SENSATION AND PERCEPTION LITERATURE PART 2
CHAPTER 8: MOTION PERCEPTION
Motion aftereffect (MAE): the illusion of motion of a stationary object that occurs after prolonged exposure to
a moving object. MAES are caused by opponent processes for motion detection. When we look at a waterfall
for a prolonged period, the detectors sensitive to downward motion become adapted. When we then switch
our gaze to a stationary object, the neurons sensitive to upward motion fire faster than the adapted downward
sensitive neurons, and we therefore perceive the object as drifting up.
Interocular transfer: the transfer of an effect from one eye to the other.
Emerging evidence suggests that the MAE in humans is caused bt he same brain region as in monkeys: the
middle temporal area (MT or V5).
Motion involves a change in position over time.
When an object such as a bug moves, it is logical to suppose that the object is perceived by the receptive fields
of separate but adjacent neurons. TO distinguish motion, the circuit needs additional neurons. The first (D)
delays neuron 1s input to M, while the second (X) fires only when both neurons are stimulated. This
combination of inputs allows M to detect motion, a single M cell fires continually as the bug moves across the
receptive fields of five distinct neurons.
One possible object to the Reichardt model is that it does not require continuous motion in order to fire.
Apparent motion: the illusory impression of smooth motion resulting from the rapid alternation of objects that
appear in different locations in rapid succession.
Aperture: a windowlike opening that allows only a partial view of an object.
Correspondence problem: in reference to motion detection, the problem faced by the motion detection system
of knowing which features in frame 2 corresponds to a particular feature in frame 1.
Aperture problem: the fact that when a moving object is viewed through an aperture, the direction of motion
of a local feature or part of the object may be ambiguous.
Every V1 cell has a limited receptive field/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.
Lesions to the magnocellular layers of the LGN impair the perception of large, rapidly moving objects. The
middle temporal area (MT) and the medial superior temporal area (MST) are considered to be the hub for
motion processing. This is also known as V5.
To detect the correlated direction, a neuron must integrate information from many local motion detectors.
Following lesions to MT, monkeys needed about ten times as many correlated dots in order to correctly identify
the orientation. The performance improved over weeks, suggesting they learned to use other brain areas to
discriminate motion.
Another experiemnd where they poked around in the mT: once they had found a group of neurons that
responded, the monkeys showed a strong tendency to report motion in the stimulated neurons’ preferred
direction.
First-order motion: the motion of an object that is defined by changes in luminance
Second-order motion: the motion in an object that is defined by changes in contrast or texture.
Texture defined/contrast-defined object: an object that is defined by differences in contrast or texture, but not
by luminance.
,Second order motion proves that matching discrete objects across movie frames is not necessary for motion
perception.
Double dissociation: the phenomenon in which one of two functions, such as first- and second-order motion,
can be damaged without harm to the other.
Optic array: the collection of light rays that interact with objects in the world that are in front of a viewer.
Optic flow: the changing angular positions of points in a perspective image that we experience as we move
through the world. Our visual system uses this to determine where we’re going.
Focus of expansion: the point in the centre of the horizon from which, when we’re in motion all points in the
perspective image seem to emanate.
Heuristics that the visual system might use to navigate
- The presence of optic flow indicates locomotion
- Outflow indicates that you are in approaching a particular destination
- Inflow indicates retreat.
- The focus of expansion tells you where you’re going to or coming from
Time to collision: the time required for moving object to hit a stationary object.
Tau: information in the optic flow that could signal time to collision without the necessity of estimating either
absolute distances or rates. The ratio of the retinal image size at any moment to the rate at which the image is
expanding is tau, and TTC is proportional to tau.
Motion can also provide information about the nature of objects.
Biological motion: the pattern of movement of living beings.
Biological motion activates a number of brain areas, including MT and other visual cortical areas. Moreover it
evokes signals in action observation networks in frontal regions of the brain considered to be premotor cortex,
leading to the suggestion that the observer’s motor system may fill in the point light displays.
Motion induced blindness seems to be somewhat related t the well-known Troxler effect in which an
unchanging target in peripheral vision will fade and disappear if you steadily fixate a central target. The result is
that the target is effectively not changing, and the underlying neurons become adapted.
