Effects of a photic input on the human cortico-motoneuron connection

Objectives: Disease manifestations such as photic cortical reflex myoclonus or myoclonus due to intermittent light stimulation rely on a pathologic in teraction between non-structured visual inputs and the corticospinal system . We wanted to assess the normal interaction, if any, between a prior photi c input and the output of the cortico-motoneuron connection. Methods: In 9 consenting healthy subjects we quantified the changes exerted by a sudden, unexpected bright light flash on (i) the motor potentials (ME Ps) evoked in the right first dorsal interosseous muscle (FDI) by transcran ial magnetic or electrical stimulation (TMS/TES) of the primary motor corte x, (ii) the FDI F-waves and (iii) the soleus H-wave. Separately, we measure d the simple reaction times to the flash itself. All determinations were re peated twice with an interval of 2-24 months. Results: When the hash preceded TMS by 55-70 ms, the MEP size was reduced, while at interstimulus intervals (ISIs) of 90-130 ms it was enlarged. Stati stical significance (P < 0.05) emerged at ISIs of 55, 70, 100, 105 and 120 ms. Conversely, the MEP latency was prolonged at ISIs of 55-70 ms and short ened at ISIs of 90-130 ms (P < 0.05 at ISIs of 55, 110 and 130 ms). Electri cal MEPs were enhanced at an ISI of 120 ms. The F-wave size showed a non-si gnificant trend of enhancement at ISIs of 90-130 ms. The soleus H-wave show ed significant enlargement at ISIs of 90-130 ms (P < 0.05 at ISIs of 100 an d 105 ms). The minimum reaction time was on average 120 ms. Conclusions: An unexpected photic input, to which no reaction is planned, c an cause an early inhibition of the responses to TMS, We think its origin l ies within the primary motor cortex, since it is not associated with change s in spinal excitability or electrical MEPs. A later facilitation persists using TES and has a temporal relationship with an enlargement of the soleus H-wave. Thus, it likely results from activation of descending (possibly re ticulospinal) fibers that excite the spinal motor nucleus. (C) 2000 Elsevie r Science Ireland Ltd. All rights reserved.