Graduate School of Systemic Neurosciences - Digitale Hochschulschriften der LMU

’Gain-field-like’ tuning behavior is characterized by a modulation of the neuronal response depending on a certain variable, without changing the actual receptive field characteristics in relation to another variable. Eye position gain fields were first observed in area 7a of the posterior parietal cortex (PPC), where visually responsive neurons are modulated by

ocular position. Analysis of artificial neural networks has shown that this type of tuning

function might comprise the neuronal substrate for coordinate transformations.

In this work, neuronal activity in the dorsal medial superior temporal area (MSTd) has

been analyzed with an focus on it’s involvement in oculomotor control. MSTd is part of

the extrastriate visual cortex and located in the PPC. Lesion studies suggested a participation

of this cortical area in the control of eye movements. Inactivation of MSTd severely

impairs the optokinetic response (OKR), which is an reflex-like kind of eye movement that

compensates for motion of the whole visual scene.

Using a novel, information-theory based approach for neuronal data analysis, we were able

to identify those visual and eye movement related signals which were most correlated to the mean rate of spiking activity in MSTd neurons during optokinetic stimulation. In a majority of neurons firing rate was non-linearly related to a combination of retinal image velocity and eye velocity. The observed neuronal latency relative to these signals is in line with a system-level model of OKR, where an efference copy of the motor command signal is used to generate an internal estimate of the head-centered stimulus velocity signal. Tuning functions were obtained by using a probabilistic approach. In most MSTd neurons these functions exhibited gain-field-like shapes, with eye velocity modulating the visual

response in a multiplicative manner. Population analysis revealed a large diversity of

tuning forms including asymmetric and non-separable functions. The distribution of gain

fields was almost identical to the predictions from a neural network model trained to perform

the summation of image and eye velocity. These findings therefore strongly support the hypothesis of MSTd’s participation in the OKR control system by implementing the transformation from retinal image velocity to an estimate of stimulus velocity. In this sense, eye velocity gain fields constitute an intermediate step in transforming the eye-centered to a head-centered visual motion signal.Another aspect that was addressed in this work was the comparison of the irregularity of MSTd spiking activity during optokinetic response with the behavior during pure visual stimulation. The goal of this study was an evaluation of potential neuronal mechanisms

underlying the observed gain field behavior. We found that both inter- and intra-trial

variability were decreased with increasing retinal image velocity, but increased with eye

velocity. This observation argues against a symmetrical integration of driving and modulating

inputs. Instead, we propose an architecture where multiplicative gain modulation is achieved by simultaneous increase of excitatory and inhibitory background synaptic input. A conductance-based single-compartment model neuron was able to reproduce realistic gain modulation and the observed stimulus-dependence of neural variability, at the same time.

In summary, this work leads to improved knowledge about MSTd’s role in visuomotor transformation by analyzing various functional and mechanistic aspects of eye velocity gain fields on a systems-, network-, and neuronal level.

Neurofascin (NF) is a cell-adhesion molecule that is found at the nodes of Ranvier. The 186

kDa isoform of neurofascin (NF186) is expressed on the axon in the exposed node, and the

155 kDa isoform (NF155) is expressed on myelinating glia at the paranode. NF186 is essential

for clustering of sodium channels to the nodes while NF155 is needed for close paranodal interactions

between myelinating glia and axons. The neurofascins are found in both the peripheral

and central nervous system (PNS and CNS).

NF-specific autoantibodies were identified in serum of multiple sclerosis (MS) patients using

a proteomics approach with two-dimensional Western blotting of human myelin glycoproteins.

A monoclonal antibody (mAb) specific for NF was shown to induce axonal injury in an animal

model of MS, experimental autoimmune encephalomyelitis. This indicated that NF is a relevant

autoantibody target in patients with inflammatory diseases of the nervous system (central and

peripheral), but actual abundance of anti-NF autoantibodies is unknown.

The objectives of the thesis were the following: 1) Develop assays to detect autoantibodies

against human NF. 2) Determine the prevalence in patients with MS and with inflammatory

diseases of the PNS. 3) Characterize the reactivity by immunoglobulin isotyping, serial dilution,

epitope mapping, and staining of nodal structures in tissue sections. 4) Affinity purify anti-NF

antibodies from plasma exchange material. 5) Determine possible in vivo effects of anti-NF

antibodies in the PNS using a neuritis animal model.

