Chapter 6

A signal is sent to the brain and motion is perceived. These signals can cancel out, therefore no signals would be sent to the brain and motion would not be perceived.

According to the corollary discharge theory, what happens when the image displacement signal reaches the comparator alone? What happens when a corollary discharge signal and image displacement signal both reach the comparator simultaneously?

induced motion is based on the assumption that the foreground figure (train sitting still in station) is moving and background is stationary (train in next track is slowly moving) Autokinetic affect describes the movement of a stationary object that is visible when presented against a totally dark background.

Compare induced motion and the auto kinetic effect

simple and complex cells. orientation in space and time.

Current approaches to visual motion suppose that the receptive field of visual motion resemble which types of cells? What is an additional difference in visual motion detectors that would not be seen in either of the two cell types?

Imagine that a spot of light moves from A to B (see Figure 6.3, top left). What might be useful is to have something that can detect the spot when it is at A, and one that detects the spot at B (two receptive fields would do the trick).We can then say that we require that both the receptor with a receptive field at A and the one with a receptive field at B must fire within a certain time period— so if our spot moves from A to B within this time period we get firing and we have detected motion—hurrah! Unfortunately this detector would respond just as well to a spot of light moving to the left (B to A) as to the right (A to B). Clearly, we must make our detector asymmetrical, and the easiest way to do this is by introducing a time delay (Figure 6.3, top right). The logic here is that if the spot moves from A to B it will excite receptor A before receptor B. If we put a time delay on the output signal from A, then it will arrive at the comparison box, the detector, at the same time as the not- delayed signal from B, provided that the delay is as long as the time taken to travel from A to B. Movement from B to A will excite detector B first; some time later, when the spot has moved to A, detector A will fire. Its response will then be delayed and there is no chance of the activities from A and B reaching the detector at the same time.

Describe the Reichardt (delay and compare) motion detector

Participant undergoes fMRI scan while viewing motion displays. control: 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. Control condition results: each dot activated a separate area of the primary visual cortex Apparent and real motion: activation of primary visual cortex from sets of stimuli was similar, suggesting the use of the same brain mechanisms.

Describe the experiment where real and apparent motion were compared (Larsen et al.). How were patients analyzed, what were control, real motion and apparent motion conditions. What were the results of each condition?

motion after effects: we have a set of cells, or filters, each tuned to a different direction of motion, and the one that is the most active signals the direction of motion of our object. Indeed, by staring at the same motion for some time we can produce direction- specific threshold elevation (i.e. it becomes harder to see an object moving in the same direction to which we just adapted, whereas one in another direction is just as easy to see as before adaptation) (Levinson and Sekuler, 1980) and a direction of motion after-effect. after you have been looking at something moving in one direction for a while, stationary things will appear to move in the oppo- site direction

Describe the motion after-effects and motion opponency

The neuron fires when the bar moves to the left across the receptive field. the neuron doesn't fire when the eyes move to the right even though this causes the bar to move across the receptive field. Real movement neurons are found in monkeys that respond only when a stimulus moves and do not respond when the eyes move.

Describe the responses of a real-motion neuron in the extrastriate cortex of a monkey: A bar sweeps across the neurons receptive field as the monkey looks at a fixation point.

One experiment seems to suggest that this is so. Subjects adapted to a set of rings that continually expanded, or adapted to a set of rings that expanded and then contracted every second or so. Both these patterns of movement excited the human area MT as expected. However, when this motion ceased, the first continuously expanding pattern caused a contracting MAE in a set of stationary rings, whereas the expanding and contract motion did not. Sure enough, the activity in area MT continued for longer (see Figure 6.15) in the first condition than in the second condition (Tootell et al., 1995a). The time course of this activity appears to mimic the time course of the after-effect motion.

Describe the results of MT (V5) activity during and after motion expansion and pure contraction of rings.

When the image of an object moves across the retina, movement of the image across the retina forms an image displacement signal. A corollary discharge signal occurs which informs the brain visual motion areas of the eye movements. yes

How is an image created, when an object moves across the retina? What happens when a motor signal (moves the eyes) is sent to the eye muscle in order to track a moving object? Do both situations giver rise to the percept of a moving object?

Under normal conditions, a stationary object would excite both the 'up' and 'down' detectors just a little, and about equally. So our device that compares the 'up' and 'down' detector responses would find no difference, and we take this as stationary Now we present our waterfall (downward motion). This should make the 'down' detector fire a lot while the 'up' detector doesn't fire much at all. Our com- parator should say 'down' and so we see downward motion (Figure 6.7b). Now pro- longed stimulation of the 'down' detector means that it gets adapted and can't fire as much as it used to. Following this adaptation, the stationary pattern that would nor- mally excite both 'up' and 'down' detectors equally will now excite the 'up' detector more than the fatigued 'down' detector. Of course this state (the 'up' firing more than the 'down') normally only occurs when there is upward motion, and so that is what is perceived We detect motion by comparing motion detector activity for one direction to activity found for the opposite direction. Motion is sensed by an opponent-motion system. Motion after effects work by fatiguing opponent motion systems.

