NEURO2020 WEEK 5

LIGHT

• Electromagnetic radiation 

• Wavelength = colour 

• Amplitude = brightness 

• Humans respond to approx. 400-700nm 

EYES

1. Light enters through pupil 

2. Refracted by cornea  

3. Fine-tuned by lens  

4. Image lands at retina 

FOVEA

• small central depression where fine-detail vision is sharpest 

• Layers are cleared aside for maximum acuity 

EYE PLACEMENT

Laterally placed eyes 

• Usually prey  

• Wider view  

• Detect predator sneaking up  

Frontal face eyes 

• Binocular disparity 

• Can break camouflage  

• Usually predator  

Cyclopean eye 

• Combining both images 

CONES

• densely packed in the fovea 

• Short, medium, long wavelength cones 

• High visual acuity  

• Not sensitive to dim lights  

• Colour vision 

• Low convergence  

  • One/two cones connected to the retinal ganglion cell 

  • Precise spatial information 

• Photopic system 

  • Peak sensitivity of 550nm 

RODS

• Non-existent in fovea 

• Many in periphery  

• Achromatic  

• Very sensitive to dim lights 

• Lower acuity 

• High convergence  

  • Many rods connected to retinal ganglion cell 

  • Vague spatial information 

• Scotopic system 

  • Peak sensitivity of 500nm 

BLIND SPOT

• Area with no photoreceptors  

• Occurs where optic nerve exits the eye 

• Brain does perceptual filling in  

SCOTOPIC SPECTRAL SENSITIVITY & BEHAVIOUR 

Behavioural measures of human sensitivity to different wavelengths closely matches the ability of rhodopsin (rod photoreceptor) to absorb those wavelengths of light.

Visual transduction is conversion of light energy into neural signals by the receptors.

ROD LIGHT ABORPTION

In the dark: 

1. Rhodopsin molecules are inactive  

2. Na+ channels kept open  

3. Na+ ions flow into the rods, partially depolarising them.  

4. Steady flow of excitatory glutamate being emitted from rod  

In the light: 

1. Light bleaches rhodopsin molecules 

2. Na+ channels close 

3. Na+ ions can’t enter rods and rods become hyperpolarised 

4. Flow of excitatory glutamate is reduced.  

 

*therefore, the presence of light is signalled by a decrease in activity 

RETINA-GENICULATE-STRIATE PATHWAY: HEMI DECUSSATION

• Anything in right visual field casts an image to the left side of the retina in the left and right eye 

• Vice versa  

• Information in the right visual field is transmitted to the left visual cortex 

• Vice versa  

RETINA-GENICULATE-STRIATE PATHWAY: RETINOTOPIC MAPPING

• Adjacent regions process features that fall on adjacent regions on the retina in the visual cortex 

• The order of stimulation on the retina is maintained all the way through the pathway to the visual cortex 

LATERAL INHIBITION

• Mechanism by which neighbouring photoreceptors mutually suppress each other's activity 

• Bright-side receptor sends strong inhibition to its dim neighbour 

• Dim-side receptor can only send weak inhibition back 

MACH BANDS

Bright side looks brighter at border, dark side looks darker in adjacent uniform rectangles 

RECEPTIVE FIELD

Region in space/on the retina, that when stimulated, affects the behaviour of a neuron that is connected to it 

RESPONSES OF ON-CENTRE + OFF-CENTRE CELLS

Responses of on-centre cell: 

• There is an 'on' response when light is shone anywhere in the centre of the field  

• There is an 'off' response when a spot of light is shone anywhere in the periphery of the field  

*off-centre process is the opposite

*if both off and on regions were illuminated together, there was little reaction  

SIMPLE CELLS

• Simple cells form multiple aligned centre-surround receptive fields to create orientation specific receptive fields.  

