Interscan Intervals Research Articles

Vigilance and predator detection vary between avian species with different visual acuity and coverage

Interspecific variations in avian visual systems have been suggested to influence antipredator strategies, yet little empirical evidence exists on how morphological and ecological factors associated with visual properties can constraint predator detection. We investigated antipredator responses (predator detection probabilities and vigilance behavior) in 2 species with different visual properties (European starlings, Sturus vulgaris, have higher acuity and wider blind areas at the rear of their heads than house sparrows, Passer domesticus) in relation to distance to the predator, body posture, and head orientation. Visual acuity may affect the distance at which a predator is detected; while the size of the blind area may influence the body postures and head orientation with the highest predator detection probabilities. Distance to the decreased predator detection probabilities of house sparrows (lower acuity) but not those of European starlings. Certain body postures and head orientations, influenced both species despite the interspecific differences in visual field configuration. Times allocated to scanning were similar, but European starlings had longer head-up scan bouts likely to enhance scanning coverage, whereas house sparrows had higher head-up scan rates probably to reduce the length of interscan intervals. We discuss alternative interpretations; however, our findings suggest that sensory configurations may limit the effectiveness of some antipredator strategies under certain ecological conditions, which has implications for understanding the evolution of different behavioral mechanisms that reduce predation risk.

Can rate of brain atrophy in multiple sclerosis be explained by clinical and MRI characteristics?

Introduction Multiple sclerosis (MS) is characterized, besides focal lesions, by brain atrophy. The determinants of atrophy rates in individual patients are poorly understood. Aim This study investigated the predictive value of clinical and magnetic resonance imaging (MRI) factors, including short-term changes thereof, for concurrent and future atrophy evolution using Spearman’s rank correlations and stepwise multiple linear regression. Methods We retrospectively identified a group of 115 active, early relapsing-remitting (RR) patients relatively homogeneous in terms of disease course and MRI activity compared to a second group of 96 patients with broader spectrum of MS phenotypes and inactive scans. All patients had undergone three MRI investigations with interscan intervals of at least 12 and 24 months, respectively. Results In the RR patients, 23% of variance in concurrent atrophy rates (over the first interval) could be explained by the combination of baseline T2 lesion volume and change in EDSS score over the first interval, whereas only 6% in future atrophy rates (over the second interval) was explained. In the heterogeneous group, 20.2% of the variance in future atrophy rates could be explained, but slightly less in concurrent atrophy rates (16.2%). Conclusion We concluded that variance in brain atrophy rates can partially be explained by clinical and MRI measures of disease. Future atrophy rates in individual MS patients are difficult to predict even when including previous atrophy rates.

Scanning pattern of greater rheas, Rhea americana: collective vigilance would increase the probability of detecting a predator

Many models using vigilance to predict the probability of detecting an approaching predator assumes that prey scanning events should be produced at random. Consequently, the length of intervals among successive scans must follow a negative exponential distribution. We analyzed the scanning behavior of the greater rhea, Rhea americana, which is a gregarious, flightless bird, in eastern Argentina. We investigated whether individual and/or collective scanning departed from random and whether this departure varied with group size. We used two simulation models based on observed scanning sequences to assess the effectiveness of vigilance on the individual and collective level when faced with an opportunistic or stalking predator. The analysis of 59 behavioral sequences of wild greater rheas foraging solitary or in groups of two to six or more individuals revealed that the inter-scan length of individual sequences significantly departed from random. In contrast, inter-scan intervals for collective vigilance were shorter than individual ones, but only fit the random expectation for groups of two and five individuals. Models showed that collective vigilance could increase the probability of detecting a predator, thereby reducing their vulnerability, independent of whether the predator uses a stalking or opportunistic approaching strategy.

