Evolving concepts of sensory adaptation

Abstract

Sensory systems constantly adapt their responses to match the current environment. These adjustments occur at many levels of the system and increasingly appear to calibrate even for highly abstract perceptual representations of the stimulus. The similar effects of adaptation across very different stimulus domains point to common design principles but also continue to raise questions about the purpose of adaptation.

Figure 1.. Color afterimages

An example of a perceptual aftereffect, similar to the “Big Spanish Castle” illusion popularized on the internet [88]. The upper image is a pseudo-negative of the original photograph (with color differences exaggerated and brightness differences reduced). Stare at the black dot in the upper image for 30 seconds or more and then quickly switch your gaze to the dot in the lower picture. The grayscale image should briefly appear colored. The illusory colors are opposite to the adapting colors, a “negative aftereffect” typical in adaptation. This occurs because adaptation reduces sensitivity to the local adapting color and, thus, biases perception toward the complementary color. Note also that the aftereffect is visible only when the afterimage is in register with the grayscale image (by fixating the dots). This occurs because the colors in the afterimage tend to fill-in between the luminance boundaries and, thus, blend together in different ways depending on the position of the borders [87]. This illustrates that adaptation effects that arise very early in the visual system can be modulated at more central levels [22].

Figure 2.. Face aftereffects

Adaptation can bias the perceived characteristics of faces in ways very similar to color aftereffects. For example, after looking at a female (or male) face, a neutral, androgynous face appears more masculine (or feminine). Studies exploiting these aftereffects have attempted to establish whether facial dimensions, like gender, are encoded by the relative differences in two broadly tuned mechanisms (upper) or by the distribution of responses across multiple narrowly tuned channels (lower). Adapted from [20].