Week 9: Vision Flashcards
What is light? What is visible to humans?
▪ Electromagnetic radiation, defined by its frequency (or wavelength measured in nanometers, nm) and amplitude
▪ Spectrum between 380 and 760 nm is visible to humans ▪ Light rays (made of photons, photons activate receptors) emitted by a luminous object can be reflected, absorbed and/or refracted
Light travels at 300,000 km/hr
Structures of the eye
▪ Enters the eye through the cornea (clear dome)
▪ Progresses through the pupil (opening in the centre of iris) ▪ Is bent by the lens (cilia muscles contract/ relax) image reversed and inverted) ▪ Continues through the vitreous humour (clear gel) ▪ Projected onto the retina
What is the process of focussing called?
A process of accommodation
Accommodation
▪ Lens shape changes to focus image/ lens on the retina
○ Focus on objects on range of distances ○ Controlled by ciliary muscles ○ Contract = zonules loosen = see shorter distances Relax = zonules contract = see longer distances
Presbyopia
as we age, the lens become stiffer = harder for lens to change shape = why as we get older vision is more likely to worsen
Layers of the retina
▪ Photoreceptors
▪ Bipolar cells
▪ Ganglion cells
▪ Horizontal and amacrine cells
Photoreceptors
convert light energy into neural activity (photopigment in cells - lamella - break down under photon energy = hyperpolarisation of rods & cones as channels close = depolarises the bipolar cells)
(more on rods and cones later)
Bipolar cells
Transmit information to ganglion cells (then send info to ganglion cells)
Ganglion cells
integrate information and send APs to brain (occipital lobe through the thamalus)
Horizontal and amacrine cells
lateral neurites influence cells close by (communicate between bipolar cells and ganglion cells, improve integration of info)
Photoreceptors: Rods: Where?
▪ 92 million (a lot more than cones)
▪ Found mainly in the retinal periphery (not the fovea)
Rods: Function? Colour? Focus?
▪ Very sensitive to light (identify light and dark)
▪ Monochromatic information (not sensitive to colour)
▪ Poor acuity (not as focussed on sharpness of objects - why peripheral is less clear)
Photoreceptors: Cones: Where?
- 4.6 million
- found mainly in the fovea
Cones: Function? Colour? Focus?
▪ Less sensitive (to light)
▪ Provide information about hue (colour/ wavelength) ▪ High acuity (very focussed on shape, sharpness of image) (There are different types of cones, sensitive to different wavelengths/ colours)
Why is there a blind spot in vision?
Optic disk - all the axons from ganglion cells here (form the optic nerve) so does not have rods & cones = blind spot
Transduction
▪ A process that converts an external stimulus to an internal stimulus
▪ Transduction of light energy into changes in membrane potential
What causes hyperpolarisation of photoreceptors?
○ Hyperpolarisation of photoreceptors because photons cause a breakdown of photopigment, so sodium & calcium channels close
= Depolarisation of bipolar cells
Receptive field
▪ The area of the visual space in which a stimulus must be presented to change the activity of a neuron
▪ Size of the neuron’s receptive field determines its acuity (smaller is better) and sensitivity (larger is better)
Receptive field size of Fovea?
Small as few photoreceptors converge on ganglion cell (parvocellular ganglion cells)
Small receptive field = sharper image, more details
(not as many cones in fovea - the centre of retina)
Receptive field of Peripheral vision
large as many receptors converge (magnocellular ganglion cells)
Periphery of retina = less detailed but better at distances
Optic chiasma
(can see different sides of the eyes picking up different sides of the visual fields)
(either nasal or temporal side)
Optic chiasma: an x, the crossover point, where half of each of the visual field is sent in different directions
Retinofugal projections
Visual information from the eye transmitted to the visual cortex
(Info that comes from the retina to the brain
What side of the retina cross at the Optic Chiasma? (only 1 crosses)
Both the nasal parts of the retina will cross (but not the temporal side info)
What helps us to see depth
Visual (hemi)field: The Left-geniculate nucleus
▪ Each eye has an optic nerve
▪ Nasal half of axons cross to opposite hemisphere
▪ Lateral half (temporal side) of axons stay on same hemisphere
- LGN is part of the thalamus
Relay station of information
What would happen if the left optic nerve was severed?
