Some issues related to perception have been already addressed. The focus here will be only on one essential question: how are nerve signals turned into perception of the world? Psychology and other related disciplines have not and are unlikely to provide a satisfactory answer as long as they operate within the presently dominant paradigm. Eccles writes:
There is a general tendency to overplay the scientific knowledge of the brain, which regretfully, also is done by many brain scientists and scientific writers. We are told that the brain "sees" lines, angles, edges, and simple geometrical forms and that therefore we will soon be able to explain how a whole picture is "seen" as a composite of this elemental "seeing". But this statement is misleading. All that is known to happen in the brain is that neurones of the visual cortex are caused to fire trains of impulses in response to some specific visual input. Neurons responding to various complications of this specific visual input are identified but there is no scientific evidence concerning how these feature-detection neurones can be subjected to the immense synthetic mechanism that leads to a brain process that is "identical" with the perceived picture. (Popper and Eccles, 1977, p.225)
It is known that retinal processing is involved in detecting intensity and wavelength contrast; early cortical areas in the brain are involved in orientation, curvature, spatial frequencies and movement; and high visual areas (in the parietal and temporal lobe) process sensations about the spatial relationships and the identity of visual objects. This, however, is not sufficient. Sherrington's comment from 1938 is still valid:
A star we perceive. The energy scheme deals with it, describes the passing of radiation thence into the eye, the little light-image of it formed at the bottom of the eye, the ensuing photo-chemical action of the retina, the trains of action potentials travelling along the nerve to the brain, the further electrical disturbance in the brain, the action-potentials streaming thence to the muscles of eye-balls and of the pupil, the contraction of them sharpening under the light-image and placing the seeing part of the retina under it. The ‘seeing'? That is where the energy-scheme forsakes us. It tell us nothing of any ‘seeing'. Much, but not that. (1940, p. 248)
In order to tackle this problem the terms sensation and perception need first to be distinguished. Putting it simply, while sensation is about touch, vision and audition, perception is about feeling, seeing and hearing. Perception can be defined as the process of transforming sensations into information or experience. Although the neuronal activity without doubt contributes to perceiving, we are not even aware of such activity - we operate with words, images and feelings, not with neurons.
The first point that needs to be made is that perception ensues from the relation between the subject and the object. If this statement sounds obvious, it should not be forgotten that in the last hundred or so years everything possible has been done (without much result) to find an alternative explanation that would exclude the one who experiences and is aware. So, reinstating the subject, as an essential ingredient that transforms sensations into perception, is necessary. Perceiving sensations as meaningful images, for example, involves, besides electro-chemical processes in the brain, also awareness, intent and the self, without which meaning would not be possible. Thus, in accord with the previous posits, it is proposed that the non-material aspect plays an essential role in the transformation of sensations into perceptions and their interpretation.
It is not controversial that when we perceive something the brain is prompted to produce coherent wave patterns, which are otherwise in a chaotic state. In the 1970s neuroscientist Walter Freeman conducted research on the olfactory perception of rabbits. He established that what distinguishes the response to one smell from another does not depend on which neurons fire or what part of the olfactory bulb (the brain region associated with smell) is affected. Rather, it is determined by the relative amplitude of the response in different parts of the bulb. If no smell is introduced, an irregular, chaotic EEG (measurement of the electrical activity of the brain) through all possible frequencies and local amplitudes can be detected. When the rabbits were exposed to a familiar odour, their EEG patterns immediately move from a chaotic to a coherent state. An unknown smell causes a modification in the collective amplitude pattern of all neurons in the olfactory bulb. Thus, the production of a coherent wave pattern is what matters, not specific neurons. A comparable principle is likely to govern vision and it is even possible that these patterns can form something similar to holograms.
