S1W2Vision Flashcards
Three stages of visual perception
Stimulus
Light reflected and transformed
Receptor processes
Lens
Controlled by cillary muscle
Allows for focus
Loses flexibility with age (glasses)
Rods and cones
Transform light energy (photons) into electrical signals.
Located at back of eye.
Rods: dim light scotopic (no colour)
Cones: full light photopic (colour)
Transduction
transformation of light energy into electrical signals
Eye to LGN processing
LGN receives all info from all senses.
Incoming data (stimuli) = bottom up
Existing knowledge = top down
Interact and meet in LGN to allow perception.
Fovea vs. peripheral
Only cones in fovea.
Peripheral contains rods.
Sharper images in fovea.
Ganglion cell to photoreceptor ratio
More photoreceptors per ganglion cell in periphery.
Makes it more sensitive to light but loses sharpness.
Sharp images (high acuity) needs low ratio of photoreceptors to ganglion cells.
4 types of cells in retina
Horizontal, Bipolar, Amacrine, Ganglion
Horizontal cells
Connect laterally to rods, cones and bipolar cells.
Lateral inhibition of neighbouring cells.
Inhibits poorly lit receptors so only well-lit signals reach ganglion cells.
Amacrine cells
Connect bipolar and ganglion cells.
Provide interaction between the two.
Bipolar to Ganglion cells
Nerve impulses from photoreceptors go from bipolar to ganglion cells.
Transmitted in graduated potentials (polarisation).
Ganglion cells
Go to optic nerve.
Visual field
Fixed point of view and what you can see around it (< 180 degrees)
Halves of retina
Temporal - outside
Nasal - inside near nose
Primary visual pathway
Partial crossing of optic nerve axons and optic chiasm.
Allows nasal and temporal halves to see whole of each eye’s visual field.
Passes info to opposite side of the brain (left occipital lobe for right visual field).
Info all goes to V1
Also goes to SCN.
SCN
Determines pupil diameter from retinal light levels.
Controls eye movements.
Helps body clock.
Brain areas
V1 - primary visual cortex
V2, 3 & 4 – additional visual areas
VP – ventral posterior
MT – middle temporal
MST – medial superior temporal
Parvocellular pathway
Ventral (what).
Extends to inferior temporal cortex.
Form and colour.
Colour coding in blobs
Deal with higher cognitie functions (imagery/memory)
Parvo have central retinal distribution.
Magnocellular neurons
Dorsal (where/how).
Extends to posterior parietal cortex.
Motion and depth.
Deals with attention, eye movements, spatial processing and object location.
Two visual systems model (Milner & Goodale, 1995)
Ventral/Parvo - Vision for perception.
Dorsal/Magno - Vision for action.
Ventral (Vision for perception)
Top-down processing from visual and semantic memory.
Perceptual representations for planning etc.
Allocentric (not person centered).
Mostly conscious awareness.
Relies on more input from fovea.
Dorsal (Vision for action)
Real time bottom up processing.
Guidance of movement.
Egocentric (observer’s perspective).
Not always conscious awareness.
Criticisms of two system model
Pathways not independent.
Interaction difficult to specify.
Dorsal stream not used on its own apart from rudimentary movements.
Most tasks use both pathways so prediction is difficult.
Landmark discrimination (lesioning)
Pick food close to a stick.
Lesioning parietal lobe made it difficult.
Both pathways interact.
Object discrimination (lesioning)
Pick correct shape.
Lesioning temporal lobe made it difficult.
Both pathways interact.
Visual Agnosia
Damage to ventral(parvo) pathway.
Errors on judging orientation of a static slot but could put card in slot.
Damage to vision for perception.
Optic Ataxia
Damage to posterior parietal cortex (dorsal/magno).
Poor at making precise visually guided movements.
Damage to vision for action.
Grasping illusion
Line 1 looks shorter but is actually longer than Line 2.
Estimate length of a line using fingers (length estimation).
Reach towards line to grasp it (grasping task).
Illusion worked in line estimation but not in grasping.
Shows when using vision for action/dorsal pathway judgement is more accurate.
Inverse projection problem
How do 2D images become 3D representations.
More than one distal stimulus to produce a proximal stimulus.
Need to work out which distal stimulus produced the correct one.
Purely retinal information would not provide the answer.
Distal and Proximal stimuli
Distal stimulus: object in environment.
Proximal stimulus: image on retina.
Theory of unconscious interference (Hemholtz)
Likelihood principle: perceive object that is most likely to have caused proximal stimulus
Perceptions are result of unconscious interferences we make about environment.
Observer’s knowledge of environment taken into account.
Figural goodness
Simplicity, order and regularity of an object.
Different perceptions for a single object but this determines which one is perceived.
Match pairs of good figures more quickly.
Remember, learn and describe them better.
Laws of grouping
Proximity: objects close together grouped together.
Continuity: organising images to produce smooth continuities not abrupt changes.
Similarity: similar objects grouped together.
Common fate: objects moving in same direction grouped together.
Law of closure
Whole shape perceived by filling in missing information.
Law of symmetry
Symmetrical objects grouped to form combined symmetrical object
Law of synchrony
Visual elements that occur at same time are perceived together
Law of common region
Objects sharing an area with a defined boundary are perceived as belonging together.
Element connectedness
Object connected by uniform visual properties perceived as more related than elements that are not.
Figure ground segregation
The figure is perceived as having a distinct form, being in front of the ground & having contours.
The ground is perceived as lacking form.
Benefits of Gestalt laws
Nearly all laws of grouping have stood the test of time.
The notion that observers perceive the simplest possible organisation of the visual environment is useful.
Law of Prägnanz
People will perceive and interpret ambiguous or complex images as the simplest form possible.
The fundamental principle of gestalt.
Criticisms of Gestalt laws
Ignores past experience.
Overfocused on people’s processes and disconnected from the environment.
Too descriptive.
Laws applied to 2D drawings not 3D objects.
Semantic regularities
Characteristics associated with the functions carried out in different types of scenes.
Scene schema
The knowledge of what a given scene ordinarily contains e.g. doctors office.
Horizontal, vertical and slanted representations
There are more cells in brain that account for horizontal and vertical orientations than slanted ones.
In life there are more horizontal and vertical orientations.
Physical regularities
Regularly occurring physical properties of the environment.
Bayesian inference
The probability of an outcome influenced by two factors:
Prior probability: our initial belief about the probability of an outcome (knowledge).
The likelihood of a given outcome.
Restates Helmholtz - we perceive what is most likely to have created the retinal image.
Conclusion of the inverse projection problem
Changing the viewing angle and distance can further strengthen the conclusion.
These processes occur rapidly and unconsciously.
The retinal image is the starting point, adding the person’s prior beliefs reduces the possible shapes that could be causing this particular image.
Experience dependent plasticity
With learning the brain’s functioning can be tuned to operate best in a specific environment.
Visual system may have been shaped to contain neurons that respond to features that are frequent in the natural environment (natural selection).
