Retinotopic organization of the guinea pig's visual cortex.

Under anaesthesia the right visual cortex of the guinea pig was investigated with 1.5 M potassium citrate-filled glass microelectrodes. Single unit and small unit cluster response showed an organized three-part representation of the contralateral visual field on the animal's visual cortex. The major central division (V1) had the representation of a nasotemporally elongated visual field. This visual field extended for about 110 degrees horizontally and for about 70 degrees vertically. The nasal visual field was represented laterally on the cortex and the lower visual field anteriorly. The visual field representation was not uniformly magnified in V1; the magnification of the nasal field was higher than the rest of the visual field. On either side of V1, the guinea pig's cortex had two additional visually responsive strips. The strip V2L, situated lateral to V1, had a condensed representation of about 40 degrees of the animal's nasal visual field and mirror-imaged the visual field representation of the adjacent V1. The strip V2M, situated medial to V1, had a similar representation of about 40 degrees of the animal's temporal visual field and mirror-imaged the visual field representation of the area V1 adjacent to it. A binocularly responsive zone of cortex was observed on either side of the boundary between the areas V1 and V2L, representing between 10 degrees and 15 degrees of the nasal visual field on each side of the boundary line. Investigation of the retinal ganglion cell distribution of the animal showed a nasotemporally oriented 'visual streak' with a high ganglion cell density per unit area of the retina, near the optic disc. Away from the 'visual streak' the density of the ganglion cells of the retina diminished progressively. The configuration of the 'visual streak' and the general pattern of the ganglion cell distribution in the rest of the retina coincided with the asymmetry in the magnification of representation of the animal's visual field on the cortex.