Our visual system continually adapts to the recent history of visual input. Although we are generally not aware of it, this adaptation causes substantial changes in our perception. A prominent effect is that of
repulsion: Adaptation exaggerates the perceived differences between stimuli and the adaptor. For example, in the well-known tilt aftereffect, adaptation to a vertical bar causes the orientation of a subsequently viewed near-vertical bar to appear further from vertical (Gibson,
1937). These changes, extensively documented in the experimental literature, provide a window into the properties of the underlying neural representations. The simplest and most accepted linking hypothesis is based on tuned populations of cells and gain changes (Blakemore, Nachmias, & Sutton,
1970; Sekuler & Pantle,
1967). Specifically, those neurons (or, more abstractly, “channels”) that are activated by the adaptor reduce their gain, which leads to a characteristic repulsive bias in the perception of subsequently viewed stimuli (Clifford, Wenderoth, & Spehkar,
2000; Maffei, Fiorentini, & Bisti,
1973; Schwartz, Hsu, & Dayan,
2007). Additional physiological changes have been observed (e.g., changes in the shape of tuning curves), which, by themselves, may lead to different effects, but altogether the net effect seems to be consistent with repulsive perceptual aftereffects (Jin, Dragoi, Sur, & Seung,
2005; Kohn,
2007; Schwartz et al.,
2007; Seriès, Stocker, & Simoncelli,
in press). As such, regardless of the physiological details, we can think of the repulsive pattern of adaptation-induced biases as providing an externally observable perceptual “signature” for the underlying adaptation mechanism.