In this chapter we will analyze the movement perception relating the vision and the cognitive processes that make us able to see and follow objects that are moving as well as the visual tracking of objects that move.

perception of movement

What is the perception of movement

The perception of movement is a much more primitive mechanism of vision than depth perception or colors. The movement is key to locate a prey and establish the precise movements to hunt or, conversely, go unnoticed by the predator and flee at the precise moment. For most animals, the fact of remaining motionless gives them a character of invisibility.

Clinical studies with patients suffering from brain injuries reveal that the perception of movement is based on the Temporal Media (TM) zone. The injury to this region is accompanied by agnosia for movement.

Definition of movement perception

We can define the perception of movement as the cognitive capacity that an organism possesses that allows it to capture, immediately, the change of place of an object or a body and, at the same time, apprehend some attributes related to this change, such as speed and direction.

What the perception of movement implies

Motion detection involves:

  1. The fine tracking of an object that moves in front of us.
  2. The function of "common destiny", in which we perceive the different elements that move accompanying our main objective, which creates a series of very useful references for the detection of the global movement and the three-dimensional perception of that objective, since it confers more keys than simple static vision.
  3. The possibility of identifying a specific object by analyzing its movement (a large bird moves the wings more slowly than a small bird, which would mean less risk as a predator).

How the movement is perceived

In general terms there are two main mechanisms through which the movement is perceived:

  • The first implies the detection of changes in the relative position of the parties that make up the visual image.
  • The second implies the use of the eyes to follow a moving target.

The first mechanism, which responds to changes in the image, we call it image systemretina and the second, which responds to movement from postural changes in the eye and head, eye-head system (Gregory 1978)

We can affirm that the analysis of movement in the primary visual areas, early processing, constitutes a fundamental property of the visual system and does not depend on the previous calculation of distance, thus enabling the execution of certain tasks at the appropriate times to guarantee our survival.

In a schematic way we can mention among these tasks:

  • Coding of the third dimension,
  • Estimation of the collision, 
  • Differentiation between figure and background, 
  • Balance control, 
  • Control of eye movements   
  • Perception of continuous movement.

From the mathematical point of view, the problem of analyzing movement and the problem of analyzing orientation in space are practically the same. The difference is that in the orientation we move in a plane of spatial coordinates (x, y), while the movement analysis takes place in the volume, in the space time, (x, y, t).

Perception of apparent movement

The reality is that it is not necessary for movement to occur for us to have the visual sensation that something is moving, Max Wertheimer (1912) was one of the first to study apparent movement, starting the School of Gestalt.

He observed that when two lights are turned on, separated by a small space and with short start intervals, the observer has the sensation of movement, which is what we know as Strobe movement. There must be an interval between 10 and 14 msec so that movement is optimally appreciated. If the interval was smaller, it has the sensation of two simultaneous flashes and, if it was higher, the sensation was that of two successive flashes. 

Sequence of successive images that create apparent movement

Following this principle, the sensation of movement that we have when we go to the cinema or watch TV is explained, in which there is a change of frames of 24 times per second, with a pixelated refresh of 30 times per second, respectively, (in the computer is x 60).

Current experiments on apparent motion establish the following relationship as intersimulation times:

  • Feeling of simultaneity: less than 20 msec
  • Partial movement: between 20 and 40 msec
  • Optimum movement or beta: between 40 and 60 msec
  • Movement fi: between 60 and 200 msec
  • Successive movement: more than 200 msec

Characteristics of the apparent movement

In the apparent movement we know that the greater the distance between the stimuli, the longer the interval between the stimuli must be (Farrell 1983).

The most important thing in apparent movement is the asynchrony between the stimuli, the time that elapses between the start of one stimulus and the next (stimulus start asynchrony, SOA), as well as the interstimular interval (end of one and beginning of the other) . For the study of apparent movement, it is necessary to differentiate between short-range and long-range movement.

Short-range movement

The typical example of the short-range movement is the cinema, with a fixed interval of sequence of stimuli, 24 per second and with a minimum displacement of the objects in each frame between 5 and 15 arc minutes.

