The classical MAE seen in natural viewing conditions involves a static test pattern; after one observes movement for a while, such as a waterfall or the view from a moving vehicle, subsequently viewed stationary objects appear to move. We shall refer to this effect as the static MAE or SMAE. In the late twentieth century laboratory researchers began using dynamic test patterns such as dynamic visual noise or counter-phase flicker to study the after-effects of motion adaptation. A dynamic visual noise (DVN) pattern contains a dense field of randomly positioned dots which are replaced by a completely new set of random dots at pre-defined time intervals, typically up to 100 times every second. DVN has the appearance of a de-tuned television display. Counter-phase flicker is created by reversing the contrast of a luminance sine-wave grating repetitively - black bars become white and white bars become black - at a pre-defined frequency (exactly the same effect can be created by spatially superimposing two gratings drifting in opposite directions). The properties of MAEs obtained using these dynamic test patterns, which we shall call dynamic motion after-effects (DMAE), were markedly different from those obtained using stationary patterns, and led to the conclusion that the two after-effects were mediated by different populations of cells. The contrasting effects produced by first-order motion and second-order motion were particularly important. First-order motion involves patterns defined by variations in the luminance of single image points, such as drifting luminance gratings or dot patterns. Second-order patterns contain features defined by variations in the luminance of pairs of image points, such as variation in texture contrast, size, orientation, or binocular disparity. In moving second-order patterns the texture elements defining the pattern are usually replaced by new texture in each animation frame, so the pattern does not contain point-by-point correspondences over time. Adaptation to second-order motion does not produce a SMAE, but it does produce a DMAE [15
]. Furthermore, first-order and second order adapting patterns differ in terms of their inter-ocular transfer. It has long been known that when the adapting stimulus is presented to only one eye, and the test stimulus to the alternate eye, an after-effect is still reported. This inter-ocular transfer (IOT) relates to the binocularity of the underlying visual neurons. The SMAE shows only partial interocular transfer (IOT) [13
], indicating that at least some of the cells involved are monocular, but the DMAE shows complete IOT [15
], indicating that all the cells involved are binocular. These and other results led to the idea that the SMAE reflects adaptation in lower level, first-order motion sensors, while the DMAE reflects adaptation in higher level second-order sensors [15
Recent research reveals that the distinction between SMAEs and DMAEs is not as simple as was once believed. Early studies of DMAEs tended to use dynamic patterns that changed at a relatively slow rate or temporal frequency. For example, Nishida and colleagues [15
] used DVN at a frequency of 2 Hz (each pattern was replaced twice each second), and their DMAE from adaptation to second-order motion decreased progressively at higher temporal frequencies. More recent studies have measured DMAEs using first-order adapting stimuli and much higher frequency dynamic test patterns. Verstraten and colleagues [21
] used test patterns that changed at rates of between 10 and 90 Hz. Some of their experiments [22
] involved adaptation to two transparently moving sets of dots, one at high velocity and the other at low velocity. In these stimuli two sets of dots drifting in different directions are spatially superimposed, and can be seen passing through each other. The direction of the resulting after-effect depended on the temporal properties of the test; stationary tests appeared to move in the direction opposite to that of the slow adapting stimulus, and dynamic tests flickering at 90 Hz appeared to move in the direction opposite to the faster adapting stimulus. Other recent work using dynamic test patterns [6
] has concluded that two low-level populations of motion sensitive cell are involved in motion after-effects, one maximally sensitive to flicker at 2 Hz and the other maximally sensitive at 8 Hz or higher ().
Figure 1 Motion after-effect duration as a function of the temporal frequency of the test pattern (abscissa) and the speed of the adapting stimulus (different plot symbols). Results are shown for four subjects. For the slowest adapting speed (2.3 deg/sec, squares), (more ...)
At least three populations of cell are required to explain the diverse empirical properties of the after-effects reviewed so far. One low-level population mediates the classical SMAEs from first-order adaptation seen using static test patterns, and perhaps DMAEs seen in very low temporal frequency dynamic test patterns. A second low-level population mediates DMAEs from adaptation to rapid first-order motion seen using high temporal frequency test patterns. A third, ‘higher level’ population mediates DMAEs from second-order motion seen using low temporal frequency test patterns. A corollary of this conclusion is that DMAEs do not tap a single population of cells, but different populations depending on the properties of the adapting and test stimuli.
Recent psychophysical results have implicated two further sites of adaptation in motion after-effects. A number of studies have found evidence for adaptation at a relatively late stage in the motion pathway, where global movements such as rotation and expansion are computed. After-effects have been reported using adapting and test patterns of varying complexity [8
]. Bex et al. [8
], for example, found that adaptation to radial and rotational patterns produced stronger MAEs than adaptation to translating patterns. Several papers report so-called ‘phantom’ MAEs, which appear when the test stimulus is projected onto a region of the retina that was not exposed to the adapting stimulus, and that did not appear to contain motion during adaptation [eg. 49
]. Meng et al. [50
], for instance, found phantom MAEs only when the adapting pattern contained radial expansion rather than translation. The presumed cortical location of phantom after-effects is MT or MST, where receptive fields are very large and sensitive to large-scale rotary or radial motion.
Culham et al. [11
] have argued that apparent motion mediated by attentional tracking can also generate an after-effect. During adaptation subjects viewed an ambiguous counter-phase grating, and were instructed to “…use attention to mentally track the bars of a radial grating in one of the two ambiguous directions…”. Tests on a static pattern showed no after-effect, but tests on a 2 Hz counter-phase grating did reveal an after-effect. Culham et al. [11
] argued that their DMAE from attentive tracking arose in relatively late cortical areas, perhaps MST. Their adapting stimulus offered equal and opposite signals for motion sensors, so it is possible that attention served to modulate these signals rather than generate its own motion signal. The fact that their effects were confined to DMAEs may indicate the site at which the attentional modulation occurred.
So far the psychophysics indicates that up to five populations of cells all potentially contribute to motion after-effects. Are these populations functionally distinct? Do they occupy different cortical locations? Perhaps recent electrophysiological and brain imaging can clarify these fundamental questions.