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PubMed1999

A virtual robot to model the use of regenerated legs in a web-building spider.

The garden cross orb-spider, Araneus diadematus, shows behavioural responses to leg loss and regeneration that are reflected in the geometry of the web's capture spiral. We created a virtual spider robot that mimicked the web construction behaviour of thus handicapped real spiders. We used this approach to test the correctness and consistency of hypotheses about orb web construction. The behaviour of our virtual robot was implemented in a rule-based system supervising behaviour patterns that communicated with the robot's sensors and motors. By building the typical web of a nonhandicapped spider our first model failed and led to new observations on real spiders. We realized that in addition to leg position, leg posture could also be of importance. The implementation of this new hypothesis greatly improved the results of our simulation of a handicapped spider. Now simulated webs, like the real webs of handicapped spiders, had significantly more gaps in successive spiral turns compared with webs of nonhandicapped spiders. Moreover, webs built by the improved virtual spiders intercepted prey as well as the digitized real webs. However, the main factors that affected web interception frequency were prey size, size of capture area and individual variance; having a regenerated leg, surprisingly, was relatively unimportant for this trait. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1999

The effect of group size on the foraging behaviour of juvenile coho salmon: reduction of predation risk or increased competition?

Animals often increase their apparent willingness to incur risk when foraging in groups, presumably because group membership reduces an individual's risk of predation. As group size increases, however, competition for resources may also increase, resulting in a decrease in the quantity of resources available to each member of the group. When resources are scarce, individuals might be expected to increase their foraging effort in an attempt to increase their share. Such increases in effort will often appear to increase an individual's risk of predation. Thus, increased competition may contribute to the frequently observed relationship between risk-taking behaviour and group size. To date, no experimental assessment of the relative importance of these two mechanisms exists. We argue that to differentiate between the hypotheses of 'risk reduction' and 'increased competition', it is necessary to quantify the effect of predation risk on the form of the relationship between group size and risk-taking behaviour, and thus, to manipulate both group size and predation risk. We conducted an experiment to determine the relative importance of risk reduction and increased competition to the foraging decisions of juvenile coho salmon, Oncorhynchus kisutch. We recorded the foraging behaviour of 18 focal individuals in the presence and absence of a predator, and in the company of zero, one and three conspecifics. As group size increased from one to four, focal fish captured more prey items, ventured closer to the feeder (and predator) to intercept them, and decreased their use of cover. Furthermore, although focal individuals captured fewer prey items and intercepted them further from the feeder in the presence of the predator than in its absence, the form of the relationship between risk-taking behaviour and group size was not affected by the level of predation risk. The results of our experiment are consistent with the interpretation that increases in risk-taking behaviour with group size occurred primarily as a consequence of increased competition for scarce resources. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed2000

Foraging behaviour of Atta cephalotes (leaf-cutting ants): an examination of two predictions for load selection.

Two mechanisms have been proposed to explain how colony-level foraging performance of leaf-cutting ants can be maximized when workers harvest leaf fragments of a size that does not maximize their individual performance. Each mechanism predicts that ants will adjust the size of leaf fragments between starting a foraging bout and establishing full traffic between the nest and foraging site, but the two models predict shifts in opposite directions. I examined fragment sizes at the start of daily foraging in five field colonies of Atta cephalotes in Costa Rica and detected an obvious shift in only one case. More shifts were detected when the small and large ends of the worker body size range were considered separately, but the direction was inconsistent among colonies. I also examined the role of returning laden workers in recruitment of nestmates by intercepting all laden workers for the first 2 h of foraging, and measuring the effect on the arrival of recruits at the foraging site. In two cases, the flow of recruits was not diminished by the interception of returning workers. The results suggest that neither mechanism correctly and consistently accounts for load size selection by leaf-cutting ants. Copyright 2000 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed2001

Association of caveolin with Chlamydia trachomatis inclusions at early and late stages of infection.