Long-range movement

The long-range movement is the one obtained with sequential ignition lights that tolerate distances greater than those of short-range movement. Also the time between the stimuli may be longer, more than 200 msec.

Perception of induced movement

We call movement induced to the illusion that appears when an object that is actually fixed is moved, by moving the frame of reference in which it is, as the example of the moon and the clouds, when these move quickly by the effect of the wind , if we are looking at the moon, crossing the clouds it seems that the moon also moves, in the opposite direction of the clouds.

Autokinetic movement

Another apparent movement is the autokinetic movement. We observe this when we have a fixed light source in a dark room.

In this circumstance, after a while of observing the light, we have the feeling that it is moving, a fact that is due to the imperceptible movement of the eyes.

Mechanism of movement perception

It seems to be accepted today that movement is a primary perception of our visual system. One of the evidences used for this statement is the illusion of the waterfall, in which after observing the water flowing in one direction, when we look at the shore, for a few moments we will see how it moves in the opposite direction to how it did. the water from the waterfall. This suggests that there are low-level neurons that have been fatigued, that are sensitive to movement, tuned to a certain direction and speed. This illusion is due to the phenomenon known as “selective adaptation".

Phases of movement perception

The movement signal begins to process in the retina in most lower animals, is from the cat and up to humans, in which the processing becomes more complex and happens to occur at the central level.

In humans, the first motion-sensitive neurons are in the superior colliculus and the primary striated cortex. Among the first studies on the way in which we detect movement, those of Reichardt stand out, who proposed a model with a minimum of three neuronal units, two simple cortical neurons, A and B, with an information delay system in a of them and, connected to a third neuron M (see figure), so that the campThe receptors of A and B are tuned to the edges of a specific orientation (vertical bars), thus, the signals they emit are compared with another signal emitted by the third neuron, M.

perception of movement

If a stimulus, such as a light bar that moves from right to left, as in the figure, reaches neuron A and then B, it happens that A's response passes into a delay system that slows down its activation , signal that is sent to the third neuron M, slowed down and thus will coincide with the arrival of the response of the second neuron B. The coincidence in M ​​of the responses of A and B, makes the discharge of M greater with respect to whether B are stimulated first and then A, where their matching responses to M would no longer reach M and the latter's response is lower. In the first case, with the high M response, it is perceived as movement and in the second case, with a lower M response, there is no longer perception of movement. Each system is tuned to a specific direction and speed of movement.

Via tectopulvinar

The study of the anatomical pathways of visual perception revealed the presence of other pathways related to movement, highlighting the tectopulvinar pathway.

The tectopaulvinar pathway is an older pathway from the evolutionary point of view with respect to the geniculate striate, although in this pathway, we also find links with the magnocellular system, more specific to the perception of movement.

Both pathways contain many more cells with temporary response than sustained, the latter more typical of the parvocellular, responsible for the details of the objects or the detection of slow movements. The temporary or transitory response is much more typical of motion detection.

Cells with a response to transient patterns are found more abundantly in the peripheral retina, while in the central retina we find cells with a sustained response, therefore slow movements are perceived with the taint, with speeds that do not exceed 1.5º per second, the presence of these cells decreasing as we move away from the macula while the cells with a transient pattern now increase, with a greater ability to detect rapid movements.

El magno system is color blind and sensitive to luminance, while the parvo It is very little sensitive to luminance and much to color, hence the magno system is more dedicated to the perception of movement.

Perception of movement and the brain

In the brain there are areas in which a large number of Reichardt detectors have been found, the primary visual cortex and the temporal lobe (temporal medial area and superotemporal medial area or V5). The cells of these zones are tuned according to the direction of the movement and in many cases, also to the speed of this. It seems that the main difference between the neurons of V1 and V5 is the size of their campReceptive, almost 5 times higher in V5.

Experiments with point coherence systems show that humans are very sensitive to the detection of movement, sometimes with only a coherence of 3% is enough to show it, especially when the speed is 2º per second and the campor visual stimulated is ampmess. In V1 we detect small movements, with respect to its amplitud, being in V5 where the global movement is detected.