Only see the right side of the visual field and the red/blue bit of middle
Left optic tract severed consequences?
won’t see any of the blue part
Optic chiasm severed consequences?
= tunnel vision
(won’t see your peripheral vision)
Structures involved in visual processing: Left geniculate nucleus
▪ Each LGN receives information from both eyes but only about the contralateral visual field
▪ 6 specialised layers, innervated by the contralateral (left) (layers 1, 4 and 6) and ipsilateral eye (right) (layers 2, 3 and 5):
LGN: Magnocellular (1&2 layers)
relay information about form, movement, depth, light-dark contrast
(a lot to do with rods)
LGN: Parvocellular (3-6 layers)
Relay information about colour (red & green) and fine detail (more to do with cones)
LGN: Koniocellular (sublayers)
relay information about colour
Structures involved in visual processing: Striate cortex (AKA The Primary visual cortex, V1)
▪ Cortical region organised into 6 layers
Structures involved in visual processing: Extrastirate cortex
▪ Surrounds the Striate cortex / primary visual cortex (V1)
▪ Combines information for perception
▪ Arranged hierarchically (V2-V8)
▪ Information moves up the visual association cortices, where it is analysed then passed on to higher centres for further analysis
▪ Hierarchy of receptive fields in the visual system: receptive fields become larger and more complex
Pathways of the extrastriate cortex (2)
- dorsal
- ventral
Pathways of the extrastriate cortex: Dorsal
“where” pathway (e.g. if image moving towards/ away)
(damage to this area = see motion less smoothly)
Pathways of the extrastriate cortex: Ventral
“what” pathway
(damage = can’t recognise/ interpret image, but can see the outline)
Why don’t we see much colour in the dark?
In the dark, you don’t see much colour because the rods take over
Colour perception: 2 systems
- Trichromatic coding (cones)
- Opponent-process coding (ganglion cells)
Trichomatic coding
▪ Retina contains three types of cones responsible for colour vision:
▪ Red (long nm)
▪ Green (medium nm)
▪ Blue (short nm)
Why do more men have colour blindness?
On x chromosome:
women have 2, so as long as one of them is okay they can see colour
(affects ~10% of males)
Opponent-process coding
▪ Three colour system is converted into opponent-colour system
▪ 2 types colour-sensitive ganglion cells, respond to colour pairs
- Yellow* - Blue
- Red - Green
*If it’s yellow, both green and red cones are activated, then
Green cone inhibits the red/green ganglion cell, and activates the yellow-blue ganglion cell = signals yellow
Reading: Sensation definition
involves the cells of the nervous system that are specialized to detect stimuli from the environment. These stimuli occur in specific forms of energy (such as light, sound, or heat), and the neurons that detect this energy transform it into action potentials
Reading: Perception
is the conscious experience and interpretation of information from the senses and involves neurons in the central nervous system
Reading: Three dimensions determine the perceived color of light
hue, saturation, and brightness
Reading: Types of eye movement
Vergence movements
saccadic movements
pursuit movement
Reading: Vergence movements
are cooperative movements that keep both eyes fixed on the same target—or, more precisely, that keep the image of the target object on corresponding parts of the two retinas. If you hold up a finger in front of your face, look at it, and then bring your finger closer to your face, your eyes will make vergence movements toward your nose
Reading: Saccadic movement
your eyes make jerky saccadic movements—you shift your gaze abruptly from one point to another. When you read a line in this book, your eyes stop several times, moving very quickly between each stop. You cannot consciously control the speed of movement between stops; during each saccade the eyes move as fast as they can.
(cont control speed)
Reading: Pursuit movement
Only by performing a pursuit movement—say, by looking at your finger while you move it around— can you make your eyes move more slowly
(in pursuit of an object)
