In the last five years, a number of detailed anatomical, electrophysiological, optical imaging and simulation studies performed in a variety of non-human species have revealed that the functional organization of callosal connections between primary visual areas is more elaborate than previously thought. Callosal cell bodies and terminals are clustered in columns whose correspondence to features mapped in the visual cortex, such as orientation and ocularity, are starting to be understood. Callosal connections are not restricted to the vertical midline representation nor do they establish merely point-to-point retinotopic correspondences across the hemispheres, as traditionally believed. In addition, anatomical studies have revealed the existence of an ipsilateral component of callosal axons. The aim of this short review is to propose how these new data can be integrated into an updated scheme of the circuits responsible for assembling the primary visual field map.

Uma série de 18 travessias, em intervalos mensais ou bimensais, na baía do Marajó nos anos 1983-1985, a fim de medir os parâmetros físico-químicos e coletar plancton, contribuiu para nosso conhecimento das condições hidrocinéticas e limnológicas durante o ciclo anual de vazão alta e baixa dos rios e das marés. Devido à diferença de vazão entre a cheia e a seca no rio Tocantins ser maior que no rio Amazonas, a entrada de água salobra na baía do Marajó é mais extensa que na foz do rio Amazonas. No período de seca há indícios de água marinha até 90 km na montante no rio Guamá. A turbidez das águas do estuário interno impede a penetração de luz, resultando em falta de produção primária, apesar dos amplos nutrientes. Na zona de encontro das águas túrbidas com as salobras (com salinidade de 2 a 4%), a matéria em suspensão agrega-se em flocos maiores que se sedimentam, o que aumenta a claridade da água (visibilidade de até 200 cm). A água nesta zona torna-se verde devido à produção de fitoplâcton, sendo que 90% dessa biomassa é constituída da diatomácea marinha Coscinodiscus. Coincidentemente, nota-se grande diminuição no teor de sílica e de outros nutrientes. Durante o período de vazão baixa, a zona de alta produtividade se encontra fora da baía do Marajó, nas águas de plataforma continental.

To understand the hydrological and limnological conditions during the annual regime of high and low river flow, as well as the action of the tides, a series of 18 collecting trips were conducted in monthly and bimonthly intervals across the Bay of Marajó during the years 1983-1985. The Tocantins River provides more than 80% of the inflow into this bay and shows a much greater difference in water volume flow between high and low water season than the Amazon. The annual displacement of brackish water influence is thus more extensive Marajó Bay than in the Amazon estuary. During the dry season of low river discharge (September-December), traces of seawater are found to penetrate up to 90 km upriver in the Guamá River. The high degree of turbidity of inner estuarine waters impedes light penetration and results in the near absence of primary production in spite of ample nutrients. Where these turbid river waters mix with brackish estuarine waters of 2 to 4‰ salinity, flocculation and subsequent sedimentation causes visibility to increase from a few to sometimes 200 cm. The water in this zone assumes a bright green color due to phytoplankton. About 90% of the biomass consists of a polyhalobic diatom species, Coscinodiscus. Concomitant great reductions in silica and other nutrient concentrations are noted. During low river flow (September to December), this high production zone is located in the central part of Marajó Bay, whereas it lies outside of the bay over the continental shelf during high river discharge (February to April).