Disease manifestations such as photic cortical reflex myoclonus or myoclonus due to
intermittent light stimulation rely on a pathologic interaction between non-structured
visual inputs and the corticospinal system. We wanted to assess the normal interaction,
if any, between a prior photic input and the output of the cortico-motoneuron connection.
In 9 consenting healthy subjects we quantified the changes exerted by a sudden, unexpected
bright light flash on (i) the motor potentials (MEPs) evoked in the right first dorsal
interosseous muscle (FDI) by transcranial magnetic or electrical stimulation (TMS/TES)
of the primary motor cortex, (ii) the FDI F-waves and (iii) the soleus H-wave. Separately,
we measured the simple reaction times to the flash itself. All determinations were
repeated twice with an interval of 2-24 months.
When the flash preceded TMS by 55-70 ms, the MEP size was reduced, while at interstimulus
intervals (ISIs) of 90-130 ms it was enlarged. Statistical 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 shortened at ISIs of 90-130 ms (P<0.05 at ISIs of 55, 110
and 130 ms). Electrical MEPs were enhanced at an ISI of 120 ms. The F-wave size showed
a non-significant trend of enhancement at ISIs of 90-130 ms. The soleus H-wave showed
significant enlargement at ISIs of 90-130 ms (P<0.05 at ISIs of 100 and 105 ms). The
minimum reaction time was on average 120 ms.
An unexpected photic input, to which no reaction is planned, can cause an early inhibition
of the responses to TMS. We think its origin lies within the primary motor cortex,
since it is not associated with changes 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
reticulospinal) fibers that excite the spinal motor nucleus.