IC-P3-171: Scan-rescan reliability of semi-automated brain region extraction (SABRE)

Background: The clinical applicability of longitudinal MRI studies requires validation of the reliability of the scan acquisition and tools used in the generation of data. A scan-rescan analysis using repeated scans with short interscan time intervals is important to accurately measure the reliability of the imaging data being acquired, and increase confidence in the results. Regional tissue classification techniques have potential to elucidate brain-behaviour relationships in both longitudinal and cross-sectional research. Our automated, T1-based segmentation protocol provides gray matter, white matter, and cerebrospinal-fluid (CSF) whole brain volumes after “head from brain” removal (Kovacevic et al.,2002). Semi-Automated Brain Region Extraction (SABRE) parcellates the brain into 26 regions of interest (ROI’s) (13 bilaterally), based on 15 manually identified landmarks (Dade et al.,2004). The output gives individual compartment volumes for gray, white, CSF and ventricular CSF for each of the 26 ROI’s. Because this method reliably captures all subdural/sulcal CSF, it is an ideal tool for measuring individualized and group atrophic changes regionally and globally while, unlike most automated methods, still respecting individual anatomical variability. Methods: Twenty-one participants, ranging in age from 22 to 91 years were scanned on a 1.5T GE signa scanner on two occasions with a mean interval of 11 days, using standardized 3D T1weighted, T2-weighted, and Proton Density acquisitions at each session. Two operators, blinded to subject identity and scan order performed the procedure on each subject. Results: Shrout & Fleiss Model-3 Intraclass Correlation Coefficients (ICC) were generated for each of the 26 ROI’s, comparing the two different scanning sessions. The mean ICC for all SABRE regions was 0.94, ranging from 0.83 (right anterior basal ganglia and thalamus) to 0.98, with 22 of 26 regions greater than 0.90, indicating highly reproducible results (Shrout & Fleiss,1979). Conclusions: The SABRE method appears highly reproducible giving confidence that it could be used in longitudinal studies. For hypothesis-driven brainbehaviour studies, this method offers a viable alternative between fullyautomated techniques, which are fast but do not account for individual variability and frequently do not include subdural CSF spaces, and fullymanual techniques which are accurate but time consuming, require extensive training, and are less reproducible. IC-P3-172 BRAIN MORPHOLOGY IN ELDERLY AFRICANAMERICANS, CARIBBEAN HISPANICS, AND CAUCASIANS FROM NORTHERN MANHATTAN

Increased hippocampal atrophy rates in AD over 6 months using serial MR imaging

We measured hippocampi on baseline-, 6- and 12-month scans in a group of AD ( n = 36) and control subjects ( n = 20). We found that mean annualised atrophy rates using 6-month intervals were comparable at a group level to those generated from a 12-month interval. Higher variance was seen using shorter intervals, although this was only significant in the control group. This has implications where shorter inter-scan intervals may be advantageous, such as rapid diagnosis, and tracking of disease progression including in a clinical trial.

Nonrandom patterns of vigilance in flocks of the greater flamingo, Phoenicopterus ruber ruber

Models of vigilance in groups usually assume instantaneous randomness in scan initiation and sequential randomness in the duration of successive interscans, thus predicting a negative exponential distribution of interscan durations and no predictability in the duration of successive interscans. However, recent models suggest that scanning should instead occur regularly rather than randomly when foragers are threatened by nonobservant predators, thus predicting interscans of constant duration and predictable interscan sequences. I examined whether vigilance patterns departed from instantaneous and sequential randomness in greater flamingos wintering in a tropical lagoon complex in Venezuela. Predation risk in this large species is almost nonexistent, and it is argued that disturbance by people, the most probable threat, should best be detected using a regular pattern of scanning. As predicted, the distribution of interscan durations showed a strong central tendency and differed significantly from the expected negative exponential distribution. Interscan intervals of similar duration occurred in succession more often than predicted by chance. Despite obvious departures from instantaneous and sequential randomness, substantial variability remained in the distribution of interscan durations, and several interscan sequences were quite unpredictable. I relate these findings to potential variation in food supply within the confine of a flock and pre-emptive scans to detect potential threats.

