina

• Retina is very thin, but it has 3 different layers

  • Sensory transduction

  • Sensory info makes it to primary visual cortex

  • Then higher cortices

Perception is about reconstruction

• This process is subjective, each brain does it differently

• Perception is all about interpretation by the brain!

Visual transduction: The process of converting living waves (sensory signal) from the external environment into an electrical signal (ex. action potentials) that the brain interprets–this process occurs in photoreceptors in the retina

Images are ambiguous

• Brain is making a decision on which color is the foreground or background

• Why is this static image moving?

  

OUr eyes 

The colors trick motion sensitive neurons in eyes

It all starts with light (electromagnetic waves)

• Light is an electromagnetic wave and particle “photons”

• We perceive “visible light” (400-700nm)

• Long wavelength “hot” light (red/orange) is lower energy than “cool” (blue/violet) short wavelength light

  • Shorter wavelengths (faster frequencies) have more energy than longer wavelengths

• We perceive different wavelengths as different colors

• Colors are themselves a perception, subjectively “colored” by the brain

Electromagnetic spectrum 

HIgher energy light repeats more often

→ We perceive as blue

Light travels very fast, lights is a particle (when you go to measure it)

Cones and color vision: The Trichromacy Theory

Cones

• Absorb light 

• Responsible for color vision

• Responsible for color vision

• Many types of cones

  • Respond differently to light of varying wavelengths, providing us with color vision

• We  humans (usually) have three different types of opsin proteins in our cones “photopsins” that are preferentially sensitive to visible light in either blue, green, or red wavelengths (left image)

• Equal mixing of the three primary colors causes equal activation of all three different cone types, resulting in a perception of “white” (right image)

Trimchromacy theory 

Humans perceive color through combined activity, not just vision and direction

Colors

Brain assigns colors

Brain perceives colors

Individual expression differences:

Blue + Black dress

→ Brain uses color constancy to interpret colors under different lighting

→ Adjusts in diff brightness etc

• If brain assumes warm yellow light, then subtracts yellow and makes the dress appear blue and black

Cross sectional anatomy of the eye

Cornea

• Outer convex/dome shaped part of the eye

• Convex tissue is transparent, light can pass through it, and refracts/bends light rays (allows light rays to enter the eye and provides most of the refractive “light-bending” power

• Cornea doesn’t change in shape unless you have lasik done

• The cornea of the eye (the curvature of which is fixed), bends light rays and is primarily responsible for forming the image on the back of the eye

• Refraction = bending of light rays (basis of eyeglasses, telescopes, and microscopes)

Cornea: has specific shape, because its a dome, when light rays enters eye it bends light (light refraction)

Pupil

• Dilation of the pupils is controlled by the sympathetic (autonomic) nervous system

• Constriction is controlled by the parasympathetic division

• Because usually both divisions are active, pupil size shows the result of the balance of both divisions

• Like the lens opening

  • Opening in the opaque disc

• In the middle of the iris

Iris

• Shades

• Lens opening

• The opaque disc

• Colored part of the eye

• Can contract and expands

• Controls the amount of light that enters the eye

Lens

• A transparent structure that can bend, which also refracts light so a clear image is formed on the retina–helps with focusing “accomodation”

• We can change the shape of our lens

• We can contact the muscles in our eyes to do what we

• Focuses light

  • Focus is adjusted by changes in the shape of the lens, controlled by the ciliary muscles inside the eye

Lens has to be more curved over a shorter working distance to see things

• Reason for needing glasses to read a book for ex. 

Retina

• Transparent & thin (0.5mm) structure, layer of light sensitive cells at the back of the eye that processes and passes visual information to the brain 

• Where the first stages of visual-information processing occurs

• Receptive surface inside the back of the eye

• Thin, but has distinct layers of light sensitive cells

• Bent or refracted by cornea, 

• Lens behind provides additional light rays, focuses views

• Parallel rays converge

• Air to liquid makes a difference, in a pool for example

• Temporal retina - towards head

• Nasal retina - towards nose

How we focus: Visual accommodation

light bent rays 

If screen is too close to you, it produces a very wide angle

So the lens has to really curve the lens, has to be big and fat for the short amount of 

The older we get, the worse it gets

• Muscles are still fine, but 

• Your ability to focus gets worse

• Old eye = presbyopia

• One of the many consequences of aging is that the lens stiffens and becomes less flexible as we age, leading to reduced visual accommodation by the lens (less curvature) and ultimately, to “cheater” glasses to read the newspaper