First, we expressed the complete human NF155 and NF186 on the surface of stable human cell

lines, produced the complete extracellular portion of the NFs in HEK293 cells, and expressed

truncated variants of the NFs in E. coli. With these reagents, we set up three antibody detection

assays: cell-based assay by flow cytometry, ELISA, and Western blot. These assays were

validated using NF-specific monoclonal and polyclonal antibodies, and optimized with a test

cohort of serum samples.

We screened 687 serum and 48 plasma exchange samples from patients with MS (n = 233),

inflammatory diseases in the PNS (n = 294), and controls (n = 208). From serum analysis,

we observed low prevalence of anti-NF reactivity (3%) by flow cytometry and/or ELISA despite

broad reactivity in almost half of the serum samples analyzed by Western blot. Reactivity observed

by flow cytometry and by ELISA were congruent only in the patients with the highest

reactivities. The anti-NF antibodies were NF-isoform specific, mainly IgG subclasses, and at

high titres in some cases. Using truncated variants of NF fused to super green fluorescence

protein (sGFP), we showed that reactivity of anti-NF Abs was largely directed towards the membrane

proximal extracellular domains that are unique to each isoform, while the membrane

distal immunoglobulin-like domains and fibronectin domains were not recognized.

A small proportion (3%; 8/254) of patients with GBS and CIDP showed reactivity to human NF

by ELISA. A few showed a particularly high reactivity (up to 1:10 000 dilution) to NF. Two CIDP

patients showed a particularly high (up to 1:10 000 dilution) anti-NF155 reactivity by FACS and

ELISA, recognized paranodes in tissue sections, and exhibited dominant IgG4 subclass usage.

Another CIDP patient who benefited from plasma exchange had a persistent anti-NF155

reactivity by ELISA in serum, and after affinity purification, anti-NF186 and -NF155 reactivity by

FACS and ELISA were detected in addition. These antibodies were mainly IgG3, with minor

contribution of IgM and IgA. To investigate possible functions of anti-NF antibodies in inflammatory

PNS diseases, we injected two different monoclonal antibodies (mAbs) into a P2-peptide

induced experimental autoimmune neuritis (EAN) animal model at disease onset. We found

that while the anti-NF mAbs prolonged and exacerbated clinical disease in these animals, they

could not induce disease on their own.

We detected NF-reactivity in a small proportion of MS samples (3%; 7/225) by ELISA and flow

cytometry. We obtained follow-up material from two NF-reactive patients and saw a persistent

NF reactivity in one of them. To increase detection sensitivity, we affinity purified anti-NF antibodies

from plasma exchange material of patients with MS (n = 8). IgG, IgM, and IgA were

isolated from most of the samples; they were found to recognize NF155 and to a lower extent

NF186 by ELISA and in a few also by flow cytometry. This indicates that low levels of anti-NF

antibodies exist in a proportion of MS patients.

In conclusion, 3% of serum samples from patients with PNS inflammatory neuropathies (GBS

and CIDP) showed reactivity by ELISA and none of the controls. In an animal model of autoimmune

peripheral nerve inflammation, we showed, using two anti-NF mAbs, that antibody

targeting of NF can enhance and prolong disease course. This suggests that antibodies to NF

may be relevant for a small group of patients with peripheral inflammatory neuropathies. In MS

patients, 3% showed anti-NF reactivity by flow cytometry and ELISA. Furthermore, low levels of

anti-NF antibodies that could be detected by ELISA and flow cytometry after affinity purification

were additionally found in some MS patient samples that were unreactive by serum screening.

This raises the possibility that low levels of antibodies to NF are present in some MS patients

and may contribute to the pathogenesis of this chronic disease.

This thesis proposes a new approach to investigate insight problem solving. Introducing magic tricks as a problem solving task, we asked participants to find out the secret method used by the magician to create the magic effect. Based on the theoretical framework of the representational change theory, we argue that magic tricks are ideally suited to investigate insight because similar to established insight tasks like puzzles, observers’ prior knowledge activates constraints. In order to see through the magic trick, the constraints must be overcome by changing the problem representation. The aim of the present work is threefold: First, we set out to provide a proof of concept for this novel paradigm by demonstrating that it is actually possible for observers to gain insight into the magician’s secret method and that this can be experienced as a sudden, insightful solution. We therefore aimed at showing that magic tricks can trigger insightful solutions that are accompanied by an Aha! experience. The proposed paradigm could be a useful contribution to the field of insight research where new stimuli beyond traditional puzzle approaches are sorely needed. Second, the present work is aimed at contributing to a better understanding of the subjective Aha! experience that is currently often relied on as important classification criterion in neuroscientific studies of insight, yet remains conceptually vague. The new task will therefore be used to further elucidate the phenomenology of the Aha! experience by assessing participants’ individual solving experiences. As a third question, we investigated the influence of insight on memory. A positive impact of insight on subsequent solution recall is often implicitly assumed, because the representational change underlying insightful solutions is assumed to facilitate the retention of solution knowledge, yet this was never tested.

