Pattern adaptation and visual channels

What can an orientation-selective aftereffect tell us about the visual processes underlying form perception? Figure 60.2A shows the kinds of measurements one might record in a study of orientation adaptation. In this plot the angle corresponds to the pattern orientation, while the distance from the origin corresponds to the pattern contrast. Note that we could represent any stimulus within the plane by taking only two “measurements” (e.g., of the component contrasts along the horizontal and vertical axes) and that these could sample contrasts along any pair of axes within the plane. But how many measurements are actually used, and along which axes do they lie?

Figure 60.2..  

Multichannel accounts of the tilt aftereffect. a, Measurements of detection thresholds (elliptical contour) or perceived tilt of a vertical test after adapting to a clockwise bar. b, Both effects can be accounted for by adaptation in orientation-selective channels that reduces sensitivity in channels tuned to the adapter and thus skews the distribution of responses to the test away from the distribution of responses to the adapter.

Adaptation experiments address this question by exploring how responses to stimuli are altered after observers are exposed to and thus adapted by different stimuli. To induce a new state of adaptation, subjects typically view the adapting stimulus for a few minutes and then make judgments about a set of briefly presented test stimuli. Two common types of judgments are used. In one case, sensitivity is probed by finding the threshold for detecting or discriminating the test stimulus. In the second, the subjective appearance of the test is assessed. One way to do this would be to match the apparent orientation of the test by physically adjusting the orientation of a nearby comparison stimulus presented to a part of the retina maintained under neutral adaptation. This asymmetric matching task assumes that the effects of adaptation are confined to the regions of the retina (or their associated pathways) that were exposed to the stimuli. A second approach is to vary the test stimulus physically in order to cancel out a perceptual change. For example, the orientation of a test could be adjusted so that it always appears vertical. This nulling method assumes that any response changes induced by adaptation are equivalent to the responses induced by a physical stimulus. Still other common measures include rating the perceived magnitude of an aftereffect or its perceived duration.

Figure 60.2A plots an idealized set of results after adapting to a bar tilted at a clockwise angle. Measures of sensitivity to different orientations would show that adaptation increases the threshold for detecting patterns that have orientations similar to the adapting pattern (Gilinsky, 1968). Measures of appearance would show that after adaptation a vertical line appears tilted counterclockwise. Both aftereffects are consistent with a selective loss in sensitivity to the adapting orientation, and thus imply that adaptation is altering the responses in something that can be selectively tuned for orientation. Results of this kind are usually explained in terms of visual channels—the notion that the visual system encodes information within a bank of filters that respond to different but overlapping ranges along the stimulus continuum (e.g., to different orientations, hues, or directions of motion). Any stimulus is thus represented by the distribution of activity across the set of channels. A further common assumption is that these channels are labeled for particular sensations, so that which stimulus is perceived (e.g., vertical or red) depends on which channels respond, while the magnitude of the stimulus (e.g., contrast or saturation) is encoded by the size of the response (Braddick et al., 1978).

Figure 60.2B shows one possible account of the tilt aftereffect based on changes in the distribution of activity across multiple channels. Suppose that adaptation reduces a channel's sensitivity according to how strongly the channel responded to the adapting stimulus. This would reduce the channel's responses to a subsequent test stimulus. The test orientations to which it is tuned would become harder to detect, and patterns that are above threshold would appear to have lower contrast. Moreover, the diminished signals would reduce its contribution to the collection of channel responses, and thus for nearby test orientations would skew the mean of the distribution away from the mean for the adapting orientation, inducing the perceived aftereffect. (However, this leaves the problem of how distortions in the pattern's features can be reconciled with their perceived retinal location; Meese and Georgeson, 1996.)

Often the studies using adaptation have not been interested in the processes of adaptation itself, but rather in the properties of the channels implied by the adaptation. One question of interest is the bandwidths or profiles of the channels. For example, an adaptation effect that influenced only a narrow range of orientations would imply that the channels are highly selective for orientation. A second commonly asked question concerns the number of channels. If the response changes are selective for the adapting axis, that implies a channel tuned to that axis. If selective aftereffects can be found for many axes, then that might imply many channels. We could thus repeat the measurements of Figure 60.2A for many adapting and test orientations in order to characterize how orientation is represented at the level at which the adaptation alters sensitivity. The results of such studies have shown that sensitivity changes appear selective for any orientation, suggesting that orientation is encoded effectively by a continuum of channels, with bandwidths (orientation range at which sensitivity falls to half the peak) on the order of roughly ±10 degrees (Blakemore and Nachmias, 1971).

However, the interpretation of these results is complicated, precisely because any inferences about the underlying channels depends on assumptions about the nature of the adaptation. For instance, the model in Figure 60.2B assumes that each channel adapts independently. Yet suppose that adaptation instead reflects an interaction between channels (Barlow, 1990; Wilson, 1975). For example, Barlow suggested that adaptation involves reciprocal inhibition between two channels that builds up whenever their outputs are correlated. The effect of this mutual repulsion is to bias the channels’ responses until they are statistically independent. An account of the tilt aftereffect based on this principle is shown in Figure 60.3. (For a comprehensive model, see Clifford et al., 2000.) In this example, orientation is encoded by pair of channels that, under neutral adaptation, are tuned to horizontal and vertical. Exposure to the clockwise adapter would produce covarying responses in both channels, leading to inhibition between them. This alters the response within each channel by subtracting a fraction of the response in the second channel. In turn, this reduces the responses to the adapting axis and tilts the tuning function for each channel away from the adapting axis, spherizing the response distribution. Thus, an important feature of this model is that adaptation could induce a selective change in sensitivity even to stimulus directions to which neither channel is tuned. Consequently, adaptation effects alone do not conclusively reveal the specific channel structure.

Figure 60.3..  

An alternative account of tilt aftereffects based on mutual inhibition between channels. a, Signals along an oblique axis produce correlated responses in channels tuned to horizontal and vertical. b, Inhibition between the channels leads to an oblique rotation of their response axes, decorrelating their outputs.

The actual neural mechanisms underlying pattern-selective adaptation remain unresolved, though it is clear that the channels defined psychophysically do not reflect passive habituation in a neuron's responses. Physiological measurements of contrast adaptation in the cortex suggest that the response changes result from a tonic hyperpolarization imposed on a separate stimulus-driven response that is unaffected by adaptation (Carandini and Ferster, 1997). At least some components of the adaptation are extremely rapid (Muller et al., 1999), and can selectively adjust to the co-occurrence or contingencies between pairs of stimuli (Carandini et al., 1997) and alter the shape of an individual neuron's tuning curve (Movshon and Lennie, 1979; Muller et al., 1999).

The fact that very different models can lead to very similar explanations of visual aftereffects shows that the implications of contrast adaptation must be interpreted with caution. On the other hand, the models illustrated share important features. Both assume that stimuli are encoded by a set of channels that are (or can be) selectively tuned to many different directions, and that adaptation alters perception by altering the distribution of responses within these channels. Thus, the presence of a pattern-selective aftereffect remains a powerful source of evidence about the nature of visual representations.