The retina: CNS tissue in the periphery

• Lots of blood vessels

• Red eye in photos = reflection of blood vessels

• There are neurons in retina

• Retina develops as part of the brain

→ Macula lutea = yellow body, oval spot in center of retina, contains yellow photopigments, area where we have highest visual acuity (sharpness, ability to discriminate fine details/sharpness)

→ Fovea

• Small “pit” at the center of the macula where visual acuity/sharpness is highest

• Area where you see a depression in the thickness of the retina

• Highest sharpness, resolution

• In the center of the maculutea

• High concentration of cones in the fovea provides high visual acuity in this region

→ Optic disc

• Located to the nasal side of the fovea

• Where blood vessels and ganglion cell axons leave the eye

• Where all our blood vessels enter our retina

• Where retinal nerve fibers (axons) leave the eye and travel to the brain via the optic nerve– this is your blindspot (no photoreceptors here!)

• Some axons leave retina through the optic disc

• Where axons leave retina

• Where blood vessels enter and leave the eye

• No photoreceptors here

  • So we have a blind spot in our vision

  • Binocural vision

  • One eye makes up for the other, both overlap

Blind spot

• There are no photoreceptors at the optic disc so there is a blind spot here that we do not normally notice

• There are many online sites that demonstrate your blind spot

Structure of the retina in cross section

• Purple and green are the rods and cones

  • Photoreceptors

• Phototransduction - initiated in the rods and cones

• All the way in the back of the retina

  • Light comes in from the bottom, photons have to pass through the layers to get to the back

• Rods and cones do not fire action potentials, but do release glutamate, they can depolarize or hyperpolarize, not considered neurons

• Blue cells: called bipolar cells, excitatory, do not fire action potentials, have graded responses

  • Receive info from rods and cones and pass info down to yellow (retinal ganglion cells), project to middle of brain (thalamus)

• All of these are excitatory

• Only retinal ganglion cells fire action potentials

• These three are the flow of visual information

• Photons get absorbed in the rods and cones, info gets passed on to the brain

• Horizontal cells

  • Make contacts among the receptor cells and bipolar cells

• Amacrine cells

  • Contact both the bipolar and ganglion cells

• These two are inhibitory, release GABA

*The rods, cones, bipolar cells and horizontal cells generate only graded local potentials, they do not produce action potentials

• These neurons affect each other through release of neurotransmitters proportional to graded changes in electrical potentials

• The ganglion cells do conduct action potentials

• Because the ganglion cells have action potentials and are relatively large, they were the first retinal neurons to have their electrical activity recorded

• From the receptor cells to the ganglion cells, enormous amounts of data converge and are compressed

• The human eye contains a lot of rods compared to cones, but there are only a little bit of ganglion cells to transmit that information to the brain

  • A great deal of information processing is done in the eye itself

Path of visual information: (top to bottom)

 Photoreceptors → Horizontal cells → Bipolar cells → Amacrine cells → Ganglion cells → Brain

Light: (bottom to top)

Pigment epithelial

• Cells that support rods and cones

• Recycle protein they use

• DIternal = awake during the day

• Ours is black in color, because it absorbs excess coors

• Absorbs excess photons, because otherwise they’d be bouncing around everywhere

• Nocturnal animals try to avoid being in environments where there is low light

  • Photons are reflected back out, these animals’ perception of the dark is maximizing every photons that come in 

Overview of retinal cells

Retina: A photosensitive sheet in the back of the eye & part of the CNS

Contains (in order of light passage):

1. Retinal ganglion cells

   1. Only source of output from the retina

   2. Axons form the optic nerve and project to the thalamus

   3. Only retinal cells that fire action potentials

      1. Release glutamate, excitatory

2. Amacrine cells

   1. Input from bipolar cells

   2. Receive input from 

   3. Influence ganglion cells (release GABA–inhibitory, no APs

3. Bipolar cells

   1. Input from the photoreceptors, output to the ganglion cells, on-center or off-center (release glutamate, excitatory, no APs)

4. Photoreceptors 

   1. The ONLY* light sensitive cells in the retina (rods & cones); output to the bipolar and horizontal cells (release glutamate, excitatory, no APs

