PYSC 211 Midterm 2 Lec 8 Vision I

Sensation

refers to the detection of a physical stimulus (such as light, sound, heat, etc.) by early sensory pathways

• how cells of the nervous system transduce (convert) these signals into a change in membrane potential and neurotransmitter release.

Perception

refers to the conscious experience and interpretation of sendory information

Sensory Neurons

specialized cells that detect a specific category of physical events such as:

• the presence of specific molecules

  • smell, taste, nausea, pain

• physical pressure

  • touch, stretch, vibration, acceleration, gravity, balance, hearing thirst, pain

• temperature

  • heat, cold, pain

• pH (whether a liquid is acidic or alkaline)

  • sour taste, suffocation, pain

• visible light (electromagnetic radiation)

  • vision

Photoreceptor Cells

• sensory cells responsible for vision

• in our eyes

• transduce visible light into a change in membrane potential which affects amount of NT released

• don’t have action potentials

• release NT in a graded fashion, dependent on their membrane potential

• detect visible light using 4 kinds of photoreceptor cells

  • around 30% of the cerebral cortex is dedicated to processing info gathered by these cells

Absorption

electrons move into higher energy orbital

Reflection

electrons oscillate within their orbital states, pushing the light backward

Transmission

electrons oscillate within their orbital states, pushing their light forward

• primarily observed with air, glass, and water

Seeing Light

• colour of an object (at room temp.) is determined by the visible light it reflects (scatters)

  • ex. object appears orange when it reflects orange light and absorbs all other visible wavelengths

• energy of absorbed visible light is usually converted into heat (molecular rotations) via near-field, non-radiative electromagnetic interactions

Vitamin A

• undergo a change in shape after absorbing visible light

  • stabilizes the excited electron in a high energy orbital

• one photon of visible light has enough energy to change 3D configuration of vitamin A

• Retinal: modified form of vitamin A; perceive visible light

Sunlight

We can perceive much but not all of the radiation that reaches us from the sun.

Retinal

• molecule synthesized from vitamin A

• is a photopigment, changes shape upon absorbing the energy of visible light

• held by the opsin proteins in the photoreceptor cells in our eyes

• what absorbs the electromagnetic energy of visible light that allows us to see

• when retinal absorbs energy of visible light it activates the opsin protein

  • this launches an intracellular g protein cascade that changes the membrane potential of photoreceptor cell, affecting how much NT it releases

Opsin

• inhibitory metabotropic receptor

• light sensitive proteins

• undergo a change in shape when the absorb the energy of visible light

  • acquire this property by holding a molecule of retinal

• embedded in the membrane of photoreceptor cells

• many opsin proteins encoded in our genome

• different ways these opsins hold onto retinal determines the wavelength of light they can absorb (e.g. red vs green light)

Sensory Transduction

• process by which sensory stimuli are transduced (converted) into receptor potentials

• sensory neurons come in all shapes and sizes

• sensory cells that transduce smell and touch are relatively normal neurons

• but many others (vision, hearing, taste) are not normal neurons, in part because they don’t have axons or action potentials

Visible Light

• has wavelength between 380-750 nm

• use 4 kinds of opsin proteins to detect visual light, each hold retinal slightly differently

Types of Photoreceptors

each expresses a different opsin protein

• red cone cells express the red cone opsin

• blue cone cells express the blue cone opsin

• green cone cells express the green cone opsin

• rod cells express the rhodopsin protein

  • last to evolve, 100x more sensitive to light than cone cells are

Cone Receptors: Trichromatic Coding

• Blue cone opsins are sensitive to short wavelengths of light (430 nm).

• Green cone opsins are sensitive to medium wavelengths of light (535 nm).

• Red cone opsins are sensitive to long wavelengths of light (575 nm).

Trichromatic Coding

• When red and green light is so close together that our eyes can’t differentiate them, we perceive the color yellow.

• There are 3 kinds of light bulbs here: red, green and blue.

• Green light (520nm) activates the green cone opsin more than the red cone opsin. Red light (640nm) activates the red cone opsin more than the green cone opsin. The combination of red and green light causes both red and green cone opsins to be activated similar amounts, which is exactly what pure yellow light does (570nm).

Brightness

intensity (total amount of light)

• if brightness is 0, there’s no light; perceived as black

Hue

principal colour (dominant wavelength in the light)

• if there’s some saturation, then the hue of the light indicates the perceived colour (from 0-360 degrees)

Saturation

purity of colour (the wavelength mixture)

• if there’s light, is it saturated with one colour or even mix of colours

• 0% saturation means that there is an even mix of all the colours, which means there is no perceived colour (gray scale, the central vertical axis of the colour cone)

Protanopia

• absences of red cone opsin (1% of males), which makes it difficult to distinguish colours in the green-yellow-red spectrum

• red cone cells can sometimes express the green cone opsin, which allows for normal visual acuity

• mutations that make the red cone opsin simply act more like the green cone opsin (in terms of the wavelength of light it can detect) produce less pronounced deficits in colour vision (1% of males)

Deuteranopia

• absence of green cone opsin 1% of males), which makes it difficult to distinguish colours in the green-yellow-red spectrum

• green cone cells can sometimes express the red cone opsin, which allows for normal visual acuity

• mutations that make the green cone opsin simply act more like the red cone opsin produce less pronounced deficits in color vision (5% of males).

