[music] Today we are going to discuss visual perception of motion. When we talk about motion perception, we should always specify: motion relative to what? We only perceive motion in relation to other objects. A car in the street is moving relative to the road, the threes, to people standing on the curb. Whe you walk, you perceive your own motion relative to other stationary and moving objects. In a situation where the object cannot be compared to other objects or to the background, we are faced with a phenomenon called the autokinetic effect. The earliest articles about the autokinetic effect were written as early as the 18th century. For example, in 1799 Alexander von Humboldt described the autokinetic effect in his article "Swinging stars". Another well-known publication on this phenomenon, written by Kaspar Gottfried Schweizer, had a name that was only slightly different: "On swinging stars". This work was published in 1859. Also, as early as 1912, Henry Foster Adams published a work dedicated to the autokinetic effect. So, what is the connection between swinging stars and the autokinetic effect? The connection is as follows: this effect of illusory motion was first discovered during star-gazing. If you look at the starry sky long enough, sometimes, the stars may appear to be moving ever so slightly against the black sky. Moreover, one person may say that the star is moving to the right, while another person says that that same star is moving to the left. The most intensive autokinetic effect can be achieved if you look at a shining dot against a dark background, for example, a flashlight in a dark room. Imagine that you are looking at a dot that is moving slowly in complete darkness. In such situations, it is difficult to detemine if the dot is moving or not. However, if we place another dot beside it, we can easily tell that one dot is moving relative to another. Surrounding objects play the most important role in such situations. Alternatively, we may be looking at a black dot against a white background, for example, a dark spot on the white wall. However, in such cases the effect is waker and does not always occur. You may also experince this effect when looking at objects that are very far from you, so that you can only see their vague contours, in thick fog, for example. In all these cases, the background should be monochrome, for example, all black or all white, or at least uniform, like sand. I would like to show you another interesting effect: the induced motion effect. Look at closely the white arrow: which direction is it moving in? In reality, the white arrow is stationary, it is the background that is moving. We are most likely to say that it is not the background that is moving along a complex trajectory to the left, but the arrow that is moving to the right. What happens if there are several objects moving at the same time? Now the background is moving to the right, and the circle is moving downwards. In this case, the dot that we are looking at will be moving to the left left relative to the rectangle, and the rectangle will be moving upwards. This effect was described by Wallach in 1959. There are enough examples of these effects in the real world: when the moon seems to be moving against the clouds or the impression that you feel when sitting in the train, and the train next to yours starts moving in the opposite direction and it seems that your train is also moving. I have just described two well-known phenomena that are both connected with motion perception. The first is the autokinetic effect - the apparent motion of a shining dot in a dark room. The second is the induced motion phenomenon, the illusory motion of a stationary object induced by viewing the motion of the objects that surround it.
