The effect of focal interictal spikes on perception and reaction time. I. General considerations.

The effect of focal interictal spikes on visual perception and reaction time (RT) was studied in 3 subjects, by means of a computerized system of spike detection, presentation of visual stimuli, and timing of a button press. A total of 8646 such trials were analyzed, comparing spike-locked and control performances within the same subject. For stimuli locked to spikes involving visual cortex, both the rate of non-responses (non-perceptions) and the RTs of responses were increased (P less than 0.0001) in all 3 subjects. The magnitude of this effect was roughly proportional to the amplitude and breadth of field of the spike. Focal spikes in cortical areas uninvolved in the visual motor task did not impair performance. These findings indicate that single focal interictal spikes transiently disrupt aspects of cortical functioning, at least in the few subjects studied so far. This may have developmental and therapeutic implications for patients with very frequent interictal spikes, even thought they may not have seizures or their seizures may be well controlled.