Chapter 10: Motion
Introduction to Motion Perception
At first glance, motion perception seems effortless. We immediately know when a car is driving down the street, when a baseball is flying toward us, or when another person is walking across a room. However, the brain faces a surprisingly difficult problem when determining whether something is actually moving.
One reason motion perception is challenging is that the image on the retina changes for two different reasons:
- Objects in the world move.
- Our eyes move.
Every time we shift our gaze from one object to another, the entire visual scene sweeps across the retina. If the brain relied only on retinal image motion, the world would appear to move every time we made an eye movement. Fortunately, this does not happen. Instead, we perceive the world as stable (Gibson, 1954; Helmholtz, 1867/1962).
How does the brain distinguish between motion caused by moving objects and motion caused by our own eye movements?
One influential explanation is the Corollary Discharge Theory (von Holst & Mittelstaedt, 1950; Sperry, 1950).
Corollary Discharge Theory
Why Does the Brain Need a Corollary Discharge?
Whenever your brain sends a command to move your eyes, it also sends a copy of that command—called a corollary discharge (or efference copy)—to other parts of the brain (von Holst & Mittelstaedt, 1950; Sperry, 1950). This extra signal allows the brain to predict how the retinal image should change as a result of its own eye movement. The brain then compares this prediction with the motion actually detected on the retina. This comparison allows us to determine whether motion is caused by our own eye movements or by objects moving in the environment.
Components of Motion Perception
There are several signals involved in this process.
1. Eye Movement Command
When you decide to move your eyes, motor areas of the brain send commands to the eye muscles. These commands produce rapid eye movements called saccades along with slower tracking movements called smooth pursuit, depending on the situation (Leigh & Zee, 2015).
At the same time that the motor command is sent to the eye muscles, a copy of that command—the corollary discharge—is sent to other parts of the brain.
This signal tells the visual system: "The eyes are about to move."
Because the brain knows that it generated the eye movement, it can predict the retinal motion that should occur.
2. Image Movement Signal
Light striking the retina creates an image movement signal whenever the visual image moves across the retina.
This retinal motion can occur because an object moves or because your eyes move.
Importantly, the retina itself cannot determine why the image is moving. It simply registers that movement has occurred.
Figure 10.1
According to the Corollary Discharge Theory, motion perception depends on comparing two signals: (1) whether an eye movement occurred (indicated by the presence or absence of a corollary discharge) and (2) whether an image moved across the retina. The red X marks the two situations in which no motion is perceived because the signals are consistent with one another. In the upper-left cell, both eye movement (corollary discharge) and retinal image movement occur together, as expected during a voluntary eye movement, so the visual world is perceived as stable. In the lower-right cell, neither eye movement nor retinal image movement occurs, so the scene is also perceived as stationary. Motion is perceived in the two remaining cells (not marked with an X), where the corollary discharge and retinal image movement signals are inconsistent..
"Conditions in corollary discharge theory." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0
Determining Motion Perception
Whether we perceive motion or not hinges on the consistency or inconsistency between the corollary discharge signal and the image movement signal. When the two are consistent (eye movement and image movement OR no eye movement and no image movement) then we do not perceive motion (shown with an X in Figure 10.1). However, when the two are inconsistent (eye movement and no image movement OR no eye movement and image movement) then we do perceive motion. Let's explore various scenarios to illustrate this
Motion Perception and Eye Movements
The Corollary Discharge Theory predicts whether we will perceive motion by comparing two pieces of information:
Did we move our eyes? (Was a corollary discharge sent?)
Did the image move across the retina?
If these two signals are consistent (both present or both absent), the brain concludes that nothing in the world is moving, and we perceive a stable scene.
If the two signals are inconsistent (one is present and the other is absent), the brain concludes that something in the environment must be moving, and we perceive motion.
For the examples below, imagine that a king is watching a swallow carrying a coconut (as shown in Figure 10.2). Each situation is depicted in animated GIFs in this online Google document.
Figure 10.2
Illustration of a king (where the eyes have been enlarged to show motion on the retina) who is looking at a swallow that is carrying a coconut. This is the scenario that is used as an example throughout. See animated GIFs of all four scenarios in this online Google document.
"Swallow carrying a coconut." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0
1. Stationary Eyes and Image Movement
Suppose the king stares straight ahead without moving his eyes. Suddenly, a swallow carrying a coconut flies across his field of view.
