Identifying the Culprit Assessing Eyewitness Identification (2014) / Chapter Skim
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4 Basic Research on Vision and Memory
4 Basic Research on Vision and Memory
Pages 45-70
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From page 45... ...
In the sections that follow the example, the different functional steps of the sequence (highlighted in italics) are dissected in some detail, with special reference to its limitations and the ways in which it may fail to deliver accurate eyewitness identification.
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From page 46... ...
VISION Functional Processes of Vision To understand the contributions and limitations of vision to eyewitness identification, it is useful to consider the workings of three functional stages of visual processing -- sensation, attention, and perception -- bearing in mind that they comprise highly interdependent elements of a continuous operation. Sensation is the initial process of detecting light and extracting basic image features.
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From page 47... ...
Perception is the process by which attended visual information is integrated, linked to environmental cause, made coherent, and categorized through the assignment of meaning, utility, value, and emotional valence. In addition, memories and emotions resulting from prior experiences with the world can influence all stages of visual processing and thus define a thread that weaves throughout the following discussions.
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From page 48... ...
In the case of the vibrating cell phone, if the distributions of tactile stimuli resulting from noise and signal overlap, as is often the case, then there will always be some cases in which you believe the phone is vibrating when it is not (noise stimuli attributed to signal source) , and there will be some cases in which the phone is vibrating and you miss the call (signal stimuli attributed to noise source)
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From page 49... ...
The example of the signal detection logic used for the vibrating cell phone applies similarly to all aspects of visual perceptual experience, including the conditions of witnessing criminal events. The uncertainty about visual events caused by manifold sources of noise will inevitably lead to inaccurate visual perceptual experiences, which result from conditions in which an observer fails to detect a critically informative stimulus as "real" (attributing the stimulus instead to a source of noise)
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From page 50... ...
to form the retinal image. Light in the image is initially "sensed" by the activation of photoreceptors, and early stages of sensory processing function to detect spatial and temporal contrast along a number of dimensions, including intensity and wavelength of light.6 These contrast measurements are integrated by subsequent processing stages in the brain to yield representations of basic image features, or primitives, such as oriented image contours.7 Several sources of noise, or factors that limit the ratio of signal to noise, can restrict the visual information accessible to these early sensory processes.
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From page 51... ...
To appreciate the impact of these limitations, consider that patients with macular degeneration are effectively blinded in the region of the visual field possessing highest acuity, and must rely instead on the much-reduced quality of visual information gained from the peripheral visual field. To compensate for this clinical loss, images and text must be greatly magnified to enable comprehension -- an option that is clearly not available to an eyewitness.
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From page 52... ...
.17 Attended image content is transiently enhanced to increase the fidelity of visual experience.18 Attention interacts with sensory processing, for example, by selectively enhancing contrast19 and potentially overcoming low signal-to-noise levels resulting from limited viewing time or illumination.20 The effects of attention on contrast enhancement can be potentiated further when attention is commanded by emotionally laden stimuli.21 Image con 13 W. James, Principles of Psychology (New York: Henry Holt, 1890)
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From page 53... ...
The signal detection framework is readily adaptable to the problem of noise in visual attention and provides some insights into the limits of attentional selection in the presence of noise.26 In essence, this signal detection approach quantifies the extent to which multiple items competing with one another for attention affect attentional enhancement for any one of the items.27 Reductions in efficiency are common under such noise conditions. Indeed, sensitivity to unattended items can be markedly reduced under conditions of high "perceptual load," in which there are many objects si 22 osner, Snyder, and Davidson, "Attention and the Detection of Signals." Y
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From page 54... ...
A related consequence of attentional noise is that competing interests can readily hijack the attentional focus. The technique of misdirection -- one of the original mainstays of performance magic -- directs attention to uninformative image content and exploits the invisibility of unattended features.31 The well-studied inattentional blindness effect is another example of this phenomenon, in which attention that is pre-directed to one behaviorally significant property of a visual scene precludes awareness of other features that also may be important.32 (For a dramatic demonstration of this effect, produced by Simons and Chabris,33 see http://tinyurl.com/ inattentional-blindness.)
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From page 55... ...
This finding suggests that naturally occurring events that briefly divert attention have the potential to markedly impair the accuracy of eyewitness identifications. Attentional hijacking is particularly characteristic of stimuli that elicit strong emotional responses, such as fear and arousal.35 Visual stimuli that trigger fear responses act as powerful external cues that command attention.36 While this potentiates sensitivity to those stimuli, at the considerable expense of sensitivity to others, it is often the case that the attended emotional stimuli are not the ones with relevant informational content.37The so-called weapon focus is a real-world case in point for eyewitness identification, in which attention is compellingly drawn to emotionally laden stimuli, such as a gun or a knife, at the expense of acquiring greater visual information about the face of the perpetrator (see also discussion of weapon focus in Chapter 5)
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From page 56... ...
complementing and interpreting the product with expectations derived from memory of prior experiences with the world, and (3) assigning meaning and emotional valence by reference to prior knowledge of function and value.40 All of these perceptual processes are affected by noise.
