Dynamic properties of the representation of the visual field midline in the visual areas 17 and 18 of the ferret (Mustela putorius).

In mammals, the visual field is split along the midline, each hemisphere representing the contralateral hemifield. We determined that, in the ferret, an 8- to 10-deg-wide strip of visual field near the midline is represented in both hemispheres. Bright squares (1.5 deg) were flashed at different azimuths within the central 20 deg of the visual field. Stimuli were flashed either alone or sequentially, and the responses were analyzed with the voltage-sensitive dye (VSD) RH 795 and/or by recording local field potentials (LFPs). In both VSD and LFP experiments, each stimulus evoked a cortical response field that extended over visual areas 17 and 18 up to a surface of 1-1.5 mm(2) and then shrank again. Amplitude of the responses decreased approaching the visual midline and the latency increased. These positional differences are likely to originate from the spatiotemporal structure of the peripheral response fields (PRFs) that form a mosaic in areas 17 and 18, interrupted near the visual midline. Unexpectedly, interhemispheric connections appear not to modify these PRFs' effects and may not contribute to the responses to discrete, flashed stimuli.