S&P exam 3 - ch 8
Lecture notes 3
Chapter 8
Overview of Motion Perception
Motion Perception: The brain’s ability to perceive movement based on visual stimuli.
Motion silencing
Akinetopsia = where motion is either very difficult or impossible to perceive
· People who have damage to parts of the brain responsible for perceiving and understanding movement. (from disease or trauma)
· aka “motion blindness”
o example: Without the ability to perceive motion following a stroke, L.M. was unable to successfully complete activities as simple as pouring a cup of tea. As she put it, “the fluid appeared to be frozen, like a glacier,” and without the ability to perceive the tea rising in the cup, she had trouble knowing when to stop pouring.
o It was difficult for her to follow dialogue because she couldn’t see the motions of a speaker’s face and mouth, and people suddenly appeared or disappeared because she couldn’t see them approaching or leaving. Crossing the street presented serious problems because at first a car might seem far away, but then suddenly, without warning, it would appear very near.
Motion
Þ Captures attention
Þ Used for object segregation
Þ Used to track self-movement
Þ Used to infer intention
Þ Overpowers other information
Þ Required to function in environment
Types of Movements
Apparent Movement: Perception of motion when there is no actual movement between a stimuli.
Type of illusory movement
Example: lights on a sign- when two stimuli in slightly different locations are alternated with the correct timing, an observer perceives one stimulus moving back and forth smoothly between the two locations.
Induced Motion: occurs when motion of one object (usually a large one) causes a nearby stationary object (usually smaller) to appear to move.
Example: the moon usually appears stationary in the sky. However, if clouds are moving past the moon on a windy night, the moon may appear to be racing through the clouds. In this case, movement of the larger object (clouds covering a large area) makes the smaller, but actually stationary, moon appear to be moving.
Motion Aftereffects: Following adaptation to one direction of motion, perceiving motion in the opposite direction after stopping.
occurs when viewing a moving stimulus causes a stationary stimulus to appear to move.
Type of illusory movement
Example: the waterfall illusion. If you look at a waterfall for 30 to 60 seconds and then look off to the side at part of the scene that is stationary, you will see everything you are looking at—rocks, trees, grass—appears to move upward for a few seconds.
Selective adaptation
Brain Responses to Movement
Study comparing real movement vs. illusory movement:
Real motion: Object moving back and forth on screen.
Actual displacement of an object in visual space, leading to a clear perception of motion.
We perceive motion when something moves across our field of view, which is an example of real motion.
- example: Perceiving a car driving by, people walking, or a bug scurrying across a tabletop.
Illusory motion: The perception of movement of stimuli that isn’t actually moving (e.g., flashing lights).
Control condition: Similar visual stimuli without perceived motion (e.g., simultaneous flashing).
in an fMRI scanner: (1) a control condition, in which two squares in slightly different positions were flashed simultaneously (figure a)
(2) a real motion display, in which a small square moved back and forth (figure b)
(3) an apparent motion display, in which squares were flashed one after another so that they appeared to move back and forth (figure c)
- The blue colored area in Figure a is the area of visual cortex activated by the control squares, which are perceived as two squares simultaneously flashing on and off with no motion between them. Each square activates a separate area of the cortex. In Figure b, the red indicates the area of cortex activated by real movement of the square. In Figure c, the yellow indicates the area of cortex activated by the apparent motion display.
- Notice that the activation associated with apparent motion is similar to the activation for the real motion display. Two flashed squares that result in apparent motion activate the area of the brain representing the space between the positions of the flashing squares even though no stimulus is presented there.
No statistical difference between real and apparent
Area MT (Middle Temporal Area)
The brain region associated with motion perception.
Control Condition: No significant brain response observed.
Real Motion Condition: Strong response detected in area MT.
Apparent Motion Condition: Reasonably strong response, no significant difference from real motion response.
Brain Imaging and Analysis
Imaging techniques used to visualize brain activity:
Enhanced images are smoothed to illustrate brain areas more clearly.
The takeaway: Apparent movement activates the same brain area as real movement.
Two ways to explain motion perception:
1. when the eye moves to follow a moving object
2. when the eye is stationary while an object moves across the visual field.
· According to Gibson’s theory, perception relies on information in the environment rather than just the retina.
o He introduced the concept of the optic array, which refers to the structure formed by surfaces, textures, and contours in the environment. When an object moves, it causes changes in this optic array.
§ example, if Jeremy walks from left to right and Maria follows him with her eyes, parts of the background are covered and then uncovered as he moves. This is called local disturbance in the optic array and helps Maria perceive Jeremy’s movement, even though his image remains stationary on her retina.
· Motion perception also depends on whether the whole scene moves.
o When Maria moves her eyes from left to right, everything in her field of view shifts in the opposite direction.
§ This phenomenon, known as global optic flow, occurs when the entire visual field moves due to the observer’s eye or body movement. Global optic flow signals that the environment is stable while the observer is moving.
· Gibson’s theory suggests that motion is perceived when part of the visual scene moves relative to the rest, but not when the entire scene moves together. However, additional factors must be considered to fully explain motion perception.
Mechanism of Motion Detection
Retinal Processing: Understanding how motion is detected via the retina when an image moves across.
Reichardt Detector: Two receptor fields connected to a single neuron in the brain, designed to signal motion.
explains motion perception that occurs when movement is viewed by a stationary eye.
