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Post-Incident
Amnesia
Most
memory results from when we "attended" to information. Every
waking second of every day our five senses flood the brain
with data, but we only attend (that is, pay attention to)
a tiny percentage of what comes in. If we do not attend
to something it is generally lost to memory (Cherry, 1953;
Moray, 1959). Intense fixation of attention on a particular
aspect of a critical incident can cause vivid memories in
some areas, but by definition this focused attending in
one area will cause a reduction in attending (and thus to
memory) in all other areas.
Sometimes
this fixation results in a kind of "flashbulb" effect. Brown
& Kulik (1977) originated the term and others (Neisser,
Winograd, & Weldon, 1991; Palmer, Schrieber & Fox, 1991)
have done significant follow-up work. In critical incidents
a Flashbulb memory is often seen when an initial image or
aspect of the critical incident will be all that is remembered.
(Other research [Haber, 1969, 1979; & Stromeyer, 1970] refers
to a similar process, which has similar effect, as "eidetic"
memory.)
This
is similar to the process that would occur if you were moving
down a familiar hallway or street, saw something new, and
thought intently about that new stimulus as you continued
to walk. At the end of the walk it is likely that you would
have a vivid memory of this one new stimulus, but could
not remember anything else that you saw or did, even though
you looked at and moved around things in your path. Most
of us have experienced something similar many times. In
critical incidents this common process can be greatly amplified
by the surprise and intensity of the initial shock. The
individual essentially functions on autopilot during the
critical incident, while the mind continues to dwell on
and try to make sense out of that initial image. Immediately
afterwards, that image may be all that is remembered.
In
addition to the failure to attend which results from fixation
and sensory overload, there is a body of research which
indicates that intense stress will result in a failure to
recall anything learned in a situation (Duncan, 1949; Squire
1986). McGaugh (1990) and Khalsa (1997) indicate that this
effect is due to the flood of stress hormones in the brain
which occurs during intense trauma. The combination of these
factors will very often result in "post-incident amnesia"
in which, immediately after a critical incident, the majority
of information will not be remembered. (This can explain,
for example, the common process by which most mothers tend
not to remember the intense pain of childbirth, and are
subsequently willing to have more children.)
The
greater the trauma, the greater the impact of post-incident
amnesia is likely to be. Key factors which will increase
the stress include: the perception of threat or danger,
the suddenness of the threat and the available time to respond
or prepare, the amount of sensory input needing to be processed,
and the degree of physical effort (aerobic and anaerobic
output) that was engaged in during the incident. If the
individual is physically wounded or injured the effect will
be even greater, and the effects of post-incident amnesia
will be greatest if the wound or injury results in unconsciousness.
Memory
Recovery
Based
on extensive combat experience, the U.S. Army has a common
understanding that, "The first report is never right." The
near universal occurrence and general understanding of post-incident
incident amnesia (although not referred to by that name)
is probably the basis for this expression. Furthermore,
combat leaders throughout history have understood that it
will be extraordinarily difficult to make sense out of a
battle until the day after combat, when the troops have
had a night's sleep, since information gathered from "shell
shocked" soldiers immediately after combat is notoriously
poor.
After
a critical incident, much of the information may still be
in the brain, but it has not been processed in such a manner
that it can be retrieved. There is increasing evidence that
it is possible for individuals to remember information that
was not attended to at the time (Treisman, 1960, 1964; Corteen
& Wood, 1972; Anderson & Pichert, 1978; Hasher & Zacks,
1984; Bennett, Davis, & Gianni, 1985; Kihlstrom et al.,
1990). But one of the key factors in being able to retrieve
this information appears to be sleep.
Sleep
(and particularly the dreaming that takes place in REM sleep
cycles during sleep) has increasingly been understood as
a time when the brain is focused on problem solving and
resolution of powerful emotional concerns (Webb & Cartwright,
1978; Cartwright, 1989, 1991) since depressed or highly
stressed individuals have longer and more focused dream
cycles. Integral to this process is the understanding that
sleep (and particularly dreaming/REM cycles) is the period
when information gathered during the day is processed into
long-term memory (Jenkins & Dallenbach, 1924; Cartwright
et al., 1975; Benson & Feinberg, 1977; Tilly and Empson,
1978; Scrima, 1982; Schoen & Badia, 1984). In particular,
it appears that REM sleep helps consolidate unusual information
that requires a good deal of adaptation in order for it
to be absorbed (Pearlman, 1982)
Evans
(1984) argues that sleep (and dreaming/REM cycles) occurs
because the brain must periodically shut out sensory input
so that it can process and assimilate new data and update
what has already been stored. Research indicates that during
sleep the brain divides new information into "wanted' and
"unwanted" categories, and makes new associations in light
of the day's experiences. Numerous research projects (Luce,
1971; Roffwarg, Muzio & Dement, 1966) support this contention
by demonstrating that REM cycles (which is when most dreaming
occurs) happen more frequently and are longer for individuals
who are placed in circumstances in which they must process
great quantities of new information.
Thus,
it can be observed that, immediately after experiencing
a critical incident, individuals have not had an opportunity
to mentally process and refine what they have experienced.
But, after a night's sleep there should be significant memory
recovery. If an individual has been kept isolated from other
sources of information, the memories at this point (approximately
24 hours after the incident) should be the most "pure" since
they have not yet integrated data from other sources.
It
should be noted, however, that if the initial night's sleep
has been disturbed, then the memory recovery which should
occur as a result of sleep will be potentially disrupted.
If the trauma encountered in the critical incident resulted
in unconsciousness, or an operation requiring general anesthesia,
then there is a good possibility that normal memory recovery
will be greatly disrupted. The ability for an officer to
convict the guilty and defend the innocent in a court of
law, or even to defend himself against spurious charges,
is greatly influenced by understanding the memory recovery
process and by safeguarding this first night's sleep.
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