visual system - bio week 5

Question 1

anatomy of the eye (general)

Answer 1

Eyes are suspended in orbits (bony pockets of the skull) and moved by 6 oculomotor muscles.

Question 2

sclera

Answer 2

tough white outer coat of the eye

Question 3

cornea

Answer 3

outermost layer of the front of the eye – transparent

Question 4

Iris

Answer 4

controls light reaching the retina and contains pupil (hole in the centre) which allows light. Light is focussed onto the retina by the lens and held in place by ciliary muscles. Pigmented (blue/brown etc)

Question 5

Retina

Answer 5

collection of neural tissue and approx. 130 million photoreceptive (light sensitive) cells.

Question 6

Optic Nerve

Answer 6

carries information to the brain, formed by the optic disk (blind spot), the exit point from the retina of the retina-ganglion cells.

Question 7

What is light

Answer 7

Waves of electromagnetic energy. Human visual only respond to a quanta of light of certain wavelength – between 380 – 760nm. These quanta are called photons. Photons enter the eye and depending on wavelength and number of quanta per second cause the visual system to respond.

Question 8

properties of light - wavelength

Answer 8

important in perception of colour

Perceive different hues (blue, green, yellow, red) due to variations in wavelength of the reflected light

Question 9

Properties of light - intensity

Answer 9

Important in perception of brightness

If intensity of electromagnetic radiation increases so does brightness

Brightness is in part created by visual system

Evidence from Lateral inhibition - Interconnected neurons in the retina inhibit their neighbours.

Result is a contrast enhancement at the edges of regions – helps us to see edges more clearly

The two patches that appear to differ, reflect the same amount of light

Question 10

Visual field

Answer 10

Everything we can see without moving the head = Visual Field

Objects in centre of VF have greatest visual acuity

Eyes mounted on front of face allows us to see what is in front through both eyes simultaneously – allows for 3D vision

Question 11

Binocular Disparity

Answer 11

Eye movements are coordinated so that each point in your visual world is projected to corresponding points on your 2 retinas. Eyes converge to achieve this.

Binocular Disparity – The difference in the position of the same image on the two retinas

Greater for close objects than for distant objects

Cocktail sausage effect

Question 12

Eye to brain connections

Answer 12

Amount of light reaching the retina is controlled by the iris, entering through the pupil.

The pupil adjusts in differing amounts of light:

Dialates in minimal light – reducing acuity.

Constricts in bright light – increasing acuity

Incoming light is focussed onto the retina by the lens.

Decreasing the tension of the eye ligamenmts brings close objects into sharper focus

Flattening the lens allows focus on objects further away (accommodation)

The retina then translates light into neural signals & sends them to CNS.

Question 13

Retina cells

Answer 13

Retina built inside-out

5 layers of neurons:

Retinal ganglion cells

Amacrine cells

Bipolar cells

Horizontal cells

Photoreceptors (rod and cones)

Light must pass through 4 layers of neurons before it reaches the photoreceptors

Question 14

retinal cells 2

Answer 14

Activated photoreceptors release Glutamate, which controls activity of bipolar neurons, which synapse onto retinal ganglion cells passing the neural message back through the retinal layers

Axons of ganglion cells form the optic nerve which exits the eyeball carrying info -> brain

Optic nerve exit leaves gap in receptor layer which creates a blind spot

We hardly notice this as the receptors around the blind spot fill in the gaps – completion.

Question 15

Retinal cells 3

Answer 15

Horizontal cells connect with receptor cells and bipolar cells.

