Page:Mind (New Series) Volume 15.djvu/361

 PHYSIOLOGICAL FACTORS OF THE ATTENTION-PROCESS. 347 the period of perception, and which rapidly passes away during the period of rest in which the other object is present to con- sciousness. The principal difference between the two cases is that in the case of the binocular rivalry of two colours the sensory content of consciousness changes, while in the case of the ambiguous figures the sensory content remains unchanged and only the mode of perception or apperception of it changes ; the corresponding difference in the neural changes in the two cases seems to be that the paths which suffer fatigue and become alternately active and passive or, in other words, alternately transmit the stream of nervous energy coming in from the sense-organ and cease to transmit it while it is diverted to the alternative path, these paths are, in the case of the binocular rivalry of colours, paths of the sensory area of the cortex, paths of the second level in the scheme on page 333 (vol. xi.) ; while in the case of the ambiguous figures these paths are paths of a higher level and the nervous energy coming in from the sense-organ is continuously transmitted by the paths of the sensory level while it penetrates alter- nately to one or other of two higher-level paths. In the discussions of fluctuations of attention authors have commonly given a prominent place to the intermittence of perception which commonly results when one attempts con- tinuously to concentrate attention on a just perceptible sound or a just perceptible difference of brightness. Some authors, notably Prof. Miinsterberg l and Prof. Heinrich,' 2 have argued that such intermittence is due to intermittence of the adjust- ment of the sense-organ, of the accommodatory mechanism of the eye and of the tension of the tympanic membrane of the ear, and they have supported their contention with ingenious experiments which seem to show that such changes in the adjustments of the sense-organs do probably occur at the moments of disappearance and reappearance of the just per- ceptible object. But, even if it could be proved that such changes in the sense-organs invariably accompany these fluctuations of the attention, it would by no means follow that the changes in state of the sense-organs, and still less that the changes of perception, are caused by peripheral fatigue and recovery. It would remain more probable that both kinds of change are the expression of central fatigue and recovery ; and this view of their causation is strongly supported by the fact that the intermittence of perception is 1 Beitrfige zur expert mentellen Psychologic, Heft ii., S. 98. 2 " Die Aufmerksamkeit u. d. Funktion d. Sinnesorganen," Zeitschrift fiir Psychologic, Bd. ix.