W5 Flashcards
(25 cards)
Where is motion processed in the human brain?
Motion is processed primarily in the visual cortex—specifically in areas V1, V2, V3, and especially V5/MT. Motion is not detected in the retina or LGN (lateral geniculate nucleus) in humans, unlike in some animals (e.g., rabbits).
What is the difference between actual motion and perceived motion in a static image?
A static image can appear to move due to illusory motion caused by eye movements (saccades). If you fixate on one point, the illusion disappears, but scanning with your eyes brings it back.
This shows motion perception can result from interaction between a stimulus and our own eye movements, not actual movement in the image.
What is the role of Area V5 (MT)?
Area V5 (also known as MT) is critical for motion detection. It contains motion-sensitive cells with large receptive fields, tuned to various directions and speeds of motion. Damage to V5 leads to motion blindness, where people cannot perceive motion at all.
What other brain areas are involved in processing motion?
V1 & V2: Respond to simple stimuli and some basic linear motion.
V3: Responds to complex motion like textures.
V5/MT: Specialized for motion detection.
MST (Medial Superior Temporal): Detects self-motion and integrates vestibular input.
STS (Superior Temporal Sulcus): Not strictly visual but involved in timing, music, and synchrony, including visual motion.
What is ‘blindsight’, and how is it related to motion detection?
Blindsight occurs when V1 is damaged (e.g., by a stroke), yet people can still detect motion without being consciously aware of it. This happens because of a bypass pathway from the LGN directly to Area V5/MT, allowing motion detection even without conscious vision.
How does the brain differentiate between motion caused by the outside world and self-motion?
The brain uses internal signals to compensate for self-movement. For example, during smooth pursuit eye movements, the brain knows the eyes are moving and adjusts perception so that the world doesn’t appear to move with them.
What are the two main theories explaining how the brain compensates for eye movement?
Inflow Theory (Sherrington): Based on muscle feedback from eye movement. (The brain uses signals from your eye muscles to know your eyes have moved.)
Outflow Theory (Helmholtz): Based on efference copy—a copy of the motor command sent to move the eye. (The brain uses a copy of the command it sent to move the eyes.)
Which theory is better supported by evidence: inflow or outflow?
Outflow theory is supported. Your brain doesn’t wait for feedback from your eye muscles—it uses a copy of the command to move to adjust your vision instantly.
Experiments (e.g., poking the eye while fixating) show that perceived motion occurs without actual intention to move, disproving inflow theory. The brain uses the intention to move (efference copy) rather than muscle feedback to compensate for self-motion.
What is an ‘efference copy’?
It’s a copy of the motor command sent to the muscles to move the eyes (or body). This copy is used by the brain to anticipate movement and adjust visual perception accordingly. It helps distinguish between self-generated and external motion.
How does a simple motion-sensitive neural circuit work?
Imagine two cells, A and B, responding to light. A moving stimulus activates A, then B.
A comparator neuron gets delayed input from A and immediate input from B.
If both inputs arrive simultaneously, it fires—signaling motion in that direction. This circuit is direction-selective.
Why does the simple motion detection circuit only detect motion in one direction?
Because the delay is on one side (e.g., from cell A), it only aligns with motion from A to B (rightward). To detect motion in the opposite direction (leftward), you’d need a mirrored circuit with the delay on B.
What is ‘opponent motion’ and how is it encoded in the brain?
Opponent motion involves neurons that are excited by motion in one direction and inhibited by motion in the opposite direction. These are found in Area V5, and help fine-tune our perception of direction by balancing conflicting inputs.
What experimental evidence supports outflow theory?
Poking the eye while fixating causes motion perception—no intention, no efference copy, so the brain doesn’t compensate.
Poking during an afterimage (which doesn’t move on the retina) still produces perceived motion.
When someone tries to move a paralyzed eye, they perceive movement despite no eye motion.
What is smooth pursuit, and how does it relate to motion detection?
Smooth pursuit is when your eyes track a moving object. The object’s image stays stable on your retina, but the brain knows your eyes are moving and still perceives the object as moving. This shows the importance of motor signals in motion perception.
What is apparent motion?
When an object appears at position A at time 1 and position B at time 2, the brain perceives motion—even if it didn’t physically move through the space in between.
If the time gap or spatial distance is too large, we perceive displacement, not motion.
What is apparent motion, and when does it occur?
Apparent motion is the illusion of movement perceived when an object is shown in two different positions across time, but not during its transition. It occurs when the time interval and spatial displacement between images are both small enough for the brain to infer smooth motion.
How does frame rate affect our perception of motion?
A frame rate of about 25 frames per second (40ms per image) is typically sufficient to create the illusion of smooth motion. Lower frame rates can lead to flickering or jerky transitions, breaking the illusion.
Why does the wagon wheel illusion occur?
The wagon wheel illusion arises when the speed of a rotating object aligns poorly with the “sampling” rate of our visual system (or a camera), causing misassignment of spoke positions across frames and making the wheel appear to slow down, stop, or reverse.
What is aliasing in the context of motion perception?
Aliasing is a mismatch between the sampling rate (e.g., video frames) and the motion being observed. If a rotor spins at a rate equal to the sampling rate, it may appear frozen or rotating backwards, due to incorrect matching of motion cues.
How does motion perception develop in infants?
Basic motion processing starts around week 6, but smooth pursuit and direction discrimination improve rapidly between 6 and 14 weeks. Interestingly, sensitivity to looming stimuli may appear even earlier and later temporarily disappears before re-emerging.
What is akinetopsia and what causes it?
Akinetopsia, or motion blindness, is the inability to perceive motion, often caused by damage to Area MT/V5 in the brain. It leads to a perception of the world as a series of static images.
How do people with akinetopsia perceive moving objects?
They see the world in snapshots and must manually compare object positions over time to infer motion. For example, to cross a street, they look at a car’s location, wait, and then look again to estimate whether it has moved.
What tool is used to study motion perception in patients with akinetopsia?
The random dot kinematogram is used, where some dots move in the same direction (coherent motion). Patients with akinetopsia require nearly 100% coherence to detect motion, while typical observers can detect it at around 10% coherence.
What does research using lesion studies and stroke patients tell us about Area MT?
Both human stroke studies and animal lesion experiments show that damage to Area MT severely impairs motion perception, confirming its critical role in detecting coherent motion.
