Isolated directional preponderance of caloric nystagmus: II. A neural network model

Hypothesis: The purpose of this study was to simulate an isolated direction al preponderance (DP) on bithermal caloric testing by constructing a realis tic neural network model. The simulation was designed to capture not only t he characteristics of the nystagmus response to caloric stimulation but als o the response to rotational stimulation in patients with an isolated calor ic DP. Background: The nature of an isolated DP-that is, a DP in the absence of a significant spontaneous nystagmus or canal paresis-is outlined in the prece ding article. In this article, the authors investigate the possible neural basis for an isolated caloric DP using the mathematic modeling technique of neural network simulation. Neural network models are typically abstract in nature; however, in this case the network was based on the known structure and function of the central vestibular system. Methods: The neural network model was based on the known neuroanatomy and n europhysiology of the horizontal vestibuloocular reflex pathway. A leftward -rightward asymmetric modification of the dynamic responses of simulated me dial vestibular nucleus type 1A neurons on one side, or of type 2 neurons o n the other side, to peripheral input would generate an isolated caloric DP . Results: The values of DP and associated canal paresis produced by the netw ork were within the same range as in the patient group. The network also pr edicted that the rotational DP would be lower than the caloric DP: between 2.5% and 56.9% of the caloric DP value. The actual rotational DP value was between 3% and 57% (average 31% of the corresponding caloric DP value. Conclusions: An isolated caloric DP can be simulated by a neural network mo del by modifying the activity of model units that represent medial vestibul ar nucleus neurons. An asymmetric dynamic response by a gain-enhancement fu nction of either type 1A neurons on one side or of type 2 neurons on the ot her was sufficient to produce an isolated caloric DP. Excitatory gain enhan cement of type 2 neurons produced a smaller rotational DP than a similar mo dification of type 1 neurons. This result indicates a potential neural locu s for the generation of an isolated DP in patients with vestibular disorder s.