Saccade: a type of eye movement, made both voluntary and involuntary. (Rapid eye movements).
Smooth pursuit: a type of voluntary eye movement in which the eyes move smoothly following a moving
object.
Six muscles are attached to each eye, arranged in three pairs. These muscles are controlled by an extensive
network of structures in the brain.
Microsaccades: an involuntary small jerky eye movement. This happens so that our view doesn’t fade (Troxler
effect).
Reflexive eye movement: a movement of the eye that is automatic and involuntary. These are known as
vestibulr eye movements and operate via the vestibulo-ocular reflex. Optokinetic nystagmus (OKN) is another
reflexive eye movement in which the eyes will involuntary track a continually moving object in pursuit of the
object moving in that same direction, and then snap back.
Vergence eye movements occur when we rotate our eyes inward or outward to focus on a near or far object.
Voluntary eye movements: saccades, smooth pursuit and vergence. We make eye movements that are based
on the content of a scene and on our specific interests in that scene.
Saccadic suppression: the reduction of visual sensitivity that occurs when we make saccadic eye movements.
Saccadic suppression eliminates the smear from retinal image motion during an eye movement.
, The motor system is thought to solve the problem of why an object is moving across the retina may appear
stationary. One copy goes to the eye muscles; another (efference copy/corollary discharge signal) goes to an
area of the visual system that has been dubbed the comparator. When we jiggle the eyeball with a finger, no
signal is sent from the eye muscles to the comparator (the eye muscles are not what move the eye in this case).
So the visual input is interpreted as our world being rocked.
Receptive fields are generally considered to be fixed in space relative to a fixation point So when the fixation
point changes, the receptive field changes along.
However, the receptive fields of some neurons in the parietal cortex actually shift to the new locations before
the saccade. (predictive remapping/updating). Updating has generally been thought to be accomplished in
higher visual areas such as parietal and temporal areas.
CHAPTER 9: HEARING
Sounds are created when objects vibrate.
Amplitude or intensity: the magnitude of displacement of a pressure wave. Is perceived as loudness.
Frequency: the number of times per second that a pattern of pressure change repeats. Perceived as pitch.
Hertz: a unit of measure for frequency.
Decibel: a unit of measure for the physical intensity of sound.
Single sine wave/pure tone: the single waveform for which variation as a function of time is a sine function. In
hearing research, this is sometimes referred to as a pure tone.
Spectrum: a representation of the relative energy present at each frequency.
Harmonic spectrum: the spectrum of a complex sound in which energy is at integer multiples of the
fundamental frequency.
Fundamental frequency: the lowest frequency component of a complex periodic sound.
Timbre: the psychological sensation by which a listener can judge that two sounds with the same loudness and
pitch are dissimilar. Timbe quality is conveyed by harmonics and other high frequencies.
Sounds are first collected from the environmeny by the pinna, the curly structure on the side of the head. The
pinna funnels sound waves into and through the ear canal. Together the pinna and the ear canal make up the
outer ear. The main purpose of the canal is to protect the structure at its end, the tympanic membrane
(eardrum), from damage.
The membrane is the border between the outer ear and the middle ear, which consists of three tiny bones, the
ossicles, that amplify sound waves.
1. The malleus is connected to the tympanic membrane and to the second ossicle
2. The incus is connected in turn to the third ossicle
3. The stapes, which transmits the vibrations of sound waves to the oval window, which forms the
border between the middle ear and the inner ear.
The ossicles amplify sound vibrations in two ways: (1) the joints between the bones are hinged in ways that
make them work like levers and (2) the concentrate energy from a larger to a smaller area.
The middle ear has two muscles: the tensor tympani (attached to the malleus_ and the stapedius (attached to
the stapes). Their main purpose is to tense when sounds are very loud. This is called the acoustic reflex.
Muscles of the middle ear are also tensed during body movement.
The inner ear is where changes in sound pressure are translated into neural signals. The cochlea is filled with
watery fluids in three parallel canals: the tympanic canal, the vestibular canal and the middle canal. The
tympanic and vestibular canals are both filled with perilymph and are connected by a small opening, the
helicotrema. The middle canals is filled with endolymph. Stria vascularis lines one side of the middle canal and
