We describe representations of the visual field in areas 18, 19 and 21 of the ferret using standard microelectrode mapping techniques. In all areas the azimuths are represented as islands of peripheral visual field surrounded by central visual field representation. The zero meridian was found at the 17/18 and 19/21 borders; at the 18/19 and anterior border of 21 the relative periphery of the visual field was found. In areas 18 and 19, elevations are represented in a smooth medio-lateral progression from lower to upper visual field. In several cases the elevations in area 21 evidenced a similar medio-lateral progression; however, in others the elevations exhibited a split representation of the horizontal meridian. Anatomically determined callosal connections coincided with the representation of azimuths near the zero meridian. Medio-lateral bands of callosal connectivity that straddle the 17/18 and 19/21 borders are connected by bridges of callosally projecting cells. Acallosal cortical islands corresponded to the peripheral visual field and were found straddling the 18/19 border and the anterior border of area 21. The results are discussed in relation to callosal connectivity and retinotopy in extrastriate visual cortex and to proposed homologies of carnivore and primate visual cortex.

Visual areas 17, 18, 19 and 21 of the ferret can be distinguished on the grounds of cytoarchitecture, myeloarchitecture and cytochrome oxidase reactivity, and with transneuronal tract-tracing from the eye. Each visual area contains callosally connected, as well as acallosal, regions. The callosal connections originate mainly from layers 2 and 3 and, more widely, from layer 6. Callosally projecting neurons and callosal terminals are organized in three roughly medio-laterally oriented bands. The posterior and intermediate bands straddle the 17/18 and 19/21 border, respectively; the third band extends along the medial bank of the lateral suprasylvian sulcus. These bands are linked by a variable number of bridges of connections that demarcate acallosal islands. The distribution of callosal connections predicts the existence of vertical meridian representations corresponding to each of the bands and of non-isotropic representations of the visual field within the bridges and islands.

No abstract

We analyzed the coherence of electroencephalographic (EEG) signals recorded symmetrically from the two hemispheres, while subjects (n = 9) were viewing visual stimuli. Considering the many common features of the callosal connectivity in mammals, we expected that, as in our animal studies, interhemispheric coherence (ICoh) would increase only with bilateral iso-oriented gratings located close to the vertical meridian of the visual field, or extending across it. Indeed, a single grating that extended across the vertical meridian significantly increased the EEG ICoh in normal adult subjects. These ICoh responses were obtained from occipital and parietal derivations and were restricted to the gamma frequency band. They were detectable with different EEG references and were robust across and within subjects. Other unilateral and bilateral stimuli, including identical gratings that were effective in anesthetized animals, did not affect ICoh in humans. This fact suggests the existence of regulatory influences, possibly of a top-down kind, on the pattern of callosal activation in conscious human subjects. In addition to establishing the validity of EEG coherence analysis for assaying cortico-cortical connectivity, this study extends to the human brain the finding that visual stimuli cause interhemispheric synchronization, particularly in frequencies of the gamma band. It also indicates that the synchronization is carried out by cortico-cortical connection and suggests similarities in the organization of visual callosal connections in animals and in man.

The development of cortico-cortical connections was studied in kittens deprived of vision by binocular eyelid suture during the formation of axonal arbors and synaptogenesis, i.e. between the second postnatal week and the end of the third postnatal month. Axons originating in area 17 and terminating either in ipsilateral or contralateral visual areas were visualized with biocytin. In ipsilateral areas 17 and 18, distinct clusters of branches begin to form, distally from the injection, during the second half of the first postnatal month, independently of pattern vision. More proximal clusters differentiate during the second postnatal month, and this seems to involve elimination of exuberant axonal branches. In kittens deprived of vision for 3 or more months, beginning before natural eye opening, the distal clusters regress and the proximal ones fail to differentiate. In extrastriate areas, distinct clusters of branches have segregated by the end of the second postnatal month, independently of visual experience; however, in kittens deprived of vision for 2 or more months, one of the clusters was selectively eliminated. In contralateral areas 17 and 18, we found stunted terminal arbors in kittens continuously deprived of vision. This was already noticeable at the end of the first postnatal month. Apparently, in the absence of pattern vision, most axons undergo only limited growth and do not form their characteristic terminal columns. Many of these axons are subsequently eliminated. In contrast, 8 days of vision beginning at natural eye opening and followed by visual deprivation caused a nearly normal development of intrahemispheric and interhemispheric connections. In conclusion, pattern vision appears to validate connections at early stages of their development; this validation is necessary for their further growth and differentiation that can then continue autonomously.