This is, however, only half of the story. As already argued, the wave oscillations produced in the brain need awareness to be perceived as meaningful images or words (a hologram too needs the interference of two light waves to be created). Moreover, perception is not passive, and therefore cannot be identified with the processes that happen in a camera, TV or computer (although there may be some resemblance on a very basic level). This is an active, creative process, involving several interrelated activities that transform sensations into information and experience:
Participating (attention, interest, curiosity, exploratory drive, seeking sensations and stimuli) is an innate drive. Its importance is highlighted by the experiments with kittens performed by Held and Hein in 1963. They created an apparatus called a ‘kitten carousel', which allows two kittens to have exactly the same visual experience, but only one of them can initiate movement. When the kittens were tested, it was found that the ‘active' one could see perfectly well, while the ‘passive' one behaved as if it was not able to see much, although there was nothing wrong with its eyes or optic nerves. So, the passive kitten could not develop a perceptual ability without active participation.
Selecting - as already discussed, selecting from all possible stimuli is an active process, although over time it becomes mostly automatic.
Organising - the materials of perception are not just received, but they are also combined and structured. Perceptual organisation groups the smaller units into larger ones. The principal organising tendency is to identify part of the world as the target (the figure) and view the rest as the background. Other organising tendencies include ‘the law of Prägnanz' (the law of simplicity), good continuation, closure, and the laws of grouping, such as proximity and similarity.
Interpreting - the sensory input is not perceived mechanically, but it is continually interpreted. One piece of evidence for this is perceptual constancy, related to size, shape and brightness. It refers to a phenomenon that the perception of invariant object properties remains constant despite changes in proximal stimulation (e.g. we always tend to perceive grass as green, although with decreased brightness it becomes, in fact, brown - which can be easily checked at dusk). Brightness constancy appears to be innate, whereas size and shape constancy are largely influenced by experience. So, the interpretation of reality is mostly achieved over time, and it takes over almost completely from un-constructed perception. This does not mean that what is perceived through our senses is not related to reality. Our perception normally corresponds (in some measure) to something real ‘out there'. After all a fly, cat or human being may perceive a table leg in different ways, but they all try to avoid bumping into it. Actually, although our interpretations may be wrong, they often reflect reality better than sensations themselves, as exemplified by shape constancy, a subcategory of the above mentioned perceptual constancy: the shape of an object such as a door, for instance, is perceived as constant, even though the retinal image changes with its movement (a rectangular shape becomes trapezoid).
Perception therefore, is not a passive process, the perceived is evaluated, modified to some extent, and interpreted on the basis of previous experience and expectations. This indicates that not only awareness, but also an active involvement (hence intent and the self) are necessary in order to perceive.
Some possible questions
Why are we aware of the external reality rather than processes in the brain?
To function efficiently, it is necessary to be aware of the outside world, rather than an intermediary. If we were aware of receiving impulses from the brain, we could not identify with the body. This identification is important because it enables correlating the materials of perception to its real source - the world outside, which makes distinguishing between the external and the internal possible.
Why do we see images, rather than energy configurations?
For the reason of simplification. We perceive a table, for example, as solid and constant, because we have learned to disregard any fluctuations that are unimportant. Narrower perception is more condensed and therefore more stable. Mystical experience, for example, can be wider, but it is hard to make sense out of it.
How do we separate what comes from the outside and what comes from the inside?
As already mentioned, identifying with the body is a necessary condition to separate the internal and external, but it is not sufficient. Distinguishing whether the waves are triggered by nerves or our own constructs (e.g. dreams) is not straightforward. In fact, what is the internal and what is the external is gradually learned, and this process relies on many factors (an ability to exercise intentional control, continuity, relative stability, confirmation by other senses, shared experiences, etc.). Without them external reality could be perceived as internal, or more often, the internal is perceived as the external.
How do we choose what is the figure and what is the background?
This depends on the characteristics of the information (e.g. the spatial unity that tells us what is in front), the habituated selection procedures that may be biologically or socially conditioned, and intent (as when we are looking for somebody, for example).
What is the difference between transpersonal experiences and seeing things as they are (behind their physical and social constructs)?
These experiences are different - the first one requires going beyond the constructs, while the second one requires going ‘below' them. Consequently, they require different methods: the first one is the result of applying transpersonal methods, the second one may be achieved by using phenomenological reduction.