Because the king does not move his eyes, no corollary discharge is generated. However, as the swallow flies by, its image moves across the retina, producing an image movement signal.
The brain detects that retinal motion is occurring without an accompanying eye movement. Because these two signals are inconsistent, the brain concludes that something in the environment is moving. As a result, the king correctly perceives the swallow flying across the sky.
Cell in Figure 10.1: upper-right cell
Corollary discharge: No
Image movement across the retina: Yes
Perception: Motion is perceived.
2. Stationary Eyes and Stationary Image
Now imagine the king is looking straight ahead at a stationary swallow that remains perfectly still.
The king's eyes do not move, so there is no corollary discharge. Because the swallow is also stationary, its image remains in the same location on the retina, producing no image movement.
Since neither signal is present, the two signals are consistent. The brain correctly concludes that nothing is moving.
Cell in Figure 10.1: lower-right cell
Corollary discharge: No
Image movement across the retina: No
Perception: No motion is perceived.
3. Eye Movement and Stationary Retinal Image (Smooth Pursuit)
Now suppose the swallow takes flight carrying its coconut. This time, instead of staring straight ahead, the king smoothly follows the swallow with his eyes.
As the king moves his eyes, a corollary discharge is generated because the brain has issued a motor command to the eye muscles.
At the same time, because the king continuously keeps the swallow centered on his fovea, the swallow's image remains nearly stationary on the retina. Consequently, there is little or no retinal image movement for the swallow itself.
Although the king's eyes are moving, there is an absence of retinal motion. This mismatch allows the king to correctly perceive that the swallow is moving through the environment.
Cell in Figure 10.1: lower-left cell
Corollary discharge: Yes
Image movement across the retina (for the tracked object): No
Perception: Motion is perceived.
4. Eye Movement and Image Movement
Finally, imagine that the swallow is sitting perfectly still with its coconut. The king decides to look from the swallow to a nearby castle.
As the king shifts his gaze, the brain sends commands to move the eyes, generating a corollary discharge. Because the eyes are moving, the entire visual scene—including the stationary swallow—sweeps across the retina, creating an image movement signal.
If the brain relied only on retinal motion, it would incorrectly conclude that the swallow was moving. Instead, the corollary discharge informs the brain that the retinal motion is a consequence of the eye movement. Since both signals are present and consistent with one another, the brain interprets the swallow as being stationary.
Even though the retinal image is moving, the king perceives the swallow as remaining stationary.
Cell in Figure 10.1: upper-left cell
Corollary discharge: Yes
Image movement across the retina: Yes
Perception: No motion is perceived.
Unusual Motion Perception Scenarios
The Corollary Discharge Theory not only explains our everyday experience of motion but also helps explain several fascinating visual illusions and laboratory demonstrations. In each example below, notice how the relationship between the corollary discharge and the retinal image movement determines what we perceive.
Afterimage in the Dark
Have you ever stared at a bright light, closed your eyes, and noticed a lingering afterimage? Now imagine moving your eyes while viewing that afterimage in a completely dark room. Although your eyes move, the afterimage is produced by temporarily fatigued photoreceptors in your retina rather than by light entering the eye. Because the afterimage is fixed on the retina, it moves wherever your eyes move. As your brain sends commands to move your eyes, it also generates a corollary discharge. However, the expected retinal motion that normally accompanies eye movements does not occur in the same way because the afterimage remains locked to the retina. As a result, the afterimage appears to move with your eyes through the dark visual field.
This illusion demonstrates that our perception of motion depends not only on retinal stimulation but also on information about our own eye movements.
Paralyzed Eye
Imagine that one eye is temporarily unable to move because its eye muscles have been paralyzed (for example, using a local anesthetic in a laboratory demonstration). When you attempt to move the eye, your brain still sends the normal motor command to the eye muscles. Consequently, a corollary discharge is generated. However, because the eye cannot actually move, the image on the retina remains stationary. Normally, eye movement and retinal image movement occur together. Here they do not. This mismatch causes the visual scene to appear to jump or jiggle each time you attempt to move your eye, even though the world itself remains stationary.
This demonstration provided early evidence that signals generated by the brain contribute to motion perception (Stevens et al., 1976).
