Optical imaging of epileptiform events in visual cortex in response to patterned photic stimulation

In a subset of patients with epilepsy, patterned visual stimuli can trigger clinical seizures. The etiology of this phenomenon, and the complex interaction between functional architecture and epilepsy, were investigated in ferret visual cortex. Optical imaging of intrinsic signals was used to visualize maps of orientation, ocular dominance and spatial frequency. Acute interictal spike foci were then induced within V1 using focal iontophoresis of bicuculline methiodide and optically mapped during presentation of patterned visual stimuli. We found that specific orientations and spatial frequencies could preferentially trigger epileptiform events, depending on the location of the epicenter of the epileptic focus within the columnar architecture of visual cortex. These data support a cortical etiology of the clinical phenomenon of pattern-sensitive epilepsy. We were not able to demonstrate a spatial correlation between the functional architecture maps and the topography of the epileptic focus. These findings implicate short-range rather than long-range horizontal excitatory connections in the lateral spread of interictal spikes, which may be specific to the epilepsy model of acute focal disinhibition. Orientation and spatial frequency maps were severely disturbed in the region of the focus but were unaltered in the surrounding cortex. Thus, optical imaging of intrinsic signals can be used to simultaneously map epilepsy and normal functional anatomy with high spatial resolution.