with the physical fact that the central excitation calls into activity
elements which have already been excited in the same way.

The nervous counterpart of the final stage of perception, the synthesis
of the sensation and the mental representation, is not clearly
ascertained. A sensation clearly resembles a mental image in quality. It
is most obviously marked off from the image by its greater vividness or
intensity. Agreeably to this view, it is now held by a number of eminent
physiologists and psychologists that the nervous process underlying a
sensation occupies the same central region as that which underlies the
corresponding image. According to this theory, the two processes differ
in their degree of energy only, this difference being connected with the
fact that the former involves, while the latter does not involve, the
peripheral region of the nervous system. Accepting this view as on the
whole well founded, I shall speak of an ideational, or rather an
imaginational; and a sensational nervous process, and not of an
ideational and a sensational centre.[12]

The special force that belongs to the representative element in a
percept, as compared with that of a pure "perceptional" image,[13] is
probably connected with the fact that, in the case of actual perception,
the nervous process underlying the act of imaginative construction is
organically united to the initial sensational process, of which indeed
it may be regarded as a continuation.

For the physical counterpart of the two stages in the interpretative
part of perception, distinguished as the passive stage of preperception,
and the active stage of perception proper, we may, in the absence of
certain knowledge, fall back on the hypothesis put forward by Dr. J.
Hughlings Jackson, in the articles in _Brain_ already referred to,