Pushing on Eyes
You can create another illusion by gently pressing on the side of one closed eyelid while looking at a stationary object with the other eye open. (Never press hard on your eye. If you try this demonstration, use only very gentle pressure through the closed eyelid.) The gentle pressure physically rotates the eyeball without your brain issuing a motor command to move it. Because no eye movement command was generated, no corollary discharge is produced. However, the mechanical movement of the eye causes the retinal image to shift. The brain interprets this unexpected retinal motion as movement in the environment, making the stationary object appear to move.
This illusion demonstrates that retinal motion alone is not sufficient for the brain to determine the true source of movement.
Smooth Pursuit Motion Suppression
Imagine watching a bird fly across the sky while you smoothly follow it with your eyes. As your eyes track the bird, your brain generates a corollary discharge informing the visual system that the eyes are moving voluntarily. Without this signal, the background would appear to sweep dramatically across your visual field. Instead, the corollary discharge helps the brain discount much of the retinal motion caused by your own eye movements. As a result, the background appears relatively stable while you continue tracking the bird. Researchers have also found that during smooth pursuit, our sensitivity to motion in the visual periphery is somewhat reduced (Schütz et al., 2011). This phenomenon is known as smooth pursuit motion suppression. By reducing the impact of retinal motion caused by our own eye movements, the visual system helps maintain a stable and coherent perception of the world.
Neural Mechanisms of Motion Perception
Motion perception depends on a network of visual areas in the cerebral cortex (Born & Bradley, 2005). Rather than relying on a single "motion center," the brain analyzes motion in stages. Information is first processed in the primary visual cortex (V1) and then passed to higher visual areas, including V3 and V5 (also called the Middle Temporal area, or MT). Each region contributes differently to our ability to perceive moving objects.
Complex Cells in V1: Detecting Local Motion
The first stage of cortical motion processing occurs in V1, the primary visual cortex (Hubel & Wiesel, 1962).
As discussed in Chapters 4, V1 contains complex cells that respond to edges or bars having a particular orientation that move in a preferred direction. For example, one complex cell may respond best to a vertical edge moving to the left, while another responds best to a horizontal edge moving upward.
Because each neuron has a relatively small receptive field, it analyzes motion within only a small portion of the visual scene. As a result, V1 neurons primarily represent local or component motion—the movement of individual edges or contours—rather than the motion of an entire object.
V3: Integrating Motion Signals
Information from V1 is transmitted to several higher visual areas, including V3.
Although V3 is less well understood than V1 and MT, it appears to play an intermediate role in motion processing (Furlan & Smith, 2016). Neurons in V3 respond to moving contours and larger motion patterns than those represented in V1. This area contributes to integrating local motion information and helps relay increasingly complex motion signals to higher visual areas.
Rather than detecting simple moving edges, V3 begins combining motion information across larger portions of the visual field, helping the brain construct more complete representations of moving objects.
V5 (MT): Perceiving Global Motion
One of the most important brain regions involved in motion perception is V5, also known as the Middle Temporal area (MT) (Zeki, 1974).
Neurons in MT have much larger receptive fields than those in V1 and receive input from many V1 neurons. This allows MT neurons to combine multiple local motion signals into a single representation of global motion (Born & Bradley, 2005).
V1 neurons respond to the local aspects of motion while many MT neurons respond to the entire pattern of movement. This ability to integrate local motion into coherent object motion is one of MT's defining characteristics.
Figure 10.3
A side view of the human brain. MT/V5 appears toward the back of the brain.
"Cortex functional areas" by Drking1234 is licensed under CC BY-SA 4.0
Comparison Structure for Motion Perception: V1, V3, and V5
The Corollary Discharge Theory proposes that the brain determines whether motion is occurring by comparing two sources of information:
- A corollary discharge generated whenever the brain sends a command to move the eyes.
- A retinal image movement signal, which is produced whenever an image moves across the retina.
The original theory, proposed independently by von Holst and Mittelstaedt (1950) and Sperry (1950), suggested that these two signals are compared within a comparison structure. If the signals are consistent (both present or both absent), the world is perceived as stable. If they are inconsistent, motion is perceived.
Although the concept of a comparison structure remains useful for understanding motion perception, modern neuroscience suggests that this comparison is not performed by a single brain region. Instead, information about retinal image motion and eye movements is processed and integrated across a network of cortical and subcortical structures. Nevertheless, studies of neurons in V1, V3, and V5 (MT) provide important clues about when this integration might take place.
V1: Detecting Retinal Motion
Neurons in the primary visual cortex (V1) respond whenever light moves across their receptive fields (Hubel & Wiesel, 1962). Importantly, this happens regardless of what caused that motion.