The Reichardt detector circuit consists of two neurons, A and B, which send their signals to an output unit that compares the signals it receives from neurons A and B.
The key to the operation of this circuit is the delay unit that slows down the signals from A as they travel toward the output unit.
In addition, the output unit has an important property: It multiplies the responses from A and B to create the movement signal that results in the perception of motion.
Apparent Movement
Process:
Light moves from one receptor to another.
Delay may be applied to synchronize signals, allowing detection of movement.
Direction Detection
Reichardt detectors are directionally tuned:
Respond to specific motion directions and speeds.
If light flashes rather than moves smoothly, it can create the perception of movement (apparent movement).
Corollary Discharge Theory
· Understanding Movement Interpretation: A combination of retinal information and eye movement is essential for accurate motion perception.
Corollary Discharge theory: The brain’s mechanism to account for eye movement when interpreting external motion.
Explains why we don’t see the scene blur when we move our eyes from place to place when scanning a scene.
the neural signals that travel from the eye to the brain.
§ Interpretations are based on a combination of retinal information and eye movement information
CD theory’s three signals:
Image Displacement Signal (IDS): Information from the retina regarding where light is on the visual field.
occurs when an image moves across the retina
Motor Signal (MS): Signal sent to muscles guiding eye movement.
sent from the motor area to the eye muscles to cause the eye to move
Corollary Discharge Signal: A copy of the motor signal sent to assist in motion interpretation.
Comparator Function
Comparator: The brain's mechanism that integrates the IDS and Corollary Discharge to determine if an object is truly moving.
According to corollary discharge theory; movement will be perceived if a brain structure called the comparator receives just one signal;
Either the image displacement signal or the corollary discharge signal. It also states that no movement will be perceived if the comparator receives both signals at the same time.
Interpretation Process:
Retinal information (IDS) is noted.
Eye movement information (MS and Corollary Discharge) is compensated for.
The resultant information dictates the perception of motion.
Motion perception in the brain
· MT (Middle temporal)
· MST (Medial superior temporal)
· STS (Superior temporal sulcus)
...and many others
Coherence: Proportion of dots moving in same direction
When the dots are all moving in random directions, coherence is 0 percent.
· Increasing coherence:
o Leads to better performance
o Increases activation in individual MT cells
· Damage to MT cells:
o More coherence required to detect direction
· Transcranial magnetic stimulation (TMS): temporarily disrupts the normal functioning of neurons.
o Example: When researchers applied TMS to the MT cortex, participants had difficulty determining the direction in which a random pattern of dots was moving.
§ Although the effect was temporary, these participants experienced a form of akinetopsia much like patient L.M.
· Stimulating MT cells: Changed interpretation of direction
o Stimulating the MT neurons shifted the monkey’s perception of the direction of movement; instead of perceiving the correct rightward motion, the monkey responded as if the dots were moving downward and to the right.
Aperture problem
· Reveals only a small portion of the scene.
· Views of a small part of a moving object can produce misleading motion information
· But all of vision is made up of views of small parts of objects
Solution to the aperture problem
· a neuron could use information about the end of a moving object (such as the tip of the pencil) to determine its direction of motion.
o neurons that could signal this information, because they respond to the ends of moving objects, have been found in the striate cortex.
· To pool, or combine, responses from across many neurons.
o Apparently, MT neurons receive signals from a number of neurons in the striate cortex and then combine these signals to determine the actual direction of motion.
MT cells
· Initial response (70 ms) follows orientation
· Later response (140 ms) reflects true motion
Conclusion :
Understanding the mechanisms of apparent and real movement can illuminate how our brains interpret dynamic visual environments.
Biological motion
· Perceiving not just that something is moving in the world but that an animal is moving (a dog or person or living creature)
o Important for our survival
o Evolved to pay attention to
· Part of brain designated to detecting living animals
o Point light walkers: only have the motion itself
§ Dots of a person walking
§ When scrambled version, can compare both and see what is different in activity in the brain
§ Superior temporal sulcus (STS): Responds strongly when seeing points that look like a person or animal walking/ moving but doesn’t respond when shown the same dots but scrambled.
· was more active when viewing biological motion than viewing scrambled motion.
· Area necessary for perceiving biological motion
§ Can use TMS to impair one part of the brain
· When researchers affected the STS: Disrupted ability to detect biological motion
· When researchers affected the Area MT: no effect on detection of biological motion
§ Can also add distractions; still the dots of person walking but also randomized moving dots around
§ Optic flow: when things get closer they look like they are moving faster
· Medial superior temporal area (MST): involved in eye movements, so it’s important for localizing a moving object in space.
o Example: ability to reach for a moving object is affected when there is impairment to the MST cortex
Brain areas involved in the perception of motion
o Motion perception in still images
§ Implied motion: Brain response when looking at images
· It is not hard to imagine the person moving to a different location immediately after this picture was taken. A situation such as this, in which a still picture depicts an action involving motion
· Strongest response to something that looks like it’s moving / expect it to move
· Still get a little bit of a response when looking at people even if they aren’t in motion but could move
§ Representational momentum: Seeing something in motion and while watching it you’re anticipating where the movement is going to be
· When you see something moving, your mind is anticipating where it stopped which is farther than it actually stopped
· Falsely recognized an image of the object a little ahead of what they’d actually seen
· Activity in areas MT and MST
o is an example of experience influencing perception because it depends on our knowledge of the way situations involving motion typically unfold.