Eyewitness testimony:
Sensation & Perception
•Seeing is believing?
•Sensation is based on physiological parameters that differs from
person to person
•Color blindness, acuity, threshold, saturation, etc.
•Hormones, level of electrolytes, teratogens (eg. Drugs)
•Environmental factors such as background also affect sensation
•Background colors and patterns
•Much of the same confounding factors apply to hearing and
other senses
Eyewitness testimony:
Sensation & Perception
•Perception is the analysis by the brain of sensory input
•It is NOT a one-to-one correlation between input and final
perception
•Perception is an active re-creation of what is an approximation of
the input
•During re-creation the brain makes assumptions and also makes
errors:
•Illusions
•Hallucinations
Eyewitness testimony:
Sensation & Perception
•Factors affecting perception:
•Past experience (Helmholtz’ glasses)
•Orienting reaction (basis for rapid changing images on TV)
•Gestalten:
•Closure
•Proximity
•Similarity
•Pregnanz
•Figure-ground attention
Eyewitness testimony:
Sensation & Perception
•Factors affecting perception continued:
•Personality
•Motivation
•Culture
•Social influences such as conformity: Asch
experiment
•The following slides contain examples of the material
presented above
Eyewitness testimony:
Sensation & Perception
Perceptual analysis slowed down to demonstrate the
process
Eyewitness testimony:
Sensation & Perception
All the color palettes are the same
Eyewitness testimony:
Sensation & Perception
Muller-Lyer illusion
Eyewitness testimony:
Sensation & Perception
Or 2 faces
A vase
A figure-ground example
Eyewitness testimony:
Sensation & Perception
Rabbit or bird: a figure-ground example
Eyewitness testimony:
Sensation & Perception
We see what we think should be seen
Eyewitness testimony:
Sensation & Perception
Perceptual error leads to impossible figures
Eyewitness testimony:
Sensation & Perception
Perceptual error creates “facts” (spirals) where none exist
Eyewitness testimony:
Memory
Memory is a construction and reconstruction process:
rehearsal
Selective
attention
Sensory input
Sensory
memory
Loss of information
Shortterm
memory
Forgetting and
interference
Long-term
memory
Eyewitness testimony:
memory (2)
•Sensory memory is part of the perception process
•During the analysis of sensory data, selective attention
determines which items will be retained for further memory
processing
•Ongoing events, personality, cultural biases, etc. will affect
selective attention
•Short-term memory is active and limited
•It requires active rehearsal or information is lost at 30 seconds
•It is limited to 7±2 bits of information
•When overloaded:
•Proactive interference: new info displaces old
•Retroactive interference: old info displaces new info
Eyewitness testimony:
memory (3)
•Transfer to long term memory requires encoding of data (consolidation)
•multitude of factors affect what info will be transferred
•Yerkes-Dodson Law shows that info stored at low or high arousal states will
be less accurate that those stored at moderate levels of arousal
•Social factors such as the need to obey, social facilitation, and cognitive
dissonance will affect memory reconstruction
•Different aspects of memory stored in different ways and locations
•Interactive memories may merge
•Remembering requires reconstruction of the encoded LTM and retrieval back to STM
•Same factors that affected storage will affect retrieval
•Retrieval mechanism itself may be a factor
•Cues and memory retrieval:
•Retrieval without cues (recall) is difficult but subject has high confidence
•Retrieval with cues (recognition) is easier but subject has low confidence
Eyewitness testimony:
memory (4)
•As a result of memory factors, eyewitness testimony is prone to error:
•If the witness is asked to describe a suspect, the subject will be sure
the description is accurate even if it is not
•If the subject is shown a series of pictures and asked to identify the
suspect, the subject may be right but inconsistent
•Selective attention factors are shown when we examine how subjects
recognize faces:
•We focus more on the details of faces that have high valence to
us…members of our own cultural group, or faces of cultural groups we
fear the most
•We focus more on upper facial features than lower facial features,
hence changes in upper facial features affect our perceptions and
memory more than other features as seen by the next slide:
Eyewitness testimony:
conclusions
Eyewitness testimony:
Conclusions
•Most eyewitnesses are in a state of high arousal during the time they
witness the event, during interrogation, and during testimony. The YerkesDodson law predicts major errors.
•Cognitive dissonance and social facilitation, predicts that once a witness
has provided their evidence, there will be a “hardening” of memory
•Hypnosis, which is a high state of suggestibility, is most prone to the
problems of reliability and validity of remembering and not acceptable as
proof (problems of recovered memories using hypnosis and age regression
hypnosis)
•Children, who have less sophisticated mental structures tend to be more
accurate when they describe events they participated in without adult
prompting, but are very inaccurate when prompted by adults or when
details are missing (closure Gestalt):
•Eg: if the suspect is missing in a lineup, children are more likely to
identify someone else