We will take up these topics in order, begin ning (1 ) with attention as a state of conscious ness. What are the essential features of the attentive state? (a) The ideas attended to are more clear and distinct than the ideas simul taneously present, but neglected by the atten tion, or. as we may say, attended from. The proceftes at the focus of attention are readily discriminated; the ideas of the background are obscure. (b) If the ideas attended to arc in trinsically weak, they are strengthened or in tensified by attention. A faint sound or light is rendered louder or brighter as we attend to it. No such intensification is observable in the case of ideas whose stimuli are intrinsically strong. (e) If the ideas attended to are fleeting and transient, they can be lengthened, made more durable, by attention. No such lengthening is observable in the ease of ideas whose stimuli are stable and permanent. Lastly, (d) the ideas attended to are more valuable for the general mental life than are the ideas attended from; they are more easily and certainly revivable, in memory and imagination. It is a commonplace of education that if we wish a child to remember something, we must make him attend to it. This. fourth characteristic is, evidently, of a different, order from the three preceding. It is a result. or after-effect of the others, and more especially of (a).
As to the physical basis of these changes in consciousness, it has long been known that cer tain nervous impulses within the central nervous system have the power of inhibiting or arresting other impulses; thus, excitation of the (pneumogastric, or tenth cranial) nerve inhibits. the beating of the heart (Weber). Now we can; account for some of our observed phenomena by a theory of inhibition. When e.g. a weak men tal process is strengthened, or a transient process lengthened, we may well suppose that the effect is due, not to any positive reinforcement or temporal extension of the process in question, but simply to the simultaneous arrest of other and conflicting excitations within the nervous system. Attention. that is to say, allows the weak or transient process to come to its full in tensive and temporal rights in consciousness by keeping down other nervous impulses whose tendency is still further to weaken or mirtail that process. Attempts have been made, in the same way, to explain the growth of clearness and distinctness in the ideas attended to by assum ing an inhibition of conflicting ideas; so that the clearness of the attentive state would repre sent the normal or natural clearness of the idea, the clearness which it can attain when its de velopment is not hindered by rival ideas. Physi ologists, however, have recently discovered that, just as certain nervous may arrest. other impulses, or block paths of nervous dis charge, so may certain impulse.s reinforce others, or open up paths of nervous discharge; the negative fact of inhibition is paralleled by the positive fact of facilitation. There seems to be no reason why we should not avail ourselves of this discovery for our theory of attention. The clearness of the idea attended to would then de pend partly upon the arrest of conflicting ideas, but partly also upon an actual enhancement of the focal idea by the reinforcing nervous im pulses. Such a view accords better with the observed mental phenomena than does the view which regards all the aspects of the attentive state as symptoms of neural inhibition. On the question of the primary seat of the inhibitory mid facilitating impulses, no more can at pres ent be said than that it is, probably, to be sought in the 'association centres' of the cerebral cor tex. (See 'NERVOUS SYSTEM AND BRAIN.)
Two special questions arise out of the fore going discussion. We may ask, in the first place, how long the state of attention continues? for how long a time an idea or group of ideas may remain poised at the apex of conscious clearness? And we may ask, secondly, how large the of ideas attended to may be; inquiring now into the range of attention, as we have before in quired into its duration. Both questions have been submitted to the test of experiment. It has been found, (a) that the state of attention is not continuous, but intermittent. Suppose that we look attentively at a faint gray patch on a. white surface.,or listen attentively to the faint hiss of a gas-flame. If the attention remains steady, the gray and the hiss will also remain steadily in consciousness. If, on the other hand, the attention fluctuates, there will be moments when the gray and the hiss undergo a loss of clearness: and since they are, at the best, only just visible and just audible, the loss of clearness will mean a loss of existence: the gray will merge into the white and the hiss will cease to be heard. The second alternative is realized. The gray appears, and is washed out, and ap pears again, at intervals of a few seconds; and the sound of the flame is heard, ceases, and then is heard again, in precisely the same way. The average length of the single 'pulse' of attention —the time counted from appearance to reappear ance, or from disappearance to the next follow ing disappearance of the stimulus—is five or six seconds. (b) The range of attention is most easily determined by the aid of visual stimuli. A number of letters, e.g. chosen at haphazard and set in a space that falls well within the ob server's range of vision, are shown upon a screen for so short a time that wandering or roving of the attention from letter to letter is prevented. Under such conditions it is possible to grasp, by a single 'act' of attention, some five or six sepa rate impressions. if short and familiar words are exhibited, in place of the mixed letters, some four or five of these will be grasped by the atten tive consciousness. The number of discriminable ideas present at the same time with the four or five focal ideas in the background of conscious ness cannot, in the nature of things, be accurate ly ascertained. (See CoxsciousNEss.) The physical basis of the intermittence of at tention may be sought in the mode of functioning of the nerve-cell. The cell is said by physiolo gis+s to 'discharge.' We may interpret the term literally, for the nerve-cell is a storehouse or reservoir of nervous energy, and gives off this energy when the appropriate stimulus arrives, not continuously or piecemeal, but all at once, by way of explosion. Now an exploded cell must be recharged before it can function again, and the period of recharging apparently corresponds to the periods of disappearance of the pale gray and the faint hiss of which we have spoken. In explanation of the limited range of attention, we can say no more than that the small group of ideas comprised within it represent the available energy of the total cortex, as directed upon the given stimuli; the fact of limitation must he considered as an ultimate fact of the psycho physical organization.