What we know so far…

There’s a central part of the retina and a peripheral part of our retina

Central is where fovea is

Fovea is where the highest acuity is

Thinner, which is why we have better acuity

The fovea (central vision) in cross section

• All other cell types Pushed out of the way]

• When you orient your eyes on an object, you are directing light onto your fovea (central vision) which has the highest visual acuity (best “sharpness” or “resolution”)

• This high acuity is in part due to the fact that in the center of the fovea, blood vessels, ganglion and bipolar cells are displaced laterally (pushed to the side) to allow light to directly strike cones (less light scattering by blood vessels and other cells

Acuity is a measure of the ability to resolve fine details from a given distance

The degree of convergence between photoreceptors and RGCs is different in central/foveal vs. peripheral vision

• Convergence:

  • Describes how many photoreceptors and BPCs (bipolar cells) signal a single retinal ganglion cell (RGC)

    • In the image, the middle ones (blue) are bipolar cells and the upper ones (orange) are retinal ganglion cells

• Center of the fovea: Low ratio of cones to RGCs(retinal ganglion cells) (1:1 in very center; good for acuity; preserves spatial information)

  • Central retina is mostly cones 

• Rods on outer periphery: High ratio “pooling” of rods to RGCs(retinal ganglion cells) (low acuity; good for detecting light in dim conditions, but spatial information is lost)

  • Peripheral retina is mostly rods

Mostly cones in the middle (central retina), rods around (peripheral retina)

• Low convergence = high acuity

• Low acuity = high convergence

High convergence is bad for acuity, but good for sensitivity

* Low convergence in the fovea is good for spatial acuity (precision), while high convergence in the peripheral retina is good for low light conditions (sensitivity), but bad for spatial acuity 

• So low convergence will give you more clear and sharp visuals in fovea

  • In the fovea, there is a 1:1 relationship where one cone connects to one bipolar cell, to one ganglion cell

  • You get maximal spatial acuity because there is just one at a time

• And high convergence will give you worse spatial acuity but better in-the-dark visibility

  • In the periphery, you get spatial summation, amplifies tiny amounts of light to help you see better in the dark, 

  • The trade-off is that the brain cannot tell which specific rod was triggered within that large group, resulting in blurry, low-acuity visuals.

  • The brain thinks that sound came from somewhere in the space, but it cannot distinguish from where

• Which of these two retinal ganglion cells (A or B) would be able to distinguish between light projected on location 1 v.s location 2?

The leftmost retinal ganglion cell cannot tell, not good for acuity but yes for low acuity

Car line analogy:

You're at the light, turns green, you're distracted, and if there's only one behind you you know who beeped, if there’s multiple you don’t know who

*Central retina can tell diff between the two, the peripheral cannot because it’s receiving multiple signals

Rods

For seeing things in periphery (more numerous than cones in the periphery of the retina)

• Long, narrow (look like rods)

• Active in low levels of light

*From textbook:

*Rods are absent in the fovea, but are more numerous than cones in the periphery of the retina

*They are also more sensitive to dim light than the cones

• Input from many rods converges on ganglion cells

• There is greater convergence of information from photoreceptors to ganglion cells in rods than in cones, so acuity is greater in the cone-rich fovea

• A cone here is more likely to have exclusive control over a single ganglion cell

• Rods provide high sensitivity with limited acuity, while cones provide high acuity with limited sensitivity

Lecture 20 Thought Questions

1. What is a cochlear implant, what are the basics of how it works (e.g. how does it take advantage of cochlear tonotopy), and what form of hearing loss can it help treat? When in life should cochlear implant surgery be done in order to have the best results?

2. Where does sensation occur in the visual system? Where does perception occur? Be as specific as possible.

3. Describe the major anatomical components of the eye. What role do the cornea, lens, and retina play in vision?

1. What is meant by the term, “visual accommodation,” and how does this process help focus light rays onto the retina? Which structure(s) of the eye are responsible for refracting (bending) light? What shape does the lens take to focus on an object far away? What about up close? Why?

Lecture 21 Thought Questions

1. Describe the major cell types of the retina, their function, and how they’re connected/wired (cytoarchitecture) Which are excitatory and which are inhibitory? In which direction does light flow through the retinal layers? In which direction does visual information flow through the retinal layers?

2. Describe the concept of visual acuity, and provide two reasons why visual acuity is greater in the fovea/central vision vs. the peripheral retina. How does visual acuity differ from visual sensitivity? Where in the retina is visual sensitivity the greatest and why?