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U. Glaeser

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FIGURE 28.16 An ideal 3-D, lowpass reconstruction filter, with cutoff frequencies determined by the spatiotemporal<br />

contrast sensitivity funtion.<br />

centered at DC, without including neighboring spectra, there is no reconstruction error. This case<br />

included no aliasing. If aliasing is included (the sample rate during acquisition is too low), the aliased<br />

components will be visible only if they fall within the passband of the CSF filter.<br />

The above frequency domain analysis explains some important aspects of human visual motion perception.<br />

Other observations are not as easily explained in this way, however. As observed in [5], perceived<br />

motion is local (different motions can be seen in different areas of the visual field) and spatial-frequency<br />

specific (individual motion sensors respond differently (selectively) to different spatial frequencies). These<br />

two observations suggest an underlying representation that is local in both the spatiotemporal and<br />

spatiotemporal-frequency domains. Examples of such representations will be discussed in the following<br />

subsection.<br />

The Effects of Eye Motion<br />

The analysis of motion perception described previously assumed a “passive” view. That is, any change in<br />

the pattern of light on the retinal surface is due to motion in the scene. That this is not the case can be<br />

seen by considering the manner in which static images are viewed. They are not viewed as a whole, but<br />

in a series of “jumps” from position to position. These “jumps” are referred to as saccades (French for<br />

“jolt” or “jerk”).<br />

Even at the positions where the eye is “at rest” it is not truly static. It undergoes very small motions<br />

(microsaccades) of 1–2 min of arc. In fact, the eye is essentially never at rest. It has been shown that if<br />

the eye is stabilized, vision fades away after about a second. The relevance of this to the current discussion<br />

is that although the eye is in constant motion, so that the intensity patterns on the retina are constantly<br />

changing, when viewing a static scene no motion is perceived. Similar behavior is observed when viewing<br />

dynamic scenes [6]. Obviously, however, in the case of dynamic scenes motion is often perceived (even<br />

though the changes in intensity patterns on the retina are not necessarily greater than for static images).<br />

Two hypotheses might explain these phenomena. The first is that the saccades are so fast that they are<br />

not sensed by the visual system; however, this does not account for the fact that motion is seen in dynamic<br />

scenes, but not static ones. The second is that the motion sensing system is “turned off” under some<br />

circumstances (the theory of corollary discharge). The basic idea is that the motor signals that control<br />

eye movement are also involved in the perception of motion, so that when intensity patterns on the<br />

retina change and there is a motor signal present, no motion is perceived. When intensity patterns change<br />

but there is no motor signal, or if there is no change in intensity patterns but there is a motor signal,<br />

motion is perceived. The latter situation corresponds to the tracking of moving objects (smooth pursuit).<br />

© 2002 by CRC Press LLC

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