Optic Flow and Perceived Speed.
The world we live in is dynamic. So are we. Even if our environment were to remain still, our own movement would cause the pattern of light that reaches the retina to fluctuate. The characteristic fluctuation caused by self motion is called the ‘optic flow field’.
The optic flow field is an integral aspect of perception. It has been shown to contain information about such things as where we are heading and the amount of time that will pass before we actually collide with something.
Consider the pictures below. These are examples of motion blur that can be captured photographically by exposing film to light while the camera is in motion. On the left we have an example of the motion blur generated when the camera is focused ahead while moving forward. On the right, we have an example of the blur that is caused by focusing to one side while moving forward. Similar situations arise when we move our eyes through the environment. The visible environment flows across a two-dimensional surface - the retina. Such movement causes the same distinctive patterns of motion across the retina as those that cause motion blur in photography.
Clifford, C.W.G., Arnold, D.H., & Wenderoth, P. (2000). Dissociable factors affect speed perception and discrimination. Clinical and Experimental Opthamology, 28, 230 - 232. - pdf
The patterns of optic flow that are analogous to the motion blurs depicted above are called expansions and translations. These patterns of optic flow are also commonly referred to as complex motion patterns.
We now know that Macaque monkeys have a cortical region that processes optic flow. Do humans? Geesaman & Qian (1996) provided evidence that suggests we do. They contrasted the perceived speed of dots within different patterns of complex motion and found that dots within expanding patterns are perceived as moving faster than dots within rotating patterns. Because of the apparent difference in perceived speed, they argued that it was unlikely that the two patterns are processed by a common mechanism. However other research seemed to contradict this.
In speed discrimination tasks, you have to identify the fastest test stimulus. From performance within these tasks we can determine how different, in terms of speed, a stimulus must be from itself for the difference in speed to be reliably detected. This difference is known as the discrimination threshold (D.T.). It has been shown that speed D.T.s for expanding, rotating and translating patterns of motion are all similar (Sekuler, 1992; Bex et al, 1998; Clifford et al, 1999). This, it was been argued, indicates that complex motion patterns are not processed within a specialized system (Geesaman & Qian, 1996; Sekuler, 1992).
So an apparent contradiction appeared to exist between dissimilar perceived speeds and similar speed discrimination thresholds. Our own research resolved this dilemma. We found that while the pattern of motion impacts upon perceived speed, it does not impact our ability to discriminate between speeds. In contrast, while the number of independent samples of motion influences our ability to discriminate between speeds, it does not influence perceived speed.
This situation can be described as a double dissociation and is often interpreted as being indicative of multiple computational mechanisms. We have suggested that this indicates that the ability to discriminate speed is restricted by activity within a relatively early computational mechanism, like V1, while the actual perception of speed is generated by a subsequent mechanism, like MSTd in the Macaque.