Sensation And Perception 2 Flashcards
Optic Ataxia
- Damage to the where stream
- Damage to the parietal lobe
- Inability to use visual information to guide movement
- Cannot match orientation of card in hand to that of a slot with the same orientation
- BUT can push the card through the slot
How do ‘what’ and ‘where’ stream relate to LGN?
- Layers 1-2 project to the dorsal pathway (where)
2. Layers 3-6 project to the ventral pathway (what)
What is Object perception
- Perceiving objects and separating them from their backgrounds seems easy and automatic
- But it’s quite difficult
- Hard for computers
- Information organized into coherent units
Images on the retina can be ambiguous, blurred or hidden
- Ambiguous because a particular shape, like a circle on the retina, can be created by objects that are aren’t circular
- Hidden meaning people understand objects continue to exist even if an object is partially covered
Gestalt psychology
- Gestalt: essence of an entity’s complete form
2. The whole is different than the sum of our parts
Gestalt principles of perceptual organization
- Similarity: similar things appear to be grouped together
- Continuity: things partially covered by other objects are seen as continuing behind the covering object
- Proximity: things that are near each other appear to be grouped together
- Common fate: things that are moving together appear as a group (same direction)
- Closure: connected region of the same visual properties, color, texture, motivation n, is perceived as a single unit
- Familiarity
- Figure-ground: when we see a separate object, it is usually seen as figure that stands out from its background, called the ground
Figure-ground segregation
- Process by which objects are separated from their backgrounds
- Borders of shape are assigned to a figure
Figural cues
- Symmetry: symmetrical regions more likely to be seen as figure
- Convexity: convex (not concave) more likely to be seen as figure
- Closure: enclosed regions more likely to be seen as figure
- Small area: regions with a smaller area more likely to be seen as figure
- More likely to perceive lower area as figure vs. upper area
- Familiarity: regions that are familiar more likely to be seen as figure
Object perception in the brain i.e. What types of neurons
- Neurons in inferior temporal cortex fire in response to whole objects
- Fire to specific objects
- IT neurons have large receptive fields
- IT neurons can prefer to have certain objects in their receptive field like an apple, square, or faces
Specificity coding
- IT neurons fire in response to objects, and fire in response to specific objects
Types of neurons in IT
- Size specific: small # of objects of a particular size
- Location specific: small # of objects of a particular location
- View specific: small # of objects shown in a particular view
- Size variant: many different sizes of a small group of objects
- Location invariant: small group of objects located in many different places in visual field. These neurons have very large receptive fields
- View invariant: small # of faces seen in many different views
Face perception
- Some neurons in IT respond selectively to faces
2. Area referred to as the fusiform face area (FFA, in temporal lobe)
Damage to FFA
- Prosopagnosia: can perceive faces but cannot recognize them
- Evidence for unconscious recognition: increased skin conductance when viewing picture of significant other
Grandmother cells
- Neurons that increase their firing rate in response to a very specific stimulus
EX. picture of grandma regardless of angle or facial expression - Experimentally tested: cell that fires to Jennifer Aniston but no other famous people or non famous people
Real motion
- When there is actually motion
2. Continuous smooth movement over space and time
Illusory motion
- When there is not actually motion
2. 4 types: apparent motion, induced motion, motion aftereffects, peripheral drift
Apparent motion
- Illusion of movement between 2 objects separated in space when the objects are flashed rapidly on and off, and separated by a time interval
- Set of discrete displacements
- Brain activation for real and apparent motion occupied in the same region of the brain
- Ex. Motion pictures
Induced motion
- Stationary object appears to be moving due to the presence of other moving objects nearby
- Ex. Clouds moving over the moon makes the moon appear like it’s moving too, but it is not.
Motion aftereffects
- The perception of motion after looking at a moving stimulus and looking away (or once the motion stops)
- Ex. Waterfall Illusion
Peripheral drift
- The Illusion of motion in the periphery when your eyes are moving or blinking
Motion perception at the retina
- Retinal neurons fire in response to motion in their RF
2. But there is motion perception beyond the retina
Where in the brain is motion processed?
