The human body is an extraordinary unit that is capable of remarkable feats. An individual's perception of external stimuli is a perfect illustration of this fact. In the following paper, I will discuss two phenomena, one visual and one auditory, which demonstrate the fascinating nature of our senses.
The concept of brightness and how it is perceived by the brain is an intriguing topic. Richard Gregory, author of the book Eye and Brain, defines brightness as "a function not only of the intensity of light falling on a given region of the retina at a certain time, but also of the intensity of the light that the retina has been subject to in the recent past, and of the intensities of light falling on other regions of the retina."" Thus, we can see that brightness is mainly dependent on three factors: the state of dark adaptation of the eye, the ratio between the retinal luminance of the object, and the retinal luminance of its background. The luminance of a surface can be thought of as the total number of photons it gives off at each wavelength, with the number at each wavelength weighted by an absorption probability factor. Although luminance gives a measure of how many photons are coming to the eye from an object, the number of photons that actually get into the eye is what truly counts. This number is jointly determined by the luminance and by the size of the pupil. .
With this background information, we can see that the perception of brightness is actually much more complicated than one initially thinks. One particularly interesting phenomenon associated with the perception of brightness is that of simultaneous brightness contrast. Simultaneous brightness contrast shows that the perceived brightness of the interior of an object depends on what is happening at its edges. There are two pieces of evidence suggesting that the brain bases its brightness judgments on information from edges.