Lecture 2 - Perception and Vision Flashcards
Why is perception important?
There is little cognition without processing stimuli.
A cognitive process. it is the fundamental cog process as it involves response to sensory stimuli and all of these other cog processes such as attention, memory, language, reasoning all derive from this.
What is sensation?
Passive process
How information from external environment is brought into the body and brain
What is perception?
Active process of selecting, organising and interpreting stimuli by senses and bringing it to the brain
What is the sensation process?
SRNB
Sense organs > (have millions of) receptor cells > (electrical signal) travels along nerve conduit > to brain area where info is processed.
What is the sensation process of vision?
Eye (involves light waves) > Detected by receptor cells called Rods and Cones > Travel along nerve conduit called Optic Nerve > To visual cortex where brain signals are processed
What do specialised receptor cells in the sense organs do?
Convert signals into electrical signals and then are processed by certain brain areas
What are the approaches in perception?
Gestalt Laws (top down) and Computational Approach: feature representation (bottom up)
What is Gestalt German for?
Form or shape.
How did Gestalt Laws emerge?
Paradigm emerging from late 19th century. Identify laws which describe how we organise our visual perception.
Describe how elements are arranged into forms and objects and are then perceived as a whole (like faces, animals)
What are the Gestalt Laws?
SPCCP(S) and CSP
Law of
Similarity, Proximity, Continuation, Closure, Pragnanz with double dots (Law of Similarity)
and
Law of
Common Fate, Symmetry, Parallelism
What is the Law of Similarity?
Elements that look similar will be grouped together and will be seen as being part of the same form
What is the Law of Proximity?
Elements closer together are grouped together and will be perceived as being meant to belong w each other
What is Law of Good Continuation?
We perceive lines as following a continuous course
What is Law of Closure?
How we view incomplete objects as a whole (can lead us to seeing illusionary lines that dont exist) we dont need boundaries to see a shape
What is Law of Pragnanz (Similarity)?
Fundamental Principle all laws assist this
We organise a scene according to its shortest / simplist explanation
What is the law of common fate?
Elements moving together can be grouped together (like moving objects)
What is the law of symmetry?
Elements symmetrical can be grouped together (tend to be)
What is the law of parallelism?
Elements that are parallel tend to be grouped together
What is a weakness of Gestalt laws?
Not scientific
What does feature representation in the visual pathway consist of?
ROT V1 V2
Retina, Optic nerve, thalamus, primary visual cortex (V1), Higher visual cortex (V2)
What is the active process of feature representation in the visual pathway?
Signal enters the eye and retina
Photoreceptors (Rods and Cones) convert lightwaves into electrical signals which pass to the optic nerve into the thalamus (brain area).
Thalamus passes info to primary visual cortex (v1)
Info passed further up and beyond to higher visual cortex (v2) for processing and Inferior temporal cortex (IT)
What is single cell recording?
Method to investigate which type of neuron is responsible for certain processing of visual stimuli.
Put electrode into brain and that directly records electrical activity w that single cell neuron
What is a limitation and strength of single cell recording?
Invasive for animal
Systematic and scientific
What are receptive fields?
Features that neurons are most responsive to eg circular images
What do neurons in the Optic nerve and thalamus respond to?
Either light or dark circular visual stimuli.
Electrical signal passes thru optic nerve, thalamus, then primary visual cortex v1
What do neurons in v1 respond to that Hubel and Wiesel found?
Neurons in v1 respond more to lines than circular visual stimuli like in thalamus and optic nerve. There are receptive fields that are responsive to particular visual fields
How can a map of the specialisations be developed?
Some activity for specific orientation that the neuron isnt attuned to,
but as u move neuron by neuron in the visual cortex, activity will increase as you move the orientations
How is edge perception created? (perceiving edges)
Putting together these orientations of activity that neurons respond to and isn’t attuned to help us perceive edges
So neurons in visual cortex are specialised for lines of a given orientation
Visual processing is _?
Hierarchical
circles become lines by combining signals from the optic nerve and thalamus to v1. combination of different receptive fields
How do the combination of different receptive fields allow us to view lines from circles?
Lower level feature info is combined by neurons
Info from receptive fields of neurons in thalamus is projected and combined w receptive fields in V1 and neuron in v1 become excitatory and fire action potential.
Project forward and activate features of a more complex level.
What does hierarchical processing describe?
As we ascend the hierarchy, the receptive field gets larger as the receptive fields get more complex and pulled from lower levels.
(This visual feature is a bottom up approach)
What is the bottom up approach?
Complexity increases as we move up, projecting up level by level
What is the inferior temporal cortex? (IT)
Large receptive fields responsive to complex stimulus (entire objects excite neurons)
What is the secondary visual cortex? (V2)
Pull orientation info together to get angles
Start to build simple outlines and shapes
What is the primary visual cortex? (V1)
Receptive fields are excited by specific orientations
What are the two separate streams of visual processing?
Bottom - up approach
Ventral stream
Dorsal stream
What is the Ventral stream?
Visual processing
The what? pathway
Identifies what you are seeing (eg shapes)
From primary visual cortex (V1) to Inferior Temporal Lobe (where language functions are localised)
To be able to communicate we need to know what objects are
What is the Dorsal stream?
The where? pathway
Locates where an object is and where object is moving to
For navigating around world
From primary visual cortex to the superior parietal lobe (Towards motor areas, Specialised for movement)
In this lecture what consists of the bottom up approach and what consists of the top down approach
Top down - Gestalt Laws/ Psychology, Context and Assumptions (Illusionary perception of ambiguous images)
Bottom - up approach - Computational Approach: Feature representation (features in pathway are built upon simple elements) and Two streams of visual processing
What is the computational approach?
Activity built up hierarchically through faithful reconstruction - built through visual inputs alone.
We reconstruct a visual scene by combination of simpler elements
What is Gestalt Psychology?
Simplified interpretations built using our assumptions and knowledge of the world
We combine elements in ways to gain a holistic understanding of a scene
Perception not always representation of reality, can be based on other factors that can affect our processing.
What is the Bottom Up approach?
Processing the stimuli influences what is perceived > data driven
Develop perception from a low level of info from visual signals, so we have very invasive low level processes which are feeding forward data about the world
What is the Top-Down approach?
Background knowledge and assumptions of the world shape what we perceive > expectation driven
from higher brain areas feed down to shape perception
Info taken from higher levels of our brain and those shape our perception
Provide us w rules, simplifying some processing and give us contextual info about those incoming signals
Why does context matter?
Our environment gives us clues when stimulus is new/ambiguous
The context in which we perceive a stimulus in can change our perception of it
What are visual illusions a consequence of?
Bottom up vs top down processing
Assumptions can cause an incorrect percept - top down
What do illusions allow us to access?
The assumptions we have
What is the Ebbinghaus Illusion?
Illusion emerges due to the size contrast between the central and surrounding circles
The orange dots are the same size
Like Deboef Illusion
Expectations of the world shape perception
eg the circle perceived as concave convex bumped depending on shading
How does the top down approach resolve ambiguities?
Use assumptions/expectations to understand what cues mean in different contexts
Decide which visual scene caused the image on the retina by combining assumptions and cues to make best guess at what we are seeing
What are assumptions?
Expectations about what we will see or what different cues mean
What are cues?
Features of the image that gives cues as to the nature of the stimulus
Which processes interact to resolve ambiguities in perception?
Assumptions and cues
What are cues based on?
Bottom up processing, physical feature and nature of stimulus