The mechanism by which the intracellular bacterial pathogen Chlamydia trachomatis enters eukaryotic cells is poorly understood. There are conflicting reports of entry occurring by clathrin-dependent and clathrin-independent processes. We report here that C. trachomatis serovar K enters HEp-2 and HeLa 229 epithelial cells and J-774A.1 mouse macrophage/monocyte cells via caveolin-containing sphingolipid and cholesterol-enriched raft microdomains in the host cell plasma membranes. First, filipin and nystatin, drugs that specifically disrupt raft function by cholesterol chelation, each impaired entry of C. trachomatis serovar K. In control experiments, filipin did not impair entry of the same organism by an antibody-mediated opsonic process, nor did it impair entry of BSA-coated microspheres. Second, the chlamydia-containing endocytic vesicles specifically reacted with antisera against the caveolae marker protein caveolin. These vesicles are known to become the inclusions in which parasite replication occurs. They avoid fusion with lysosomes and instead traffic to the Golgi region, where they intercept Golgi-derived vesicles that recycle sphingolipids and cholesterol to the plasma membrane. We also report that late-stage C. trachomatis inclusions continue to display high levels of caveolin, which they likely acquire from the exocytic Golgi vesicles. We suggest that the atypical raft-mediated entry process may have important consequences for the host-pathogen interaction well after entry has occurred. These consequences include enabling the chlamydial vesicle to avoid acidification and fusion with lysosomes, to traffic to the Golgi region, and to intercept sphingolipid-containing vesicles from the Golgi.

Animals↗
PubMed1999

Enthalpy-Entropy Compensation Phenomenon Observed for Different Surfactants in Aqueous Solution.

Based on previously reported thermodynamic data such as changes of the Gibbs energy (DeltaG(m)( degrees )), the enthalpy (DeltaH(m)( degrees )), and the entropy (DeltaS(m)( degrees )) on micelle formation of more than 15 species of surfactants (including nonionic, anionic, and cationic surfactants), plots of DeltaH(m)( degrees ) vs DeltaS(m)( degrees ) (not of DeltaS(m)( degrees ) vs DeltaH(m)( degrees ), as is usually done) were made. For each surfactant, a linear relation having almost the same slope (1/307 K(-1)) within a small error (+/-2.3%) but a different intercept (varsigma) depending on the surfactant species was obtained, i.e., DeltaS(m)( degrees ) = (1/307)DeltaH(m)( degrees ) + varsigma, where 1/307 (K(-1)) means that the so-called compensation temperature (T(C)) is 307 K. Strictly speaking, T(C) ranges from 299 to 315 K, depending on the species. The intercept corresponds to the entropy change at a specific temperature giving DeltaH(m)( degrees ) = 0, at which the driving force of micelle formation comes only from the entropy term; this temperature is characteristic of the surfactant species. On the other hand, the compensation temperature has no significant meaning other than a mean temperature studied. Copyright 1999 Academic Press.

Journal Article↗
PubMed2002

Estimating recreational trout fishing damages in Montana's Clark Fork River basin: summary of a natural resource damage assessment.

This paper summarizes a natural resource damage assessment for the State of Montana. Mining wastes have caused significant reductions in trout stocks in a 145-mile stretch of Montana's Silver Bow Creek and Clark Fork River. To estimate economic damages from decreases in catch rates, we develop and estimate an individual-based utility-theoretic model of where and how often an angler will fish as a function of travel costs, catch rates, and other influential characteristics of the site and individuals. The model includes resident and nonresident anglers who currently fish in Montana, and allows them to have different preferences. Demand parameters and expected catch rates are simultaneously estimated. The value of time is endogenously estimated as a proportion of the wage rate. Catch rates are linked to trout stocks through a stock-catch function. Collection of the angler data involved a three-step process: anglers were intercepted at 26 study sites, a subsample of anglers was selected to reflect the population trip-taking proportions to the study sites, and these anglers received follow-up surveys through the fishing season. Avidity weights are used to correct for the higher level of avidity inherent in intercept samples.

Animals↗
PubMed2000

Electrostatic dependence of the thrombin-thrombomodulin interaction.

