Chapter 8 Perceiving Motion PDF

Preview

Summary

Summary notes on c8 Goldstein...

Description

Perceiving Motion Functions of Motion Perception Motion Helps Us Understand Events in Our Environment  



E.g. Heider and Simmel (1944): participants explain patterns of shapes moving and interacting with each other in a human way, explaining stories and relationships One source of information when navigating the environment is how objects in the environment flow past us as we move (optic flow); this provides information about the walker´s direction and speed Case study of akinetopsia (blindness to motion): she could not pour tea/coffee as she could not see the cup fill up, struggled with dialogue as she could not see motion of the face/mouth, could not see cars/people approaching – they suddenly appeared very close

Motion Attracts Attention  

Attentional capture: the ability of motion to attract attention May happen when you see someone wave at you etc., may also happen when you are not consciously focusing on/looking for the moving object o Also seen in animal survival; animals freeze in place to avoid predation

Motion Provides Information About Objects  

Movement perceptually organises elements of an object, so they can be perceived as a whole Movement of an observer around an object (and of the object itself) can reveal hidden characteristics of the object

Studying Motion Perception When Do We Perceive Motion? 

 

Difference between real motion and illusory motion (def: the perception of motion using stimuli that are not moving) o One example of illusory motion is apparent motion Induced motion: when motion of one object causes a (usually smaller) object to appear to move (e.g. clouds moving makes the moon appear to move) Motion aftereffects: when viewing a moving stimulus for 30 to 60 seconds causes a stationary stimulus to appear to move

Comparing Real and Apparent Motion 



Larsen et. al (2006): showed 3 displays to someone in an fMRI scanner o Control condition: two dots in different places flash simultaneously o Real motion display: a dot moving back and forth o Apparent motion display: dot flashed one after another, so it appears to move Found that activation associated with apparent motion is similar to the activation for the real motion display (therefore researchers do not treat the two as particularly separate)

What We Want to Explain 

Difference between watching someone walk past when eyes are stationary (projection changes across retina), following someone with eyes (projection is the same on the retina

but movement is perceived), and looking around an empty room (projection changes on retina but movement is not perceived)

Motion Perception: Information in the Environment    

J. J. Gibson: ecological approach; information for the perception is located in the environment, not the retina Optic array: the structure created by the surfaces, textures, and contours of the environment (Gibson studied how movement of the observer changes this) Local disturbance in the optic array: when parts of the optic array are covered/uncovered by movement (provides information that something is moving) When someone scans an empty room, everything moves in relation to the observer´s eyes/body; this is called global optic flow

Motion Perception: Retina/Eye Information The Reichardt Detector 

An early neural explanation for motion perception – excitation and inhibition are arranged so movement in one direction creates inhibition that eliminates neural responding, whereas movement in the opposite direction creates excitation that enhances neural responding o Therefore, this neuron will only respond if the light is moving in a certain direction o This, however, does not explain what happens if the eyes are following movement

Corollary Discharge Theory Signals from The Retina and the Eye Muscles 

 

Three signals that may show motion: o Image displacement signal (IDS): generated when the image moves across receptors o Motor signal (MS): generated when a signal is sent from the brain to the eye muscles – e.g. when eyes follow movement o Corollary discharge signal (CDS): generated as a copy of the motor signal that is sent to a different part in the brain If only the IDS or CDS is sent to the brain, motion is perceived, but if both are, then no motion is perceived Brain contains a comparator: this is a structure that receives both the IDS and CDS. This is likely to be made of a number of structures

Behavioural Evidence for Corollary Discharge Theory  

When you can see an afterimage, it may appear to move as the image is bleached on your retina but if you move your eyes, your brain will receive the CDS but no IDS If you push on your eyelid but keep your eyes still, scene may move because the motor signal to keep eyes still sends a CDS, but no IDS is generated

Physiological Evidence for Corollary Discharge Theory 



Case study of R.W.: experienced vertigo every time he moved his eyes o Found he had a lesion in his medial superior temporal area; as he moved his eyes, the environment also appeared to move – when he did so, the brain sent an IDS but no CDS Monkey studies – extrastriate cortex neuron responds strongly when monkey looks at fixation point as a moving bar moves across neuron´s receptive field. However, if the monkey

follows the bar with its eyes, neuron does not fire – real-motion neuron (also observed in many other areas of the cortex)

