Senses 2: Visual pathways

Question 1

Visual processing

Answer 1

takes up a large proportion of the human brain

important part of the evolution of the human brain has been ways to improve how we use vision to guide our actions (and understand the world)

large proportion of neuroscience research has been into vision so we understand the neural processing quite well

understanding visual pathways is probably the best way to understand how our brain works

Question 2

what is vision?

Answer 2

detecting and interpreting patterns of electromagnetic radiation

differences in intensity – can see in bright and low light conditions

differences in wavelength – can see colours

Question 3

evolution of vertebrate eyes

Answer 3

eyes evolved through a gradual sequence of improvements for detecting directions and forming an image

advanced types of eyes have evolved several times in the animal kingdom

fossil records date back to the Cambrian explosion

faster movement and navigation in animals required better vision

Question 4

regulation of phys processes during day and night

Answer 4

light levels detected through eye are sent to the SCN (suprachiasmatic nucleus)

keeps the circadian clock in the SCN accurately timed with natural daily light cycles

Question 5

what is the pineal gland?

Answer 5

unpaired midline structure near epithalamus

evolutionary old part of the brain that is found in nearly all vertebrates (the third or parietal ‘eye’)

Question 6

how is vision used to make sense of the world?

Answer 6

if your brain can makes sense of what it sees, it can initiate or guide (hopefully) appropriate actions

eye and other areas of the visual brain codes and analyse regularities and patterns in spatial and temporal differences in light intensity and wavelengths

Question 7

projections of light onto the retina

Answer 7

rod and cone cells form an array in the retina

human eyes contain 95 million receptor cells

Question 8

steps in image processing?

Answer 8

retina could be compared to 95 Megapixel camera but with a larger, curved sensor chip and with much more sophisticated processing circuits.

lens to focus image

aperture to control light entering (Iris)

pixels to register image (photoreceptors)

filtering media (glass body, macula, pigment)

filter to protect lens (cornea)

lens cover for when not in use (eye lid)

cleaning mechanism (tears)

processing algorithms (retinal interneurons

Question 9

light refraction in the eye

Answer 9

light passes through cornea, aqueous humor, lens and vitreous humor on the way to the retina.

at each boundary it is refracted

Question 10

shape of lens is adjustable

Answer 10

the nearer the object, the stronger the lens needed to form a focussed image

lens become stiffer from age 40+

Question 11

accomodation

Answer 11

changing the strength of the lens to form a focussed image

Question 12

why do humans and animals move their eyes

Answer 12

saccades (jumps) and fixations

2-3 saccades per second

direct fovea to collect information about the visual scene

move centre of attention to centre of visual field

Question 13

fovea

Answer 13

the central portion of the retina, packed with the most photoreceptors and therefore the center of our gaze.

Question 14

eye movements in everyday actions (Land et al., 1999)

Answer 14

three subjects (a-c) showed similar sequences and locations where they fixated on objects

predictive saccades in anticipation of the next movement

saccades moved to particular locations when eyes engaged in visual search, more precisely if the subject knew what to find where

eyes disengaged from fixating hand and/or object before an action was completed

Question 15

controlling movement of eyes

Answer 15

the field of view

we can move eyes and heads separately. Many animals cannot move their eyes (insects, birds) and they have to move their head and/or body to be able to see.

Question 16

stabilising gaze for better vision

Answer 16

movement can be described as combination of three directions of translation and three directions of rotation

movement of head renders vision blurry

Question 17

diff types of eye movements

Answer 17

saccades

smooth pursuit moevemnts

optokynetic nystagmus

vestibulo-ocular movements

Question 18

saccades

Answer 18

move the eye very quickly to a new position between periods of gaze stabilisation (fixations) in order to scan the scene across the entire field of view

Question 19

smooth pursuit movementd

Answer 19

slower, keeps a moving stimulus on the fovea

Question 20

optokynetic nystagmus

Answer 20

brings the eye back from a peripheral to a more central position after it has followed a large-scale moving stimulus (whilst head is still)

Question 21

vestibulo-ocular movements

Answer 21

compensate for the movement of the head by moving the eye the same distance but in the opposite direction in order to maintain a constant field of view

Question 22

brain circuit for saccadic eye movements

Answer 22

conscious control of eye movements comes from the cortical frontal eye fields (FEF)

automatic control of eye movements comes from the superior colliculus

both use input from vision, but also from auditory and somatosensory systems

Question 23

coping with changing light levels

Answer 23

two types of visual receptors: rod and cone cells

both types of receptors detect light in basically the same way
(opsins, cell shapes, regulatory processes are slightly different)

Question 24

why have 2 types visual receptors?

