5 Flashcards
Why is motion perception important?
- many (risky) objects are moving (predators, cars etc)
- social interactions involve movement at many levels (greetings, accept/refuse an offer), use movement cues to determine a person’s intentions
What is real and illusory motion?
Real motion
* an object is physically moving on the retina, moves across our field of view
Illusory motion
* the stationary stimuli are perceived as moving
What are 3 types of illusory motion?
- apparent motion
- Induced motion
- motion aftereffects
What is apparent motion?
- 2 stimuli in different locations are alternated with the correct timing, we perceive one stimulus moving back and forth smoothly
- basis for motion we perceive in movies and TV, stop motion
What are the rules of apprent motion?
the shortest-path constraint = objects will take the shortest path between flashed positions
even if its physically impossible
What is the threshold of interval detection for apparent motion?
~0.1 second
if interval is short enough, then the 2 stationary stimuli will be perceived as one moving stimulus during this period
What did Larsen experiment find out about real and apparent motion using fMRI?
- 3 conditions during fMRI scanning
- control, apparent motion condition, real motion condition
- fMRI show similar neural responses to real and apparent motion
What is induced motion?
motion of one object (usually larger) causes a nearby stationary object (usually smaller) to appear to move
e.g. clouds are moving past the moon, the moon may appear to be racing through the clouds
What are motion aftereffects
a visual illusion occurs after viewing a moving stimulus for a long time, and then viewing a stationary stimulus and it appears to move
e.g. waterfall illusion: if you look at it for 30s then look at a rock, the roc will appear to move in he opposite direction
What is the Reichardt detector?
a neural circuit that creates a neuron that responds to movement in a specific direction
What does a Reichardt detector consist of?
- 2 adjacent neuron (A and B): both sends signals to the output unit
- 1 delay unit: slow down the signals from A as they travel toward the output unit
- 1 output unit: compare and multiple signals from A and B
How does a Reicardt detector work?
- when a stimulus moves from left to right
- it reaches A, then activates the delay unit
- then the stimulus travels to B, signals from A and B reach to the output unit at the same time
- motion detected
How is a Reichardt detector direction-sensitive?
- it is determined by the position of the delayed unit
- a right motion detector is insensitive to left motion
- if stimulus goes from B to A, and B doesnt have the delayed unit, the stimulus does not arrive at the output unit together and thus no signal
How do Reichardt detectors contribute to movement across the visual field?
there are many circuits like this in the visual system, each tuned to a different direction of motion, working together they can create signals that indicate the direction of movement across the visual field
What is corollary discharge theory
- movement perception is dependent on 3 signals
1. Image movement signal (IMS): signal generated by an image that moves across the retina
2. Motor signal (MS): signals sent from the motor cortex to the eye muscles to cause the eye to move
3. Corollary discharge signal (CDS): a copy of the motor signal - cancels out IMS and CDS to estimate the likelihood of real motion in the world
- movement will be perceived if the comparator (a brain structure) receives IMS or CDS
- movement signal can be cancelled if both IMS and CDS reeach the comparator
- Movement will be perceived if the comparator (a number of brain structure) receives just one signal (IMS or CDS)
Explain how we perceive motion when eyes follows a moving stimulus
- A MS is initiated to move the eyes rightward
- the CDS is (copy of the MS) sent to the comparator
- no moving image on retina, no IMS
- |IMS-CDS| > 0
- comparator therefore receives just one signal, so perceives movement
Explain how we perceive motion when eye is stationary; stimulus is moving
- no eye movement = no MS and CDS
- but stimulus is moving across retina = yes IMS
- |IMS-CDS|>0
- there is movement
Explain how we perceive motion when eye is scanning a scene from left to right?
- MS due to eyes moving
- CDS sent to comparator
- Moving image on retina generates IMS
- |IMS-CDS| = 0
- no motion
What cells in the brain is sensitive to motion?
- V1
- complex cells: sensitive to moving bars at some orientation, some are directionally-selective
- end-stopped cells: sensitive to moving corners/angles/bars of a specific length in a particular direction
Where is the visual motion processing “hub”
- middle temporal (MT) area
- aka M5
- contains many directionally selective neurons
What is the coherent motion task?
Coherence: the degree to which the dots move in the same direction
- used moving dot displays in which the direction of motion of individual dots can be varied
- monkeys were trained to judge the direction in which dots were moving
- Record the MT neurons during he task
- As the dots coherence increase
- the monkey judged the direction of motion more accuraetly
- MT neurons showed a higher firing rate
- demonstrated close relationship between monkeys perception of motion and neural firing of MT cortex
What happens after lesioning the MT cortex?
monkeys motion perception performance decreased
= strong causal evidence
How can we replicate lesioning studies in humans?
- using TMS
- non invasive brain stimulation via means of electromagnetic induction
- the magnetic field can generate an electrical current inside the skull
- can temporarily inhibit or activate the neural activity of a brain region
What was the case of patient LM
- MT cortex damaged due to stroke
- lost the ability to perceive motion
- “the fluid appeared to be frozen, like a glacier”
- could not perceive the tea rising in the cup, she had toruble knowing when to stop pouring
- Akinetopsia (aka motion blindness)
What are the limitations of human lesion studies?
cannot control size of damage, size of damage is often larger than the region we are interested in
What is the effect of TMS on MT cortex during a stimulation and sham group?
- stimulation group: the neural activity of the MT cortex was inhibited by TMS, showed worse direction judgment performance than the sham group
- Sham group: the angle of the stimulating coil was changed, so no TMS to the MT cortex
What is the aperture problem?
- a limitation of V1 neuron
- the direction of a moving stimulus can be ambiguous, because the size of the RF of a V1 neuron is limited
- RF of a neuron is like a aperture: the circular region, only reveals small portion of the scene
- Because the motion of the edge was the same in both situations, a single directionally selective neuron would fire similarily in a and b so based on the activity of this neuron it isnt possible to tell whether the pole is moving horizontally to the right or upward at an angle
What are the solutions to the aperture problem?
- the high-level global-motion detectors in the MT cortex can solve this problem
- it receives info from lower-level local-motion detector (e.g. in V1)
- then combine these signals to determine the actual direction of motion
2 solutions
* Can use information about the end of a moving object (i.e. tip of pencil) to determine its direction of motion, neurons that can respond to this are found in the striate cortex
* Pool or combine responses from a number of directionally selective neurons
What is biological motion?
- the self-produced motion of a person (or other living organism)
- by connecting dots and motions
What is a point light walker stimulus
placing small lights on peoples joints then filming the pattern created by these lights when people move
the movement of the dots will be organised in to a “moving person”
stationary lights are meaningless, but once it starts moing, it is immediately perceived as being caused by a walking person
What is scrambled motion compared to biological motion?
same number of fots moved similarily to the point light walker dots, but were scrambled so they did not resylt in a “person walking”
Is there a region that selectively responds to biological motion?
found superior temporal sulcus (STS) is more active when viewing biological motion compared to scrambled
What are the brain regions involved in motion perception?
- V1 (striate cortex) = direction of motion across small RF
- MT (middle temporal area) = direction (and speed) of object motion across large RF
- STS (superior temporal sulcus) = perception of motion related to animals and people (biological motion)
