Samenvatting boek SEP – deeltentamen 2
Chapter 8: Visual Motion Perception
Motion aftereffect (MAE) = the illusion of motion of a stationary object that occurs after
prolonged exposure to a moving object. After viewing motion in a constant direction for a
sustained period of time, we see any stationary objects that we view subsequently as
moving in the opposite direction.
The emerging evidence suggests that the MAE in humans is caused by the same brain
region shown to be responsible for global-motion detection in monkeys: the middle
temporal area of the cortex, an area commonly referred to as MT or V5.
When an object such as a ladybug moves, it is logical to suppose that the object is perceived
by the RFs of separate but adjacent neurons 1 and 2. Motion-detection neuron M responds
identically to a single moving ladybug and to two separate stationary ladybugs. To
distinguish motion, the circuit needs additional neurons.
Cell D receives input from neuron 1 and delays
transmission of this input for a short period of time.
Cell D also has a fast adaptation rate. That is, it fires
when cell 1 initially detects light, but it quickly stops
firing if the light remains shining on 1’s receptive
field. Neurons 2 and cell D are connected to cell X,
which is a multiplication cell. This multiplication cell
will fire only when both cells 2 and D are active. By
cell D delaying receptor 1’s response and then cell X
multiplying 1’s input by receptor 2’s input, we create
a mechanism that is sensitive for motion.
One possible objection to the Reichardt model is that it does not, in fact, require continuous
motion in order to fire. Although it raises a valid concern, this observation turns out to be a
virtue rather than a liability, because it provides an excellent explanation for a visual illusion,
called apparent motion.
Apparent motion = the illusory impression of smooth motion resulting from the rapid
alternation of objects that appear in different locations in rapid succession.
Other problem Reichardt model: Aperture problem = the fact that when a moving object is
viewed through an aperture (or a single RF), the direction of motion of a local feature or part
of the object may be ambiguous.
Every V1 cells sees the world through a small aperture. Therefore, none of the V1 cells can
tell with certainty which visual elements correspond to one another when an object moves,
even when no mask is present. The solution to this problem is to have another set of
neurons, each of which ‘listens’ to several V1 neurons and integrates their potentially
conflicting signals. Such an integrative neuron is known as a global-motion detector.
Information from magnocellular neurons feeds into V1 and is then passed on to the middle
temporal area of the cortex, an area commonly referred to as MT, and then to the medial
superior temporal area, MST. MT (hMT+ or V5 in humans) and MST are considered to be the
hub for motion processing.
, After researchers lesioned the monkey’s MT areas. The monkeys needed about ten times as
many correlated dots in order to correctly identify the direction of motion. However, the
monkey’s ability to discriminate the orientation of stationary patterns was generally
unimpaired. These results make a strong case that the MT is critically involved in the
processing of global motion.
Akinetopsia = disorder in which the affected individual cannot perceive motion.
First order motion = the motion of an object that is defined by changes in luminance
(reflected light).
Second order motion = the motion of an object that is defined by changes in contrast or
texture, but not by luminance.
First and second order motion can be damaged without harm to the other -> double
dissociation.
Biological motion = the pattern of movement of living beings that helps us identify both the
moving object and its actions. Biological motion activates a number of brain areas, including
the middle temporal (MT/V5) and other visual cortical areas.
Because high-acuity vision falls off rapidly with eccentricity (= the distance between the
retinal image and the fovea), we must constantly move our eyes to fixate the object of
interest with our fovea, and to follow that object as it moves from place to place. We make
very fast eye movements known as saccades to change fixation from one place to another.
Smooth pursuit = a type of voluntary eye movement in which the eyes move smoothly to
follow a moving object.
The brain sends two copies of each command to move the eyes. One copy (the motor signal)
goes directly to the extraocular muscles; the other (known as the efference copy or the
corollary discharge signal) goes to the comparator, which compares the image motion signal
with the eye motion signal and can compensate for image changes produced by the eye
movement.
Chapter 9: Hearing
Sounds are created when object vibrate. The vibrations of an object cause molecules in the
object’s surrounding medium to vibrate as well. This vibration causes pressure changes in
the medium. These pressure changes are best described as waves.
Sound waves are simply fluctuations in air pressure across time. The magnitude of
the pressure changes in a sound wave – the difference between the highest pressure and the
lowest pressure of the wave – is called the amplitude or intensity. Amplitude is perceived as
loudness: the more intense a sound wave is, the louder it will sound.
In reference to sound, the frequency, is the number of times per second that a pattern of
pressure changes repeats. Frequency is perceived as pitch: low frequency sounds correspond
to low pitches, and high frequency sounds correspond to high pitches. Hertz is a unit of
measure for frequency; 1 hertz equals 1 cycle per second.
