Slides Week 4 Flashcards
From Light to Neural Signals
- We see objects when we detect light reflected from the through our eyes
- A narrow band of electromagnetic radiation that can be demonstrated as a wave of a stream of photons
From Light to Vision
- Light enters the eye throught the Pupil
- Lens refracts light and focusses it on the back of the eye
- Lens focuses an image on the Retina
- Retina sends signals to brain through the Optic Nerve
Rods & Cones
- Light is Transduced into neural energy by photoreceptors
- Located in the Retina
- There are two types
- Rods
- Cones
Photoreceptors - Rods
- Specialised for night vision
- Do not process colour
Photoreceptors - Cones
- Specialise in Daytime Vision
- Fine visual Acuity
- Detect Colour
Photopic and Scotopic Vision Table
- Photopic
- relating to vision in daylight or other bright light, believed to involve chiefly the cones of the retina
- Cone cells are nonfunctional in low visible light.
- Scotopic
- The vision of the eye under low-light levels
- Produced exclusively through rod cells,
Photoreceptors - Horizontal and Bipolar Cells
- Bipolar cells effectively transfer information from rods and cones to ganglion cells.
- Horizontal cells introduce lateral inhibition to the dendrites
- Horizontal cells give rise to the center-surround inhibition which is apparent in retinal receptive fields.
- Ganglion Cells have axons that leave the retina through the Optic Disk (Blind Spot)
Lateral Inhibition
- The capacity of an excited neuron to reduce the activity of its neighbors
- Disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction
Receptive Field
- The region on the retina in which stimuli influence a neuron’s firing rate
- We can map receptive fields by recording signal from the optic nerve
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ON-Centre Ganglion Cells
- Have centre-surround receptive fields
- ON-centre ganglion cells are excited by light that falls on their centre
- They are inhibited by light that falls in their surround.
OFF-centre Ganglion Cells
- Inhibited when light falls in their centre
- Excited when light falls in their surround
Mach Bands
- Illusions created by the visual system
- A bright or dark stripe illusion in gradiated colour scheme
- Occurs because ganglion cells near the transitional areas have parts of their receptive fields in another shade.
The Path of image processing from Eyeball to Brain
- Eye
- Photoreceptors
- Bipolar Cells
- Retinal Ganglion Cells
- Lateral Geniculate Nucleus
- Striate Cortex
Flow Chart of Visual Pathways:
Retinal Ganglion Cells
What can Retinal Ganglion Cells Respond to
- Respond vigorously to spots of light
- Also respond well to certain stripes and gratings
- Striped pattern is called a Sine Wave Grating
- The visual system ‘samples’ the grating discretely
Ganglion Cells can detect stripes because they are sensitive to frequency and phase.
Flowchart of Visual Pathways:
Thalamus LGN
Lateral Geniculate Nucleus
- Where axons of Retinal Ganglion Cells synapse
- We have two LGNs
- each is retinotopically organised
- maps vision contralaterally to each eye
- Have retinal radiofrequencies next to each other
Magnocellular Cells
- also called M-cells
- Neurons located within the Adina magnocellular layer of the lateral geniculate nucleus of the thalamus.
- Large RF Fast Response
- High Sensitivity
- Process Motion
Parvocellular Cells
- Also called P-cells
- Neurons located within the parvocellular layers of the lateral geniculate nucleus (LGN) of the thalamus.