Amacrine cells connect with bipolar and ganglion cells

Fovea: a thinning of the retina ganglion cell layer at the centre of the retina (high acuity vision)

Question 16

Photoreceptors (Rod and Cones)

Answer 16

Duplexity theory of vision - rods and cones mediate different kinds of vision

Cones – photopic vision
High visual acuity in good lighting
Colour vision
High density in fovea

Rods – scotopic vision
High sensitivity allowing for low acuity vision in dim lighting
Lacks detail and colour
No rods in fovea

Question 17

Eye to brain connections

Answer 17

Receptors convert light to neural signals (visual transduction)

Rods contain a red photopigment - rhodopsin which responds to light

In the dark

Rhodopsin is inactive -> sodium ions flow into cell -> rods depolarise -> release glutamate continuously

In the light

Light bleaches rhodopsin ->sodium channels close ->rods hyperpolarise -> glutamate release reduces

Reduction in glutamate triggers adjacent bipolar cell to depolarise, which triggers action potential in adjacent Ganglion cell.

Question 18

Ganglion cells then …

Answer 18

Ganglion cells then pass this information into the brain along the optic nerve

Ganglion cells project to various parts of the brain including:
- Lateral geniculate nuclei (LGNs)
- Superior colliculus

Each LGN has 6 layers of neurons

4 x Parvocellular (small cells) - colour, fine detail, cones mainly input here
2 x Magnocellular (large cells) - movement, rods mainly input here

Projections from the LGN travel into the visual
cortex (striate cortex)

Question 19

Retina-geniculate-striate pathway

Answer 19

Retina-geniculate-striate pathway is contralateral with cross over at optic chiasm.
Each LGN receives visual input from both eyes but only from one VF.

Info from right VF goes to left LGN and then left PVC.

Info from left VF goes to the right LGN and then right PVC

Question 20

Retina-geniculate-striate system

Answer 20

Retina-geniculate-striate system is retinotopic

each level is a map of the retina
Point-to-point correspondence between neighbouring parts of visual space is maintained – map-like projection

More cortex devoted to areas of high acuity

Question 21

Visual areas

Answer 21

Basic visual processing takes place in areas V1 (primary visual cortex) and V2 (secondary visual cortex)

Higher visual areas undertake more specialised tasks:
V3 is associated with form
V4 with colour perception
V5 (MT) with motion

Question 22

Dorsal stream

Answer 22

V1 to the dorsal prestriate cortex to the posterior parietal cortex

Cells respond to spatial stimuli (location, direction)

Question 23

Ventral stream

Answer 23

V1 to the ventral prestriate cortex inferotemporal cortex

Cells respond to characteristics of stimuli (colour and shape)

Question 24

Two theories of function

Answer 24

1) “What”/”Where” Theory (Ungerleider & Mishkin, 1982)

2) Control of behaviour vs. Conscious Perception Theory (Goodale and Milner, 1992)

Question 25

1) “What”/”Where” Theory (Ungerleider & Mishkin, 1982)

Answer 25

Dorsal stream – perception of “where” objects are

Damage to dorsal stream disrupts visuo-spatial perception

Ventral stream– perception of “what” objects are

Damage to ventral stream disrupts visual pattern recognition

Question 26

2) Control of behaviour vs. Conscious Perception Theory (Goodale and Milner, 1992)

Answer 26

Dorsal stream – visually guided behaviour (vision for action)

Patients with bilateral (both sides of the brain) lesions to dorsal stream can see objects, but cannot interact with them under visual guidance

Ventral stream– conscious visual perception (vision for perception)

Patients with bilateral lesions to ventral stream report no conscious experience of seeing, but can interact with objects with visual guidance

Question 27

Prosopagnosia

Answer 27

Failure to recognise faces

Not attributable to visual deficit or verbal or intellectual impairments

Recognise that a face is a face but not whose it is

Associated with damage to the ventral stream (fusiform gyrus - junction between occipital and temporal lobes)

Question 28

Akinetopsia

Answer 28

Deficit in movement perception (motion blindness)

Associated with damage to area of dorsal stream – V5/MT (middle temporal) (junction of temporal, parietal and occipital lobes)

Inhibiting activity in MT with TMS produces motion blindness (e.g. Beckers and Zekki, 1995)

Electrical stimulation of MT induces visual motion perception (e.g. Blanke et al., 2002)