Cognitive Semester 1 Week 2: Perception I - Vision Flashcards
What is sensation?
The passive process of bringing information from the outside world into the body and to the brain.
What is perception?
The active process of selecting, organising and interpreting the information brought to the brain by the senses.
How does sensation work?
Sense organ -> receptor cells -> nerve conduit -> brain area
What is gestalt psychology?
Gestalt - ‘form’ or ‘shape’
Gestalt psychology explains how parts are arranged into forms and objects, and perceived as a whole.
Gestalt law: Similarity
Elements that look similar will be perceived as being part of the same form.
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Gestalt law: Proximity
Elements that are close together will be perceived as belonging together.
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Gestalt law: Good continuation
We perceive lines as following a smooth course.
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Your brain sees this as a continuous zigzag line rather than separate discontinuous elements.
Gestalt law: Closure
A boundary isn’t necessary for us to perceive a shape. When small elements are arranged in groups we tend to perceive them as larger figures. This can lead to us seeing illusory lines that do not exist.
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This is an incomplete rectangle, but your brain still recognises it as a rectangle by ‘closing’ the gaps.
Gestalt law: Prägnanz (simplicity)
We organise a scene according to its simplest (shortest) explanation. “Simplifying complex visuals”
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We may see this as a collection of diamonds and triangles, rather than a random assortment of lines.
Gestalt law: Common fate
Elements that move together tend to be grouped together.
Gestalt law: Symmetry
Elements that are symmetrical tend to be grouped together.
Gestalt law: Parallelism
Elements that are parallel tend to be grouped together.
Are gestalt laws still relevant?
- measure responses when systematically varying by a Gestalt law, e.g. to test symmetry
- Gestalt laws influence designers e.g. app design
The visual pathway
- Retina
- Optic nerve
- Thalamus
- Primary visual/striate cortex (V1)
- Higher visual cortices (V2, IT etc.)
How do we investigate which features in a visual scene are represented in each area of the visual pathway?
Idea: record from a single neuron, present different visual stimuli, check which ones elicit a response.
Invasive single cell recording by placing electrode in brain that is closest to that single neuron. If a high response is detected to a specific stimulus (e.g. diagonal line) this tells us that neuron is specialised for that feature.
What are the receptive fields of the primary visual cortex sensitive to?
Line orientation
What are the receptive fields of the optic nerve and thalamus sensitive to?
- They are most responsive to circular stimuli
- Centre-surround organisation (dark centre white surround or vice versa)
What is hierarchical processing/visual hierarchy?
- Information higher up in the visual pathway is built up upon information from the earlier areas.
- Receptive fields in v1 (lines) are built up by combining receptive fields of neurones in the thalamus (dots)
What is the ventral pathway?
- The ‘what’ pathway
- Language functions (e.g. object recognition)
- Ventral, V1 to inferior temporal lobe
What is the dorsal pathway?
- The ‘where’ pathway
- Motor functions (e.g. spatial awareness)
- Dorsal, V1 to superior parietal lobe
What is bottom-up processing?
- low level -> high level
- processing the stimuli influences what is perceived -> data driven
- developing perceptions from low level information from visual inputs
- computational visual perception
What is top-down processing?
- high level -> low level
- background knowledge and expectations influence what is perceived -> expectation driven
- heavily influenced by environmental context
- Gestalt psychology
Visual illusions
- Many illusions are a consequence of top-down or bottom-up processing
- Assumptions can cause an ‘incorrect percept’.
How do we resolve ambiguities?
- Assumptions and cues combine to make a best guess
- Cues: features of the image that give clues as to the nature of the stimulus (bottom-up)
- Assumptions: expectations about what we will see or what different cues mean (top-down)
