similar illusions will be described in the other senses.Taken together, all these facts would force us to admit that the subjective difference between imagined and felt objects is less absolute than has been claimed, and that the cortical processes which underlie imagination andsensation are notquite as discrete as one at first is tempted to suppose.That peripheral sensory processes are ordinarily involved in imagination seems improbable; that they may sometimes be aroused from the cortex downwards cannot, however, be dogmatically denied.
The imagination-process CAN then pass over into the sensation-process.In other words, genuine sensations can be centrally originated.When we come to study hallucinations in the chapter on Outer Perception, we shall see that this is by no means a thing of rare occurrence.At present, however, we must admit that normally the two processes do NOT Pass Over into each other; and we must inquire why.One of two things must be the reason.Either 1.Sensation-processes occupy a different locality from imagination-processes; or 2.Occupying the same locality, they have an intensity which under normal circumstances currents from other cortical regions are incapable of arousing, and to produce which currents from the periphery are required.
It seems almost certain (after what was said in Chapter II.pp.49-51) that the imagination-process dryers from the sensation-process by its intensity rather than by its locality.However it may be with lower animals, the assumption that ideational and sensorial centres are locally distinct appears to be supported by no facts drawn from the observation of human beings.After occipital destruction, the hemianopsia which results in man is sensorial blindness, not mere loss of optical ideas.Were there centres for crude optical sensation below the cortex, the patients in these cases would still feel light and darkness.
Since they do not preserve even this impression on the lost half of the field, we must suppose that there are no centres for vision of any sort whatever below the cortex, and that the corpora quadrigemina and other lower optical ganglia are organs for reflex movement of eye-muscles and not for conscious sight.Moreover there are no facts which oblige us to think that, within the occipital cortex, one part is connected with sensation and another with mere ideation or imagination.The pathological cases assumed to prove this are all better explained by disturbances of conduction between the optical and other centres (see p.50).In bad cases of hemianopsia the patient's images depart from him together with his sensibility to light.
They depart so completely that he does not even know what is the matter with him.To perceive that one is blind to the right half of the field of view one must have an idea of that part of the field's possible existence.
But the defect in these patients has to be revealed to them by the doctor, they themselves only knowing that there is 'something wrong' with their eyes.What you have no idea of you cannot miss; and their not definitely missing this great region out of their sight seems due to the fact that their very idea and memory of it is lost along with the sensation.A man blind of his eyes merely, sees darkness.A man blind of his visual brain-centres can no more see darkness out of the parts of his retina which are connected with the brain-lesion than lie can see it out of the skin of his back.He cannot see at all in that part of the field; and he cannot think of the light which he ought to be feeling there, for the very notion of the existence of that particular 'there' is cut out of his mind.
Now if we admit that sensation and imagination are due to the activity of the same centres in the cortex, we can see a very good teleological reason why they should correspond to discrete kinds of process in these centres and why the process which gives the sense that the object is really there ought normally to be arousable only by currents entering from the periphery and not by currents from the neighboring cortical parts.We can see, in short, why the sensational process OUGHT TO be discontinuous with all normal ideational processes, however intense.For, as Dr.Münsterberg justly observes:
"Were there not this peculiar arrangement we should not distinguish reality and fantasy, our conduct would not be accommodated to the facts about us, but would be inappropriate and senseless, and we could not keep ourselves alive....That our thoughts and memories should be copies of sensations with their intensity greatly reduced is thus a consequence deducible logically from the natural adaptation of the cerebral mechanism to its environment."
Mechanically the discontinuity between the ideational and the sensational kinds of process must mean that when the greatest ideational intensity has been reached, an order of resistance presents itself which only a new order of force can break through.The current from the periphery is the new order of force required; and what happens after the resistance is overcome is the sensational process.We may suppose that the latter consists in some new and more violent sort of disintegration of the neural matter, which now explodes at a deeper level than at other times.
Now how shall we conceive of the 'resistance'