For example, V1 neurons respond when a moving object passes through the visual field, and this also happens when an observer moves their eyes across a stationary objec.
In other words, V1 signals that motion has occurred on the retina, but it does not distinguish whether the motion originated in the external world or resulted from the observer's own eye movements.
V3 and V5 (MT): Integrating Motion Information
Higher visual areas, including V3 and especially V5 (also called the Middle Temporal area or MT) (Furlan & Smith, 2016), receive input from V1 along with information related to eye movements.
Experiments using single-cell recordings in monkeys have shown that neurons in these areas respond robustly when a moving bar passes across their receptive fields while the animal maintains fixation. However, many neurons in MT—and to a lesser extent V3—respond much less to retinal image motion that is produced solely by the animal's own eye movements.
This finding suggests that these higher visual areas are not responding simply to retinal image motion. Instead, their activity integrates eye movements.
A Distributed Comparison Process
Rather than a single comparison center, the brain appears to perform this computation through interactions among several regions (Crapse & Sommer, 2008). By the time motion information reaches MT, retinal motion has been integrated with information about eye movements, allowing MT neurons to represent the motion of objects in the external world more accurately than neurons in V1.
Double Dissociation Between Object and Motion Perception
One of the strongest pieces of evidence that object perception and motion perception rely on different neural systems comes from studies of people who have had brain damage that results in a double dissociation (Shallice, 1988).
A double dissociation occurs when damage to one brain region disrupts one function while leaving another relatively intact, and damage to a different brain region produces the opposite pattern. This provides strong evidence that the two functions depend on different neural mechanisms.
Blindsight
Individuals with damage to the primary visual cortex (V1) often report that they cannot consciously see objects presented in the affected part of their visual field. Surprisingly, however, they can often detect visual stimuli—and in many cases guess the direction of motion—at levels well above chance, even though they insist they are simply guessing (Weiskrantz et al., 1974).
This condition, known as blindsight, suggests that some visual information, including motion information, can reach higher visual areas such as MT (V5) through pathways that bypass V1 (Sincich et al., 2004). Although conscious visual awareness is absent, some motion perception remains intact.
Motion Agnosia (Akinetopsia)
Damage to V5 leads to the inability to consciously perceive motion while retaining the ability to perceive objects (Zihl et al., 1983). This highlights the importance of V5 in motion perception.
Together these conditions illustrate a double dissociation between object perception and motion perception.
Overlapping Moving Gratings
Imagine looking at two sets of parallel lines (called gratings) that are superimposed on one another, creating a pattern often called a plaid. One set of lines moves downward and to the left, while the other moves downward and to the right.
At first glance, it might seem that you would perceive two separate motions occurring at the same time. Instead, most people perceive the entire plaid as moving straight downward (Adelson & Movshon, 1982) . Although the individual gratings continue to move in different directions, the visual system combines these separate motion signals into a single, coherent pattern of motion. This process is known as motion integration.
Figure 10.4
Two gratings moving in different directions produce a coherent perception of motion when viewed together. The left panel shows one grating moving downward and to the right, and the middle panel shows a second grating moving downward and to the left. When the gratings are superimposed to form a plaid (right panel), observers perceive the pattern as moving straight downward. This illustrates that the visual system integrates separate component motion signals into a single global motion direction. .
"Global pattern straight down." by Kahan, T. A. is licensed under CC BY-NC-SA 4.0.
How the Brain Creates the Perception of Global Motion
Different areas of the visual cortex contribute to motion perception by analyzing motion at different levels.
V1 (Primary Visual Cortex): Detecting Component Motion
Neurons in V1 are sensitive to basic visual features, including edges, orientation, and motion direction (Movshon et al., 1985). When viewing overlapping gratings, many V1 neurons respond primarily to the motion of the individual gratings, often called the component motions. In this example, one population of neurons responds best to motion downward and to the left, while another responds best to motion downward and to the right. At this stage of processing, the brain represents the separate motion signals rather than the overall movement of the plaid.
V5/MT (Middle Temporal Visual Area): Computing Pattern Motion
Information from V1 is sent to area V5 (also called MT), a region of the visual cortex that plays a central role in motion perception. Many neurons in MT combine the separate motion signals arriving from V1 to compute the global pattern of motion (Movshon et al., 1985). For the overlapping gratings shown here, many MT neurons respond as though the entire plaid is moving straight downward, even though neither grating is physically moving in that direction.