The rate constants for the binding interaction between thrombin and a fully active fragment of its anticoagulant cofactor, thrombomodulin, have been determined by surface plasmon resonance. At physiological ionic strength, the k(a) was 6.7x10(6) M(-1) s(-1 )and the dissociation rate constant was 0.033 s(-1). These extremely fast association and dissociation rates resulted in an overall binding equilibrium constant of 4.9 nM, which is similar to previously reported values. Changing the ionic strength from 100 mM to 250 mM NaCl caused a tenfold decrease in the association rate while the dissociation rate did not change significantly. A similar effect was observed with tetramethylammonium chloride. A Debye-Hückel plot of the data had a slope of -6 and an intercept at 0 ionic strength of 10(9) M(-1) s(-1). The same slope and intercept were obtained for data that was collected in the presence of glycerol to slow the association rates. These results show that the thrombin-TM456 interaction is extremely rapid and nearly completely electrostatically steered. An association model is presented in which TM456 approaches thrombin along the direction of the thrombin molecular dipole.

Amino Acid Chloromethyl Ketones↗
PubMed1998

End-systolic pressure-dimension relationship of in situ mouse left ventricle.

The increasing popularity of genetically engineered mice in cardiovascular research has made it important to evaluate cardiac function in small animals. We have developed a system to enable simultaneous pressure-dimension analysis of the mouse left ventricle. The chest was opened under anesthesia, and a 1.4 F micromanometer catheter was inserted into the left ventricle through the apex. A pair of sonomicrometry crystals were attached to the anterior and posterior walls using tissue adhesive. Pressure and dimension were recorded simultaneously at baseline and after isoproterenol injection (1 micro g, intraperitoneally). The ascending aorta was occluded transiently to estimate the end-systolic pressure-dimension relationship (ESPDR), which was parameterized subsequently by the quadratic equation: Pes=C2 X (Des-D0)2+E0 X (Des-D0), where Pes is end-systolic pressure, Des is end-systolic dimension, D0 is the dimension axis intercept, E0 is the local slope at D0, and C2 is the curvilinearity coefficient. The maximum and minimum external dimensions at baseline were 5.82+/-0.50 (s.d.) mm and 5.49+/-0.46 mm with fractional shortening of 0.057+/-0.014 (n=12). The ESPDR was significantly curvilinear and increased convexity after isoproterenol injection (C2, -444+/-281 to -1113+/-780 mmHg/mm2, P<0. 05; E0, 536+/-175 to 889+/-276 mmHg/mm, P<0.001), while the dimension axis intercept remained relatively constant (D0, 5.39+/-0. 46 to 5.37+/-0.52 mm). In conclusion, the combination of miniature piezo-electric crystals and a micromanometer enables continuous measurement of pressure and dimension of in situ mouse left ventricle. This technology may be useful in evaluating the cardiac phenotype of genetically engineered mice.

Animals↗
PubMed1994

Effect of erythrocyte heat treatment on pulmonary vascular resistance.

The effect of red blood cell deformability on the pulmonary vascular resistance was studied in isolated dog and rat lungs. Blood cells were incubated at 49 degrees C for 1 hr, to render them rigid. The resistance to blood flow in the lung was assessed either by calculating the pulmonary vascular resistance (PVR = arterial - venous pressure difference divided by flow rate) or by examining the vascular pressure-flow relationship for changes in slope and intercept. The resistance in the lung was first assessed during perfusion with normal blood and again during perfusion with rigid cells. The results showed that PVR in dog lungs increased by 15% during perfusion with heat-treated blood and that this increase in PVR was associated with a significant increase in the middle segment resistance (arterial-venous occlusion technique) and with an increase in critical closing pressure (pressure intercept of the pressure-flow curve). In contrast to the small effect in dog lungs, the PVR in rat lungs rose more than 400% during perfusion with heat-treated blood. The marked increase of PVR in rat lungs was prevented with papaverine (PVR increased only 58%), suggesting that vasoconstriction was a primary event in rat lungs. The rise in vascular resistance in rat lungs was further shown to be primarily due to the presence of rigid erythrocytes (RBC). The increase in PVR in the rat lungs was not due to mechanical obstruction of the vasculature but rather to constriction of arteries and veins (double occlusion technique). The conclusion from this study is that RBC deformability plays an important role in the pulmonary vasculature, primarily because of release of vasoactive substances and partially because of the potential mechanical obstruction of capillaries. These events are apparently species dependent and are attributed mostly to red blood cell deformability which decreases during heat treatment.

