Chapter 8: Perceiving Motion

Figure 8.1 Motion perception occurs (a) when a stationary observer perceives moving stimuli, such as this couple crossing the street; and (b) when a moving observer, like this basketball player, perceives moving stimuli, such as the other players on the court. Figure 8-1 p176

Functions of Movement Perception Survival in the environment Predators use movement of prey as a primary means of location in hunting Attentional capture - motion attracts attention to the moving object Thus if prey remains motionless, it is less likely to be noticed. Akinetopsia – blindness to motion

Figure 8. 2 One image from the film used by Heider and Simmel (1944) Figure 8.2 One image from the film used by Heider and Simmel (1944). The objects moved in various ways, going in and out of the “house” and sometimes interacting with each other. The nature of the movements led subjects to make up stories that often described the objects as having feelings, motivations, and personalities. Figure 8-2 p176

Functions of Movement Perception - continued Perceiving objects and events Movement of objects or the observer’s movement through objects assists in organization of stimuli

Figure 8.3 The bird becomes camouflaged when the random lines are superimposed on it. When the bird is moved relative to the lines, it becomes visible, an example of how movement enhances the perception of form. Figure 8-3 p177

Figure 8.4 (a) The shape and features of this car are revealed as different aspects of it become visible as it moves. (b) Moving around this “horse” reveals its true shape. Figure 8-4 p178

Studying Motion Perception Real motion - an object is physically moving Illusory motion Apparent movement - stationary stimuli are presented in slightly different locations Basis of movement in movies and TV Induced motion - movement of one object results in the perception of movement in another object

Figure 8.5 Apparent motion (a) between two dots when they are flashed one after the other; (b) on a moving sign. Our perception of words moving across a lighted display is so compelling that it is often difficult to realize that signs like this one are simply dots flashing on and off. Figure 8-5 p179

Studying Motion Perception - continued Motion aftereffect 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 is an example of this.

Figure 8. 6 The waterfall movement aftereffect Figure 8.6 The waterfall movement aftereffect. (a) Observation of motion in one direction, such as occurs when viewing a waterfall, can cause (b) the perception of motion in the opposite direction, indicated by the arrows, when viewing stationary objects in the environment. Figure 8-6 p179

Comparison of Real and Apparent Motion Experiment by Larsen et al. 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

Comparison of Real and Apparent Motion - continued Results showed that Control condition - each dot activated a separate area of visual cortex Apparent and real motion - activation of visual cortex from both sets of stimuli was similar Thus the perception of motion in both cases is related to the same brain mechanism.

Figure 8.7 Three conditions in Larsen’s (2006) experiment: (a) control condition; (b) real motion; (c) apparent motion (flashing dots). Stimuli are shown on top, and the resulting brain activation is shown below. In (c), the brain is activated in the space that represents the area between the two dots, where movement was perceived but no stimuli were present. Figure 8-7 p180

What We Want to Explain An object moves, and the observer is stationary. Movement creates an image that moves on the observer’s retina. An object moves, and the observer follows the object with his or her eyes. Movement is tracked so that the image is stationary on the retina.

What We Want to Explain - continued An observer moves through a stationary environment. Image of environment moves across retina but environment is perceived as stationary. What mechanism explains all three situations?

Figure 8.8 Three motion situations: (a) Maria is stationary and looks straight ahead as Jeremy walks past; (b) Maria follows Jeremy’s movement with her eyes; (c) Maria scans the room by moving her eyes to the right. (The optic array and optic flow are described in the next section.) Figure 8-8 p181

Table 8-1 p181

Motion Perception: Information in the Environment Ecological approach (Gibson) Information is directly available in the environment for perception. Optic array - structure created by surfaces, textures, and contours, which change as the observer moves through the environment.

Motion Perception: Information in the Environment - continued Local disturbance in the optic array Objects relative to background such that it is covered and uncovered. Global optic flow Overall movement of optic array Indicates that observer is moving and not the environment.

