visual system - bio week 5 Flashcards
anatomy of the eye (general)
Eyes are suspended in orbits (bony pockets of the skull) and moved by 6 oculomotor muscles.
sclera
tough white outer coat of the eye
cornea
outermost layer of the front of the eye – transparent
Iris
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)
Retina
collection of neural tissue and approx. 130 million photoreceptive (light sensitive) cells.
Optic Nerve
carries information to the brain, formed by the optic disk (blind spot), the exit point from the retina of the retina-ganglion cells.
What is light
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.
properties of light - wavelength
important in perception of colour
Perceive different hues (blue, green, yellow, red) due to variations in wavelength of the reflected light
Properties of light - intensity
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
Visual field
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
Binocular Disparity
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
Eye to brain connections
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.
Retina cells
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
retinal cells 2
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.
Retinal cells 3
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)
Photoreceptors (Rod and Cones)
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
Eye to brain connections
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.
Ganglion cells then …
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)
Retina-geniculate-striate pathway
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
Retina-geniculate-striate system
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
Visual areas
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
Dorsal stream
V1 to the dorsal prestriate cortex to the posterior parietal cortex
Cells respond to spatial stimuli (location, direction)
Ventral stream
V1 to the ventral prestriate cortex inferotemporal cortex
Cells respond to characteristics of stimuli (colour and shape)
Two theories of function
1) “What”/”Where” Theory (Ungerleider & Mishkin, 1982)
2) Control of behaviour vs. Conscious Perception Theory (Goodale and Milner, 1992)
1) “What”/”Where” Theory (Ungerleider & Mishkin, 1982)
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
2) Control of behaviour vs. Conscious Perception Theory (Goodale and Milner, 1992)
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
Prosopagnosia
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)
Akinetopsia
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)
