How to Explain Tunnel Phenomena 

Tunnel phenomena some times are observed in OBEs, they are dominant observations in NDEs and they are used as standard OBE method in shamanism.
Though the tunnel phenomena are well known nevertheless there is no satisfying theoretic explanation about their origin. Let us say at the start - we can't explain it either. Nevertheless it is useful to publish a collection of different theories and to discuss about.

Medical explanation:
As a result of brain distortion the visual field can be diminished starting from periphery and producing a disc like field of vision (open eye experiences and extrapolated to altered states of consciousness).
From the point of view of an OBE practitioner these medical observed features are different in some aspects: The tunnel observed in OBE is three dimensional and not a disc. In OBE you are moving in the tunnel and not only looking at a visual field.

Explanation in the sense of a dream symbol:
A well known class of dream symbols are the so called "border symbols". These are archetypical symbols (see C.G. Jung) which are observed when you pass the border of different states so as: unconscious to conscious dream to awakening allowed to forbidden in the sense of moralism present time to the past (or future) common consciousness to spiritual consciousness emotional to mental body based to emotional etc.
The usual symbols beside of tunnels are : rivers, ports, walls, mountains, stairs etc.
Counter argument: the tunnels observed in dream symbols are not as pregnant as observed in OBEs. In dreams you pass a tunnel, sitting in a train for instance or you are crawling through a mine. The tunnels in dreams are analogic symbols derived from common life features - not so the tunnels observed in NDEs and OBEs.

Tunnels explained from the point of view of NDEs and OBEs:
Well, you are passing some border between the physical and the transcendent plane. The origin of this border however is not explained up to now.

The main thing I would emphasise regarding my experience of "tunnel" type projections is how absolutely exhilerating it is - ultra "intensity" of the experience!
It is not a wispy, dream-like two dimensional experience, but as Alfred says ... it is absolutely three dimensional regarding one's perception of it and as "real" as any trip down a tunnel in the physical dimension.

My first projection in this regard was accompanied by a loud whirring noise like an electrical dynamo increasing in pitch and the rushing sensations as I went through the tunnel and bright geometric patterns were fascinating in themselves...

More than likely, as Alfred has suggested in other places, that this was the Ajna or Crown Chakras operating with a high level of energy .... entirely spontaneously I should add since, despite a lot of energy raising work and meditation practice, these things "happen" spontaneously at least as far as I am concerned and, again, this is confirmed by Alfred in many of his responses to letters regarding his own experiences. It must be marvellous to bring them on at "will".

It would be interesting to speculate as to why people who are NOT experiencing a Near Death Experience should be fortunate enough , in an "ordinary" OBE to experience the same "Tunnel" effect which predominates when death occurs.
Sceptical materialist based investigators such as Susan Blackmore have produced ingenious brain orientated explanatory structures which seem plausible to those who have not directly experienced the phenomena. But it has to be said that the overwhelming intensity and "reality" of the experience overwhelms one and if "reality" has any true meaning or definition then the tunnel experience is as real as anything one experiences in the physical dimension.