PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 335 the muscular adjustments, reinforce the excitation-pro- cesses initiated by those favoured sense-stimuli and tend to determine their penetration to some one system of higher- level paths. We may usefully characterise these as accessory internal conditions of attention, to distinguish them, on the one hand, from such factors as the relative intensities of the sense-stimuli, their novelty, their sudden incidence or con- trasted characters which we may call external conditions, and, on the other hand, from the intrinsic conditions of attention which we have now to consider. These consist in the ever-varying states of the organised neural systems of the higher brain-levels. At any moment of waking life some one such system is predominantly active, is the main path of discharge of neurin from the afferent to the efferent side of the brain, and as the current shifts from system to system it leaves each one in a condition of subexcitement which only gradually passes away and which we may conceive as consisting in the presence in it of a residual charge of free energy. Let us suppose that at any moment of waking life a number of objects are simultaneously affecting the various sense-organs, and that the external and the accessory internal conditions are equally favourable to all of them (a condition of course never realised) ; if one of them excites a sensory tract which is intimately connected with some upper-level system that is in a condition of excitation or subexcitation it, rather than any other, will become the object of attention, and the mode of its perception will depend upon the nature of that system. Which object shall become the object of attention and the way in which it shall be perceived are thus determined by the interplay of these three kinds of factors, the external, the accessory neural and the intrinsic neural factors. Instances of the selective influence of the state of excitement or residual excitement of a neural system are frequent in common life, as when the expectation of the arrival of a friend leads one to catch the sound of his voice in the distance, or leads one to perceive him in the approaching figure of a stranger who perhaps bears but little resemblance to him ; in fact all the familiar instances of pre-perception which have been adequately treated by a number of authors. An important point to notice is that the idea of the expected object need not be present to consciousness in order that the corresponding mental system shall exert its selective action upon the sensory processes ; .it suffices that it shall have recently been present to consciousness so that the corre- sponding neural disposition remains in a condition of residual excitement.
