Psyc 211 Lecture 9 Vision part 1

  • Sensation- refers to how cells of the nervous system detect stimuli in the environment (light, sound, heat) and how they transduce these signals into a change in membrane potential and a NT release

  • Perception- refers to the conscious experience and interpretation of sensory information

 

Sensory neurons

  • Sensory neurons- are specialized cells that detect a specific category of physical events such as:

    • The presence of specific molecules

      • Small,t aste, nausea, pain

    • The presence/ absence of physical pressure (ion channels)

      • Touch, stretch, vibration, acceleration, gravity, balance, hearing, thrist pain

    • The temperature

      • Ion channels gated by the temperatutre: heat, cold, pain

    • The pH of a liquid is it liquid (acidic or alkaline)

      • Sour taste, suffucation, pain

    • Electromagnetic radiation (light)

      • Vision

 

Sensory Transduction

  • Sensory neurons have specialized receptors that transduce sensory stimuli into a change in the membrane potential

  • Sensory neurons come in all shapes and sizes

  • Many sensory neurons do not have axons or action potentials but they all release neurotransmitter

  • Sensory neurons that do not have action potentiatls release NT in a graded fashion, dependent on their membrane potential

  • The more depolarized the more NT they release the less, the less they release- non action potential sensory neruons are not all or none

 

Photoreceptors

  • Photoreceptor cells

    • Sensory neruons responsible for vision

    • The cells do not have action potentials

    • These cells transduce the electmagnetic energy of visible light into a change in the membrane potential, which affects how much NT they release

  • Opsins- light- sensitive proteins

    • Needed to transduce light into a change in membrane potenbtial

    • The opsisna cells are metabotropic receptors

    • They are only sensitive to light because they bind a molecule of retinal which changes shape in repones to light

    • The change in the shape of retinal is what activates this metabotropic receptor

  • Retinal- small molecule (syntheiszed from vitamin A) that attaches to the opsin proteins in the photoreceptor cells in our eyes

    • This retinal molecule is what technically absorbs the electromagnetic energy of visible light that allows us to see

 

The two configurations of the retinal molecule

  • When retinal absorbs a wavelength of visible light it actitvates the opsin protein

  • This launches an intracellular g protein singaligh cascade that changes membrane potential of the photoreceptor, affecting how much NT is released

 

Neural Transduction of Light

  • 4 types of photoreceptor cells contribute to our concious preception of vision

  • Each one expresses a different type of metabotropic opsin protein

    • Red cone cells expresses red cone opsin

    • Green cone cells expresses the green cone opsin

    • Blue cone cells express the blue cone opsin

    • Rod cells expresses the rhodoosin opsin

  • Each of these opsin protines are sensitive to different wavelengths of light according to how they hold the retinal molecule

  • Rod cells were the last to evolve: they are 100t imes more sensitive to light than the cone cells

  • Concious vision perception come from these four proteins

 

What the Eyes detect

  • Visible light refers to electromagnetic energy that has a wavelength between 380 and 760 nm. We detect this light using four kinds of pr cells

  • Light is considered to be a distrubance the electromagnetic field of space

  • Gamma rays- high energy light

    • Short wavelength

    • Unlikely to interact with physical matter

    • Ionizing radiation- happens in the case that gamma rays do interact with atoms (unlikely) but can cause a distrubnace in the system- cancer

  • Xrays

    • Longer wave lengths, dense electrons

    • High probability of hitting atoms

    • Less dangerous than gamma rays but in a high enough concentration can cause cell death and cancer

    • Ionizing radiation

  • Ultraviolet rays

    • Long enough to interact with physical matter

    • No ionizing radiation

    • Doesn't pass through skin

    • Can cause skin damage (sun, sunburn)

    • Melonin- a protein designed to absorb uv light

  • Visible light spectrum

  • Infrared rays

  • Radar

  • Telvesion and radio broadcast bands

 

Cone Photoreceptors: Trichromatic Coding

  • Blue light cone opsins are most sensitive to short wavelength light

  • Green cone opsisna re most sensitive to medium

  • Red cone opsins are most sensitive to long wl

  •  colour perception is a function of the relative rates of activity acorss the three types of cone cells

  • Each of opsin proteins have different sensitivity to light

 

Additive versus Subtractive Light

  • Three primary colours of light

    • Red

    • Green

    • Blue

  • Sunlight is white light since it contains an equal mixture of all colours

  • The primary colors of paint

    • Yellow

    • Cyan

    • Magenta

  • Paint absorbs light and doesn't reflect it therefore the way it expresses light is different

 

Perceptual Dimensions of Colour and Light

  • Our perception of light and color has three dimensions

    • Brightness- intensity (luminance, amount)

    • Saturation- purity (in terms of wavelength mixture)

    • Hue- dominant wave length (color)

  • If brightness is zero the image is completely black. Hue and saturation have no impact if there is no brightness

  • If there is brightness and saturation is 0. you are in the middle of the color cone where there is an equal distribution from all visible wavelengths these are gray scale colours (black and white) all wavelegnths are equally present

