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The sheer variety of motion phenomenon that can be perceived argues for the necessity of having more than one detection system, perhaps located at different levels in the nervous system. It is not the case that an image moving across the retina is sufficient stimulus for the perception of movement. For example, the retinal image moves whenever a saccadic eye movement is executed, yet we are unaware of any motion. Conversely, we can track a moving object against a featureless without necessitating changes in the retinal image, yet a strong sense of movement results (p251).
As with other aspects of visual perception it is possible to locate specialised areas. Anderson (1989) points out that:
A substantial body of evidence suggests that visual motion analysis is treated by specific brain regions. There are several accounts of brain lesions in humans that produced deficits in motion perception without deficits in other forms of vision (p383).
Anderson locates the pinnacle of a prescribed hierarchy in motion processing in the posterior parietal cortex Brodman areas 5 and 7. Zeki (1990) confirms that, in macaque monkeys:
The funnel for the motion pathways of the visual cortex is area V5 . . . All its cells are responsive to motion in the field of view and the overwhelming majority are directionally selective . . . The fact that none is wavelength selective and the majority are not orientation selective either . . . led me to propose that it is a cortical visual area specialised for the detection of motion in the field of view (pp321-322).
From their examination of healthy and brain-damaged patients, Schenk and Zihl (1995) conclude that there are two distinct mechanisms involved in motion detection. One of these is concerned with the analysis of global motion. The other deals with extracting form from motion.
In their discussion of human motion perception and the physiological nature of motion detectors, Burr and Ross (1986) point out that accurate perception of form in motion involves visual integration of a type that eliminates smear. Image motion does not produce the same problems for the eye as it does for the camera. People can clearly see objects in motion, and they can see motion on cinema or TV when what they are shown is actually a sequence of stills. They look at mechanisms that allow for understanding when there is real motion, and interpolation when motion is sampled as in cinema. They suggest that such visual abilities may be explained by specialisations of visual neurons.
In examining moving images of a Mondrian-like nature, Zeki and Lamb (1994) conclude three things:Using PET scanning DuPont et al (1994) confirm that several brain areas are involved in the perception of movement. In addition to sites on either side of the brain at the border between Brodmann areas 19 and 37, a V1/V2 focus and a focus in the cuneus, they observed activity in other visual areas in the cerebellum and in two presumed vestibular areas at the posterior part of lateral sulcus and at the border of Brodmann Areas 2 and 40 (Figure 1).
Fig 1 Diagrammatic view of the brain areas identified by DuPont et al (1994) as being involved in the perception of movement
He demonstrated that motion could be perceived from two stationary images (sparks of electricity in his case) presented in quick succession a fact exploited by television and movies . This had been known for some time previously . . . However, Exner s great insight was that this perception of movement could be elicited from two sparks that were so close together in space that they could not be distinguished. Under these conditions it seems impossible that the observer could infer (consciously or unconsciously) motion from a knowledge of position and time. It therefore follows that motion perception must be a sensation in its own right, not one derived from a sense of position and time (p5).
Previous to Exner s studies, others had commented on aspects of motion such as optical illusions but little detailed study had been carried out. However, the stroboscope for examining objects in motion had been in existence since the 1830s (see Boring, 1942 pp588-596). About that time, too, Robert Addams (1834) reported on a perceptual anomaly about motion that he had encountered on a trip to Scotland:
During a recent tour through the Highlands of Scotland, I visited the celebrated Falls of Foyers on the border of Loch Ness, and there noticed the following phenomenon.
Having steadfastly looked for a few seconds at a particular part of the cascade, . . . , and then suddenly directed my eyes to the left, to observe the vertical face of the sombre age-worn rocks immediately contiguous to the waterfall, I saw the rocky surface as if in motion upwards, and with an apparent velocity equal to that of the descending water, which the moment before had prepared my eyes to behold this singular deception.
In parenthesis it is worth noting the style of language used in this description of what has now become known as the waterfall illusion . It was written for a scientific journal and yet the style does not differ markedly from the sort that was used a few short years later by, for example, John Ruskin (1819-1900), in his descriptions of natural phenomena in his books on art. The differences we now see between scientific reporting and writing on art seem to have come about in this century: the two cultures of CP Snow are clearly a modern concept.
Despite some earlier work and remarks on motion phenomena, it was not until the late nineteenth century that systematic studies of motion occurred. We should also remember that it was in the period 1870-1890 that the English photographer, Eadweard Muybridge (1830-1904) in the US and Etienne-Jules Maray (1830-1904) in France, carried out and published studies on the movement of humans and animals. As well as leading in the mid-1890s to the development of the motion picture, these studies must have also spurred an interest in the phenomenon of movement perception.
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