How is motion detected?

performance is very poor after lesions.

What are the effects of MT (V5) lesions on motion perception?

the viewer fixates on a stationary object and the moving object therefore moves across the retina. On the right side, the viewer 'tracks' the moving object hence the moving object remains stationary upon the retina and the stationary background moves in the opposite direction across the retina. Both situations give us the perception of a moving object on a stationary background. But what happens if we let our gaze follow the moving spot of light? This action is called tracking, which we achieve with smooth pursuit eye movements. Now, because the spot of light is kept on the fovea (our central vision) at all times, there is no movement of the spot across our retina—and yet we still see the spot as moving. Aha, you cry, but if the moving spot is tracked by the eye then there is still movement on the retina—of the actually stationary background moving in the opposite direction to that of the spot (see Figure 6.1). This is true (and very clever of you to spot it), but what if we can repeat the experiment, tracking a moving dot against a totally dark back- ground? The spot still appears to move. So we must have two routes to perceiving movement, one that detects movement across the retina (we shall term this the retinal movement system), and one that detects movements of the eyes in the head (we shall term this the eye-head movement system).

What are two ways of seeing movement?

viewing duration and location of motion in the visual field. he speed of a rotating disc (or indeed any other moving pattern) is altered by both how close it is to the fixation point, and for how long we look at it. If we look at the disc on the left, then the ones to the right (falling in our peripheral retina) appear to be rotating more slowly (the effect gets bigger as the disc is moved further into the periphery). As we carry on observing this, the discs in the periphery get slower and even stop As we look at a moving pattern, for a time it slows down quite dramatically, and in some cases can actually come to a complete stop (see Figure 6.9). Indeed, this illusion of 'slowness' is often experienced after driving quickly for a while. Notice, too, that the ganglion cell in Figure 6.8 also shows what happens to the cell when the motion stimulus comes to a halt: the firing rate drops, not back to its resting noise level of about 10 impulses per second, but right down to zero for a few seconds

What can perceived speed depend on?

Stimulus contrast. Perceived speed is reduced (car in fog)

What else does perceived speed depend on? What happens to perceived speed when the stimulus contrast is low?

This leads to a stationary environment perceived as moving when the eyes are moved voluntarily.

What happens the medial superior temporal area (MST) is damaged in the human brain?

It causes akinetopsia, an inability to perceive motion.

What happens when area MT (V5) is lesioned in a primate brain?

It responds to a bar moving across it from left to right, but it would work equally well if we flashed a stationary object at A and then another stationary object at B a little later—we don't actually need to have the object moving in between. Does this work in the real world? Fortunately it does, otherwise we would have no television or cinema, both of which create an illusion of motion by very rapidly presenting a series of stationary images—what we term apparent motion. Obviously, to generate apparent motion from a series of stationary images requires us to present the images at an appropriate speed—if the images are presented too slowly no motion is seen, whereas if they are presented too quickly they appear simultaneous.

What is apparent motion?

The firing rate of a directionally selective ganglion cell in the rabbit retina (note that rabbits have directionally selective cells in the retina, whereas we do not encode direction until the cortex). In the absence of any moving stimulus, the cell fires at some 'resting level'. When a motion stimulus is shown that matches the preferred direction of the cell, the firing rate soars to over 60 impulses per second. But note what happens as the stimulus motion continues: the firing rate of the cell drops steadily (adapts) until a plateau is reached and it settles down. Stimulated by motion in its preferred direction, the cell starts firing vigorously, but its response drops over time. When the motion stops, the cell's firing rate drops below its resting level. See text for details. Motion in the opposite direction has no effect on the cell's response

What is direction selectivity

Idea that we take a copy of the signal to move our eyes (sometimes known as an efferent copy or collorary discharge). Retinal movement system: a moving object viewed by stationary eyes provides movement in the retinal image - an image displacement signal Eye-head movement system: Tracking using smooth pursuit eye movements- the object perceived as moving provides a retinal image that does not move. Initiating a voluntary movement of the eyes generates a corollary discharge of the motor common that is fed back to the brain areas concerning visual motion. In effect, areas of the brain concerned with motion are told about voluntary eyes movements.

What is the corollary discharge theory? What kind of motion is detected by the retinal moving system? What kind of motion is detected by the eye-head movement system? How did Helmholtz propose that these two systems worked together?

The feeling of self-motion caused by the movement of things we see, rather than out actual motion. An optokinetic drum is a large cylinder that can be rotated around a person to create vection.

What is vection? What is an optokinetic drum and how can this be used to create vection?

Primate V1 and area MT (Area V5). In frogs, rabbits, and many other species, directionally selective neurons can be found at the retinal level.

Where can directionally selective neurons be found?