Dynamical mechanism for sharp orientation tuning in an integrate-and-fire model of a cortical hypercolumn

Orientation tuning in a ring of pulse-coupled integrate-and-fire (IF) neurons is analyzed in terms of spontaneous pattern formation. It is shown how the ring bifurcates from a synchronous state to a non-phase-locked state whose spike trains are characterized by clustered but irregular fluctuations of the interspike intervals (ISIs). The separation of these clusters in phase space results in a localized peak of activity as measured by the time-averaged firing rate of the neurons. This generates a sharp orientation tuning curve that can lock to a slowly rotating, weakly tuned external stimulus. Under certain conditions, the peak can slowly rotate even to a fixed external stimulus. The ring also exhibits hysteresis due to the subcritical nature of the bifurcation to sharp orientation tuning. Such behavior is shown to be consistent with a corresponding analog version of the IF model in the limit of slow synaptic interactions. For fast synapses, the deterministic fluctuations of the ISIs associated with the tuning curve can support a coefficient of variation of order unity.<br/

Aspects of the biology of the Amazonian dolphins genus Inia and Sotalia fluviatilis

SIGLEAvailable from British Library Document Supply Centre-DSC:D063625 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Number, seasonal movements, and residency characteristics of river dolphins in an Amazonian floodplain lake system

The size and structure of a community of Amazon river dolphins or botos, Inia geoffrensis (de Blainville, 1817), was investigated using boat surveys and long-term observations of recognisable animals. Year-round, some 260 botos occurred in or near the 225-km(2) Mamiraua varzea floodplain lake system, of which half were permanent residents by our definition. Seasonal variation in water levels influenced distribution between habitats but not the overall number of botos. Ninety percent of marked botos encountered within the lake system were permanent residents. There appeared to be a cline in site fidelity between those that always live in or near the system and those that visit at intervals of years. We estimated that 270 botos were "significant users" of the lake system (i.e., occurred within it for sufficient periods in a year to be observed at least once) and that many others visited for short periods. Individuals moved many tens to hundreds of kilometres along the rivers, but there was no broad-scale seasonal migration. The boto population of the central Amazon, at least, may be structured on the basis of floodplain lake systems, with extensive animal movement between systems. We estimate that 13 000 botos occur in the 11240 km(2) Mamirauna Sustainable Development Reserve, which covers an estimated 11%-18% of varzea habitat in Brazil

Amazon river dolphins ( Inia geoffrensis ) modify biosonar output level and directivity during prey interception in the wild

Toothed whales have evolved to live in extremely different habitats and yet they all rely strongly on echolocation for finding and catching prey. Such biosonar-based foraging involves distinct phases of searching for, approaching and capturing prey, where echolocating animals gradually adjust sonar output to actively shape the flow of sensory information. Measuring those outputs in absolute levels requires hydrophone arrays centred on the biosonar beam axis, but this has never been done for wild toothed whales approaching and capturing prey. Rather, field studies make the assumption that toothed whales will adjust their biosonar in the same manner to arrays as they will when approaching prey. To test this assumption, we recorded wild botos (Inia geoffrensis) as they approached and captured dead fish tethered to a hydrophone in front of a star-shaped seven-hydrophone array. We demonstrate that botos gradually decrease interclick intervals and output levels during prey approaches, using stronger adjustment magnitudes than predicted from previous boto array data. Prey interceptions are characterised by high click rates, but although botos buzz during prey capture, they do so at lower click rates than marine toothed whales, resulting in a much more gradual transition from approach phase to buzzing. We also demonstrate for the first time that wild toothed whales broaden biosonar beamwidth when closing in on prey, as is also seen in captive toothed whales and bats, thus resulting in a larger ensonified volume around the prey, probably aiding prey tracking by decreasing the risk of prey evading ensonification

Riverine habitat preferences of botos (Inia geoffrensis) and tucuxis (Sotalia fluviatilis) in the central Amazon

The distribution and density of the Amazon's two contrasting endemic dolphins-boto, or Amazon river dolphin, Inia geoffrensis, and tucuxi, Sotalia fluviatilis-were examined on two adjoining large rivers in western Brazil. Using a 17-m river boat as a sightings platform, strip transects were used to cover areas within 150 m of the river margin and line transects were used in all other areas. Highest densities of both dolphins occurred near the margin, and lowest in the center of rivers. Seven different habitats were identified along river margins. The boto and the tucuxi differed in some elements of habitat choice, but they shared a preference for areas with diminished current and where two channels joined. Neither species favored the two most common edge types in this region of the Amazon-mud banks and flooded forest margins. Overall, the most preferred habitat type was the least common, and known as "meeting of the waters." In these areas a channel of sediment-rich white water meets one carrying acidic black water;, the resultant mixing produces particularly productive, and obviously attractive, conditions for dolphins. These results demonstrate that Amazonian dolphins selectively Occur in areas known to be favored for gill net deployment by local fishermen, and may explain why entanglement is apparently a common cause of mortality

Object carrying as socio-sexual display in an aquatic mammal

Amazon river dolphins or botos (Inia geoffrensis Blainville) were observed carrying objects in 221 social groups over a 3-year study period. Sticks, branches and clumps of grass were taken from the water surface and often repeatedly thrashed or thrown. Lumps of hard clay were collected from the river bed and held in the mouth while the carrier rose slowly above the surface and submerged again. Carriers were predominantly adult males and less often subadult males. Adult females and young dolphins rarely carried objects. Groups of dolphins in which object carrying occurred were differentially large and comprised a greater proportion of adult males and adult females. Aggression, mostly between adult males, was significantly associated with object carrying. The behaviour occurred year-round, with peaks in March and July. A plausible explanation of the results is that object carrying by adult males is aimed at females and is stimulated by the number of females in the group, while aggression is targeted at adult males and is stimulated by object carrying in the group. We infer that object carrying in this sexually dimorphic species is socio-sexual display. It is either of ancient origin or has evolved independently in several geographically isolated populations

Are birth weight and maternal smoking during pregnancy associated with malnutrition and excess weight among school age children?

In the late 1980's child malnutrition was still prevalent in Brazil, and child obesity was beginning to rise in the richest regions of the country. To assess the extent of the nutritional transition during the period and the influence of birth weight and maternal smoking on the nutritional condition of schoolchildren, we estimated the prevalence of excess weight and malnutrition in a cohort of Brazilian schoolchildren from 1987 to 1989. We calculated the body mass index (BMI) of 8- to 10-year-old schoolchildren born in Ribeirão Preto in 1978/79. We considered children with a BMI <5th percentile (P5) to be malnourished, children with P5³BMI<P85 to be thin and normal, and children with BMI ³P85 to be overweight. We evaluated the association of these nutritional disorders with birth factors (infant weight, sex, preterm delivery, number of pregnancies, maternal smoking during pregnancy, marital status, and schooling) and type of school using nominal logistic regression. A total of 2797 schoolchildren were evaluated. There was a significant prevalence of malnutrition (9.5%) and excess weight already tended to increase (15.7%), while 6.4% of the children were obese. Excess weight was more prevalent among children attending private schools (odds ratio, OR = 2.27) and firstborn children (OR = 1.69). Maternal smoking during pregnancy protected against malnutrition (OR = 0.56), while children with lower birth weight were at higher risk for malnutrition (OR = 4.23). We conclude that a nutritional transition was under way while malnutrition was still present, but excess weight and related factors were already emerging