This illustrates an important principle of visual perception: the brain does not simply record the motion of individual image features. Instead, it combines multiple local motion signals to infer the movement of the object or pattern as a whole.
Motion Can Alter Conscious Awareness
Up to this point, we have examined how the visual system detects and interprets motion. However, motion does more than simply tell us that objects are moving. Research has shown that motion can also influence what we consciously perceive.
In everyday life, we generally assume that if an object is directly in front of us and receives enough light to stimulate the retina, we will consciously see it. Surprisingly, this is not always the case. Under certain conditions, moving visual patterns can cause highly visible objects to disappear from conscious awareness or prevent us from noticing changes that would otherwise be obvious.
These demonstrations illustrate an important principle of perception: our conscious visual experience is an active construction of the brain rather than a direct copy of the information reaching the eyes. Motion can influence not only how we perceive movement but also whether we consciously perceive objects and changes occurring around us.
Two striking examples of this phenomenon are motion-induced blindness (Bonneh et al., 2001) and motion-induced silencing (Suchow & Alvarez, 2011).
Motion-Induced Blindness
Imagine staring at three bright yellow dots positioned around a small fixation point. Behind the dots is a field of blue crosses that slowly rotates around the center of the display. At first, all three yellow dots are clearly visible. However, after several seconds, one or more of the dots may suddenly disappear from conscious awareness. Moments later the missing dot reappears, while another may disappear. Remarkably, nothing has changed in the physical stimulus. The yellow dots remain continuously present on the screen, yet your conscious experience changes dramatically.
This phenomenon is known as motion-induced blindness (MIB) (Bonneh et al., 2001). Motion-induced blindness demonstrates that a moving background can temporarily suppress our conscious awareness of stationary objects, even when those objects produce strong retinal stimulation. The disappearing dots continue to stimulate the retina throughout the demonstration; it is our conscious perception that changes.
Researchers believe that motion-induced blindness results from interactions among motion processing, attention, and visual competition within the cortex. Although the precise neural mechanism remains an active area of research (Bonneh et al., 2001), motion-induced blindness provides compelling evidence that seeing an object depends on more than simply having its image fall on the retina. Instead, conscious perception reflects ongoing competition among neural representations within the visual system.
Try it yourself. Motion-induced blindness demonstrations are widely available online. Search for "motion-induced blindness demonstration" using Google and click “videos”. Most observers experience one or more of the stationary dots disappearing, despite knowing that they remain physically present.
Motion-Induced Silencing
Motion can also prevent us from noticing changes occurring in objects that remain visible. In motion-induced silencing, numerous dots move continuously around the display while some of the dots repeatedly change color, brightness, size, or shape. When the dots are stationary, these changes are immediately obvious. However, once all of the dots begin moving together, many observers fail to notice the changes in the dots altogether. Although the changes continue to occur, they seem to disappear from conscious awareness.
This illusion demonstrates that motion can suppress awareness not only of entire objects but also of changes occurring within those objects. The effect suggests that the visual system may prioritize processing the overall motion of the scene at the expense of detecting local feature changes. Rather than analyzing every individual object's changing characteristics, the brain appears to emphasize the coherent movement of the entire group. Motion-induced silencing highlights an important limitation of conscious perception: even highly noticeable changes can escape awareness when they occur within moving displays (Suchow & Alvarez, 2011).
Try it yourself. Search online for "motion-induced silencing demonstration" or “silencing illusion” using Google and click “videos”. Before the dots begin moving, pay attention to which dots are changing color or size. Once motion begins, notice how surprisingly difficult it becomes to detect changes that were previously obvious.
Conclusion:
Motion perception is far more complex than it appears. To determine whether something in the world is moving, the brain must distinguish motion caused by external objects from motion caused by our own eye movements. It accomplishes this by combining retinal image motion with information about eye movements and integrating these signals across a network of visual areas, including V1, V3, and MT (V5). Studies of blindsight and motion agnosia demonstrate that object perception and motion perception depend on different neural systems, while phenomena such as motion-induced blindness and motion-induced silencing show that motion can even influence what reaches conscious awareness. Together, these findings illustrate that our perception of motion is not a direct recording of the visual world but an active construction of the brain that allows us to perceive a stable, coherent, and dynamic environment.
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