Organization of the inferotemporal cortex in the macaque monkey: Connections of areas PITv and CITvp

Visual cortex of macaque monkeys consists of a large number of cortical areas that span the occipital, parietal, temporal, and frontal lobes and occupy more than half of cortical surface. Although considerable progress has been made in understanding the contributions of many occipital areas to visual perceptual processing, much less is known concerning the specific functional contributions of higher areas in the temporal and frontal lobes. Previous behavioral and electrophysiological investigations have demonstrated that the inferotemporal cortex (IT) is essential to the animal's ability to recognize and remember visual objects. While it is generally recognized that IT consists of a number of anatomically and functionally distinct visual-processing areas, there remains considerable controversy concerning the precise number, size, and location of these areas. Therefore, the precise delineation of the cortical subdivisions of inferotemporal cortex is critical for any significant progress in the understanding of the specific contributions of inferotemporal areas to visual processing. In this study, anterograde and/or retrograde neuroanatomical tracers were injected into two visual areas in the ventral posterior and central portions of IT (areas PITv and CITvp) to elucidate the corticocortical connections of these areas with well known areas of occipital cortex and with less well understood regions of inferotemporal cortex. The locations of injection sites and the delineation of the borders of many occipital areas were aided by the pattern of interhemispheric connections, revealed following callosal transection and subsequent labeling with HRP. The resultant patterns of connections were represented on two-dimensional computational (CARET) and manual cortical maps and the laminar characteristics and density of the projection fields were quantified. The laminar and density features of these corticocortical connections demonstrate thirteen anatomically distinct subdivisions or areas distributed within the superior temporal sulcus and across the inferotemporal gyrus. These results serve to refine previous descriptions of inferotemporal areas, validate recently identified areas, and provide a new description of the hierarchical relationships among occipitotemporal cortical areas in macaques. ^

Developmental Changes in the Structure and Composition of the Postsynaptic Density

The development of the brain and its underlying circuitry is dependent on the formation of trillions of chemical synapses, which are highly specialized contacts that regulate the flow of information from one neuron to the next. It is through these synaptic connections that neurons wire together into networks capable of performing specific tasks, and activity-dependent changes in their structural and physiological state is one way that the brain is thought to adapt and store information. At the ultrastructural level, developmental and activity-dependent changes in the size and shape of dendritic spines have been well documented, and it is widely believed that structural changes in spines are a hallmark sign of synapse maturation and alteration of synaptic physiology. While changes in spine structure have been studied extensively, changes in one of its most prominent components, the postsynaptic density (PSD), have largely evaded observation. The PSD is a protein-rich organelle on the cytoplasmic side of the postsynaptic membrane, where it sits in direct opposition to the presynaptic terminal. The PSD functions both to cluster neurotransmitter receptors at the cell surface as well as organize the intracellular signaling molecules responsible for transducing extracellular signals to the postsynaptic cell. Much is known about the chemical composition of the PSD, but the structural arrangement of its molecular components is not well documented. Adding to the difficulty of understanding such a complex mass of protein machinery is the fact that its protein composition is known to change in response to synaptic activity, meaning that its structure is plastic and no two PSDs are identical. Here, immuno-gold labeling and electron tomography of PSDs isolated throughout development was used to track changes in both the structure and molecular composition of the PSD. State-of-the-art cryo-electron tomography was used to study the fine structure of the PSD during development, and provides an unprecedented glimpse into its molecular architecture in an un-fixed, unstained and hydrated state. Through this analysis, large structural and compositional changes are apparent and suggest a model by which the PSD is first assembled as a mesh-like lattice of proteins that function as support for the later recruitment of various PSD components. Spatial analysis of the recruitment of proteins into the PSD demonstrated that its assembly has an underlying order.

The effects of population density on the spread of disease

The objective of this study is to identify the relationship between population density and the initial stages of the spread of disease in a local population. This study proposes to concentrate on the question of how population density affects the distribution of the susceptible individuals in a local population and thus affects the spread of the disease, measles. Population density is measured by the average of the number of contacts with susceptible individuals by each individual in the population during a fixed-length time period. The term “contact with susceptible individuals” means sufficient contact between two people for the disease to pass from an infectious person to a susceptible person. The fixed-length time period is taken to be the average length of time an infected person is infectious without symptoms of the disease. For this study of measles, the time period will be seven days. ^ While much attention has been given to modeling the entire epidemic process of measles, attempts have not been made to study the characteristics of contact rates required to initiate an epidemic. This study explores the relationship between population density, given a specific herd immunity rate in the population, and initial rate of the spread of the disease by considering the underlying distribution of contacts with susceptibles by the individuals in the population. ^ This study does not seek to model an entire measles epidemic, but to model the above stated relationship for the local population within which the first infective person is introduced. This study describes the mathematical relationship between population density parameters and contact distribution parameters. ^ The results are displayed in graphs that show the effects of different population densities on the spread of disease. The results support the idea that the number of new infectives is strongly related to the distribution of susceptible contacts. The results also show large differences in the epidemic measures between populations with densities equal to four versus three. ^