Animals↗
PubMed1998

Investigation of the positive response of ethyl acrylate in the mouse lymphoma genotoxicity assay.

To develop a better understanding of the relationship between ethyl acrylate (EA)-induced cytotoxicity and mutation frequency in the mouse lymphoma assay (MLA) we measured the effects of EA treatment to ML cells on: (1) nonprotein sulfhydryl (NPS) levels; (2) mitochondrial rhodamine 123 (Rh123) uptake; (3) the DNA elution slope (single-strand breakage) and Y intercept of fitted curves (cytotoxicity and double-strand breakage) in the alkaline elution assay; (4) the appearance of apoptosis; and (5) the pulsed-field gel electrophoretic resolution of high-molecular-weight DNA. EA reduced NPS in both a time- and concentration-dependent manner. By 30 min, > or = 20 micrograms/ml EA reduced NPS by 50% or greater. By 4 h, > or = 10 micrograms/ml markedly decreased both NPS cell content (> or = 71.5% reduction) and mitochondrial Rh123 uptake (10-50 micrograms/ml; 9-62%), the latter effect being further enhanced by washing and incubation for an additional 2 h (12-85%). EA did not induce single-strand breaks in the alkaline elution assay. Only highly cytotoxic EA concentrations (80-87% reduction in RCG at 40-50 micrograms/ml) caused both increases in the elution slope and parallel drops (Y intercept) in the elution curve in the alkaline elution assay. Conventional agarose gel electrophoretic analysis of the DNA neutral fraction of these high dose preparations showed evidence for both apoptosis (180-bp oligonucleosomal DNA laddering effect) and random smearing of DNA (necrosis). Pulsed-field gel electrophoresis of directly loaded high dose cell preparations revealed both high- and low-molecular-weight DNA double-strand breaks, but only at the highest concentrations. These observations indicated that the EA-induced mutagenic response correlated best with cellular cytotoxicity mediated by NPS depletion and mitochondrial membrane impairment.

Acrylates↗
PubMed1984

The dependence of force and velocity on calcium and length in cardiac muscle segments.

The segment length (SL) dependence of force (F) and light load shortening velocity (VL) was determined for central segments of ferret papillary muscles at different extracellular calcium concentrations. Muscles were maintained at 27 degrees C in a physiological solution which contained in mM: NaCl 140; KCl 5.0; MgSO4 1.0; NaH2PO4 1.0; acetate 20; the pH was 7.4. Calcium concentrations were 1.125, 2.25, 4.5 and 9.0 mM. Total force-segment length relations were determined from both muscle length isometric ( auxotonic ) and segment isometric contractions, and were found to be the same for each contraction mode. The peak force generated at a particular segment length was independent of both the amount of shortening during a contraction and the initial SL. Increasing extracellular Ca2+ shifted the F-SL relation toward greater force and the SL axis intercept toward shorter SL. Maximum peak twitch tension was achieved in 9.0 mM Ca2+. Calcium variations also changed the shape of the total F-SL relation from linear in high Ca2+, to concave in low Ca2+. In order to estimate the active F-SL relations, corrections were made for passive force by two methods. The first assumed that passive force was related to SL, and yielded F-SL relations which were nearly identical to those found for total force. This similarity included the curvature changes observed in different Ca2+ concentrations, a finding which is consistent with the hypothesis that length dependent activation is the cause of force decline at short SL. The second method assumed passive force to be related to muscle length, an approach which would be appropriate if, for example, a connective tissue sheath on the muscle dominated passive behavior. These F-SL curves displayed a plateau above 90% SLmax and appeared to be vertically shifted versions of each other. Such characteristics are consistent with the possible role of an internal load in causing the decline of force at short SL. VL-SL relations were obtained from load clamps to 1 mM, imposed at various times during a segment isometric twitch. The results indicate that 1) VL declines linearly with SL below 90% SLmax and 2) VL-SL relations are shifted to higher velocity and shorter SL axis intercepts by increasing Ca2+. The slopes of the VL-SL relations obtained in different calciums are similar. Although an internal load could explain the calcium dependence of VL, it would not explain the similarity of the slopes of the VL-SL relations found in different calciums .