Motion detection

A first point to analyze movement is to ask about how much displacement of the image is necessary to be able to perceive it, that is, we ask ourselves about the minimum threshold of movement.

Hermann Aubert (1886) was the first to observe that the movement of a point of light could be detected in the dark, located 50 cm from the eye and moving at 2.5 mm per second, which is equivalent to 10-20 arc minutes or 1 / 5 part of a degree of visual angle per second.

Measurement of visual perception of movement

The precise way to detect movement is by using methods that compare an element that moves with others that remain motionless, which is what we call "fixed visual context"Thus, the minimum perceived movement is of 0.25 mm per second or, 0.3º of visual angle per second, in comparison to the 0.02º that are needed with fixed context.

It seems that this ability to discriminate movement appears in the early stages of life, with only eight weeks.

Visual illusion of movement

The frame of reference is also important to detect the acceleration of an object, which can lead to errors of appreciation.

If we have a fixed point inside a moving rectangle, even if the velocity is constant, when the point approaches the edge of the rectangle, the velocity seems to increase, it accelerates, although it is only an illusion.

Another important relationship between the object and the frame are the sizes of these. An object in a large frame has to move faster than if it is in a small frame. The feeling of speed is very different. 
Investigations in the perception of movement, show a series of problems that generate ambiguity in perception, the most prominent are:

Movement correspondence

The movement correspondence arises when the "frame" of the visual scene is presented in time t1 and the one corresponding to a later moment, t2, where the objects of the scene have changed and we must assemble them (matching), so that we have a fluid vision of the sequence.

The problem arises as to which part of the first image corresponds to which part of the second image. Dawson (1991), proposes the principle of the closest neighbor, the principle of softness and the principle of integrity of the element, to achieve a uniform effect on the sequential passage of the frames.

Opening problem

The opening problem related to the perception of movement, we see how a Reichardt detector can be activated with different stimuli, both speed and direction.

A sensing unit is stimulated with a horizontal movement of a vertical bar, but it will also stimulate it with a slanted bar that moves horizontally or with a vertical bar that moves obliquely. The answers will be the same, which supposes a problem of ambiguity in the detection of that movement, it is as if we saw a figure moving through a very narrow window, we would see movement, something moves through it, but we could not say in which direction does it.

The experimental works indicate that the V1 cells would be sensitive to the opening movement, while the global movement occurs in the V5 cells. In V1 there is a vision of linear movement, you can see what the retina sees, while in V5, the movement is structured and in V3, you can see the dynamic form, the shape of the object in motion.

Movement speed problem

The way our perceptual system has to analyze the speed at which a stimulus moves is not so clear.

We know that the inclined stimuli are perceived slower than the vertical stimuli. At the present time there are several hypotheses but, it seems that the one of the weighted average of Castet, 1993, is the most accepted.

Another interesting aspect is that of speed record. If we see an object moving at a constant speed and moving away, the greater distance determines that the angular velocity at campor visual decrease, however, we do not have the sensation that its travel speed is reduced. There is a mismatch between the retinal speed and the central speed, which allows us to maintain the real vision of an object that follows a constant speed, regardless of the distance it is from us.

We leave the topic of "Collision time" as a specific section that we will deal with when we talk about vision and action, especially in the cases of vision and sport.

The eye movement

A common occurrence of daily life, is that we can follow with the eyes an object that moves in front of us, we follow it and we keep it fixed in the retina, it does not move on our retina and nevertheless we know that this object moves.

In the same way, when we look for something, for example in a room, we move our eyes over a scene that remains fixed, so that the objects move in our retina but we know, we perceive them, as motionless. In both cases the eyes move but the scene remains stable. To explain this, the corollary discharge theory has been proposed.

Theory of unloading carolaria

This theory proposes that the perception of movement depends on three types of signals, associated with the movement of the eyes or the images that cross the eyes:

  1. Motor signal (SM): That is sent to the eye muscles when the observer moves or tries to move his eyes.
  2. Una corollary discharge signal (SDC): What is a copy of the motor signal
  3. Una signal movement of the image (SMI): That occurs when an image stimulates the receptors while moving through the retina.

