Lecture 9 - Vision Flashcards
The Visible Spectrum
Light is electromagnetic energy.
One nm = one billionth of a meter
Properties of light
- hue
- saturation
- brightness
hue
determined by wavelength.
saturation
relative purity of light.
brightness
variation in intensity.
Distance b/t 2 peaks =
wavelength
As wavelength increases, frequency _____
decreases
In order to see things in greatest detail our eyes are moved so that the object being looked at falls on the ____.
FOVEA
Fovea
is a central portion of the retina with the greatest visual acuity.
Cones
are responsible to BRIGHT light
are responsible for COLOUR vision & our ability to see FINE detail
Rods
are more NUMEROUS than cones & are more SENSITIVE to DIM light
are mainly used for NIGHT VISION
Photoreceptors
- rods and cones contain photopigment that provides input to bipolar and horizontal cells.
- photoreceptors and bipolar cells do NOT produce action potentials – instead release neurotransmitters to the ganglion cells.
- ganglion cells connect with the optic nerve. (& travel out the back of the eye)
Blind spot
Optic disk – where the optic nerve joins the retina – transmits retinal information to the occipital lobes
(have blind spot in our eye but its masked by parts of our eye)
Blind spot
close your LEFT eye and move head closer to or further away from the screen until the central red circle disappears - always fixate the CROSS
Visual Fields
animals that tend to be prey (eyes on side of head) - have WIDE visual field (HAVE LARGE MONOCULAR FIELD)
animals that are hunters (eyes in front of head - concerned with whats in front of them)- have LARGE BInocular field
Visual Fields cross over
cross over @ optic chiasm
- info from LEFT visual field ends up making its way to the RIGHT part of the brain
- info from the RIGHT visual field ends up making its way to the LEFT part of the brain
CONTRALATERAL organization in visual system due to crossing over
Visual Field Temporal vs Nasal
TEMPORAL portion of LEFT visual field falls on the NASAL portion of the eye
NASAL portion of RIGHT visual field falls on the TEMPORAL portion of the retina
Primary geniculostriate visual pathway
(main visual pathway for vision)
crossing over of NASAL & TEMPORAL sections
- which is why in each eye, you can see diff. parts of the visual field are present
BLUE - LEFT visual field
- falls on temporal portion of R eye & nasal portion of LEFT eye
- LEFT eyes info crosses over to R side of brain in optic chiasm
- RIGHT eyes info stays where it is & travels down the geniculate nucleus & through the optic radiations to the primary visual cortex
YELLOW - RIGHT visual field
- goes to LEFT side of brain
- info projects to nasal portion of RIGHT eye, but temporal portion of LEFT eye stays on LEFT while nasal portion of RIGHT eye crosses over & goes to lateral geniculate nucleus through optic radiations to primary visual cortex
CONTRALATERAL
Primary Geniculastriate Pathway
main visual pathway
- retina
- optic nerve
- optic chiasm
- optic tract
- dorsal lateral geniculate nucleus (LGN)
- optic radiations
- striate (aka primary) visual cortex
Lateral Geniculate Nucleus
The LGNd has six layers each of which gets independent input from either the left or the right eye but not both.
There are two major classes of projections, parvocellular (small) and magnocellular (large) projections (known as the P and M pathways).
Lateral Geniculate Nucleus
There are two major classes of projections:
parvocellular (small) and magnocellular (large) projections (known as the P and M pathways).
Magnocellular
Large ganglion cells
Centre/Surround
Colour insensitive
Large RFs (tend to be in the perifery)
Fast, transient (if something is coming toward, it picks it up)
High contrast sensitivity
Parvocellular
Small ganglion cells
Centre/Surround
Colour sensitive
Small RFs
Slow, sustained (reading a book - keeping it still while looking at it)
Low contrast sensitivity
Primary Visual Cortex
The LGNd projects to primary visual cortex (striate cortex or area V1) in the occipital lobe.
The magno and parvo projections are still somewhat segregated in V1.
