Chapter 8: Perceiving Motion

Overview of Questions

• Why do some animals freeze in place when they sense danger?

• How do films create movement from still pictures?

• When we scan a room, the image of the room moves across the retina, but we perceive the room and the objects as remaining stationary. Why does this occur?

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.

Functions of Movement Perception - continued

• Perceiving objects and events

– Movement of objects or the observer’s movement through objects assists in organization of stimuli

• Motion agnosia

– Damage to the cortex resulting in inability to perceive movement

– Extremely debilitating and dangerous for the patient

Four Ways to Perceive Motion

• 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

Four Ways to Perceive Motion - 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.

Comparison of Real and Apparent Motion

• Experiment by Larson 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

Experiment by Larson et al. - 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 The three conditions in Larson’s (2006) experiment: (a) control; (b) real motion; and (c) apparent motion (flashing dots). Stimuli are shown on top, and the resulting brain activation below. In (c) the brain is activated in the space that represents the area between the two dots, indicating that movement was perceived even though no stimuli were present.

Three Situations that Lead to Motion Perception

• 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.

Three Situations that Lead to Motion Perception - 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 observes Jeremy walking past; (b) Maria follows Jeremy’s movement with her eyes; (c) Maria walks through the room.

Table 8.1 Conditions for perceiving and not perceiving motion depicted in Figure 8.8

Two Explanations of Motion Perception

• 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.

Ecological Approach to Motion Perception

• 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.

Physiological Approach to Motion Perception

• 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 8.9 (a) The red area at the back of the eye represents the receptive field of a neuron in the cortex that responds to movement of vertical bars to the right. (b) when the image of the vertical bar sweeps across the receptive field, the neuron in the cortex fires.

Figure 8.10 (a) The pole’s overall motion is horizontally to the right (blue arrows). The circle represents the area in Maria’s field of view that corresponds to the receptive field of a cortical neuron. The pole’s motion across the receptive field (which is located on Maria’s retina) is also horizontal to the right (red arrows).

Figure 8.11 In this situation the pole’s overall motion is up and to the right (blue arrows). The pole’s motion across the receptive field is horizontal to the right (red arrows), as in Figure 8.11. Thus, the receptive field “sees” the same motion whether the overall motion is horizontal or up and to the right.

Figure 8.12 Moving a pencil between an aperture. See text for details.

Physiological Approach to Motion Perception - continued

• 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.

Figure 8.14 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 8.13 Human brain, showing the location of a number of the structures we will be discussing in this chapter. MT = medial temporal cortex (motion perception); MST = medial superior sulcus (motion perception); STS = superior temporal sulcus (biological motion); FFA = fusiform face area (face perception); EBA = extrastriate body area (perceiving bodies).

Determining Direction of Fields of Moving Dots

• 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.

Figure 8.15 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. From Newsome, W. T., & Pare, E. B. (1988). A selective impairment of motion perception following lesions of the middle temporal visual area (MT). Journal of Neuroscience, 8, 2201-2211.

Figure 8.16 The perceptual cycle from Chapter 1. Newsome measured relationship PH2 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 relationship PH1.

Determining Direction of Fields of Moving Dots - 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%.

Determining Direction of Fields of Moving Dots - continued

• 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 8.17 (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.

Effect of Eye Movements on Motion Perception

• Corollary discharge theory - movement perception depends on three signals

– Motor signal (MS) - signal sent to eyes to move eye muscles

– Corollary discharge signal (CDS) - split from the motor signal

– Image displacement signal (IDS) - movement of image stimulating receptors across the retina

Physiological Approach to Movement Perception - 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 8.18 According to the corollary discharge model (a) 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. (b) When the CDS reaches the comparator, it sends a signal to the brain that the eye is moving, and motion is perceived.

Figure 8.19 (a) When a stationary observer watches a moving object, movement of the image across the retina creates an image displacement signal (IDS). (b) When the IDS reaches the comparator, it sends a signal to the brain, and motion is perceived, (c) if both a CDS and IDS reach the comparator simultaneously, as would occur if a person is scanning a stationary scene, then no signal are sent to the brain,and no motion is perceived.

Figure 8.21 When the eye moves in the dark, the image remains stationary (the bleached area on the retina), but a corollary discharge signal is sent to the comparator, so the afterimage appears to move.

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 8.23 Responses of a real-movement neuron in a extrastriate cortex of a monkey. In both cases, a bar sweeps across the neuron’s receptive field. (a) The neuron fires when the bar moves to the left across the retina. (b) The neuron doesn’t fire when the eye moves to the right past the bar. Adapted from Galletti, C., & Fattori, P. (2003). Neuronal mechanisms for detection of motion in the field of view. Neuropsychologia, 41, 1717-1727.

Perception of Biological Motion

• 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 8.25 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.

Perception of Biological Motion - 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 8.26 (a) Biological motion stimulus; (b) scrambled stimulus; (c) stimulus from a, with “noise” added (dots corresponding to the walker are indicated by lines, which were not seen by the observer); (d) how the stimulus appears to the observer. From Grossman, E. D., Batelli, L., & Pascual-Leone, A. (2005). Repetitive TMS over posterior STS disrupts perception of biological motion. Vision Research, 45, 2847-2853.

Implied Motion

• Pictures that are stationary depict an action that involves motion.

• Representational momentum - observers show that the implied motion is carried out in the observer’s mind.

• Experiment by Reed and Vinson

– Observers saw a series of still pictures.

– The third picture was called the memory picture.

Experiment by Reed and Vinson

• The fourth picture was the test picture and was in the same position, slightly lower, or higher than the memory picture.

• Observers were to indicate whether the memory and the test pictures were in the same position.

• Results showed the tendency for representational momentum was affected by expectations about the object.

Figure 8.28 Stimuli used by Reed and Vinson (1996) to demonstrate the effect of experience on representational momentum. In this example, the test pictures are lower than the memory picture. On other trials the rocket would appear in the same position as or higher than the memory picture.

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 8.29 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 bar below each picture indicates the average fMRI response of the MT cortex to that type of picture.

Apparent Motion: The Occlusion Heuristic

• Experiment by Ramachandran and Anstis

– Two dots flashed on left followed by single dot on right.

– Observers report one dot moves horizontally and one diagonally to the dot on right.

– Change one stimulus to a square

– Perception changes so both dots move horizontally to the right with the bottom dot sliding behind the square.

Figure 8.30 Stimuli from the Ramachandran and Anstis (1986) experiment. (a) “The initial stimulus condition. Both dots move to the position of the dot on the right. (b) Placing a square in the position shown changes the perception of the movement of the lower dot, which now moves to the right and under the square.