According to the theory of corollary discharge, our perception of movement would be determined, either by the motor signal of the ocular muscles and the corollary discharge or by the signal of the movement of the image on the retina, or even by both when forming jointly, which would arrive at a structure called “comparator”. In it, stimuli are received from the neurons that conduct these two signals.

The movement is perceived when the comparator receives the signal of the movement of the image or the signal of the corollary discharge separately.

However, when the two signals reach the comparator at the same time, cancel each other, there is no vision at that moment, so no movement is perceived. Let's see what this means.

vision of moving objects
Diagram of the corollary discharge model.

The motor area sends the motor signal to move the eyes to the eye muscles and sends the signal of the corollary discharge to a structure called a comparator. The movement of a stimulus through the retina generates a signal of movement of the image, which also reaches the comparator. The comparator sends its signal to the visual cortex so that the sensation of movement is generated or not.

Neuroanatomical studies have identified cells responsible for internal flow and external flow in the brain. These mechanisms serve to stabilize the campor visual like this, before the movement of the eyes or the head, we do not see the visual scene move in front of us, our external perception is not de-structured, as we have said before.

This can be verified by pressing the eye with a finger, the eye moves and the external scene moves in the opposite direction, because the eye movement does not come from an order from the brain, it is not voluntary, there is no signal from the muscles eyepieces and that is why the external image is destabilized, everything moves. This set of facts is what constitutes the basis of the Corollary Discharge Theory.

How we see the movement

The nature of this process is that a copy of the motor order is made.

The corollary discharge would inform the voluntary movement of the eyes, establishing a movement comparator system by taking into account this signal in relation to the afferent information that would arrive from the retina. If said afferences about the movement of the visual scene, fit with the prediction of movement deducible from the movement of the eyes, there will be no perception of movement. On the other hand, if a certain disparity is perceived between the sensory afference and the corollary discharge, in the case of following a moving object with the gaze, the retinal afferences of the object will not fit with the displacement that the whole visual scene must undergo according to the movement of our eyes, so the comparator informs the relevant sensory areas of the existing disparity and, despite the retinal clues, we will end up seeing a moving object on a static background.

Movement of the eyes and the brain

The medial premotor frontal cortex generates a replica of the efferent order, being able to address the putative comparator system or it could be directly integrated into certain cortical areas involved in the perception of movement. The corollary discharge is a copy of the motor command associated with voluntary movements that goes directly to the cortical perceptual centers. The anatomical neural substrate where this function is carried out would be the superior colliculus, since there is clinical evidence that indicates this. A discharge from its neurons is registered when the eye remains immobile before a certain stimulus that passes through its campor receiver but, they are inactive when, due to the movement of the eyes, it is theampor peripheral that moves in relation to a static stimulus. The inactivity of this last situation would be due to the fact that, although the stimulus moves in the retina, and in the campor receptor of said neurons, the motor copy of the order to move the muscles that cause the ocular displacement, would neutralize the retinal afferent.

Recent works also relate the pulvinar-thalamic nucleus to these tasks, it would receive information about eye movement from the superior colliculus and compare it with the visual image received from V1. The output resulting from this comparison sends it to the pre-striated cortical zones specialized in the perception of movement, V3 and V5, so these areas can distinguish between the displacements of the retinal image due to a moving object, from those produced by eye movements.

More recent studies in monkeys suggest that certain neurons in the MST area combine visual information with that of eye movements, constituting what has been called a “comparator neocortical system” (Barinaga 1996 and Andersen 1997). In humans there would be a similar mechanism but, lodged in the upper parieto-occipital area, SPO, which indicates that a corollary discharge type signal would be integrated at this level. At this level, other information would come from extra-striated, vestibular, kinesthetic areas, etc., which may be useful for the comparator system. A study by Brandt, 1998 indicates that there is a reciprocal inhibition between the SPO area and a region of the vestibular cortex, a parietal area that is considered a multisensory integration center for the perception of body orientation and automotive. We understand this when we go in a car and it accelerates or decelerates, there is an activation of the vestibular cortex that induces a deactivation of the visual cortex of the PSO area, in this way we perceive that we are the ones who move and not the landscape that we see out the window.

