Perception Chapter 9: Event Perception
Event Perception: an event is defined as a change in both time and space.
Thus far we have discussed how our visual system processes very simple static
qualities of the environment such as color, distance, pattern, etc. But, the visual
system was not designed just to encode and understand these static events,
instead, and even more impressively, our visual system was designed to encode and
understand the meaning of "events", changes that are dynamic, that visually unfold
over time and space.
Structure from motion: perceiving three-dimension shape from object movement
Biological motion
Gunnar Johansson (1975) point-light
studies
Biological Motion: it appears that the
human visual system is keenly
sensitive to certain properties of
movement which indicate biological
movement vs. inanimate movement.
What cues to biological motion are
present here?
self-propelled- regularity predictability - fluidity
Certain parts moving in certain ways
in relationship to other parts are
indicators of the movement of a
living organism
200ms exposure; 4month infant
preference; Brain area: Visual cortex,
MT and STS.
Visually guided locomotion
Optic flow: changing pattern of visual stimulation
associated with movement through a textured environment
Focus: least expansion, target of movement; expansion
increases further into periphery
Retinal image size/expansion
At a distance, focus
corresponds to target
of movement,
minimal retinal image
expansion
As target nears, rate
of retinal image size
expansion increases
rapidly. Visually
system uses this
expansion rate to
calculate time of
arrival or collision.
Eye movements
Types of eye movements:
1) saccadic: these are the jerky, jumping
movements made by the eye in order to bring a
target into the foveal region for intense detailed
encoding. It takes about 200 milliseconds to
initiate a saccade, fifth of a second; saccades are
ballistic -- cannot be readjusted during
movement; take about 100 milliseconds to
complete, 10thof a second during a second, the
visual world is blurry
If the visual world is blurry during a saccade, and
thousands of saccades are made in a typical
hour, why do we not constantly see a blurry
world?
Visual Masking: the ability to see one stimulus is
reduced by the presence of another stronger
stimulus presented temporally close to the
original.
Visual masking
• Backward masking: the masking stimulus
is presented after the original
• Forward masking: the masking stimulus is
presented before the original.
• In laboratory studies (which more often
use backward masking) the interval in
which masking takes place is around 100
milliseconds, just about exactly the time
of the typical saccade. Since the blurry
visual scene is sandwich between two
more stable images, it is not surprising
that it is masked and never noticed.
Experiments have shown that if you turn
on lights only during saccade, blurry world
will be seen.
Eye movements and perceived motion
• Corollary discharge hypothesis: Eye movements signals
automatically cause a second signal to be sent to visual system to
discount scene movement in opposite direction of eye movement.
Eye movements: 2nd type
2) Tracking movements: (or
pursuit or smooth
movements) these are
movements made to maintain
foveation on a target as it
moves. Tracking movements
require two types of
information for successful
execution – sensory
information about speed and
direction of movement.
Successful up to 1/3 of
millimeter per second across
retina. Faster require
predictive saccadic
movements.
Motion sensitive cells
Probably in PVC
Response strongest to
equally intense visual
stimulus that moves at
certain rate across visual
scene. Delay line in
earlier stimulated region
specifies speed of
movement to which cell
response most strongly.
In diagram on right,
delay line would be
leftmost connection
from white-coded cell to
“on” motion detecting
cell.
Apparent motion: perceiving movement from static images
Note that motion sensitive cells are based on
timing, no actual movement. Thus they can be
fooled.
Role of MT lobe in movement perception
Possess motion sensitive cells tuned to optic flow patterns. As
little as 3-5% correlated movement in random-dot stimuli
will result in motion perception. Damage to MT requires
10x greater correlation
Motion After Effect
Selective adaptation or fatigue of motion sensitive cells
can produce illusion of motion in opposite direction;
waterfall effect
Lesson: movement is always a aggregate response
across different motion sensitive cells or systems.