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What is implied is that the same mechanism that grants the certainty of perceptual experience in the face of noise and ambiguity is also capable of implicitly fabricating content that does not correspond to external reality and yet is experienced with no less certainty. Performance magic relies on this constructive nature of perceptual experience, and that nature is also the foundation for many visual illusions and forms of visual art.46 In a classic experiment that drives home the point, Bruner and Postman looked at the ability of observers to recognize ‘‘trick'' playing cards.47 The trick cards were created by altering the color of a given suit (e.g., a red seven of spades)
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L McClelland, "A Computational Model of Semantic Memory Impairment: Modality Specificity and Emergent Category Specificity," Journal of Experimental Psychology: General 120 (4)
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MEMORY Functional Processes of Memory Conscious visual perceptual experiences, rendered by the processes described in the previous section on vision, are commonly stored as declarative memories, meaning that they can be consciously accessed and expressed as knowledge about the world (as distinct from procedural memories, such as motor skills) .53 Declarative memories are of two types, semantic and episodic, reflecting a distinction between memories of meanings, facts, and concepts versus memories of events (such as those witnessed during a crime)
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Memory Encoding Memory encoding refers to the process whereby perceived objects and events are initially placed into storage. The encoding process involves two stages, which are commonly distinguished by the quantity of information stored, the duration of storage, and the susceptibility to interference.57 Short-term or working memory is the conscious content of recent perceptual experiences or information recently recalled from long-term storage.
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From page 61... ...
The structure of an individual's full library of long-term declarative memories can be thought of as a collection of associations between items of specific semantic (e.g., the fact that that person X is a 34-year-old female) or episodic content (e.g., the fact that person X was at location Y on the night of the witnessed crime)
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We forget, qualify, or distort existing memories as we acquire new perceptual experiences and encode new content and associations into memory.65 Forgetting can be partially mitigated, and memories stabilized, by habits of retrieval (or reactivation) and reconsolidation, which happen whenever we tell the story of our experiences.66 Reactivation is not perfect.
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From page 63... ...
Research on false memories shows that it is possible to plant fabricated content in memory, which leads us to recall things we never experienced.70 The emotional content of stored memories is a factor that appears to promote long-term retention; memories of highly arousing emotional stimuli, such as those associated with a witnessed crime, tend to be more enduring than memories of non-arousing stimuli.71 Highly salient, unexpected, or arousing events -- such as the Kennedy assassination or the Space Shuttle disaster -- are commonly more strongly stored in memory, and their later retrieval is often associated with the subjective experience 69 .
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which is triggered by arousing content and has potentiating effects on the neuronal processes underlying memory consolidation and storage.73 Despite the vividness and the sense of reliving that characterizes retrieval of emotional memories, there are many indications that such memories are just as prone to errors.74 This may reflect, in part, memory enhancements, of the sort described above, which accompany frequent re-consolidation or re-telling of the story of the emotional experience, and often include details (some true to fact, some not) learned after the experience.75 Although emotional memories are often inaccurate in detail, one important corollary of their vividness is that they are frequently 72 .
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From page 65... ...
These same retrieval processes can also be engaged internally -- a verbally triggered stream of thought related to a witnessed crime may readily bring to mind visual features of the perpetrator. A corollary of this association-based phenomenon is that memory retrieval is often context dependent; a memory may be more 76 .
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L McClelland, "A Compu T tational Model of Semantic Memory Impairment: Modality Specificity and Emergent Category Specificity," Journal of Experimental Psychology: General 120(4)
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From page 67... ...
,84 which is accomplished by verbally or physically placing him or her in the same context, and may offer a valuable investigative tool for probing eyewitness accounts.85 Recognition Memory Recognition memory is a specific type of declarative memory retrieval in which a sensory stimulus (a "cue" stimulus) elicits a memory of the stimulus stored following a prior encounter and often the sequence of events involving the stimulus, the spatial context in which the stimulus was experienced, and the presence of other objects, people, or thoughts that had appeared with the stimulus during the event.86 Recognition memory decisions are based on the retrieved memory evidence, which can be triggered by the stimulus and can also emerge from an active search of items 83 .
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From page 68... ...
Recognition memory decisions can thus be thought of as the final stage in the process of eyewitness identification. Because it is a form of memory retrieval, recognition memory is susceptible to all of the factors summarized above that are known to interfere with retrieval.
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From page 69... ...
The committee has reviewed much of this research, as it pertains to eyewitness identification, and has identified restrictions on what can be seen under specific environmental and behavioral conditions (e.g., as poor illumination, limited viewing duration, viewing angle) , factors that impede the ability to attend to critically informative features of a visual scene (e.g., the deleterious effect of an attention-grabbing element, such as a weapon, on the ability to correctly perceive the features of the assailant's face)
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From page 70... ...
70 IDENTIFYING THE CULPRIT from a faithful record of what was perceived through the sense of sight: its contents can be forgotten or contaminated at multiple stages, it can be biased by the very practices designed to elicit recall, and it is heavily swayed by emotional states associated with witnessed events and their recall. From this analysis, the committee must conclude that there are insurmountable limits on vision and memory imposed by our biological nature and the properties of the world we inhabit.
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