Motion Perception: Information in the Environment - continued Reichardt detectors are neurons that fire to movement in one direction

Figure 8. 9 Reichardt circuit Figure 8.9 Reichardt circuit. Green indicates excitation; orange indicates inhibition. (a) and (b) When the receptors are stimulated from left to right, neuron I does not fire. (c) and (d) When the receptors are stimulated from right to left, neuron I fires. Figure 8-9 p182

Motion Perception: Retina/Eye Information Corollary discharge theory - movement perception depends on three signals Image displacement signal (IDS) - movement of image stimulating receptors across the retina Motor signal (MS) - signal sent to eyes to move eye muscles Corollary discharge signal (CDS) - split from the motor signal

Motion Perception: Retina/Eye Information - continued Movement is perceived when comparator receives input from: corollary discharge signal. image displacement signal. Movement is not perceived when comparator receives input from: both corollary discharge and image displacement signals at the same time.

Figure 8.10 (a) When the image of an object moves across the retina, movement of the image across the retina creates an image displacement signal (IDS). (b) When a motor signal (MS) to move the eyes is sent to the eye muscles, so the eye can follow a moving object, there is a corollary discharge signal (CDS), which splits off from the motor signal. Figure 8-10 p183

Figure 8.11 According to corollary discharge theory, (a) when the IDS reaches the comparator alone, a signal is sent to the brain and motion is perceived; (b) when the CDS reaches the comparator alone, a signal is sent to the brain and motion is perceived; (c) if both a CDS and an IDS reach the comparator simultaneously, they cancel each other, so no signals are sent to the brain and no motion is perceived. Figure 8-11 p184

Figure 8.12 Afterimage stimulus. Figure 8-12 p184

Figure 8. 13 Afterimage demonstration Figure 8.13 Afterimage demonstration. When the eye moves in the dark, the image remains stationary (the bleached area on the retina indicated by the red oval), but a CDS is sent to the comparator, so the afterimage appears to move. Figure 8-13 p184

Physiological Evidence for Corollary Discharge Theory Damage to the medial superior temporal area in humans leads to perception of movement of stationary environment with movement of eyes. Real-movement neurons found in monkeys that respond only when a stimulus moves and do not respond when eyes move.

Figure 8.15 Responses of a real-motion neuron in the extrastriate cortex of a monkey. In both cases, a bar (B) sweeps across the neuron’s receptive field (RF) as the monkey looks at a fixation point (FP). (a) The neuron fires when the bar moves to the left across the receptive field. (b) The neuron doesn’t fire when the eye moves to the right even though this also causes the bar to move across the receptive field. Figure 8-15 p185

Motion Perception in the Brain Solution to aperture problem Responses of a number of V1 neurons are pooled This may occur in the medial temporal (MT) cortex, which is located in the where/action stream. Evidence for this has been found in the MT cortex of monkeys. Neurons on the striate cortex respond to movement of ends of objects.

Motion Perception in the Brain - continued Firing and coherence experiment by Newsome et al. Coherence of movement of dot patterns was varied. Monkeys were taught to judge direction of dot movement and 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.

Motion Perception in the Brain - continued - continued Lesioning experiment by Newsome and Paré Normal monkeys can detect motion with coherence of 1 or 2%. Monkeys with lesions in MT cortex cannot detect motion until the coherence is 10 to 20%.

Figure 8.16 Moving dot displays used by Newsome, Britten, and Movshon (1989). These pictures represent moving dot displays that were created by a computer. Each dot survives for a brief interval (20–30 microseconds), after which it disappears and is replaced by another randomly placed dot. Coherence is the percentage of dots moving in the same direction at any point in time. (a) Coherence = 0 percent; (b) Coherence = 50 percent; (c) Coherence = 100 percent. Figure 8-16 p186

Figure 8. 17 The perceptual cycle from Chapter 1 Figure 8.17 The perceptual cycle from Chapter 1. Newsome measured the physiology–perception relationship by simultaneously recording from neurons and measuring the monkey’s behavioral response. Other research we have discussed, such as Hubel and Wiesel’s receptive field studies, have measured the stimulus–physiology relationship. Figure 8-17 p187

Effect of Lesioning and Microstimulation Microstimulation experiment by Movshon and Newsome Monkey trained to indicate direction of fields of moving dots. Neurons in MT cortex that respond to specific direction were activated. Experimenter used microstimulation to activate different direction sensitive neurons. Monkey shifted judgment to the artificially stimulated direction.