- Area MT (medial temporal)
2. Located in dorsal ‘where’ pathway
Newsome et al. Study on monkeys
- Presented monkeys with moving dot display
- Asked monkeys to indicate direction of motion
- Goal: investigate relationship between monkeys ability to perceive motion and the response of a neuron in MT
- Higher motion coherence = greater activity in MT
- Result: Area MT is involved in motion perception
What if MT is damaged: Newsome study with monkeys
- Monkey can detect motion at 1-2% coherence
2. With MT lesion, monkey can’t detect motion until 20% coherence
MT damage in humans looks like?
- Bilateral MT damage = akinetopsia: motion blindness
- All perceptual/ cognitive functions intact…only motion perception is impaired
- Photographic snap shots of motion
What if MT is stimulated?
- Electrodes implanted into the brain of an awake monkey
2. Stimulation to MT neurons changes perception (may change direction of motion)
When we follow something with our eyes, is there movement on the retina/ cortex? And what 2 bits of info are needed?
- No movement on retina or cortex
2. 2 bits of information: whether eyes are moving and what the image on the retina is
Corollary discharge theory
- 2 types of signals sent to the brain regarding motion
- Corollary discharge signal (CDS): signal that tells the brain an eye movement has occurred.
- to initiate eye movements, brain sends motor signals (ms) to eye
muscles (moving your eyes to follow a moving object) - the ms causes the CDS to be sent to the cortex
- to initiate eye movements, brain sends motor signals (ms) to eye
- Image displacement signal (IDS): signal that tells the brain that something has moved across the retina (no eye movement)
- movement across retinal receptors sends IDS to the cortex
What happens when CDS or IDS signals are sent?
- Motion is perceived
What happens when CDS and IDS signals are sent?
- No motion is perceived
Neural evidence for CDT
- Bar moves across receptive field
- Bar stationary and eye movement causes receptive field to move across bar
- Results: different neural responses in visual cortex
- Neurons fire in response to real movement, not any movement
- Real motion neurons
Biological motion
- Movement of an animate object (like a human) EX. walking, skipping
Neural correlates of biological motion
- Superior temporal sulcus (STS) responds more to biological motion than scrambled motion
- Knocking out STS via TMS = impaired biological motion perception
Area MT: major findings from Newsome study with monkeys
- Area MT is involved in motion perception: higher activity with higher motion coherence
- Area MT is necessary in motion perception: impairment to MT= can’t perceive motion
Distributed Attention
- Broad focus on no particular object
Divided attention
- Focus on a few particular areas/objects
- We can not multitask, we are only switching back and forth between tasks quickly
- Exception is supertaskers
- Ex. Driving and talking on phone = delayed brake time, impaired object perception, increased accident rates.
Selective attention
- Focus on a particular area/object
Why is attention important/necessary?
- Need help dealing with all the mess of info in the world
2. Processing everything at one is bad i.e autism
Saccades
How does it affect attention?
- Selective attention task
- Eye movements that can occur unconsciously (200 ms)
- Eye tracker: detects contrast between pupil and cornea in order to track saccades
- Can have saccades without attention
- Can have attention without saccades
Bottom up influences involved in attention
- Stimulus salience: the features that stand out due to physical properties (contract, brightness, familiarity, drawing our attention)
Top down influences in attention
- Task: where we focus our attention depends on the task.
- Experience: where we focus our attention depends on how much we know about the situation i.e someone who knows a lot about sports
Eye movements and autism
- Autistic children have trouble detecting facial expressions
- Saccades when viewing faces much different than individuals without autism
- Perhaps due to differences in eye movements
Can attention enhance perception?
- Yes, attention improved reaction times to a target
Object based attention
- Attention is automatically drawn to objects and spreads within them
- Attention enhancing perception
Can attention hinder perception?
- Yes, attention is limited: you can only focus on one thing at once
- Attention is a spotlight
Change blindness
- Failure to perceive something due to breaks in your visual scene
Inattentional blindness
- Failure to perceive something due to your attention being allocated elsewhere
- Ex. Magic trick thief or following an assigned task
Processing unattended stimuli
- Using interocular suppression: 2 different images shown to each eye
- Results: attention unconsciously directed towards opposite gender
- For men, attention was directed away from same gender (but not for women)
- Attention can be directed unconsciously, we process stimuli unconsciously
Binding: function of attention (what is the purpose of attention?)
- Process by which features (individual lines, colors, motion) are assembled to form a coherent object
- Attention helps to assemble or bind these features
- To perceive objects, need to put features together that are processed by V1 (location, depth, color, form)