  • If saturation is 0< the hue indicates the light it is saturated with

 

Color Vision Deficiency (colour blindness)

  • Protanopia- absence of red cone opsins

    • Males are more predisposed

    • Females have no copies (2 chromosomes)

    • People with this condition have trouble distinguishing colrs in green- yellow red spectrum

    • Some people will have simple mutations of the red cone opsins produces less pronounces deficits in color vision- mutations in red cone opsin hinger color vision if they make it act more lije the green cone opsin

    • Visual acuity is normal because red cone cells swtich to using the green cone opsin

  • Deutranopia- absence of green cone opsin (1% of males)

    • Have trouble distinguishing colors in the green- yellow specturm

    • Visual acuity is normal because green cone cells switch to using red cone opsins

    • Simple mutations in the green cone opsin (6% of males) produce less pronounced deficits in colour)

  • Tritanopia- absence of blue cone opsin (1% of the population)

    • Blue cone opsins do not compensate fir this in any way

    • Blue cone opsin not really sensitive to light anyway

    • Visual acuity is not normally affecte

  • Achromatopsia- comeplte color blindness

    • Mutations in the g protein signaling cascade that is used by all the cone opsins

 

What the Eyes Detect

  • Rhodopsin protein- 100 times more sensitive to light (rod cells)

  • Cone cells concentrated in the middle of the eye- this is where we see colour

  • Peripheral vision is all rod cells

    • Not sensitive to colour

 

Anatomy of the eye

  • Conjactiva- muous membrane that line the eyelid (this is what prevents things like contact lenses from fully penetrating the eye)

  • Cornea- the outer layer of the eye. Focuses incoming light in a fixed amount

  • The sclera- opaque and does not permit entry of light

  • Iris- the ring of muscle. The contraction and relaxation of this muscle determines the size of pupil which determines how mught light enters the eye

  • Lens- consists of several transparent layers. We change the shape of this lens to focus near versus far, process known as accomodation

  • Retina- the interrior lining of the eye. Photoreceptor cells are located in the furthese back layer of the retina

    • The periphery of the retina only contains rod cells

  • Fovea- the centre of the retina. Primaily contains cone cells

  • Optic disk- where the blood vessels enter and leave the eye. It is also where the optic nerve exits the eye carrying visual information to the brain. There are no photoreceptors in this spot, so it is a blindspot.

 

Movement of the eye

  • Eyes are suspended in bony sockets called orbits

  • Six extraocular musckles of the sclera: the tough outer white of the eye. These muscles rotate and hold it in place

  • Saccadic eye movements- rapid, jerky shifts in gaze from one points to another

  • Pursuit movements- when we maintain focus on object that is moving. This is the only time our eyes appear to be calm and move smoothly, slowly

 

Organization of the Retina

  • Visual informatin propogates from photoreceptor cells- bipolar cells- retinal ganglion cells- brain

  • Light must pass through each of the cell layers in the retina before it can reach the opsin proteins in PR cells

  • There does seem to be a good reason for this awkward arragement

 

Retina Fovea Versus Periphery

  • In the fovea there is an equal number of PR cells, bipolar cells, and retinal ganglion cells

  • There is no compression of information

  • The fovea is the only part of our retina where our visual acuity is good to read text

  • Fovea is mostly cone cells, which suport color vision, so the fovea supports high resolution, colour vision

  • Fovea supports high resolution colour vision but only when there is a sufficient amount of light

  • Outside of the fovea (in the periphery of our retina) there is a massive compression (averaging of information) our visual acuity is blurry

    • High density of rod cells

  • Peripheral vision is very sensitive tp dim light but provides low reolustion grayscale images. What we see in peripheral vision 20 feet away is what we see in our fovea 200 feet away (20/ 200)

 

Neurons in the Retina

  • Photoreceptor cells- located in the furthest part of the reinta. Exppress the opsin proteins that transduce light. Photoreceptor cells synapse on bi[polar cells

  • Bipolar cells- realy information form photoreceptor cells to retinal ganglian cells

  • Retinal ganglian cells are the only cells the send information out of the eye. The axons from the optic nerve which exits the reitna through the optic dic

 

 

Visual Information Pathways

  • Thalumus

    • Specifically in the lateral geunculate nucleus, projects to the primary visual cortex, where the information enters conciousness

    • The retina, thalumus, V1, creates an internatl mental representation of your entire visual space

  • Midbrain

    • Specifically in the superioir colliculi

    • Visual infromation is used to control fast visually guided reflexive movements

    • The mid brain doesn't know what you're looking at but it can fraw attention to unexpected visual events

  • Hypothalumus:

    • Visual information is used here to control circadiam rhytms, such as sleep- wake cyles

    • Doesn’t know what you are looking light but it know how much light is present in the environment

 

Visual Cortex Wiring Diagram

  • Oversimplification of pathways 

  • Predictive coding theory- theory of sensory proecessing. Idea is that each node in the networkd tries to predict what its ascending inputs will ooke like in the next movement based on previus expereince.

  • Top down activity represents sensory predictions that neutralize any correctly predicted bottom up ascending signals