Nissl, Golgi and fibre preparations were made of the cerebral cortex of the lateral gyrus of the bottlenose dolphin (Tursiops truncatus)in the region where visual evoked potentials have been reported (Sokolov et al., '72; Ladygina et al., '78). In the adult the visual cortex is relatively thin (average about 1,300 μm) for so large a brain (fixed brain weight for a typical adult in our series was 1,330 g). Layers I, III, and VI are wide and represent three-quarters of the total cortical thickness. Layer I contains few cell bodies, while III and VI have a variety of pyramidal and nonpyramidal neurons. Layers II and V are narrow and contain striking palisades of darkly staining pyramidal cells that are particularly large in layer V. No clearly demarcated layer IV is present in the adult dolphin visual cortex. Many of the neurons identified with the Golgi technique are typical of pyramids in other mammals, with a single apical dendrite and a bouquet of basal dendrites, mostly highly spiny. Others are unusual in having bifurcated or oblique apical dendrites. Typical large and small spiny and nonspiny stellates are also found, mainly in layers III and VI. In addition various forms of spindle-shaped, bipolar and multipolar neurons are found in most layers.
An 18-day-old brain shows signs of immaturity in its visual cortex. It is thinner (970 μm) and on average its neurons are smaller, paler, and more densely packed. Especially the pyramids of layer V are much smaller than in the adult. Also, a distinct “granular” band occurs between layers III and V and seems to be a rudimentary layer IV. At 3 years of age most of the adult features have developed, but layer IV is still detectable.
No striking differences were observed in cell and fibre architecture between the cortex of the lateral gyrus and that of the so-called “calcarine” area that has also been considered as “visual.”
We conclude that, although different in many respects from other mammalian visual cortices, that of the dolphin is apparently well developed and differentiated.