Optical illusions

**Figure 2.16:** Optical illusions present an unusual stimulus that highlights limitations of our vision system. (a) The *Ponzo illusion* causes the upper line segment to appear larger than the lower one, even though they are the same length. (b) The *checker shadow illusion* causes the B tile to appear lighter than the A tile, even though they are the exactly the same shade of gray (figure by Adrian Pingstone).
$\begin{figure}\begin{center} \begin{tabular}{cc} \psfig{file=figs/PonzoIllusion.... ...Squares.ps,width=2.7truein} \\ (a) & (b) \end{tabular}\end{center} \end{figure}$

One of the most popular ways to appreciate the complexity of our perceptual processing is to view optical illusions. These yield surprising results and are completely unobtrusive. Each one is designed to reveal some shortcoming of our visual system by providing a stimulus that is not quite consistent with ordinary stimuli in our everyday lives. Figure 2.16 shows two. These should motivate you to appreciate the amount of work that our sense organs and neural structures are doing to fill in missing details and make interpretations based on the context of our life experiences and existing biological structures. Interfering with these without understanding them is not wise!

**Figure 2.17:** A classification of the human body senses.
$\begin{figure}\begin{tabular}{llll} Sense & Stimulus & Receptor & Sense Organ \\... ...emical composition & Chemoreceptors & Mouth, nose \\ \end{tabular} \end{figure}$