- “Parvus” is Latin for “small”, and the name “parvocellular” refers to the small size of the cell compared to the larger magnocellular cells
- Small RF (High Resolution)
- Slow Response
- Low Sensitivity
- Process Colour
Koniocellular Cells
- Also known as K cell is a neuron with a small cell body
- Located in the koniocellular layer of the lateral geniculate nucleus (LGN)
- Not fully understood
- Process blue/yellow colour
- Large RF
- High contrast sensitivity
- Low Spatial frequency
- Projects directly onto the Extrastriate Cortex
- Implicated in Blindsight
Primary Visual Cortex
- Transforms visual information in the Striate Cortex or V1
- Circular receptive fields found in Retina and LGN replaced with elongated stripe receptive fields in cortex
- Contains about 200 Million Cortical Cells
Two Important features of Striate Cortex
- Retinotopic Mapping
- Cortical Magnification
Striate Cortex: Retinotopic Mapping
Different parts of the visual field are mapped onto different parts of the visual cortex
Striate Cortex: Cortidal Magnification
- Proportionally much more cortex devoted to processing the fovea than the periphery
Fovea
- A tiny pit located in the macula of the retina
- Provides the clearest vision of all.
- Layers of the retina spread aside to let light fall directly on the cones, the cells that give the sharpest image
Cortical Magnification - Visual Crowding
- The harmful effect of clutter on peripheral object detection
- Stimuli seen in isolation in peripheral vision becoes hard to detect when other stimuli are nearby
eg: child on road but much crowded traffic and clutter; can’t see the child
Hubel & Wiesel’s Model
- Circular receptive fields in LGN transformed into elongated fields in Striate Cortex
- A cortical neuron that responds to oriented bars of light might receive input from several retinal ganglion cells
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Orientation Tuning
- Tendency of neurons in Striate Cortex to respond to most bars of certain orientation
- Response rate falls off with angular difference of bar from preferred orientiation
Striate Cortex - Simple Cells
- Respond primarily to oriented edges and gratings and they only respond to light OR dark
Striate Cortex - Complex Cells
- Also respond primarily to oriented edges and gratings
- have a degree of spatial invariance because the respond to light AND dark
- Receptive field cannot be mapped into fixed excitatory and inhibitory zones.
- It will respond to patterns of light in a certain orientation within a large receptive field, regardless of the exact location.
Striate Cortex - Hypercomplex Cells
Like complex cells, but prefer stimuli with an end within the RF
How are cells organised in the Visual Cortex?
- Column
- A vertical arrangement of neurons
- within each column all neurons have the same orientation tuning.
- Hypercolumn
- A block of striate cortex with cells necessary to respond to everything the visual cortex is responsible for in a part of the visual world
Damage to the visual system
- Damage can occur anywhere along the visual system and can result in a unique pattern of impairment
- From the eye: myopia, hyperopia, astigmatism
- To the Brain: Anto-babinski Syndrome, Neurological visual impairment without conscious awareness.
- Charles Bonnet Syndrome: Visual hallucinations in blind individuals
How do we perceive Depth?
- Depth Cue
- Monocular Depth Cue
Depth Perception - Depth Cue
Information about the third dimension (depth) of visual space
Depth Perception - Monocular Depth Cue
A depth cue that is available even when the world is viewed with one eye alone.
Monocular Cues to Three-Dimensional Space: Occlusion
- A cue to relative depth order in which one object partially obstructs the view of another
- Sometimes Occlusion could be an accidental view of the objects in the world
Monocular Cues to Three-Dimensional Space: Relative Size
All things being equal, we assume that smaller objects are farther away from us than larger objects
Monocular Cues to Three-Dimensional Space: Relative Height
- For objects touching the ground, those higher in the visual field appear to be farther away
- In the sky above the horizon, objects lower in the visual field appear to farther away.
Monocular Cues to Three-Dimensional Space: Familiar size
- A cue based on knowledge of the typical size of objects
- When you know the typical size of an object you can guess how far away it is based on how small it appears
- works on cues in conjunction with the cue of relative size.
Monocular Cues to Three-Dimensional Space: Texture Gradient
- Depth cue basedon the geometric fact that items of the same size form smaller, closer spaced images the farther away they get.
