Wednesday, November 23, 2005

Hypnosis & Witnesses & Subconscious Inference

There is a considerable body of law, much of it dating from the 1980s, about the question of the admissibility or inadmissibility of the testimony of witnesses who have (allegedly) been hypnotized. A recent article in the New York Times describes recent studies of hypnosis by neuroscientists. See Sandra Blakeslee,This Is Your Brain Under Hypnosis, in SCIENCE, NYTimes Online (Nov. 22, 2005). One of the major conclusions of these recent studies parallels the general conclusion of many earlier studies of human perception and human memory: the mind is not a tabula rasa; the mind constitutes, or creates, perceptions (or conclusions based on perceptions) from the sensory signals that are fed into the human brain and the human organism. But there are important differences in detail between the older and newer studies of perception -- there are differences in theories about precisely how the brain processes sensory data and converts them into perceptions -- and the article describes some of the mechanisms that are, it is now thought, in play.

Some of the recent studies described in the article involve the study of the brain activity during hypnosis. According to some current neuroscientific theory, "top-down" processing is very important in the brain's interpretation of sensory inputs:

One area that it may have illuminated is the processing of sensory data. Information from the eyes, ears and body is carried to primary sensory regions in the brain. From there, it is carried to so-called higher regions where interpretation occurs.

For example, photons bouncing off a flower first reach the eye, where they are turned into a pattern that is sent to the primary visual cortex. There, the rough shape of the flower is recognized. The pattern is next sent to a higher - in terms of function - region, where color is recognized, and then to a higher region, where the flower's identity is encoded along with other knowledge about the particular bloom.

The same processing stream, from lower to higher regions, exists for sounds, touch and other sensory information. Researchers call this direction of flow feedforward. As raw sensory data is carried to a part of the brain that creates a comprehensible, conscious impression, the data is moving from bottom to top.

Bundles of nerve cells dedicated to each sense carry sensory information. The surprise is the amount of traffic the other way, from top to bottom, called feedback. There are 10 times as many nerve fibers carrying information down as there are carrying it up.

These extensive feedback circuits mean that consciousness, what people see, hear, feel and believe, is based on what neuroscientists call "top down processing." What you see is not always what you get, because what you see depends on a framework built by experience that stands ready to interpret the raw information - as a flower or a hammer or a face.

Some of the studies described in the NYTimes article show that such top-down processing can be so powerful that it can make a human beings "see" the color red when in fact some other color is plainly before the human actor's eyes. This sort of one finding is one reason why students of the law of evidence should be interested in these recent neuroscientific studies. Such studies may have a bearing on controversies about the appropriate treatment of witnesses who have been hypnotized. But the studies may also have implications for the more general question of the "suggestibility" of witnesses and the implantation or development of false memories and beliefs. However, as always, whether such studies have any immediate practical implications depends very much on the details. The mere insight that the human mind is an active participant in perception -- like the insight, or conclusion, that expectations are capable of altering or falsifying perceptions (or conclusions based on perception) -- is not new. One needs to know, for example, under what circumstances various kinds of changes in (conclusions based on) perceptions occur, what the magnitude of such changes is, and so on.

There is more in the article -- much more. I strongly suggest you read it -- and then dip into further literature in this area.

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