Predator detection and avoidance by starlings under differing scenarios of predation risk

Practically all animals must find food while avoiding predators. An individual's perception of predation risk may depend on many factors, such as distance to refuge and group size, but it is unclear whether individuals respond to different factors in a similar manner. We tested whether flocks of foraging starlings responded in the same way to an increased perception of predation risk by assessing three factors: (1) neighbor distances, (2) habitat obstruction, and (3) recent exposure to a predator. We found that in all three scenarios of increased risk, starlings reduced their interscan intervals (food-searching bouts), which increased the frequency of their vigilance periods. We then examined how one of these factors, habitat obstruction, affected escape speed by simulating an attack with a model predator. Starlings were slower to respond in visually obstructed habitats (long grass swards) and slower when they had their head down in obstructed habitats than when they had their head down in open habitats. In addition, reaction times were quicker when starlings could employ their peripheral fields of vision. Our results demonstrate that different sources of increased risk can generate similar behavioral responses within a species. The degree of visibility in the physical and social environment affects both the actual and perceived risk of predation.

Three-dimensional reconstruction of the biliary tract using spiral computed tomography.

Spiral computed tomography (SCT) is a recently introduced technique which enables scanning of one large volume with no interscan intervals. Three-dimensional reconstruction using this technique is emerging as an effective means of identifying complex anatomical relationships. Ninety-two patients with cholecystolithiasis including ten with choledocholithiasis were investigated. All underwent SCT after oral and intravenous infusion cholangiography (IVC-SCT) and 22 were also examined by endoscopic retrograde cholangiography (ERC). IVC-SCT showed significantly more anatomical detail than conventional intravenous infusion cholangiography (IVC). The junction between the cystic duct and the common bile duct could be identified before surgery in 19 of 22 patients undergoing ERC, in 18 of 22 receiving IVC-SCT and in five of 22 undergoing IVC. Significant differences were noted between IVC-SCT, ERC and conventional IVC. Three-dimensional surface reconstruction of the biliary tract is useful in the evaluation of anatomical relationships between the cystic duct and the common bile duct (clearly seen in 86 per cent of cases). Three-dimensional reconstruction using SCT scanning is useful for anatomical assessment before laparoscopic cholecystectomy.

Evolutionary stability of vigilance coordination among social foragers.

Coordination can greatly improve the efficiency of anti-predatory vigilance scans by increasing predator detection for a constant proportion of time spent vigilant. However, it has been rarely found in nature and most studies have detected or assumed independent scanning by group members. In this study, we analysed the functional consequences of the coordinated alternation of vigilance scanning by group foragers. We introduce coordination by assuming that interscan intervals (ISIs) follow a modified gamma distribution. Depending on the parameters of the distribution, successive scans can be evenly spaced (coordinated scanning) or may present a high overlap (uncoordinated scanning). Comparing evolutionarily stable strategies for animals that do not coordinate their scanning with animals that do coordinate their anti-predator behaviour shows that coordination has a marked effect on survival probability. Moreover, the coordinating strategy is quite robust against mutants that scan independently with exponential distributions of ISIs. However, coordination breaks down when animals can continuously adjust their level of coordination by deciding the proportion of time they spend monitoring the behaviour of other group members. In this case, coordination is only evolutionarily stable if it can be very easily achieved.

Why are scanning patterns so variable? An overlooked question in the study of anti‐predator vigilance

Anti‐predator vigilance describes how animals scan their environments for predators. For mathematical simplicity, theories of anti‐predator vigilance have generally assumed that animals initiate scans with a constant probability for each unit of time spent feeding. Initiating scans in such a fashion would produce a wide distribution of intervals between scans. Because so much variation was assumed by theory, observations of variation have received little consideration. We show that the original theories of vigilance contain a conceptual discrepancy between the assumed predator and anti‐predator tactics. Modified versions of the theory allow variation only under restrictive circumstances that seem unlikely to apply in nature. Plausible reasons for the observed variation in interscan interval are that theories of vigilance assume the wrong predator tactics or the wrong details of predator detection. These possibilities lead to testable predictions about how distributions of interscan intervals should change under different threat and detection conditions.

