| 1. |
- Johansson, Fredrik, 1983-
(författare)
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Microscopic Simulation of Pedestrian Traffic
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There has recently been a renewed interest in planning for pedestrian traffic, primarily in connection to public transport interchange stations, since these are important for public transport to constitute an attractive alternative to car usage. This thesis concerns microscopic simulation of pedestrian traffic, which is a promising tool for analyzing and predicting the traffic situation in a given pedestrian facility; particularly powerful when the traffic is congested. Important applications of microscopic simulation include comparison of possible infrastructure designs such as proposed interchange stations, and evaluations of various traffic management solutions, for example information systems.The purpose of this thesis is to advance the capabilities of pedestrian microsimulation toward a level at which it can be reliably applied for quantitative analysis by practitioners in the field. The work is based on an established microscopic model of pedestrian dynamics, the Social Force Model (sfm), and the advances are made in a number of different areas.To be able to evaluate and compare simulated traffic situations suitable performance measures are needed. A set of local performance measures are proposed that quantifies the local delay rate density and estimates the discomfort perceived by the pedestrians.The sfm is extended to include waiting pedestrians through the introduction of a waiting model, demonstrated to be stable and free from oscillations. The inclusion of waiting pedestrians in the model is critical for accurate modelling of public transport interchange stations, where large groups of waiting pedestrians may hinder passing pedestrians if the design of the station is poor.The relaxation time of the adaptation to the preferred velocity is an important parameter in force based models of pedestrian traffic since it affects several behaviors of the simulated pedestrians, two of which are linear acceleration and turning movements. A comparison of observations of accelerating pedestrians reported in the literature and new observations of turning pedestrians indicates that no value of the relaxation time can give model behavior consistent with both sets of observations. This indicates that modifications of the model is needed to accurately reproduce the observed behavior.An important input to simulations is the preferred speed of the simulated pedestrians. The common assumption that the preferred speed distribution at a location does not vary during the day is tested through observations of pedestrian traffic at Stockholm Central Station. The results demonstrate that the preferred speeds are lower in the afternoon than in the morning, implying that the preferred speed should be treated as a source of uncertainty when applying pedestrian microsimulation.Finally, a sensitivity analysis of a simulation of the lower hall of Stockholm Central Station is performed to find the most important sources of uncertainty in the model predictions, given the available data. The results indicate that the uncertainty related to calibration is the largest of the considered potential error sources.
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| 2. |
- Edberg, Niklas J. T., et al.
(författare)
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Spatial distribution of low-energy plasma around comet 67P/CG from Rosetta measurements
- 2015
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:11, s. 4263-4269
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Tidskriftsartikel (refereegranskat)abstract
- We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be approximate to 1-210(-6), at a cometocentric distance of 10km and at 3.1AU from the Sun. A clear 6.2h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260km from the nucleus falls off with radial distance as approximate to 1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet.
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| 3. |
- Gallistel, Charles Randy, et al.
(författare)
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Quantitative properties of the creation and activation of a cell-intrinsic duration-encoding engram
- 2022
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Ingår i: Frontiers in Computational Neuroscience. - : Frontiers Media SA. - 1662-5188. ; 16, s. 1-27
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Tidskriftsartikel (refereegranskat)abstract
- The engram encoding the interval between the conditional stimulus (CS) and the unconditional stimulus (US) in eyeblink conditioning resides within a small population of cerebellar Purkinje cells. CSs activate this engram to produce a pause in the spontaneous firing rate of the cell, which times the CS-conditional blink. We developed a Bayesian algorithm that finds pause onsets and offsets in the records from individual CS-alone trials. We find that the pause consists of a single unusually long interspike interval. Its onset and offset latencies and their trial-to-trial variability are proportional to the CS-US interval. The coefficient of variation (CoV = σ/μ) are comparable to the CoVs for the conditional eye blink. The average trial-to-trial correlation between the onset latencies and the offset latencies is close to 0, implying that the onsets and offsets are mediated by two stochastically independent readings of the engram. The onset of the pause is step-like; there is no decline in firing rate between the onset of the CS and the onset of the pause. A single presynaptic spike volley suffices to trigger the reading of the engram; and the pause parameters are unaffected by subsequent volleys. The Fano factors for trial-to-trial variations in the distribution of interspike intervals within the intertrial intervals indicate pronounced non-stationarity in the endogenous spontaneous spiking rate, on which the CS-triggered firing pause supervenes. These properties of the spontaneous firing and of the engram read out may prove useful in finding the cell-intrinsic, molecular-level structure that encodes the CS-US interval.
