Cited by T. Quick, 'Disciplining Physiological Psychology: Cinematographs as Epistemic Devices, 1897-1922', Science in Context 30 (4), pp. 423-474.
Description:

'Following Hering, Münsterberg had proposed that the alternation of attention between different sense-organs could be explained in terms of a pair of generally-acting antagonistic forces within the body: the motor functions of the eyes, for example, strained against one another to apprehend their surroundings (Munsterberg 1900, 532-537). McDougall re-interpreted Münsterberg's conclusions in terms of a fluidic 'X-substance' within the retina. The transition of attention from one eye to another was not due to the temporary predominance of assimilatory over dissimilatory forces, as Hering had claimed, but rather to the interaction between the stimulatory effects of this substance as it was conveyed via the neural fluid. The alternation of images between the eyes could be explained by the interaction between differently-originating streams of this fluid competing for synaptic influence. Moreover, if an area of the retina was exposed to light for a long time, the X-substance in that area would be used up, allowing another stream of fluid to predominate (McDougall 1901a, 91, 242). McDougall thereby characterized spatial interactions between the visual fields in terms of the inhibitory activity or 'drainage' of vital fluids that coursed from the retina through the nervous system. Though he opposed Hering's conclusions, McDougall did not disagree with the former's explanatory invocation of phenomena specific to life: visual sensation was for McDougall the manifestation of autonomously-acting vital substances, rather than the more strictly physiological 'forces' that underpinned Hering's conclusions, and to which Bergson would also appeal.'

 

Relevant passage from McDougall:

'As an example of the numerous phenomena that can be explained by my theory of induction and contrast, at least equally as well as by Hering's, I will mention the "Dunkelhof" or dark halo of Henng. When a small S patch is fixated on a W ground for about one minute and then suddenly removed, the area of W which it had covered appears brighter than the general W ground and is surrounded by a narrow zone of W or grey darker than the ground. This narrow darker zone is the "Dunkel-hof" of Hering, which he accounts for as being a contrast effect of the brighter W of the area that it surrounds, i.e., as due to increased assimilation caused by the increased dissimilation of the neighbouring area. On the other hand I regard it as due partly to contrast (in my sense of the word, i.e., inhibition by the brighter after-image), and partly to the fact that during fixation the X-substances diffuse into the area of the retina affected by the S patch from the immediately surrounding parts, and so leave these parts relatively poorer in the X-substances.' (91)