The basic conceptual idea underlying most of modern Cognitive Psychology is that of information processing. With that in mind, perception deals with how information gets into the brain-mind. The perceptual systems receive patterned energy and must represent, transform, act upon, and/or experience it.

Senses and sense organs

Specific nerve energies: This is the idea that the different senses have a totally different quality of experience. If a sense organ is stimulated, and it activates a nerve, it may trigger a sensory experience; the experience is different dependent upon which sense organ is stimulated.
        The several sense organs, e.g., eye, ear, nose, tongue, and skin, have within them sensory receptors, which selectively are excited by particular energy or chemical stimulation or changes of stimulation. Thus, the rods and cones in the retina are usually stimulated by light energy, the hair receptors in the ear are stimulated by sound energy, and the receptors on the tongue are stimulated by certain dissolved molecules. When excited, the receptors initiate a chain of events that culminates in the excitation of an electric potential in nerve cells (neurons). The conscious experience of hearing, seeing, tasting, cold, warmth, etc. must be due to the location in the brain that the sensory nerves finally connect to, or possibly the pattern of nervous firing that results, or both.
        "The sensory modalities are separate within the cerebral cortex. Thus an individual does not hear light, taste sound, or smell pressure. How the cerebral cortex converts virtually identical nerve impulses into specific and distinct sensations is still a mystery, but the nature of the sense organ that is stimulated and the area of the brain to which the impulse is sent clearly play major determining roles." ("Perception," Grolier's Multimedia Encyclopedia)

Visual information processing

The anatomy and physiology of the eye (Britannica article)
        "Although the rods and cones may be said to form a mosaic, the retina is not organized
 in a simple mosaic fashion in the sense that each rod or cone is connected to a single
 bipolar cell that itself is connected to a single ganglion cell. There are only about
 1,000,000 optic nerve fibres, while there are at least 150,000,000 receptors, so that
 there must be considerable convergence of receptors on the optic pathway. This
 means that there will be considerable mixing of messages. Furthermore, the retina
 contains additional nerve cells besides the bipolar and ganglion cells; these, the
 horizontal and amacrine cells, operate in the horizontal direction, allowing one area of
 the retina to influence the activity of another. In this way, for example, the messages
 from one part of the retina may be suppressed by a visual stimulus falling on another,
 an important element in the total of messages sent to the higher regions of the brain."
Thus the pattern of visual stimulation as well as the local intensity affects which neurons in the optic nerve fire.

The visual system. Drawing of primary visual system by Netter

Receptive fields. Microelectrorecordings from the thalamus and area V1 have identified receptive fields. These are specific brain locations in which the neurons respond specifically to particular visual stimulation. For example, some cells respond to vertical lines in the right location, but not horizontal ones, other cells respond to horizontal lines. Some respond to lines of certain thickness, but not others, some cells respond to small objects moving to the left, but not the right. Certain (on-off) cells are inhibited if the appropriate stimulus is near, but not exactly on the right location, others (off-on) are excited if the stimulus is near and inhibited if it is right on. These activities from the receptive fields are thought to be some of the first building blocks of the information processing system. These receptors respond more to changes of stimulation than to stimulation. If the stimulus doesn't change the receptors gradually return to their resting state of activity.

Hemidecussation: The right visual field projects to the the primary visual area (V1) in the  left hemisphere, and the left visual field projects to V1 in the right hemisphere. The same perceptual areas from both visual fields abut one another.
        Overlap in brain areas of the same visual field by the two eyes is illustrated here.
Later projections, After the stimuli leave V1, different parts of the cortex seem to do different processing in secondary visual projection areas. One area seems to recognize color; another area recognizes shape, and another that identifies the location of the object in space.

Pattern Perception. All the physical and physiological properties of the eye and the brain strongly suggest that its primary function is to identify different meaningful patterns of stimuli. Psychological data confirm this. We are remarkably skillful in making order out of visual stimuli.
        Oliver Sacks (1987 in The man who mistook his wife for a hat, New York: Harper and Row, wrote an important book containing case studies of people with neurological problems of one sort or another. The first case study is about a man who could see all of the features of an object, but could not integrate them into a perceptual whole. This illustrates the same kind of problem as Anderson's first discussion in Chapter 2. Sacks's case studies are very interesting and informative.
Gestalt laws. e.g. Laws of common fate, similarity (similarity of texture and similarity of size are good grouping cues), proximity, constancy, good continuation.Top
Perception of depth. Many cues lead to the perception of depth. Many are in this picture, e.g., texture gradients, perspective. relative size, etc.
        Organisms have evolved and developed so that they can use their receptors to learn about the world. The organism interprets many of the patterns of ambient energy as they come into contact with the receptors. Cognitive and perceptual psychologists, and neuroscientists have tried to identify some of the principles by which this is done. Once we discover the principles we may be able to fool the perceptual and cognitive systems with unlikely stimuli as if they represent real world phenomena, for example, the Ames room. Some natural phenomena simulate other phenomena in natural illusions. That is some natural phenomena may be interpreted as other natural phenomena. For example, on hot days we can see water on the road in the distance, or we may see the end of the rainbow nearby.
Relative motion. Faster things are closer. When we move, near things move in the opposive direction, far things in the same direction. Top
Phi or apparent motion Movies, Moving signs, etc.
Loom. Things that are expanding away from a central poin are coming toward us.
Discovering contours. We can find contours by integrating over relatively random patterns. See Dalmation Integrating over time using systematically varying pictures (random dots) and integrating over eyes because of retinal disparity (e.g. Magic Eye). Some drawings lead to illusory contours.

The auditory system and ear,  (article from Encyclopedia Britannica.)( Chart of ear from Encyclopedia Britannica)
The ear does not seem to have as obvious a mechanism to pick up patterns as the eye. Hearing is more obviously a temporal sense than a spatial one. It can to some extent in the inner ear. Drawing of primary auditory system by Netter
Most of the studies of pattern perception have been studied with speech, although there is work being done with music. It is very clear from this work, that the patterns of auditory stimulation over time and over the spectrum of sounds are major factors involved in hearing. Much of the important processing is localized somewhere other than the primary auditory areas. Broca's and Wernicke's areas in the frontal and temporal lobes (From Britannica).  are usually thought to be the primary speech areas in the brain.

Information processing mechanisms for perception.

According to David Marr (Vision, 1980), there are three relatively independent steps in understanding how an information processing system does its job. First, we must clearly know what the system is doing. That is, what exactly is the input to the system, and what is the output. Second, We must find some algorithm that can actually go from the input to the output. Third, we must find out how the system can implement that algorithm. What are the properties of its units? How are they interconnected? Top

The problem of recognition. How do we identify what we perceive?
Template matching. (Image) There is an attempt to wholistically match the input with memory.
Feature detection. Break the input up into components and then somehow use the combination to identify it in memory. (Lines, bars, intersections, geons)
Some form of feature detection seems to be preferable. However, context within which the object is perceived plays a critical role. One more set of examples.Top
 
 Speech perception.Speech shows a rather large disparity between conscious experience and the particular patterns of stimulation which trigger those experiences. We cannot localize where a noise occurs, even when it masks important sounds. What makes sense can be used to perceive incoming stimuli. Stimulus theory that prevailed until @ 1945 was that there was a filter theory of phoneme perception
Examples from speech. Breaks in the continuity of the stimulus are not where gaps are heard. Example 1 Different sounds are heard as the same, and same sounds are heard to be different.Example 2.  Minor differences make major differences in categorical perception. Example 3.   Top
 
 

Click for Britannica article on perception