By means of a computerized system of spike detection, presentation of visual stimuli, and timing of a button press, we showed previously that single posterior interictal spikes resulted in transient prolongation of reaction time and increased non-response rate in 3 subjects. By varying the response hand and the visual field of stimulus, conditions with maximum spike effect were defined more precisely, in relationship to the location of the spike. In general, spike-induced dysfunction was most pronounced when either response hand or visual field of stimulus was contralateral to the spike. This not only was true across all 3 subjects, but held even for independent right and left occipital spikes within the same subject. Also, perceptual versus motor aspects were differentially affected by the anterior-posterior location of the spike. These findings indicate that focal interictal spikes may transiently disrupt aspects of cortical functioning corresponding to their neuroanatomical location.