Retinotopic map in striate visual cortex
means ways your field maps out & falls onto your retina is represented as separate areas in cortex
fixation point - stuff falling on fovea/central vision (central vision)
small circle gets quite a bit of real estate devoted to it
- b/c that small circle of info is falling onto the fovea that’s packed full of high acuity receptors –> tons of info coming from that area so it needs a lot of processing power in the brain so it gets more area of the brain devoted to it
large part of lower visual field gets projected on a relatively smaller section of cortex
- b/c there’s bigger receptive fields in the periphery, fewer cells can cover a much larger area, so it takes less processing power, less real estate space then the v. small section of the visual field that’s covered by the fovea that has a ton of cells recording info from the space
BOTTOM PART OF VISUAL WORLD GETS MAPPED UP ON UPPER PART OF VISUAL FIELD & VV
Retinotopic organization…
basically, the way are visual map is layed out in our retinal world, maps on to how its layed out across the brain
Visual receptive fields
- receptive fields of retinal ganglion cells correspond to specific regions in space – hence a retinotopic map of the world in the occipital cortex.
- receptive fields in visual cortex also respond selectively to other stimulus properties (e.g., orientation, brightness).
piece of visual cortex
- if you were to stick an electrode down the orientation columns along the side, all of the cells in that column will all respond maximaley to a cell that’s oriented almost perfectly horizontantly, if you go next to it, all of the cells in that column will respond maximately to a cell that’s angled up more
therefore, cells are organized by the amount of excitation that they have to a partic. stimulus orientation
Centre – surround organization
- TUNING – different types of cells are “tuned” to respond to different aspects of visual information
e. g., brightness, location, direction of motion, colour etc…
Coding information at the retina - brightness
- centre / surround ORGANIZATION
- ON, OFF and OFF/ON cells
- ON/OFF cells project primarily to the superior colliculus (midbrain)
- the SC is important for directing reflexive saccades
Coding information at the retina - colour
the 3 types of cones are called “blue,” “green,” & “red.”
these names loosely refer to the frequencies of light to which each cone is maximally sensitive (the peak sensitivities are 419, 531, & 559 nm respectively)
- trichomatic sensitivity AND colour opponency
- red – green
- blue – yellow
- on/off surround organization
impossibility of seeing a redish green colour!
Adaptation – negative afterimages
- after staring at the green Canadian flag you see a red one because the “green” component of red/green cells has adapted to the stimulus.
- some red/green cells are inhibited for a long period.
- when looking at neutral light (white light) these cells “rebound” due to the absence of inhibition creating the afterimage.
- Big Spanish Castle
- can get afterimages for motion – waterfall illusion .
Striate cortex
- 6 layers (bands or striations).
- input from magno and parvocellular information processed at layer IV.
- disproportionate representation of the fovea (brain would weigh over 30,000 pounds (≈13,600 kg) if the whole visual field had as many neurons dedicated to it as are dedicated to the fovea!!!).
Orientation and movement
- cells in striate cortex sensitive to specific orientations.
- simple cells – opponent system.
- complex cells – no inhibitory surround – direction specific movement detectors (also in MT).
- cells organized in columns.
Simple cells
opponent system.
Complex cells
no inhibitory surround – direction specific movement detectors (also in MT).
Spatial frequency
many of the cells in striate cortex are actually tuned to different spatial frequencies.
everything you see in the world can be described in terms of spatial frequency.
Information not lost at low spatial frequencies
Gender and can still be extracted from the low frequency image (right) but identity requires the high frequency image (left).
Modularity in vision
Different “modules” sensitive to different visual processes
V4 – colour MT – motion FFA – face perception PPA – place recognition IT – object recognition
A unique feature of the fovea is that it
A) contains mostly rods. B) contains mostly cone photoreceptors. C) is devoid of photoreceptors. D) mediates vision in dim light. E) has very poor acuity.
B) contains mostly cone photoreceptors.
The reason for a “blind spot” in the visual field is that
A) rods are less sensitive to light than are cones.
B) blood vessels collect together and enter the eye at the blind spot.
C) the lens cannot focus all of the visual field onto the retina.
D) retinal cells die with age and overuse, resulting in blind spots.
E) there are no photoreceptors in the retina where the axons exit the eye.
E) there are no photoreceptors in the retina where the axons exit the eye.
Action potentials in the visual system are first observed in the
A) bipolar cells. B) horizontal cells. C) ganglion cells. D) photoreceptors. E) axons leaving the internal surface of the retina.
C) ganglion cells.
Select the correct sequence for processing of information in the primary visual pathway.
A) Retina - > dorsal lateral geniculate (DLG) -> striate cortex
B) Retina -> striate cortex -> extrastriate cortex -> inferior temporal cortex
C) DLG -> retina -> striate cortex -> primary visual cortex
D) Retina -> DLG -> inferior temporal cortex -> amygdala
E) DLG-> frontal cortex -> amygdala -> extrastriate cortex
A) Retina - > dorsal lateral geniculate (DLG) -> striate cortex
