Lecture 14 - Objects in Motion

Question 1

Four ways we perceive motion

Answer 1

1) real motion
2) illusory motion
3) induced motion
4) motion after effects

Question 2

illusory motion

Answer 2

where you have stationary stimuli that are displaced in time that the visual system interprets as motion

– no motion present, but it is perceived.
– Apparent movement - stationary stimuli are presented in slightly different locations
– Basis of movement in movies and TV

Question 3

induced motion

Answer 3

some other object is moving but you’re misplacing it (sitting at a stop in a car and a car next to you moves and you feel like your car is moving)

• movement of one object results in the perception of movement in another object (moving clouds may make the moon
  appear to move).

Question 4

real motion

Answer 4

an object is physically moving

Question 5

Motion after-effect

Answer 5

THIS IS NOT ILLUSORY MOTION

after an observer has perceived some sort of movement for some period of time (and that perceived motion could have been illusory) and looking at a stationary object the movement seems to go in the opposite direction

Observer looks at movement of object for 30 to 60 seconds.

– Then observer looks at a stationary object.
– Movement appears to occur in the opposite direction from the original movement.
– The waterfall illusion seen in lab 3 is an example of this.

Question 6

different at the brain level of real vs. apparent motion

Axel Larson experiment

Answer 6

– Participant is scanned by an fMRI while viewing three displays:

• Control condition – two dots in different positions are flashed simultaneously
• Real motion - a small dot is moved back and forth
• Apparent motion - dots are flashed so they appear to move

Results:

• Control condition: each fixed dot (FD) activated a separate area of visual cortex.

– Apparent and real motion: activation of visual cortex (V1) from both sets of stimuli was similar.
- the experience is just about the same: something is causing that activity

• Thus, the perception of motion in both cases is related to the same brain mechanism: can’t be cause by bottom up because there is no actual distal stimulus with apparent motion
• But what is that mechanism?

Question 7

Reichardt detectors

Answer 7

are neurons that fire to real movement in a
specific direction.

one neuron has a receptive field looking for motion in a particular direction

This is done through one-way lateral inhibition.

– receptor A causes inhibition in its neighbor as light move to the right [inhibits F as the light is arriving at B].

– receptor C will inhibit activity from D, but only after the light has passed. As the
light moves leftward, the neuron (I) continues firing - - - get’s to a new excitatory field before inhibition can take place

Question 8

How can a neuron have a

receptive field that detects motion?

Answer 8

Reichardt detectors

Question 9

Complex cortical cells…

Answer 9

……respond preferentially to an oriented bar moving in a specific direction.

– This can be accomplished with Reichardt detectors and/or convergence of simple cortical cells.

– However, each cell has a small receptive field. It can’t “see” what a large object is doing.

– So, the cell can only detect local activity as a stimulus (e.g. bar) moves across the receptive field on the retina.

Question 10

each receptive field by itself can’t see…

Answer 10

…. the larger object

can only detect the local activity (one little bar of light)

Question 11

Aperture problem

Answer 11

you have a large stimulus leading across a tiny receptive field

• observing a small portion of a larger stimulus may lead to misleading (ambiguous) info about the direction of movement

– Activity of a single complex cell does not provide accurate information about direction of movement.

Question 12

solution to aperture problem

Answer 12

lots of little apertures that converge their inputs in the dorsal pathway (where/how pathway) to higher regions: more hierarchical organization

outputs from V1 converge on area MT that goes to area MST

Question 13

dorsal pathway

Answer 13

where/how pathway spatial reasoning = movement

Question 14

solution to aperture problem again

Answer 14

Responses of a number of V1 neurons are pooled via hierarchical convergence in ‘higher’ areas.

• This may occur in the middle temporal (MT) cortex, which is located in the where/how stream and then later converges on MST
• Evidence for this has been found in the MT cortex of monkeys.

Question 15

Firing and coherence experiment by Newsome et al. (1989)

Answer 15

• Coherence of movement of dot patterns was varied.

– Monkeys were taught to judge direction of dot movement while measurements were taken from MT neurons.

– Results showed that as coherence of dot movement increased, so did the firing of the MT neurons and the judgment of movement accuracy.

• the responses in MT and the judgment (how accurate they were) were corrleated to the level of activation in MT;
  more coherent motion meant there was more feedforward activation from V1 (all the same direction, lots of little apertures all going in the same direction) converged in MT and the cells in MT therefore had more activation, and thus the monkeys were more accurate
• the activity in MT seems to be associated with what was happening in the distal stimuli and the perception of the distal stimuli

Question 16

Microstimulation experiment by Movshon and
Newsome (1992)

what was causally happening? was MT just responding to the input or did it have a more causal

follow up experiment

Answer 16

– Demonstrated how neural activation can play a causal role in altering perception.

– Monkey trained to indicate direction of fields of moving dots.

– Neurons in MT cortex that respond to specific direction were activated (electrically stimulated).

– Experimenter used microstimulation to activate different direction sensitive neurons.

– Monkey shifted judgment to the artificially
stimulated direction.