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| 4. |
- Hesslow, Germund, et al.
(författare)
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Classical conditioning of motor responses: What is the learning mechanism?
- 2013
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Ingår i: Neural Networks. - : Elsevier BV. - 1879-2782 .- 0893-6080. ; 47:Mar,28, s. 81-87
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Tidskriftsartikel (refereegranskat)abstract
- According to a widely held assumption, the main mechanism underlying motor learning in the cerebellum, such as eyeblink conditioning, is long-term depression (LTD) of parallel fibre to Purkinje cell synapses. Here we review some recent physiological evidence from Purkinje cell recordings during conditioning with implications for models of conditioning. We argue that these data pose four major challenges to the LTD hypothesis of conditioning. (i) LTD cannot account for the pause in Purkinje cell firing that is believed to drive the conditioned blink. (ii) The temporal conditions conducive to LTD do not match those for eyeblink conditioning. (iii) LTD cannot readily account for the adaptive timing of the conditioned response. (iv) The data suggest that parallel fibre to Purkinje cell synapses are not depressed after learning a Purkinje cell CR. Models based on metabotropic glutamate receptors are also discussed and found to be incompatible with the recording data.
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| 5. |
- Jirenhed, Dan Anders, et al.
(författare)
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Learned response sequences in cerebellar Purkinje cells
- 2017
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 114:23, s. 6127-6132
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Tidskriftsartikel (refereegranskat)abstract
- Associative learning in the cerebellum has previously focused on single movements. In eyeblink conditioning, for instance, a subject learns to blink at the right time in response to a conditional stimulus (CS), such as a tone that is repeatedly followed by an unconditional corneal stimulus (US). During conditioning, the CS and US are transmitted by mossy/parallel fibers and climbing fibers to cerebellar Purkinje cells that acquire a precisely timed pause response that drives the overt blink response. The timing of this conditional Purkinje cell response is determined by the CS-US interval and is independent of temporal patterns in the input signal. In addition to single movements, the cerebellum is also believed to be important for learning complex motor programs that require multiple precisely timed muscle contractions, such as, for example, playing the piano. In the present work, we studied Purkinje cells in decerebrate ferrets that were conditioned using electrical stimulation of mossy fiber and climbing fiber afferents as CS and US, while alternating between short and long interstimulus intervals. We found that Purkinje cells can learn double pause responses, separated by an intermediate excitation, where each pause corresponds to one interstimulus interval. The results show that individual cells can not only learn to time a single response but that they also learn an accurately timed sequential response pattern.
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| 6. |
- Johansson, Fredrik, et al.
(författare)
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Absence of Parallel Fibre to Purkinje Cell LTD During Eyeblink Conditioning
- 2018
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Long-term depression (LTD) of parallel fibre/Purkinje cell synapses has been the favoured explanation for cerebellar motor learning such as classical eyeblink conditioning. Previous evidence against this interpretation has been contested. Here we wanted to test whether a classical conditioning protocol causes LTD. We applied a conditioning protocol, using a train of electrical pulses to the parallel fibres as the conditional stimulus. In order to rule out indirect effects caused by antidromic granule cell activation or output from Purkinje cells that might produce changes in Purkinje cell responsiveness, we focused the analysis on the first pulse in the conditional stimulus, that is, before any indirect effects would have time to occur. Purkinje cells learned to respond with a firing pause to the conditional stimulus. Yet, there was no depression of parallel fibre excitation after training.