Animals↗
PubMed2000

Time course of trace of diffusion tensor [Trace(D)] and histology in brain edema.

We examined the correlation between changes in the trace of diffusion tensor [Trace(D)], regional water content and tissue ultrastructure relating to cellular (cytotoxic) and vasogenic brain edema. Cellular edema was induced by left middle cerebral artery occlusion in cats (Kuroiwa T et al., 1998). Vasogenic edema was induced in the white matter of cats by a cold lesion (Kuroiwa T et al., 1999). In cellular edema, the water content increase correlated linearly with the Trace(D) decrease in both the gray and white matter. However, both the slopes and intercepts of the correlation lines were significantly different. Hydropic astrocytic swelling was seen in both structures, and in the white matter, oligodendrocytic and myelinated axonal swelling were observed. In vasogenic edema, the increase in Trace(D) showed a significant linear correlation with the increase in tissue water content. Histologically, nerve fibers were dissociated and the extracellular space was markedly enlarged with protein-rich fluid. These result showed that the different slopes and intercepts of the water content--Trace(D) correlation lines for different subtype of brain edema, which reflect different ultrastructural localization of water, should be taken into account when evaluating brain edema using Trace(D) mapping.

Animals↗
PubMedDate not supplied

Multiple phenotypic divergence of mammary adenocarcinoma cell clones. II. Sensitivity to radiation, hyperthermia and FUdR.

We have shown that, with in vitro passage, subclones derived from clonal cell populations of 13762NF mammary adenocarcinoma undergo phenotypic drift and diversification in their cellular properties. Here we examine whether phenotypic divergence of 13762NF cell clones extends to therapeutic treatments used in eliminating mammary tumors and whether the apparent rates of phenotypic divergence vary for different treatments. Six subclones isolated from low passage clone MTF7 (T11; tissue culture passage 11) cells were compared to a similar number of subclones isolated from high passage clone MTF7 (T35; tissue culture passage 35) cells. Subclones derived from clone MTF7 (T11) were relatively homogeneous (not significantly different) in their inherent sensitivities to ionizing radiation, extrapolation coefficients and quasithreshold dose values (Do = 1.61-1.99 Gy; n = 0.89-3.42; Dq = 0-2.34). When the MTF7 (T11) subclones were examined for their sensitivities to 45 degrees C hyperthermic treatment, the inherent sensitivities and dose-response curve parameters (Do = 5.24-10.05 min; n = 1.08-10.47; Dq = 0.78-12.31) were heterogeneous (significantly different). In addition, the MTF7 (T11) subclones were heterogeneous (significantly different) in their sensitivities and dose-response curve parameters to 5-fluoro-2'-deoxyuridine (FUdR) treatment (slope = -0.70 to -1.59; y-intercept = 1.31 X 10(2) to 47.80 X 10(2]. The LD50 values for FUdR ranged from 14-150 nM for the MTF7 (T11) subclones. At high passage MTF7 (T35) subclones were heterogeneous in their dose-response parameters to ionizing radiation (Do = 1.17-2.05 Gy; n = 0.80-41.18; Dq = 1.79-4.94), hyperthermia (Do = 3.57-6.32 min; n = 2.08-13.54; Dq = 3.68-9.30) and FUdR (slope = -0.77 to -0.93; y-intercept = 4.64 X 10(2) to 8.83 X 10(2); LD50 = 50-160 nM). The results indicate that clonal cells diverge for distinct phenotypic properties at differing rates to form heterogeneous cell populations with unique sensitivities to various therapeutic treatments.

Adenocarcinoma↗
PubMed1988

Nicorandil shortens action potential duration and antagonises the reduction of Vmax by lidocaine but not by disopyramide in guinea-pig papillary muscles.