- Result from a combination of the cues of relative sizenand relative height
Monocular Cues to Three-Dimensional Space: Aerial Perspective
- Depth Cue based on implicit understanding that light is scattered by the atmosphere
- More light is scattered when we look thorught more atmosphere
- As such distant objects appear fainter, bluer and less distinct
Monocular Cues to Three-Dimensional Space: Linear Perspective
- lines that are parallel in three dimensional world appear to converge in a two dimensional image as they extend into the distance
eg: a road heading off into the distance
Monocular Cues to Three-Dimensional Space: Vanishing Point
The apparent point at which parallel lines receding in depth
Monocular Cues to Three-Dimensional Space: Motion Paralax
- Images closer to the observer move faster across the visual field than images further away
- The brain uses this information to calculate the distances of objects in the environment
Oculomotor Monocular Depth
- Cues based on the ability to sense the position of our eyes and the tension in the eye muscles
- Accomodation
- Convergence
- Monster Illusion
Oculomotor Monocular Depth: Accommodation
- The process by which the eye changes its focus
- The lens gets fatter as gaze is direct toward nearer objects
Oculomotor Monocular Depth: Convergence
- The ability of the two eyes to turn inwards
- Often used on nearer objects
Oculomotor Monocular Depth
- Depth cues indicate right monster is further away
- Both monsters subtend at the same angle on retina
- Brain deduces right monster is bigger
- Both monsters are the same size
Subtend
Form an angle at a particular point when straight lines from its extremities are joined at that point.
Depth Perception: Binocular Cues
A depth cue that relies on information from both eyes
Depth Perception: Stereopsis
The perception of depth produced by the reception in the brain of visual stimuli from both eyes in combination
Depth Perception: Horopter
- Location of objects whose images lie on the corresponding points
- The surface of zero disparity.
- Objects on the horopter are seen as single images when viewed with both eyes.
- Objects significantly closer to or farther away from the horopter fall on noncorresponding points in the two eyes and are seen as two images
Depth Perception: Binocular Disparity
- The differences between the two retinal images of the same scene
- Disparity is the basis for Steropsis a vivid perception of the three dimensions of the world not avaliable with monocular vision
Stereoblindness
- An inability to make use of binocular disparity as a depth cue
- Most infants are stereoblind before 3 months
- Can show a sudden onset of stereopsis at 3-5 months
- They may have acuity matching that of adults by about 6 months (birch & Petrig, 1996)
Define Acuity
- Mental acuity is sharpness of the mind
- Things considered in determining a person’s mental acuity are memory, focus, concentration, and understanding.
Depth Perception: Correspondence Problem
- In binocular vision the problem of figuringout which bit of the image in the left eye should be matched with which bit in the right eye
How is Stereopsis implemented in the human brain
Input from two eyes must converge onto the same cell
Binocular Rivalry and Suppression
- Sometimes we have different stimuli presented to the two eyes
- How does our visual system decide what to see?
Do you see what I see? Antorini et al., 2017
- Perception depends on personality.
- Participants in binocular rivalry studies occasionally have moments of ‘rivalry suppression’ where both images are available at once.
- This is more likely to occur for people high on the openness personality trait
Do you see what I see? Tulviste 2019
- Visual perception depends on culture.
- Tribes in South Africa could not perceive depth in 2D images
- There are difference in colour, face and object processing
Is vision of psychological interest? Jure et al., 2016
- ASD is highly prevalent in blind children
- > 30% times more likely
- But it is underdiagnosed
Is vision of psychological interest? Silverstein et al., 2013
Congenital Blindness is a protective factor for schizophrenia
Is vision of psychological interest? Martinez et al., 2008
- Schizophrenia is characterised by deficiencies along all aspects of the visual pathway, particularly the magnocellular pathway
Is vision of psychological interest? Kaliuzhna et al., 2020
Research is beginning to examine whether deficits in motion perception could be responsible for many of the symptoms of schizophrenia
Is vision of psychological Interest? Teeple et al., 2009
- Visual hallucinations are common in a number of conditions including:
- Parkinson’s (50%)
- Dementia (20%)
- Depression (27%)
- Migraines (31%)