Single-Trial Variability in Event-Related BOLD Signals

Most current analysis methods for fMRI data assume a priori knowledge of the time course of the hemodynamic response (HR) to experimental stimuli or events in brain areas of interest. In addition, they typically assume homogeneity of both the HR and the non-HR “noise” signals, both across brain regions and across similar experimental events. When HRs vary unpredictably, from area to area or from trial to trial, an alternative approach is needed. Here, we use Infomax independent component analysis (ICA) to detect and visualize variations in single-trial HRs in event-related fMRI data. Six subjects participated in four fMRI sessions each in which ten bursts of 8-Hz flickering-checkerboard stimulation were presented for 0.5-s (short) or 3-s (long) durations at 30-s intervals. Five axial slices were acquired by a Bruker 3-T magnetic resonance imager at interscan intervals of 500 ms (TR). ICA decomposition of the resulting blood oxygenation level-dependent (BOLD) data from each session produced an independent component active in primary visual cortex (V1) and, in several sessions, another active in medial temporal cortex (MT/V5). Visualizing sets of BOLD response epochs with novel BOLD-image plots demonstrated that component HRs varied substantially and often systematically across trials as well as across sessions, subjects, and brain areas. Contrary to expectation, in four of the six subjects the V1 component HR contained two positive peaks in response to short-stimulus bursts, while components with nearly identical regions of activity in long-stimulus sessions from the same subjects were associated with single-peaked HRs. Thus, ICA combined with BOLD-image visualization can reveal dramatic and unforeseen HR variations not apparent to researchers analyzing their data with event-related response averaging and fixed HR templates.

Prey scan at random to evade observant predators.

Anti-predator scans by animals occur with very irregular timing, so that the initiation of scans resembles a random, Poisson-like, process. At first sight, this seems both dangerous (predators could exploit the long intervals) and wastefull (scans after very short intervals are relatively uninformative). We explored vigilance timing using a new model that allows both predators and prey to vary their behaviour. Given predators that attack at random with respect to prey behaviour, constant inter-scan intervals minimize predation risk. However, if prey scan regularly to minimize their risk from randomly attacking predators, they become more vulnerable to predators that initiate attacks when the inter-scan intervals begin. If, in order to defeat this tactic, prey choose extremely variable inter-scan intervals, they become more vulnerable to predators who wait for long intervals before launching attacks. Only if predators can monitor the variability of inter-scan intervals and either attack immediately (if variability is too low) or wait for long intervals to attack (if variability is too high) does the empirically observed pattern of Poisson-like scanning become the optimal prey strategy.

Randomness, chaos and confusion in the study of antipredator vigilance

The study of antipredator vigilance underwent a rapid and relatively recent synthesis 15–20 years ago. During the rise of behavioral ecology and sociobiology, researchers began to measure how often animals looked up from feeding. Subsequently, the field of vigilance crystallized quickly around a few striking results and an elegant theory. The convenient mathematical assumptions of this original theory continue to channel researchers' attentions today. Although data tend to match these assumptions — flock members scan independently, vigilance sequences are essentially unpredictable, and interscan intervals are highly variable — the assumptions themselves are difficult to justify. Some of our basic ideas about vigilance require detailed re-examination.

Comparison of PET [15O]Water Studies with 6- and 10-Minute InterScan Intervals: Single-Subject and Group Analyses

Comparison of PET [15O]Water Studies with 6- and 10-Minute InterScan Intervals: Single-Subject and Group Analyses

Does harvesting arable crops influence the behaviour of the European hare Lepus europaeus?

Behavioural changes of 20 radio‐tagged European hares Lepus europaeus were analysed in relation to the harvesting of summer crops under intensive large‐scale farming in northeastern France. Focal sampling was also performed to estimate activity budgets and to analyse vigilance sequences according to group size and pre/post‐harvest factors. Night‐time ranges were 40% larger than day‐time ranges; the total home range size was 190 ± 53 ha. Both night‐time and day‐time range sizes were independent of the pre/post harvest factor, although pre‐harvest range centres shifted after harvest and the overlap areas were avoided. Habitat use within ranges was independent of the day/night and pre/post harvest factors, and hares made a larger use of cultivated areas than expected by chance (P < 0.01). Landscape diversity in the home range of hares was lower after harvest than before, but the animals did not maximise their access to crop diversity. Time spent scanning was negatively correlated with group size (r = ‐0.32, P < 0.005) and vigilance levels averaged lower after than before harvest (P < 0.05). Inter‐scan intervals (ISIs) were non‐randomly distributed and periodicity in the scanning behaviour was identified using spectral analysis. Hares foraging alone displayed a stronger cyclic pattern in vigilance sequences (i) than hares foraging communally (P = 0.043), and (ii) after than before harvest (P = 0.047). These results were analysed in connection with increased predation risks and shifts in resource distribution.

