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:

'each colour-system is continuously excited by the appropriate X-substance in the retina, but, owing to the mutual antagonism of the cortical activities of the three systems, that of one inhibits that of the other two systems, the conditions being favourable to 'complete fading,' until in turn it is inhibited by the activity of the more rested of the other two.' (242)