Conventional microelectrode techniques were used to examine whether or not nicorandil, which shortens action potential duration (APD), modifies the lidocaine- or disopyramide-induced time-dependent reduction of Vmax in guinea-pig papillary muscles. First, effects of 0.1 and 1 mmol/l nicorandil were examined on the frequency dependence of Vmax and on the recovery process of Vmax. Second, the frequency-dependent reduction of Vmax by 20 mumol/l antiarrhythmic drugs was examined in the presence and absence of 1 mmol/l nicorandil at stimulation frequencies of 1/120 Hz - 5 Hz. Third, the recovery process of Vmax in the presence of 20 mumol/l antiarrhythmic drugs was examined, with and without 1 mmol/l nicorandil, by applying test stimuli at various diastolic intervals after conditioning stimuli. 1 mmol/l nicorandil greatly shortened APD90 to 30-40% of control without changing the frequency dependence of Vmax, the recovery process of Vmax, and the resting potential. The lidocaine-induced, frequency-dependent reduction of Vmax was significantly antagonised by 1 mmol/l nicorandil, but the disopyramide-induced reduction was not. The recovery process of Vmax slowed in the presence of lidocaine was antagonised by 1 mmol/l nicorandil as follows: the time to get the full recovery of Vmax was shortened by nicorandil with a significant decrease in the zero time-intercept (from 0.54 to 0.38) but with an insignificant change in the recovery time constant (from 130 ms to 121 ms). In contrast, the recovery process of Vmax slowed in the presence of disopyramide (a zero time intercept of 0.13 and a recovery time constant of 50 s) was not significantly antagonised by 1 mmol/l nicorandil. In conclusion, nicorandil having an action potential-shortening action antagonises the lidocaine-induced, time-dependent reductions of Vmax, but not the disopyramide-induced reductions. These results suggest that: (1) lidocaine and disopyramide preferentially bind to inactivated and activated sodium channels, respectively, because lidocaine's effects are dependent on and disopyramide's effects are independent of APD (during which sodium channels are in the inactivated state); and (2) nicorandil is a useful drug for estimating whether a sodium channel-blocking action of class I antiarrhythmic drugs is due to an inactivated channel block or an activated channel block. These time-dependent reductions of Vmax by both lidocaine and disopyramide were well simulated by the guarded receptor hypothesis.

Action Potentials↗
PubMed1992

Estimation of chromium-51 ethylene diamine tetra-acetic acid plasma clearance: a comparative assessment of simplified techniques.

Chromium-51 ethylene diamine tetra-acetic acid (51Cr-EDTA) total plasma clearance was evaluated using a multi-sample method (i.e. 12 blood samples) as the reference compared with several simplified methods which necessitated only one or few blood samples. The following 5 methods were evaluated: terminal slope-intercept method with 3 blood samples, simplified method of Bröchner-Mortensen and 3 single-sample methods (Constable, Christensen and Groth, Tauxe). Linear regression analysis was performed. Standard error of estimate, bias and imprecision of different methods were evaluated. For 51Cr-EDTA total plasma clearance greater than 30 ml.min-1, the results which most approximated the reference source were obtained by the Christensen and Groth method at a sampling time of 300 min (inaccuracy of 4.9%). For clearances between 10 and 30 ml.min-1, single-sample methods failed to give reliable results. Terminal slope-intercept and Bröchner-Mortensen methods were better, with inaccuracies of 17.7% and 16.9%, respectively. Although sampling times at 180, 240 and 300 min are time-consuming for patients, 51Cr-EDTA total plasma clearance can be accurately calculated for values greater than 10 ml.min-1 using the Bröchner-Mortensen method. In patients with clearance greater than 30 ml.min-1, single-sample techniques provide a good alternative to the multi-sample method; the choice of the method to be used depends on the degree of accuracy required.

Adult↗
PubMed1996

Coordination between posture and movement in a bimanual load-lifting task: is there a transfer?