Answer 24

cones are specialised for vision during the day

(1-100 million times brighter in sunlight than moonlight)

rods are specialised for vision during the night

Question 25

what do humans have?

Answer 25

duplex retina with rods and cones

Question 26

dim-light vision (rods) doesn’t use central fovea

Answer 26

acuity is proportional to

eye movements position the fovea in those positions of the visual field where it is most important to collect high-acuity information

at night high acuity is sacrificed for sensitivity, and it is more advantageous to have no rods in the fovea

Question 27

visual transduction

Answer 27

light causes graded hyperpolarisation of receptor membrane

conformational change in rhodopsin activates G-protein (transducin)

activated rhodopsin activates a (G-protein) messenger (transducin). Through a series of steps, it causes Na channels to close. The membrane therefore becomes more polarised. – hyperpolarised

signal amplification in G-protein cascade

sequence of steps means that the absorption of single photon can close up to 200 Na channels

but the amount of amplification can be regulated so that fewer channels open - useful in bright light

Question 28

types photopigments in rods and cones

Answer 28

opsin – light-sensitive protein (G-protein coupled receptor molecule) in the membrane of photoreceptors

opsins are covalently bound to a chromophore

three functional classes of cones

one functional class of rods

Question 29

first steps in processing in retina

Answer 29

receptor cells have graded (non-spiking_ responses

3 layers on analogue processing (graded responses)

retinal ganglion cells send APs to brain

Question 30

visual pathways

Answer 30

geniculate-striate pathway:

extrageniculate pathway:

Question 31

primary visual cortex of rhesus monkey

Answer 31

ca. 1200 mm2 (1/3 of a credit card)

15% of area of whole cortex

Question 32

disproportionate large foveal projection area in V1

Answer 32

given the higher density of cones in V1 (higher acuity), more processing power is required for information originating from the fovea

Question 33

visual functions in blind humans and primates

Answer 33

damage to V1 (primary visual cortex) causes cortical blindness, the loss of conscious vision. Patients are able to perform visually-guided behaviours, like grasping or pointing to the location of objects, or avoiding obstacles, correctly at a level above chance.

Question 34

what does the pineal gland do?

Answer 34

produces melatonin during darkness

Question 35

what does vision require?

Answer 35

forming and processing an image in the eye

Question 36

rod cells

Answer 36

are specialised for high sensitivity to see in dim light

large cells containing large amounts of photopigment

G-protein cascade produces high amplification

Question 37

cone cells

Answer 37

specialised for high acuity and high speed of response to see in bright light

smaller in size than rods and contain less photopigment per cell

not saturated at higher light levels (e.g. low amplification)

photoreceptor recovers rapidly from change

Question 38

where is acuity of vision highest

Answer 38

in fovea and decreases towards the periphery of the retina

Question 39

what is acuity proportional to?

Answer 39

the density of receptor cells

Question 40

in mammals what is the chromophore?

Answer 40

retinal - absorption of light causes a conformational change in retinal molecule to the activated form (all-trans retinal

Question 41

functional classes of cones

Answer 41

(S-, M- and L-cones):

Cone opsins differ in their wavelength-specific affinity to absorb light (S, M and L opsins)

Only one opsin type is expressed per cone.

Question 42

functional classes of rods

Answer 42

All rods express the same type of opsin (RH1, or rhodopsin)

Question 43

geniculate striate pathway

Answer 43

retina

LGN (lateral geniculate nucleus) of the thalamus

V1 (primary visual cortex)

areas of the higher visual cortex (90% of retinal projections)

Question 44

extrageniculate pathway

Answer 44

retina

Superior colliculus (SC)

several projections to areas of the higher visual cortex and the pulvinar nucleus of the thalamus (eye movement control and visual attention) (10% of retinal projections)

Question 45

the field of view

Answer 45

is defined by the position and orientation of the eye ball, of the head and of the body