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The branching structure of neurones is thought to influence patterns of connectivity and how inputs are integrated within the arbor. Recent studies have revealed a remarkable degree of variation in the branching structure of pyramidal cells in the cerebral cortex of diurnal primates, suggesting regional specialization in neuronal function. Such specialization in pyramidal cell structure may be important for various aspects of visual function, such as object recognition and color processing. To better understand the functional role of regional variation in the pyramidal cell phenotype in visual processing, we determined the complexity of the dendritic branching pattern of pyramidal cells in visual cortex of the nocturnal New World owl monkey. We used the fractal dilation method to quantify the branching structure of pyramidal cells in the primary visual area (V1), the second visual area (V2) and the caudal and rostral subdivisions of inferotemporal cortex (ITc and ITr, respectively), which are often associated with color processing. We found that, as in diurnal monkeys, there was a trend for cells of increasing fractal dimension with progression through these cortical areas. The increasing complexity paralleled a trend for increasing symmetry. That we found a similar trend in both diurnal and nocturnal monkeys suggests that it was a feature of a common anthropoid ancestor.

Previously it has been shown that the branching pattern of pyramidal cells varies markedly between different cortical areas in simian primates. These differences are thought to influence the functional complexity of the cells. In particular, there is a progressive increase in the fractal dimension of pyramidal cells with anterior progression through cortical areas in the occipitotemporal (OT) visual stream, including the primary visual area (V1), the second visual area (V2), the dorsolateral area (DL, corresponding to the fourth visual area) and inferotemporal cortex (IT). However, there are as yet no data on the fractal dimension of these neurons in prosimian primates. Here we focused on the nocturnal prosimian galago (Otolemur garnetti). The fractal dimension (D), and aspect ratio (a measure of branching symmetry), was determined for 111 layer III pyramidal cells in V1, V2, DL and IT. We found, as in simian primates, that the fractal dimension of neurons increased with anterior progression from V1 through V2, DL, and IT. Two important conclusions can be drawn from these results: (1) the trend for increasing branching complexity with anterior progression through OT areas was likely to be present in a common primate ancestor, and (2) specialization in neuron structure more likely facilitates object recognition than spectral processing.

The primate hippocampus is needed for spatial memory tasks in which the location of objects must be remembered, and in which the location of places where responses must be made are to be remembered. It is also involved in some non-spatial memory tasks such as recognition memory for visual stimuli. Some single neurons in the hippocampus of macaque monkeys performing these memory tasks respond to the positions in space of the stimuli; or to a combination of a non-spatial stimulus with a spatial response when the monkey must learn to make spatial responses to visual stimuli, with the responses of these neurons becoming modified during this type of learning; or to only novel visual stimuli in the recognition memory task. On the basis of these and related findings the hypothesis is suggested that the importance of the hippocampus in spatial and other memories is that it can rapidly form “episodic” representations of information originating from many different areas of the cerebral association cortex. These neurophysiological findings have been combined with information on the microanatomy of the hippocampus, and on long-term potentiation in the hippocampus to produce a neuronal network theory of the operation of the hippocampus. A key aspect of the theory is that the CA3 pyramidal cells with their 4% interconnectivity and Hebb-modifiable synapses implement an autoassociation memory which provides the basis for “episodic” memories which are required for many spatial and non-spatial memory functions.