Senses 2: Visual pathways Flashcards Preview

PSY1202 Introduction to biological psychology > Senses 2: Visual pathways > Flashcards

Flashcards in Senses 2: Visual pathways Deck (45)
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1
Q

Visual processing

A

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

2
Q

what is vision?

A

detecting and interpreting patterns of electromagnetic radiation

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

differences in wavelength – can see colours

3
Q

evolution of vertebrate eyes

A

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

4
Q

regulation of phys processes during day and night

A

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

5
Q

what is the pineal gland?

A

unpaired midline structure near epithalamus

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

6
Q

how is vision used to make sense of the world?

A

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

7
Q

projections of light onto the retina

A

rod and cone cells form an array in the retina

human eyes contain 95 million receptor cells

8
Q

steps in image processing?

A

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

9
Q

light refraction in the eye

A

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

at each boundary it is refracted

10
Q

shape of lens is adjustable

A

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

lens become stiffer from age 40+

11
Q

accomodation

A

changing the strength of the lens to form a focussed image

12
Q

why do humans and animals move their eyes

A

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

13
Q

fovea

A

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

14
Q

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

A

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

15
Q

controlling movement of eyes

A

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.

16
Q

stabilising gaze for better vision

A

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

movement of head renders vision blurry

17
Q

diff types of eye movements

A

saccades

smooth pursuit moevemnts

optokynetic nystagmus

vestibulo-ocular movements

18
Q

saccades

A

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

19
Q

smooth pursuit movementd

A

slower, keeps a moving stimulus on the fovea

20
Q

optokynetic nystagmus

A

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)

21
Q

vestibulo-ocular movements

A

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

22
Q

brain circuit for saccadic eye movements

A

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

23
Q

coping with changing light levels

A

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)

24
Q

why have 2 types visual receptors?

A

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

25
Q

what do humans have?

A

duplex retina with rods and cones

26
Q

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

A

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

27
Q

visual transduction

A

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

28
Q

types photopigments in rods and cones

A

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

29
Q

first steps in processing in retina

A

receptor cells have graded (non-spiking_ responses

3 layers on analogue processing (graded responses)

retinal ganglion cells send APs to brain

30
Q

visual pathways

A

geniculate-striate pathway:

extrageniculate pathway:

31
Q

primary visual cortex of rhesus monkey

A

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

15% of area of whole cortex

32
Q

disproportionate large foveal projection area in V1

A

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

33
Q

visual functions in blind humans and primates

A

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.

34
Q

what does the pineal gland do?

A

produces melatonin during darkness

35
Q

what does vision require?

A

forming and processing an image in the eye

36
Q

rod cells

A

are specialised for high sensitivity to see in dim light

large cells containing large amounts of photopigment

G-protein cascade produces high amplification

37
Q

cone cells

A

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

38
Q

where is acuity of vision highest

A

in fovea and decreases towards the periphery of the retina

39
Q

what is acuity proportional to?

A

the density of receptor cells

40
Q

in mammals what is the chromophore?

A

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

41
Q

functional classes of cones

A

(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.

42
Q

functional classes of rods

A

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

43
Q

geniculate striate pathway

A

retina

LGN (lateral geniculate nucleus) of the thalamus

V1 (primary visual cortex)

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

44
Q

extrageniculate pathway

A

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)

45
Q

the field of view

A

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