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| 7. |
- Johansson, Fredrik, 1974-
(författare)
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Essays on Measurement Error and Nonresponse
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Essay 1 deals with measurement errors. Matching survey data with administrative records provides unique opportunities to evaluate the statistical properties of respondents' answers. This paper focuses primarily on home ownership and wealth variables. For all analyzed variables we find that the classical assumption of no correlation between measurement error and true value is violated. In an empirical application explaining the housing tenure choice, LPM and Probit models are estimated using both survey and register data on gross financial wealth.Essay 2 (with N. Anders Klevmarken) also analyzes measurement errors. Survey measures of wealth are error prone with a relatively large error variance. The errors are not uncorrelated with the true values but tend to have a negative correlation, which implies that wealthy people tend to under-report and less wealthy to over-report their responses. There is no general tendency of survey data to under-estimate mean wealth with the exception of the last percentile. The under-estimate of the wealth of the very rich is however not due to under-reporting but rather to selective nonresponse. Using simple models this paper discusses consequences of error prone wealth data.Essay 3 (with N. Anders Klevmarken) studies the problem of nonresponse in survey data. Using rich register data to analyze response behavior in a survey on health and economic standard, a model to explain contact and participation probabilities is estimated. Previous attempts to build such models have been constrained by the very limited information available in the sampling frames. One main result is that both probabilities are lower among respondents out of the labor market, who are immigrants and on benefits.Essay 4 analyzes different approaches to adjust for nonresponse bias. When a survey response mechanism depends on the variable of interest measured within the same survey and observed for only part of the sample, the situation is one of nonignorable nonresponse. If the nonresponse is ignored it will most likely generate significant bias in the estimates of the model parameters of interest. To solve this, one option is the joint modelling of the response mechanism and the variable of interest. Another option is to calibrate each observation with weights constructed from auxiliary data. In an application where earnings equations are estimated these approaches are all applied and compared with reference estimates. These reference estimates are based on a large data set without any nonresponse.
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| 8. |
- Johansson, Fredrik, et al.
(författare)
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Memory trace and timing mechanism localized to cerebellar Purkinje cells
- 2014
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Ingår i: Proceedings of the National Academy of Sciences. - : Proceedings of the National Academy of Sciences. - 1091-6490 .- 0027-8424. ; 111:41, s. 14930-14934
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Tidskriftsartikel (refereegranskat)abstract
- The standard view of the mechanisms underlying learning is that they involve strengthening or weakening synaptic connections. Learned response timing is thought to combine such plasticity with temporally patterned inputs to the neuron. We show here that a cerebellar Purkinje cell in a ferret can learn to respond to a specific input with a temporal pattern of activity consisting of temporally specific increases and decreases in firing over hundreds of milliseconds without a temporally patterned input. Training Purkinje cells with direct stimulation of immediate afferents, the parallel fibers, and pharmacological blocking of interneurons shows that the timing mechanism is intrinsic to the cell itself. Purkinje cells can learn to respond not only with increased or decreased firing but also with an adaptively timed activity pattern.
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| 9. |
- Rasmussen, Anders, et al.
(författare)
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Golgi Cell Activity During Eyeblink Conditioning in Decerebrate Ferrets.
- 2014
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Ingår i: Cerebellum. - : Springer Science and Business Media LLC. - 1473-4230. ; 13:1, s. 42-45
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Tidskriftsartikel (refereegranskat)abstract
- Golgi cells have a central position in the cerebellar cortical network and are indirectly connected to Purkinje cells, which are important for the acquisition of learned responses in classical conditioning. In order to clarify the role of Golgi cells in classical conditioning, we made extracellular Golgi cell recordings during different stages of conditioning, using four different conditional stimuli. Our results show that forelimb and superior colliculus stimulation, but not mossy fiber stimulation, evokes a short latency increase in Golgi cell firing. These results suggest that Golgi cells are involved in modulating input to the cerebellar cortex. There were however no differences in Golgi cell activity between naïve and trained animals, which suggests that Golgi cells are not intimately involved in the plastic changes that occur during classical conditioning. The absence of long latency effects of the conditional stimulus also questions whether Golgi cells contribute to the generation of a temporal code in the granule cells.
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| 10. |
- Rasmussen, Anders, et al.
(författare)
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Number of spikes in climbing fibers determines the direction of cerebellar learning.
- 2013
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Ingår i: The Journal of Neuroscience. - 1529-2401. ; 33:33, s. 13436-13440
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Tidskriftsartikel (refereegranskat)abstract
- Cerebellar learning requires context information from mossy fibers and a teaching signal through the climbing fibers from the inferior olive. Although the inferior olive fires in bursts, virtually all studies have used a teaching signal consisting of a single pulse. Following a number of failed attempts to induce cerebellar learning in decerebrate ferrets with a nonburst signal, we tested the effect of varying the number of pulses in the climbing fiber teaching signal. The results show that training with a single pulse in a conditioning paradigm in vivo does not result in learning, but rather causes extinction of a previously learned response.
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