Tritanopia

• absence of blue cone opsin

• blue cone cells don’t compensate for this deficit in any way

• but there are relatively few blue cone cells, and they are less sensitive to light that the other cone cells, so visual acuity is often not noticeably affected

Cone monochromacy

there is no colour vision when only one of the three cone opsins are functional

Achromatopsia

there is no colour vision when none of the cone cells work

• because of a problem with the intracellular signaling cascades shared by all three of the cone cells)

Rod Cell

• express the rhodopsin protein

• Rhodopsin is 100x more sensitive to light than other cone opsins

  • is in diff category, not used for colour vision

Lens

consist of several transparent layers

• adjust the shape of this lens to focus near or far

Iris

ring of muscle

• contraction and relaxation of this muscle determines the size of the pupil

Pupil

determines how much light enters the eye

-coloured part of the eye

Cornea

outer, front layer of the eye

• focuses incoming light a fixed amount

• laser eye surgery reshapes the cornea

Conjuctiva

mucous membrane that lines the front of the eye

• connects to eyelids, preventing things from getting behind the eye

Retina

lines the back part of the eye

• contains several layers of cells

• photoreceptor cells are located in the furthest back layer in the retina

• periphery of the retina only contains rod cells

Fovea

central region of the retina

• primarily contains cone cells

• equal number of photoreceptor cells, bipolar cells, and retinal ganglion cells

  • no compression of info

• only part if our retina where our visual acuity is good enough to read text (20/20 vision)

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

Optic Disk

where blood vessels leave/enter the eye

• also where the optic nerve exits the eye, carrying visual info to the brain

• there are no photoreceptor cells in this spot in our retina; our blind spot

Movements of the Eye

• eyes are suspended on bony sockets in the front of the skull

  • six muscles rotate the eye and hold it in place

• our eyes scan a scene by making saccadic eye movements

• when we maintain focus on an object that is moving (relative to us) our eyes exhibit pursuit movements

Saccadic Eye Movements

rapid, jerky shifts of gaze from one point to another

Pursuit Movements

only time our eyes appear to calm down and move smoothly, slowly

Orgnization of the Retina

• light passes through multiple layers of the retina before reaching the photoreceptor cells

• activated photoreceptor cells pass visual info forwards (towards front of retina

• Visual info (via NT release) goes from

  • photoreceptor cells → bipolar cells → retinal ganglion cells → brain

• Opsin proteins are in the furthest back of the retina

Periphery of our Retina

• massive compression (averaging) of visual info ss there are 100x more photoreceptor cells than retinal ganglion cells

• our visual acuity in our peripheral vision is about 20/200, which is quite blurry

  • we can make out general shapes but not details

• what we see in our peripheral vision 20 ft away is as detailed as what we see in our fovea 200 ft away

• contains high density of rod cells

• allows us to easily detect dim light and movement of light, although without any colour info

Peripheral Vision

• people have poor visual acuity and are not great at identifying colours in their peripheral vision

• we can still make out general shapes, but images look grey, blurry, and unfocused

• high visual acuity needed for reading is only possible in the fovea

Cones

• most prevalent in the central retina; found in the fovea

• sensitive to moderate-to-high levels of light

• provide info about hue

• provide excellent acuity

Rods

• most prevalent in the peripheral retina; not found in the retina

• sensitive to low levels of light

• provide only monochromatic info

• provide poor acuity

Neurons in the Retina

• Photoreceptor Cells: synapse on bipolar cells

• Bipolar Cells: relay visual info from photoreceptor cells to retinal ganglion cells

Retinal Ganglion Cells (RGCs)

only cells that send info out of the eye

• RGCs have axons and action potentials (unlike photoreceptor cells and bipolar cells)

• Their axons form on the optic nerve, which exits the retina through the optic disc (blind spot of retina)

Visual Info Pathway: Thalamus

• thalamus, specifically the lateral geniculate nucleus

• projects to primary visual cortex (V1) in the occipital lobe where visual info enters consciousness

• Retina → thalamus → V1 pathway

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creates an internal (mental) representation of your entire visual space: the objects in it, their position, and their attentional value

Visual Info Pathway: Midbrain

• specifically the superior colliculi

• visual info is used here to control fast visually guided reflexive movements

• midbrain doesn’t know what you’re looking at, but it can draw your attention to unexpected visual events

Visual Info Pathway: Hypothalamus

• visual info is used here to control circadian rhythms such as sleep-wake cycles

• hypothalamus doesn’t know what you are looking at, but it knows how much light is present in your environment

Visual Cortex Wiring Diagram

• was understood in the 1970s

• oversimplification as there are actually tons more pathways most bidirectional (bottom-up and top-down)

• Predictive Coding Theory: idea that each node in the network tries to predict what its ascending inputs will look like in the next movements, based on previous experience

• idea is that top-down (descending) activity represents sensory predictions that neutralize (cancel out) any correctly predicted bottom-up (ascending) signals

• Thus, what propagates up through the network may only be prediction error signals. which inform the brain of how the current movement differs from what was expected

• Prediction error signals that ascend through the network would drive learning to improve future predictions