Motion Perception and the Brain The Movement Area of the Brain   



Middle temporal area also contains many directionally sensitive cells Coherence (def): the degree to which dots move in the same direction; 0% coherence when all dots moving in different directions, 100% when they all move in the same direction, etc. Newsome et. al: study on monkeys, when coherence increases, the monkey is able to judge the direction of motion more accurately and the MT neuron fires more rapidly (very strong relationship between the 3) Three relationships can be drawn from this: o Stimulus-perception relationship: presenting a stimulus and measuring whether motion is perceived; e.g. when an array of dots is moving in the same direction, we see movement in that direction o Stimulus-physiology relationship: presenting a movement stimulus and measuring neural responding o Physiology-perception relationship: the relationship between perception and physiological responding – e.g. measuring the rate of MT firing and the perception of the monkey of the moving dots

Effect of Lesioning and Microstimulation   

If a monkey has an intact brain, it can detect the direction dots are moving when coherence is from 1-2%; however, if the MT is lesioned, the coherence needs to be at least 10-20% Microstimulation of a column of MT neurons that indicate downward motion, monkeys saw dots moving to the right as moving down and to the right Medial superior temporal (MST) area is also involved in motion perception (but also activates a number of other areas across the brain)

Motion from a Single Neuron´s Point of View   



The aperture problem: a neuron only detects a small part of what is on a visual field; cannot tell if the stimulus is moving up and down as well as to the left/right This is solved by the visual system by pooling the responses of a number of neurons Pack and Born (2001): MT original response to the stimulus (70ms after onset) was determined by the orientation of the bar, and was the same whether it was a vertical bar moving horizontally or a vertical bar moving upwards and to the right o However, 140ms after onset, neurons began to respond to the actual direction – shows that MT neurons receive signals from neurons in the striate cortex and combine these to determine actual direction of motion Another way to tell whether an object is moving upwards would be to look at the tip as it would move up. Neurons that respond to the ends of moving objects have also been found in the striate cortex

Motion and the Human Body Apparent Motion of the Body 

Apparent motion is perceived as the shortest path between two stimuli (shortest path constraint)



If two pictures (one of a woman with her hand in front of her head, and one with her hand behind her head) are alternated rapidly, it is perceived that her hand goes through her head o However, if these images are alternated slower than five times per second, it is seen as her hand going around her head; this shows: 1. The visual system needs time to process information in order to perceive the movement of complex, meaningful stimuli 2. There may be something meaningful about the meaning of the stimulus that influences how motion is perceived o Also found that if other objects (e.g. boards) were used as stimuli, a lower rate of alternation does not mean that the longer path of movement is perceived o Stevens et. al (2000) found that both perceptions were associated with activation in the parietal cortex (associated with movement); however, the latter perception also activated the motor cortex

Motion of Point Light Walkers 

Point light walkers: small lights placed on the joints of a person and filming the patterns made by these lights when people are in motion and all the lights are turned out

Perceptual Organisation 

When the person is still, looks like a random pattern of lights, but quickly seen as a person when they start walking etc. – biological motion (we are good at detecting biological motion as we see it all the time)

Brain Mechanisms 

 

Proposed there may also be a brain area that responds to biological motion – a small area in the superior temporal sulcus (STS) was more active when viewing biological motion that with random patterns (N=8) Extrastriate Body Area (EBA) does not respond more to biological motion, but FFA does TMS (transcranial magnetic stimulation) of the STS decreases ability to perceive biological motion but TMS on the MT area had no effect

Representational Momentum: Motion Responses to Still Pictures  





Implied motion: where a still picture depicts an action involving motion Freyd (1983): showed participants a photo of a person in midair and jumping, and then a photo of (a) the same picture, (b) a “time-forward” photo (person was closer to the ground), (c) a “time-backward” photo – asked participants to judge whether the photo was the same or not o Found that it took longer to judge whether the time-forward was different because they anticipated this movement – motion depicted in a picture tends to continue in the mind of the observer (representational momentum) o This is an example of experience influencing perception fMRI response in MT and MST cortex to pictures of motion and areas of the brain that respond to actual motion also respond to pictures of motion, and implied-motion pictures cause a greater response than no-implied-motion pictures When viewing pictures with implied motion, there has also been found to have a motion aftereffect (more likely to see dots with 0% coherence moving in the opposite direction after,

as the prolonged viewing of implied motion decreases the sensitivity to neurons corresponding to this direction, so more neuron activity in the other direction)...