Figure 8.18 (a) A monkey judges the motion of dots moving horizontally to the right. (b) When a column of neurons that prefer downward motion is stimulated, the monkey judges the same motion as being downward and to the right. Figure 8-18 p188

Motion for a single Neuron’s Point of View Complex cortical cells respond preferentially to an oriented bar moving in a specific direction. Aperture problem - observation of small portion of larger stimulus leads to misleading information about direction of movement Activity of a single complex cell does not provide accurate information about direction of movement.

Figure 8.19 The pole’s overall motion is horizontally to the right (blue arrows). The ellipse represents the area in an observer’s field of view that corresponds to the receptive field of a cortical neuron on the observer’s retina. The pole’s motion across the receptive field is also horizontal to the right (red arrows). Figure 8-19 p188

Figure 8.20 In this situation, the pole’s overall motion is up and to the right (blue arrows). However, the pole’s motion across the receptive field is horizontal to the right (red arrows), as in Figure 8.19. Thus, the receptive field “sees” the same motion for motion that is horizontal and motion that is up and to the right. Figure 8-20 p189

Figure 8.21 Moving a pencil behind an aperture in the “Movement of a Bar Across an Aperture” demonstration. See text for details. Figure 8-21 p189

Figure 8. 22 The circle represents a neuron’s receptive field Figure 8.22 The circle represents a neuron’s receptive field. When the pencil is moved up and to the right, as shown, movement of the tip of the pencil provides information indicating that the pencil is moving up and to the right. Figure 8-22 p190

Motion and the Human Body Apparent Motion of the Body Biological motion - movement of person or other living organism Point-light walker stimulus - biological motion made by placing lights in specific places on a person. Structure-from-motion takes place with point-light walkers. Neurological studies show biological motion is processed by STS and FFA.

Figure 8.24 A point-light walker is created by placing lights on a person’s joints and having the person walk in the dark so only the lights can be seen. Figure 8-24 p191

Motion and the Human Body - continued Grossman et al. Participants viewed point-light stimuli for activities. Task was to determine whether motion was biological or scrambled. Noise was added to dots so they could only achieve 71% accuracy. Transcranial magnetic stimulation applied to STS caused a decrease in ability to detect biological motion.

Figure 8. 25 Frames from the stimuli used by Grossman and Blake (2001) Figure 8.25 Frames from the stimuli used by Grossman and Blake (2001). (a) Sequence from the point-light walker stimulus. (b) Sequence from the scrambled point-light stimulus. Figure 8-25 p192

Figure 8.26 TMS coil positioned to present a magnetic field to the back of the person’s head. Figure 8-26 p192

Figure 8. 27 (a) Biological motion stimulus. (b) Scrambled stimulus Figure 8.27 (a) Biological motion stimulus. (b) Scrambled stimulus. (c) Biological motion stimulus with noise added. The dots corresponding to the walker are indicated by lines (which were not seen by the observer). (d) How the stimulus appears to the observer. Figure 8-27 p193

Representational Momentum: Motion Responses to Still Pictures Implied motion are still pictures that depict an action that involves motion. Representational momentum - observers show that the implied motion is carried out in the observer’s mind.

Figure 8.28 A picture that creates implied motion. Figure 8-28 p193

Figure 8. 29 Stimuli like those used by Freyd (1983) Figure 8.29 Stimuli like those used by Freyd (1983). See text for details. Figure 8-29 p194

Experiment by Kourtzi and Kanwisher fMRI response was measured in MT and MST to pictures with Implied motion No-implied motion At rest Houses Results showed areas of brain responsible for motion fire in response to pictures of implied motion.

Figure 8.30 Examples of pictures used by Kourtzi and Kanwisher (2000) to depict implied motion (IM), no implied motion (no-IM), at rest (R), and a house (H). The height of the bars below each picture indicates the average fMRI response of the MT cortex to that type of picture. Figure 8-30 p194

Event Perception Event is defined as a segment of time at a particular location with a beginning and end Event boundary is the point where one event ends and another begins