A real-time response of vigilance behaviour to changes in group size

Sequences of vigilance behaviour were obtained for preening crested terns, Sterna bergii. Paired comparisons were made between the vigilance of individuals before and after flock size changes. As predicted, individuals responded to flock size increases by increasing their inter-scan intervals (reducing vigilance), and to flock size decreases by decreasing inter-scan intervals. Scan durations did not vary with flock size. However, arrivals and departures of birds were immediately followed by long scans. The study represents a more powerful test of flock size effects than has previously been presented in correlational studies.

When to scan: an analysis of predictability in vigilance sequences using autoregression models

Abstract. The temporal organization of vigilance behaviour was examined. The durations of inter-scan intervals of preening sanderlings, Calidris alba , were not predictable from any of the preceding inter-scan interval durations. However, examination of the differences between successive inter-scan interval durations provided a means of describing the sequences: the differences were predictable using models based on negative autocorrelations. Similar results were obtained for sequences from other species. The predictability of vigilance sequences is therefore dependent upon the level of analysis. Some causal and functional interpretations of the derived autoregression models are discussed. Simulations showed that the models could account for periodicities previously reported without invoking oscillatory mechanisms.

Relationships between scan and interscan durations in three avian species

Foraging birds interrupt a feeding bout to scan the environment for potential predators; their behaviour can therefore be described as an alternating sequence of scans (S) and interscan intervals (I). The ratio of I to S oscillates rhythmically during a foraging period; this is consistent across three avian species (Barbary dove, Streptopelia risoria; chough, Pyrrhocorax pyrrhocorax; and goldfinch, Carduelis tristis) and in a variety of social and foraging contexts. Thus, although there are differences in the mean ratio (possibly related to differences in food-handling time), all samples show the same pattern of cycling in the ratio of I to S. In addition to these oscillations, the length of an interscan interval tends to vary in direct proportion to the length of the preceding scan in solitary individuals (Barbary doves); however, in flock foragers (choughs and goldfinches) there is an inverse relationship between I and S.

Temporal and Sequential Structure of Vigilance Behavior of Wild Boars (Sus scrofa)

It has been argued that animals alternating between feeding and vigilance behavior should scan randomly. However, the spectral analysis of individual sequences of scans and interscan intervals obtained from wild boars ( Sus scrofa ) shows the existence of significant periodicities; the scans and interscan intervals oscillate regularly among long and short events. For all the examined series, the serial patterns were similar. However, the temporal patterns vary strongly among individuals and depend upon the number of conspecifics present. These results support the assumption of the existence of general behavioral processes, presumably involving self-sustaining endogenous oscillators.

Pharmacokinetic Parameters in CNS Gd-DTPA Enhanced MR Imaging

Dynamic MR imaging can be used to study tissue perfusion and vascular permeability. In the present article a procedure for dynamic MR is presented, which (a) accurately resolves the fast kinetics of tissue response during and after intravenous infusion of the paramagnetic contrast medium Gd-DTPA and (b) yields a linear relationship between the measured MR signal and the Gd-DTPA concentration in the tissue. According to these features, the measured signal-time curves can be analyzed within the framework of pharmacokinetic modeling. Tissue response has been parameterized using a linear two-compartment open model, with only negligible effects of the peripheral compartment on the central compartment. The three model parameters were fitted to the signal-time data pixel by pixel, based on a set of 64 rapid SE images (SE 100/10 ms, image scan time 13 s, interscan intervals 11 s). This makes it possible to construct parameter images, whereby structures become visible that cannot be distinguished in conventional Gd-DTPA enhanced MR. As a clinical example, the approach is discussed in a case of glioblastoma.