So prominent is the position of attention in the scientific discourse on behaviour, that Titchner (1908) regarded it as 'the nerve of the whole psychological system', and added that 'as men judge of it, so shall they be judged before the general tribunal of psychology'. James (1890) was convinced that 'everybody knows what attention is' and described it as '... the taking possession by the mind in clear and vivid form of one of what seem several simultaneous objects or trains of thought'. James was right in stating that intuitively we know what attention is, but, probably because the concept is so inclusive, a consensus on its definition is not easy to attain.
Not always was attention at the focus of attention of psychology. "Behaviourism intentionally ignored postulated inner faculties of the mind, including attention. The interest was renewed only after the Second World War, with the application of information processing theory, originally developed for warfare purposes, to the cognitive sciences (Broadbent 1958). A large body of work on attention has been accumulated since then, both in psychology and neurobiology. It ranges from investigation of the orienting reflex ("sensitization) to auditory and visual perception.A substantial part of what we currently know on attention stems from the analysis of vision in primates, at "levels ranging from behaviour via "functional neuroimaging and neuroanatomy to single cell activity (Posner and Petersen 1990; Desimone and Duncan 1995; Egeth and Yantis 1997; Kanwisher and Wojciulik 2000).
'Attention' refers to multiple mental states and activities, involving vigilance, orientation, and selection of information. The spectrum of activities thus ranges from the distributed to the selective and to the focused in time and space. These activities engage to various degrees on-line information (percepts of sensory attributes, location and timing) as well as off-line information (i.e. lasting internal representations). Similarly, attention could be "stimulus-driven (a bottom-up process) or task-driven (a top-down process). The latter dichotomy is illustrated in vision. Here selective attention was explained in terms of two consecutive, partially overlapping processes. The first is stimulus-driven, automatic, instantaneous and transient. The second is task-driven, slower, sustained and requires cognitive effort (Sperling and Weichselgratner 1987). Early stimulus-driven processing is frequently referred to as 'preattentive' (Neisser 1967), because it involves parallel processing of primitive features over the sensory space in the apparent absence of mental-resource limitation (Julesz 1981; Treisman 1985). Indeed, central to the notion of attention is resource-limited 'selection' (Norman and Bobrow 1975), which is detected at multiple points between post-receptor input and response (Desimone and Duncan 1995). Hence, lack of resource competition is taken by some authors to indicate lack of 'real' attention. More recent findings suggest, however, that even 'preattentive' vision is constrained by mental resources (Joseph et al. 1997).
A common connotation of attention is "conscious awareness (definition 2). Does this mean that attentive nonhuman species can be consciously aware of their dids, and if so, which species? Definitions 1 and 3 above fit situations in which conscious awareness cannot be proven or even assumed. Another definition, suggested by Hebb, also does not specify consciousness: 'central facilitation of the activation of one assembly by the previous one' (Hebb 1949); this view of attention depends, however, on the validity of the notion of "cell assembly. As far as the relationship of attention to conscious awareness is concerned, it is noteworthy that on the one hand, even humans may not be aware of activity in a cortical area assumed to be involved in some atten-tional tasks (Crick and Koch 1995); on the other hand, some degree of conscious awareness is expected to exist in other species as well (example in "classical conditioning). It is therefore useful to regard attention as involving a spectrum of awareness. Attention has been proposed to be the "binding agent of consciousness, and it is tempting to speculate that it has been a driving force in the emergence of consciousness. Seen that way, one could not escape the humble conclusion that the most precious niches of our inner world owe their existence to the emergence in evolution of the primitive, elementary orienting reflex.
Developments in two "methodologies have contributed much to the contemporary research on brain mechanisms of attention. One is cellular physiology, used in the "monkey, the other is functional neuroimaging, used in research on human "subjects (Desimone and Duncan 1995; Kawashima et al. 1995; Kastner et al. 1998; Reynolds et al. 1999; Kanwisher and Wojciulik 2000). The combination of both methodologies has led to the identification of brain circuits and cellular processes that are engaged in attention either correlatively or casually ("criterion).
At the system level, research on visual attention shows that areas in the frontoparietal, inferotemporal and occipital "cortex are involved. Among the visual processing areas, high-order cortex is particularly engaged, but there is also evidence for attentional activity already at the primary visual cortex. Attending a stimulus modulates the activity in cortex, even when the subject only expects to attend the stimulus before stimulus onset1 (Chawla et al. 1999). This is taken to reflect the task-driven, top-down attentional facilitation of the processing in the area that expects the signal. There is also evidence for hemispheric lateralization, with a right hemispheric bias for tasks involving attention to locations in space and left hemispheric bias for tasks involving attention to timing (Coull and Nobre 1998). As to the frontal cortex, it is considered to subserve a 'supervisory attentional system' or 'central executive system', which co-ordinates and prioritizes attention across sensory and internal modalities (Shallice 1988; Baddeley 1993). This is the same cortex involved in "working memory. This should not be surprising, since clearly, attention and working memory are complementary and closely related (James 1890;
Cowan 1988; Baddeley 1993). Attention identifies where the action is (a popular "metaphor likens it to a searchlight, Crick 1984a); working memory then immediately takes note of that action for further use. By so doing, it not only permits an instantaneous "plastic response, but also prevents superfluous exploitation of attentional resources. Whereas some of the automatic-ity in stimulus-driven attention is innate ("a priori), it is clear that the system has to be capable to quickly compare stimuli with use-dependent internal representations in order to decide whether focused attention and further processing and action are warranted. This interplay of attention and memory takes place within a fraction of a second of perception. Working memory is therefore also 'working attention' (Baddeley 1993).
At the cellular level, attention was found to increase the magnitude of the response of neurons in higherorder visual cortex to the attended stimulus in the receptive field;2 when multiple stimuli are within the receptive field, the activity is larger when attention is directed at the target stimulus (Moran and Desimone 1985; Reynolds et al. 1999). This gain and gating control could involve multiple circuit and system mechanisms, including the action of diffused neuromodulatory systems ("neurotransmitter). The function of these neuromodulatory systems in learning is assumed to involve regulation of gain and gating control as well; hence at the "synaptic level, certain molecular mechanisms of learning and attention merge.
A variety of pathologies impair attention. Among these are parietal and frontal lesions (Shallice 1993), schizophrenia (Andreasen et al. 1994), and attention-deficit/hyperactivity disorder, one manifestation of which is learning difficulties (Shaywitz et al. 1997). It has been suggested that attention and memory are also co-impaired in chronic fatigue syndrome, and the hypothetical 'central executive' was implicated (Joyce et al. 1996). In "real-life, multiple methods could be used to enhance attention, and, good news, some of these methods are clearly devoid of any side effect: a comparison of memory for humorous and non-humorous versions of sentences shows that the humorous ones are remembered better, probably because they are associated with increased attention (Schmidt 1994).
Selected associations: Binding, Homunculus, Metaphor, Percept, Working memory
'Expecting to attend is actually an 'attentional set'; for more on what is meant by 'set', see *learning set.
2A receptive field is that sector of the sensory space that could be sensed by the neuron.
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