• Respond most vigorously when static bars with an approximate orientation falls onto the 'on' subfield of the receptive field 

• Regions don't need to be uniform 

SPATIAL FREQUENCY - SIMPLE CELLS

Low spatial frequency: 

• Activates simple cells with widely separated subfields  

High spatial frequency: 

• Activates simple cells with less separated subfields 

COMPLEX CELLS

• Larger receptive fields  

• They respond to oriented contour anywhere within their receptive field  

• Receive input from both eyes although prefer one eye over the other 

• Respond more vigorously when both eyes stimulated simultaneously 

ORGANISATION OF PRIMARY VISUAL CORTEX (V1) 

V1 is organised into functional columns perpendicular to cortical surface  

Columns alternate in eye dominance across the cortical surface 

Hypercolumn: 

• A full set of orientation columns for both eyes 

Within column cells share: 

• Same retinal location 

• Same eye dominance 

• Same preferred orientation 

SCOTOMAS & CORTICAL FILLING-IN

• Damage to V1 causes scotoma (cortical blind spot) 

• Brain fills it in automatically 

BLIND SIGHT

Patients with large V1 scotomas report no conscious awareness of objects in blind field yet can still interact with objects there  

• Incomplete damage leaving residual processing 

• Parallel visual pathways that reach secondary visual cortex without passing through V1 

DORSAL STREAM (PARIETAL CORTEX)

• Specialises in visual spatial perception 

• Specialises in visually guided behaviour 

• Reaching/intercepting/ interacting with objects 

• Damage impairs motor interaction but not verbal description 

VENTRAL STREAM (INFEROTEMPORAL CORTEX)

• Specialises in visual pattern recognition 

• Specialises in conscious visual perception 

• Damage impairs description/recognition, but not motor interaction 

PULFRICH ILLUSION

Illusion where a 2D moving object is perceived as 3D (moving in depth) dye to a delay in signal processing between eyes.

EX. ball swinging left to right it perceived as swinging in a circle

PULFRICH ILLUSION COMPONENTS

1. Stereoscopic Vision

2. Simple harmonic motion of the pendulum

3. The filter/lens

STEREOSCOPIC VISION

• 2 eyes get slightly different images of the world

• brain combines these 2 images to get a single perception

• important for depth perception

FIXATION:

fixate → eyes focus and align → image falls on fovea

PERIPHERAL:

image falls beside fovea, on retina.

UNCROSSED DISPARITY

Focusing/fixating on an image BEHIND what you were initially fixated on. Object appears further away in depth

• farther than fixation point

• image appears further to left in space to left eye

• image appears further to right in space to right eye

CROSSED DISPARITY

Focusing/fixating on an image IN FRONT what you were initially fixated on. Object appears nearer in depth

• nearer than fixation point

• image appears further to right in space to left eye

• image appears further to left in space to right eye

SIMPLE HARMONIC MOTION

Ball accelerates as travelling down due to gravity. At bottom it is maximum gravity. Ball slows down as it travels back up, due to going against gravity.

DIMMING FILTER/LENS

• Signal transduction from retina to brain is faster for brighter images

• filter over your eyes dims the image that falls on your retina

• filter in ONE eye, means that signals from one eye are slightly delayed

COMBINATION: FILTER OVER LEFT EYE

• ball appears further away on the same side as filtered eye

• delay in filtered eye causes ball to always be perceived as behind

• ball appears to move in clockwise direction

LEFT TO RIGHT MOTION

• uncrossed disparity

• further away in depth

• illusory backward arc

RIGHT TO LEFT MOTION

• crossed disparity

• nearer in depth

• illusory forward arc

COMBINATION: FILTER OVER RIGHT EYE

• ball appears further away on the same side as filtered eye

• delay in filtered eye causes ball to always be perceived as behind

• ball appears to move in anti-clockwise direction

LEFT TO RIGHT MOTION

• crossed disparity

• nearer away in depth

• illusory forward arc

RIGHT TO LEFT MOTION

• uncrossed disparity

• further in depth

• illusory backward arc

MONOCULAR DEPTH CUES (ONE EYE)

1. OCCLUSION - chair blocks persons leg therefore, person behind chair

2. RETINAL SIZE - object gets ‘bigger’ when closer

3. AMES WINDOW - using angles and shading

4. LINEAR PERSPECTIVE - parallel lines converge in the distance

5. TEXTURE GRADIENT - more detailed closer to

6. SHADING - light generally comes from above (convex vs concave)

7. LIGHT SCATTER - distant objects appear hazier & bluer

8. MOTION PARALLEX - objects further away appear to move slower