The present experimental series was designed to test the possibility that an anticipatory postural adjustment learned during the performance of a bimanual load lifting task may be transferred between the upper extremities. Eight seated subjects were asked to maintain horizontally one forearm (postural arm) loaded with a 1-kg load, which was fixed to the arm by means of an electromagnet. The unloading was triggered either by the experimenter pressing a switch (control) or by the subjects making a voluntary movement with their other arm (moving arm). In the latter case, the subject lifted a 1-kg load resting on a force platform with the moving hand, and the switching off was triggered when the force level reached a threshold of 0.5 kg. The maximum amplitude (MA) and the maximum velocity (MV) of the postural forearm elbow joint rotation occurring after the unloading were measured at each trial. The learning process was estimated by performing a regression analysis on each series of trials, using an exponential model, and from the intercept of the regression curve with the ordinate. 1. During the original learning session (three series of 20 trials), a decrease in MA and MV was found to occur both within the series and between the series during a session. 2. After the initial learning session, the sides of the postural and moving arm were interchanged to test whether any transfer had occurred. The first series of trials in the second session (transfer) and the last series of trials in the original learning session were compared and found to be significantly different in terms of the intercept (seven subjects in the case of MA, five subjects in the case of MV) and the slope (five subjects), indicating a lack of transfer. 3. The data recorded during the second transfer learning session indicated that learning occurred in all eight subjects in the case of MA and in six subjects in the case of MV. It was observed that the original learning session did not facilitate the second one. 4. The lack of transfer of the anticipatory postural adjustment observed in this task is discussed with reference to the data in the literature.

Conditioning, Psychological↗
PubMed1993

Ocular limit cycles induced by delayed retinal feedback.

Lisberger's and Robinson's models of smooth pursuit predict very different results from altering retinal feedback delay. We have therefore investigated the effects of increasing the retinal feedback time delay in three normal human subjects by means of an artificial feedback paradigm. When additional delays were incorporated into the retinal feedback path a threshold was reached beyond which the eye exhibited sustained self-excited oscillations or "limit cycles". The oscillation period increased linearly (as the added delay was increased) with slopes ranging from 1.41 to 1.6 with zero-delay intercepts of between 212 and 306 ms. Contrary to our experimental findings the Robinson and Lisberger models predict that the plot of oscillation period against added delay should have a slope of 3.4 and 2.7 and an intercept of 479 and 554 ms, respectively. Neither model produced comparable limit cycles, both being unstable at delays greater than 280 ms. Our results imply that the models of smooth pursuit need to incorporate predictive control.

Adult↗
PubMed1987

Cognitive spatial-motor processes. 1. The making of movements at various angles from a stimulus direction.

Naive human subjects (N = 18) were asked to move a manipulandum on a phase in directions other than going straight towards a visual stimulus. They were instructed verbally to generate a movement at an angle from a stimulus direction which varied in 2-dimensional (2-D) space from trial to trial in a pseudorandom fashion. Each subject performed eight sets of twenty consecutive trials: one for moving in the stimulus direction and seven for moving in directions at an angle from it. The angles were 5, 10, 15, 35, 70, 105 and 140 degrees. Nine subjects were instructed to move in the clockwise (CW) departure and 9 to move in either (EI) the clockwise or the counterclockwise (CCW) departure, as they wished. The direction of the movement in 2-D space and the reaction time (RT) were measured. The mean angle achieved in a given set overshot the instruction angle, especially in the lower range (5-35 degrees). The reaction time, RT phi, of movements made at an angle from the stimulus direction showed two kinds of change; first, a step increase from the reaction time, RT0, of movements in the stimulus direction, and second, superimposed upon it, a linear increase with the amplitude of the angle. The slope of the line was similar for the CW (2.37 ms/degree) and the EI case (2.28 ms/degree), but the step increase (y-intercept) for the EI case (84 ms) was substantially less than that of the CW case (155 ms). The linear increase of the RT with angle is compatible with the idea that performance in the task may involve a mental rotation of the imagined movement vector about its origin. The rotation would begin from the stimulus direction and end when the required angle is judged to have been reached; in addition, corrections of this angle at the end of the rotation could be made. The slope of 2.37 ms/degree observed in the CW case would correspond to a rotation rate of 422 degrees/s. The finding of a similar rate for the EI case indicates a similarity in strategy with regard to achieving a desired angle. In contrast, the lower intercept observed for the EI case suggests significant savings in processing information which is unconstrained with regard to angular departure. Assuming this model of internal motion, we analyzed the amplitude-accuracy relations using Fitts' (1954) approach to real movements.(ABSTRACT TRUNCATED AT 400 WORDS)

Cognition↗