1. Recorded from a dissected immobilized animal, or from an unrestrained animal which is quiescent, the descending contralateral movement detector (DCMD) neurone shows an exponential decremental response to a repetitive stimulus (habituation), reaching a plateau level characteristic of the stimulus conditions. The process is site-specific on the retina, and movement to a new area of retina gives a complete recovery. In the absence of stimulation responsiveness returns over minutes or hours.
2. Immediate recovery without a rest (dishabituation) can be obtained by a variety of strong sensory stimuli of several different modalities (‘extra-stimuli’) or by non specific electrical stimulation of parts of the CNS. The dishabituating efficacy of all these wanes with repetition. When the habituating stimulus is moved to a new retinal site the previous site is not dishabituated.
3. Dishabituation is not site-specific but affects the whole retina simultaneously. It appears to reverse the original decremental process (‘re-set’) rather than to produce an independent enhancement elsewhere in the pathway, as it does not increase the response from a submaximally stimulated, but unhabituated, retinal site.
4. In unrestrained animals dishabituating extra-stimuli also cause behavioural arousal or other motor activity. When motor activity starts, the DCMD is dishabituated and shows no regular decremental trend thereafter until movement ceases. DCMD background activity is also increased. These effects are not due to the visual stimulus of the moving appendages.
5. The association between motor activity and dishabituation suggests that the latter derives either from motor system collaterals or from mechanoreceptive reafference. Stimulation of the antennal nerve of a totally de-efferented brain cause some dishabituation; this eliminates the lower motor system (below command-fibre level) as the source of dishabituation and suggests it is purely sensory.
6. The activity of a thoracic cord unit (of possibly a wide-field mechanoreceptor interneurone) precedes by 5-20 sec, and closely correlates with, changes in responsiveness of the DCMD. It is either an important input to, or an output from, the dishabituating system.
7. Progressive reduction of sensory input to the brain affects DCMD responsiveness as follows: (i) spontaneous dishabituation is less frequent, (ii) dishabituation is less easily induced and smaller, (iii) rate of habituation is increased, (iv) plateau response level after habituation is lower.
8. Electrical stimulation of the circumoesophageal connective can depress DCMD responsiveness for many minutes.
9. The probable anatomical and physiological bases for modulation of DCMD responsiveness are discussed.
Understanding Variable Motor Responses to Direct Electrical Stimulation of the Human Motor Cortex During Brain Surgery