Human eye
Rods are highly sensitive to light and allow us to see under low levels of illumination. However, they are unable to resolve fine detail and are subject to saturation. This is the reason for the temporary blindness that is caused when we move from a dark room into sunlight. There are approximately 120 million rods per eye which are mainly situated towards the edges of the retina.
Cones are less sensitive to light than rods and can tolerate more light. There are three types of cones, each of which is sensitive to a different wavelength of light. These cones which number about 6 million are mainly concentrated on the fovea, a small area of the retina on which images are formed.
Visual perception
The information that is that is gathered by the eye must be filtered and passed
to the brain, which in turn generates coherent images. We shall look at different aspects of the perceptual process.
Reflected light from an object forms an inverted image on the retina of the eye. The size of that image is specified as a visual angle. The visual angle is affected by both the size of the object and its distance from the eye. Therefore, if two objects are at the same distance, the larger one will have the larger visual angle. Hweover, our perception of an object's size remains the same even if its visual angle changes. This is the the law of size constancy.
Perceiving brightness forms another aspect of visual perception. Brightness is a subjective reaction to levels of light and is affected by the amount of light emitted by an object. This is known as luminance. Visual acuity increases with increased luminance. This is probably one of the reasons for using high luminance.
The perception of color is also an important part of the perceptual process. Colour is usually regarded as being made up of three components: hue, intensity, and saturation. Hue is determined by the spectral wavelength of the light entering the eye. An average person can distinguish 150 different hues. Intensity is the brightness of the color, and saturation is the amount of whiteness in the color.
The eye perceives color because the cones are sensitive to lights of different wavelengths. Color vision is best in the fovea, and worst at the periphery where rods predominate.
Visual processing compensates for the movement of the image on the retina which occurs as we move around and as the object which we see moves. Although the retinal image is moving, the image that we perceive is stable. Similarly, color and brightness of objects is perceived as constant, in spite of changes in luminance. This can also create optical illusions.
During reading, the eye makes jerky movements called saccades followed by fixations. Perception occurs during the fixation periods, which account for approxiamtely 94 percent of the time elapsed, The eye moves backwards over the text as well as forwards, in regressive movements. Although a child reads words by breaking them into constituent letters, it is not true of adults. They instead form visual patterns which they associate with certain words. Hence, removing word shape cues (such as capitalizing words) is detrimental to reading speed and accuracy.
The outer part is the visible part of the ear. It has two parts: the pinna which is attached to the sides of the head, and the auditory canal through which sound waves pass to the middle ear. The pinna and auditory canal serve to amplify sounds.
The middle ear is a small cavity, connected to the outer ear by the tympanic membrane or ear drum, and to the inner ear by the cochlea. Within the cavity are the ossicles, the smallest bones in the body. Sound waves pass along the auditory canal and vibrate the ear drum which in turn vibrates the ossicles. The ossicles concentrate and amplify the sound waves they receive and transmit the vibrations to the cochlea. Cilia which are small hair inthe cochlea bend due to the vibrations in the cochleal fluid and release a chemical transmitter which causes impulses in the auditory nerve.
Despite its usefulness, the ear has not been used to its potential in user interface design.
Touch or haptic perception is an important means of feedback. The apparatus of touch differs from that of sight and hearing as it is not localized. The skin contains three types of sensory receptors: thermoreceptors which respond to heat and cold, noiceptors respond to which intense pressure, heat and pain, and mechanoreceptors which respond to pressure. Only the last one is of interest to us.
There are two kinds of mechanoreceptors, which respond to different types of pressure. Rapidly adapting mechanoreceptors respond to immediate pressureas the skin is indented. They also react more quickly to increased pressure. However, they stop responding if continuous pressure is applied. Slowly adapting mechanoreceptors respond to continuously applied pressure.
Another aspect of haptic perception is kinesthesis: awareness of the position of the body and the limbs. This is an important aspect as it affects both comfort and performance. For a touch typist, for example, awareness of the relative positions of the fingers and feedback from the keyboard are very important.