Along with these facts we could add the transitory blindness that occurs when we perform a rapid eye movement, a saccade. Under these circumstances the optokinetic system inhibits the retinal signal, so that there is no displacement of the external scene. The mechanism involved would be similar to that of the corollary discharge.

Movement of the optical matrix

In the perception of movement involved stimuli that activate the characteristic detectors or the corollary discharge, but also influences the way in which a thing moves in relation to others in the environment.

Gibson at 1979, was the first to defend this fact and coined the term "optical matrix"To refer to the structure created by surfaces, textures and contours. The most important element of the optical matrix with respect to the perception of movement, is the way in which it changes when the observer moves or when something moves in the environment. These changes are fundamental for know if the observer is moving or if the surrounding objects are moving.

There are several situations that offer clues, when moving an object isolated from the environment, alternately covering and uncovering the fixed background. This corresponds to the phenomena of elimination and accretion. We see it when we look at a person walking through a scene, it is what is called "local disturbance of the optical matrix", And it tells us that that person is moving. In parallel, we know that it is moving by the signals that send the eye muscles to follow its movement and by the fact of the phenomenon of occlusion and de-occlusion of the objects of the scene, more distant from the person who walks and who "covers" and "Uncover" as you walk. 

When the entire visual scene moves, the entire matrix is ​​called "global optical flow", And this indicates that it is the observer who is moving around the environment.

Perception of movement and our biological movement

We have already seen in other chapters that the objects in a scene do not appear independently, they are grouped into larger objects following other precepts, as proposed by the Gestalt laws, and one of the grouping criteria was movement. For example, the law of common destiny that stimuli that move in the same direction appear to belong to the same object.

Another example of how movement contributes to the perceptive organization, we see it in the detection of a human being or an animal. If we put lights on a person or an animal and only perceive those lights in a dark room, where we do not see the figure that moves, at the moment that the movement starts, in most cases we will guess that it is a person, of a human being or an animal.

The biological movement is a very powerful stimulus that becomes patent in a very obvious way. The detection of the human figure obeys to a mixed mechanism, of bottom-up and, very especially, from top to bottom, that is to say, the previous experience to see walk to human beings, is fundamental to appreciate this type of movement, to identify it.

Biological movement

From the biological movement, it has been seen that the perception of movement is linked to the whole of visual perception.

Most of the time we see objects in movement, generally we are in a changing scenario, either because we move ourselves or because there are elements of the scene, alien to us, that move. The combination of particular stimuli of objects, their shape, texture, etc., is combined with their movement characteristics, which makes them easier to identify and with that, to predict what will happen to them.

If we detect a table, it is likely to remain in place and we will avoid colliding with it, whereas if we identify a vehicle, a moving car and we want to cross a street, we will establish an action plan so that it does not run over us, calculating the time of collision, by virtue of size, approximation speed (expansion in retina, etc.) and other factors.

The influence of top-down mechanisms is evident in experiments such as the one conducted by Shiffrat Freyd (1993).

In them, they projected two photos of a person with the right arm raised and flexed at the height of the head. In the first photo, the hand is behind the head and in the second, in front. If the presentation time between the two photos was less than 200 msec, there is no conscious processing time and the two photos are perceived as the hand going through the head, but if they are presented with times greater than 200 msec, most of the observers perceived a movement of the hand around the head, passing from back to front. This indicates the strength of the cognitive effect, the hand cannot pass through the head, to pass from back to front, it must surround the head and, although it is not what was projected, the observers perceived this fact.

This shows that the perception of movement follows the same precepts that we saw in the perception of static forms, where Gestalt, cognitive mechanisms from top to bottom, played a fundamental role.

Summary
Perception of movement
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Perception of movement
Description
We explain the perception of movement and cognitive processes in detail. This is one of the chapters on vision, the eye and how we see.
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Área Oftalmológica Avanzada
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