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Memory
Influences Before The Critical Incident
All
individuals have a set of schemas, inferences, and expectations
that they bring into a situation, and which have significant
potential to distort their memory of a critical incident.
Some of the most classic and fundamental memory research
(Bartlet, 1932; Allport and Postman, 1947; Carmichael, Hogan
& Walter, 1932), research which has been built on and replicated
for over half a century, demonstrates that human memory
is not like a camera taking precise photographs of new experiences.
Instead, it has long been understood that memory is a process
of active construction in which old knowledge, beliefs,
prejudices, and expectations are constantly shaping (and
potentially distorting) our memories.
This
research demonstrates clearly how a witness who was physically
abused as a child may see a standard or necessary police
action as violently abusive, or someone who has racist beliefs
or stereotypes may perceive a minority suspect as having
a weapon when, in fact, that was not the case. Understanding
the potential for memory distortion based on the baggage
that the witness "brings to the table" is "step one" in
understanding what is outlined from this point on, and this
should be considered as a possible explanation any time
a single witness has a testimony that is significantly different
from that of several others.
Fixation
and Perceptual Distortion
Data
perceived through each of the five senses is combined to
create what we call "memory." Each of the sensory systems
provides the brain with a constant source of information
about the environment through a complex network of neural
receptors. Memory is created when data perceived through
the sensory network is collated and then "hardwired" into
neurons in the brain. Since memory is a product of perception,
it is clear that memory can be disrupted when perception
becomes disrupted.
In
stressful situations, there is almost always an overwhelming
flood of information in a very short period of time. At
lower levels of stress this commonly results in a state
of excessive alertness in which the subject scans the entire
environment in order to take in information. Such an individual
may have good memories of some aspects, but in this state
it is common to become over aroused and never really settle
on or process any of the sensory stimuli into long-term
memory (Horiwitz, 1976; Janis & Mann, 1977).
In
extraordinarily stressful situations this sensory overload
often results in "fixation" on a particular aspect of the
critical incident, resulting in a very vivid memories of
that aspect of the experience but severely limiting memories
of anything else (Hockey, 1970; Bacon, 1974; Mandler, 1982).
Indeed, individuals who experience these memories (such
as a face, a weapon, or even a particular sound) often referred
to them as being "burned" into their memory.
This
fixation is due to "perceptual narrowing" in which the five
perceptual senses collapse around a central point of focus
as arousal or stress increases (Easterbrook, 1959; Schmidt,
1991). This powerful process will generally result in a
situation where only vision is processed, and even very
powerful cues from other senses (such as loud sounds or
injuries) may be diminished or completely filtered out.
Perceptual
narrowing is an effect of sympathetic nervous system (SNS)
activation. The SNS is activated anytime the brain perceives
a threat to survival, resulting in an immediate discharge
of stress hormones, which is designed to prepare the body
for what has been recognized as the "fight or flight response."
The activation of the SNS is automatic and virtually uncontrollable.
Once the SNS is activated, the visual system goes through
a series of automatic changes.
Breedlove
(1995) has found that SNS excitement causes vasoconstriction
to the blood vessels on the periphery of the retina, resulting
in a collapse of the peripheral field which is referred
to as "peripheral narrowing" or "tunnel vision." Thus, not
only is an individual in a critical incident likely to be
limited to only one sense (usually vision) but that one
sense will also be greatly reduced or "narrowed." Breedlove
states that the visual field can be expected to narrow by
70%.
Cannon
(1915) found that SNS excitement triggers pupil dilation,
leading to the loss of near vision, and SNS activation also
disrupts the ability to focus, which results in a loss of
depth perception and the ability to focus on close objects.
How
does this research apply to critical incident amnesia? First,
memory is a function of collating perceptions, and in combat
the visual system is the mother of all senses. But if the
visual system is disrupted or narrowed, the amount of information
to collate a complete picture will be incomplete. Second,
the SNS causes vasoconstriction to the periphery of the
retina resulting in a significant collapse of the visual
field. In other words, the officer will fixate on major
threat cues, but cues on the periphery of the visual system
may not be processed into memory. This explains why individuals
sometimes fail to remember "seeing" individuals or cues
immediately adjacent to the threat.
Finally,
the loss of depth perception and near vision accounts for
why individuals often fail to accurately identify distances
after a survival incident.
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