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have provided significant empirical evidence in support of the 'i*

■H

basic concepts of the- present theory. This theory suggests 1

a new approach to problems investigated over the last twenty

five years or so, under the general category of ’’vigilance”. Further, analysis of experimental work in this area carried out to date appears to provide strong support for the present theory. Comprehensive reviews of all recent work and the history of the extensive experimental research into vigilance phenomena are provided by Welford (1968) and Broadbent (1971).

In typical laboratory vigilance experiments, (v/hich were originally derived directly from applied situations), subjects

are required to detect and act upon the presence of faint and/or infrequent signals of low probabilities of occurrence over

extended time periods. Welford (1968, p.19) states that the laboratory studies of the past two decades ’’have built up a complex, but reasonably coherent, picture of the association between, on the one hand, sensitivity and responsiveness as measured by behaviour, and on the other, physiological variables such as level of autonomic activity, and of the relationships of both to environmental conditions.” He notes that these

investigations have emphasised the importance of studying ’’continued performance as a function of time. ” (ibid)

Under certain conditions subjects often fail to detect the presence of target signals, even though the experimental

situation requires that their primary attention he continuously

focused on the potential signal source, (e.g. a. radar screen).

Recordings of eye movement patterns show clearly that many 5

signals may be missed by subjects in spite of the fact that these signals occur well within the subject’s visual field, the sensory information indicating the presence of a signal therefore reaching at least the retinal receptors. (Mackworth, Kaplan and Metlay, 1964). In a one-dial display task situation in the Mackworth et al experiment it was found that ’’every missed

signal was fixated without being recognised” (ibid., p.397). Broadbent (1971) has termed this phenomenon "looking without seeing” (p. 42).

Prom the premises of the present theory, this finding is particularly significant. It shows that subjects fixating a specific visual target display pattern may not be consciously

aware of portions of sensory information emanating from this

pattern. In many instances, sensory information concerning the ÿ presence of a signal is physically received by, and available to

the subject, but it is not utilised. In terms of the present

approach it is considered that, in general, this sensory information does not become consciously available without an

internalised reorganisation of the information flow dynamics of the cognitive system.

Investigators in this area do not appear to have overtly

considered the vigilance problem from the particular cognitive viewpoint of the present approach. It is considered that the

fundamental phenomena of vigilance-type situations are amenable to an analysis in terms of the present model. This analysis suggests new explanations and theoretical propositions which may be experimentally investigated. These explanations complement existing theoretical work, and deal with aspects of vigilance which

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existing theories do not attempt to explain. In very general terms, most existing theories are concerned with the "crucial question"

(Welford, 1968, p. 277) of v/hy lapses of attention occur in

vigilance tasks. In some contrast, the present theory is concerned with the explanation of what happens when these lapses occur - i.e.

the manner in which the altered dynamics of the information flow | within the organism may cause environmental sensory input to be

disregarded.

The conscious and continued attention of subjects to the primary task in vigilance situations lapses because the tasks

typically employed in such experiments are unstimulating, cog­ nitively undemanding, and repetitive or monotonous. Post- experimental evidence from subjects indicates that during such

tasks they start thinking as the task becomes increasingly boring. Thought processes involving daydreaming, imagining and fantasising

are often concomitant with the monotony of the task situation (Antrobus, Singer, and Greenberg (1966)).

A reasonable explanation for subjects’ failure to maintain high signal detection performance in vigilance tasks is that such

tasks are essentially "laboratory artifacts", quite unlike, and with sustained performance requirements different from any task commonly encountered or likely to be undertaken in a subject’s normal environment - i.e. in the evolutionary sense, man is not biologically adapted to the performance requirements of vigilance tasks. Therefore, when it does become necessary for human

operators to perform vigilance-type tasks in real situations - such as manufacturing industry, defence, flying and driving, for example - applied psychological problems arise because of man’s apparent inability to fulfil consistently the performance re­ quirements of these tasks.

In terms of the present model, high levels of signal detection in vigilance task situations effectively require that

to the continuous analysis of his entire visual sensory inform­

ation input. As discussed previously (p. 34 ), research

evidence indicates that the conscious information processing

capacity of this system is restricted. Thus, if part of this limited capacity system is utilised in thought, (even if the

content of such thought is concerned with the task situation

itself), the concomitant capacity available for analysis of visual sensory data is reduced. The two picture memory experiments reported in chapters IV and V have provided strikingly effective empirical demonstrations of this phenomenon.

In typical vigilance task environments, once the subject has become fully accustomed to the static visual sensory pattern of the task situation, especially those relating to the target display, and has stored this rapidly overlearned information in memory, such memory information can be utilised via the imagery system to meet the fundamental perceptual requirements of the task situation. At any given instant, the total amount of

information concerned with conscious awareness of the basic task display may comprise phenomenally indistinguishable proportions of pattern information from memory and from the real sensory environment. The relative sizes-of these proportions are

related to the degree to which the subject’s cognitive capacities

are occupied by internalised thought processes, i.e. the degree to which his attention to the primary experimental task has lapsed.

The subjective probability of occurrence of the basic patterns concerning the target display (but obviously not the possible signal itself) very rapidly becomes, in effect, unity. From the theory, this means that a minimum amount of the

sensory information environment needs to be sampled to confirm

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the presence of the expected pattern, thus allowing the bulk of the appropriate pattern information passed to conscious

awareness to emanate directly from memory by means of the imagery system. In this sense, subjects in vigilance tasks continuously "see" the task display environment, i.e. in that they consciously experience its presence perceptually. However, while the nature of this conscious perceptual awareness may

remain constant, the composition of the information content

forming this awareness may vary substantially over time. The

conceptual notions under consideration may be represented graphically as in Fig. 7.

This diagram schematically represents the information

content of conscious perceptions of the same basic complex visual pattern A (e.g. a radar set, a T.V. screen etc...) at several consecutive instances in time - T^, Tg, Tg, T^. The shaded areas of these graphs represent the proportions of the pattern information content derived directly from the analysis of retinal sensory input data. The remainder of the pattern information (the unshaded area) comes from memory=

At T-j^, the subject is utilising his maximum conscious

capacity in the analysis of the sensory data input. At Tg, Tg, T^, because the subjective expected pattern probability is

extremely high, and because the capacities of the cognitive

system are to an extent focused elsewhere, the greater proportion of this pattern information is coming from memory, direct sensory

analysis having ceased when the presence of the expected pattern is confirmed. Clearly, then, if the same signal should occur at the "position" X shown in the displays, it will be detected at T^ and Tg, but not at T2 or T^. Detection occurs at T^ because of the contextual juxtaposition of the portion of the sensory

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r.

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information input being analysed and the signal occurring at that instant. Thus, at times when the subject’s conscious perceptual awareness is largely comprised of memory information, with minimal analysis of the concurrent sensory input, signals are less likely to be detected,

6,2 The foregoing analysis of vigilance-type situations leads to quite specific propositions amenable to experimental investi­ gations, vis:-

Condition 1; If over an extended time period a subject is repeatedly presented with the same complex visual pattern, which may or may not contain within it a given infrequent signal. Si, for the reasons outlined above, his detection performance should show the decrement over time normally observed in experiments in which the signal to be detected is a ’blip' on a blank cathode ray tube.

Condition II: However, if the task is restructured so that