Chapter 10: Motion

Introduction to Motion Perception

Motion perception may seem straightforward, but the process of perceiving motion is quite intricate and relies on several factors. In this chapter, we will explore motion perception, starting with a fundamental theory known as the Corollary Discharge Theory.

Corollary Discharge Theory

Understanding the Essence of Motion Perception

The Corollary Discharge Theory plays a pivotal role in elucidating when and how we perceive motion. This theory explains the conditions necessary to perceive motion, as it goes beyond the simple movement of visual information across the retina.

Components of Motion Perception

Motion perception involves various components, each contributing to our ability to discern movement:

1. Eye Movements: When we move our eyes, signals are sent from our brain to instruct our eye muscles. However, along with this signal a corollary discharge signal is sent to a comparison structure in the brain.
2. Corollary Discharge Signal: This is a signal that is sent to a region of our brain (which might be V3 or V5) whenever we move our eyes.
3. Image Movement Signal: This signal is generated when light moves across our retina. It can occur due to external objects moving in front of us, or our eyes moving which in turn causes light to move across the retina.

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. 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:

Figure 10.1

Motion Perception and Eye Movements

Motion perception is intricately linked with eye movements. Let's explore various scenarios and how they affect our perception:

1. Stationary Eyes and Image Movement When our eyes remain stationary, and an object or light source moves across our retina, we perceive motion. This scenario involves no eye movement but significant image movement. This would happen if a bird flew in front of us while we looked straight ahead.

2. Stationary Eyes and Stationary ImageIf our eyes and the object or light source both remain stationary, we do not perceive any motion. In this case, there is neither eye movement nor image movement. This would happen if we looked straight ahead at a stationary object.

3. Eye Movement and Stationary ImageIf we move our eyes to follow an object, then there is eye movement but no image movement across the retina. There is no image movement because we keep the object at our fovea. This would happen if a bird flew in front of us and we followed the bird with our eyes keeping it in the center of our gaze.

4. Eye Movement and Image MovementWhen our eyes move, causing image movement across the retina, we do NOT perceive motion. This scenario involves both eye movement and image movement. This would happen if we moved our eyes around to look at stationary objects.

Unusual Motion Perception Scenarios

Afterimage in the Dark

Staring at a bright light and then closing our eyes and moving our eyes around in a dark environment can create the illusion of motion in the afterimage. This happens because there is eye movement, but there is no image movement. Here it looks like we are moving our eyes to track the afterimage, keeping it in our fovea.

Paralyzed Eye Juggling Effect

Temporary paralysis of the eye muscles can lead to a peculiar illusion where an object appears to "jiggle" each time we try to move our paralyzed eyes. In this situation, there is a corollary discharge signal (because we try to move our eyes), but no image movement because we cannot move our eyes.

Pushing on Eyes

Gently pushing on one's own eye can create the perception of motion. This happens because the slight movement of the eye generates image movement, even though there is no eye movement (i.e., no corollary discharge).

Smooth Pursuit Motion Suppression

While tracking a moving object with our eyes, the perception of motion in our periphery may decrease. This phenomenon, known as smooth pursuit motion suppression, occurs because our eye movements create a corollary discharge signal that reduces the influence of the image movement (this leads to a reduction in the amount of perceived motion).

Neural Mechanisms of Motion Perception

The brain regions involved in motion perception include V1 (primary visual cortex), V3, and V5 (also known as the medial temporal region or MT/V5). These regions respond differently to visual motion:

Physiological Basis of Motion Perception

Complex Cells in V1

As discussed earlier, the primary visual cortex (V1) contains complex cells that respond to specific orientations of edges moving in particular directions. These cells play a crucial role in detecting motion-related information. However, V1 primarily responds to the component parts of a moving object rather than the overall coherent pattern of motion.

Role of V5 in Motion Perception

Functional brain imaging studies, including fMRI and PET scans, consistently show activation in V5 when individuals are engaged in motion perception tasks. This activation corresponds to the processing of visual motion stimuli.

Figure 10.2

Comparison Structure for Motion Perception: V1, V3, and V5

Response to Bar Movement

• V3 and V5 have cells that respond well when a bar moves across the receptive field. When a monkey is observed with its eyes facing forward, and a bar moves, cells in V3 and V5 respond, indicating their role in motion perception.

• V3 and V5 cells do NOT respond when the eyes of an animal move, which causes light to move across the receptive field.

• Cells in V1 respond whenever light moves across the retina (irrespective of whether the eyes move).

• Together this indicates that V3 or V5 may be the comparison structure because unlike V1 cells in V3 and V5 take into account eye movements.

Double Dissociation Between Object and Motion Perception

The concept of double dissociation in motion perception is exemplified aith various neurological conditions:

• Blindsight: Damage to V1 results in the inability to consciously perceive objects, yet the individual can still perceive motion. This showcases the role of V1 in object perception.

• Motion Agnosia (Akinetopsia): Damage to V5 leads to the inability to consciously perceive motion while retaining the ability to perceive objects. 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 two sets of parallel lines or gratings, like the stripes on a zebra, moving in different directions. Let's say the first set is moving downward to the left, while the second set is moving downward to the right. When these two sets of lines overlap, something interesting happens. From a physiological standpoint, both sets of lines are still moving in their respective directions, which we can refer to as their "component motions." However, our conscious perception of the overlapping gratings is straight downward motion, or "perceptual integration".

Figure 10.3

Role of V5 (MT) and V1 in Perception of Moving Gratings

1. V1 (Primary Visual Cortex):

• Cells in V1 are sensitive to basic features such as edges, orientations, and direction of motion. These cells are known as complex cells.

• In the context of our overlapping gratings, cells in V1 would respond to the component motions. Some cells would signal the downward left motion, while others would signal the downward right motion.

2. V5 (MT - Medial Temporal region):

• V5, on the other hand, is specialized for processing motion information at a higher level of abstraction.

• Cells in V5 are not concerned with the individual component motions; instead, they respond to the overall coherent pattern of motion.

• In the scenario of overlapping gratings, V5 cells would register straight downward motion.

Discrepancy Between Neural Processing and Perception

The crucial point here is that there is a mismatch between how neural cells in V1 and V5 respond to motion and our conscious perception. While cells in V1 dutifully report the individual components of motion, our perception integrates these components into an overall coherent pattern.

This phenomenon underscores the complexity of motion perception in the human brain. Our conscious experience of motion is not a direct reflection of neural responses but is instead the result of intricate processing that occurs in higher-order visual areas like V5.

Conclusion:

In conclusion, corollary discharge theory has provided valuable insights into how we recognize the difference between self-induced eye movements and external motion. Notably, distinct brain regions, such as V1 and V5 (or the medial temporal region), have emerged as pivotal players in motion perception. While V1 processes component pieces of motion, V5 responds to the overall coherent pattern, highlighting a discrepancy between neural responses and our conscious experience. The chapter has also showcased double dissociations, exemplified by blindsight and akinetopsia, which demonstrate that damage to specific brain areas can selectively impact object and motion perception.