A stimulus set of magic tricks was developed in collaboration with a professional magician, covering a large range of different magic effects and methods. After recording the tricks in a standardized theatre setting, pilot studies were run on 45 participants to identify appropriate tricks and to ensure that they were understandable, surprising and difficult. In the main experiment, 50 participants watched the final set of 34 magic tricks, with the task of trying to figure out how the trick was accomplished. Each trick was presented up to three times. Upon solving the trick, participants had to indicate whether they had found the solution through sudden insight (i.e. with an Aha! experience) or not. Furthermore, we obtained a detailed characterization of the Aha! experience by asking participants for a comprehensive quantitative (ratings on a visual analogue scale with fixed dimensions) and qualitative evaluation (free self-reports) which was repeated after 14 days to control for its reliability. At that time, participants were also required to perform a recall of their solutions.

We found that 49% of all magic tricks could be solved and specifically, that insightful solutions were elicited in 41.1% of solved trials. In comparison with noninsight solutions, insightful solutions (brought about by representational change) were more likely to be correct and reached earlier. Quantitative evaluations of individual Aha! experiences turned out to be highly reliable since they remained identical across the time span of 14 days. Qualitatively, participants reported more emotional than cognitive aspects. This primacy of positive emotions was found in qualitative as well as in quantitative evaluations, although two different methods were used. We also found that experiencing insight leads to a facilitated recall of the respective solutions since 64.4% of all insight solutions were recalled correctly, whereas only 52.4% of all noninsight solutions were recalled correctly after a delay of 14 days.

We demonstrated the great potential of our new approach by providing a proof of concept for magic tricks as a problem solving task and conclude that magic tricks offer a novel way of inducing problem solving that elicits insight. The reliability of individual evaluations of Aha! experiences indicates that, despite its subjective character, it can be justified to use the Aha! experience as a classification criterion. The present work contributes to a better understanding of the phenomenology of the Aha! experience by providing evidence for the occurrence of strong positive emotions as a prevailing aspect. This work also revealed a memory advantage for solutions that were reached through insight, demonstrating a facilitating effect of previous insight experiences on the recall of solutions. This finding provides support for the assumption that a representational change underlying insightful solving experiences leads to long-lasting changes in the representation of a problem that facilitate the retention of the problem’s solution. In sum, the novel approach presented in this thesis is shown to constitute a valuable contribution to the field of insight research and offers much potential for future research. Revealing the relationship between insight and magic tricks, the framework of the representational change theory is applied to a new domain and thus enlarged. Combining the novel task domain of magic tricks with established insight tasks might help to further elucidate the process of insight problem solving which is a characteristic and vital part of human thinking and yet so difficult to grasp.

Animals require cognitive maps for efficiently navigating in their natural habitat. Cognitive maps are a neuronal representation of their outside world. In mammals, place cells and grid cells have been implicated to form the basis of these neuronal representations. Place cells are active at one particular location in an environment and grid cells at multiple locations of the external world that are arranged in a hexagonal lattice.

The hippocampus is one of the regions in the mammalian brain that is associated with memory of events in their spatiotemporal context. Sequences of neuronal activity in the hippocampus are the chief candidate for a neurophysiological correlate of such contextual, or episodic memory. Simultaneously to replaying these behaviorally-related activity sequences, the hippocampus engages in a powerful and fast oscillation known as sharp-wave ripples (SWR). Ripples in turn participate in a brain-wide pattern of activity and may orchestrate the local strengthening of memories and their broadcasting to the cortex. In this Thesis, both memory sequences and ripple oscillations are studied in the light of the unifying hypothesis that the coordinated activation of a neuronal assembly represents an individual memory item in the sequences, and is at the same time responsible for the individual cycles in the oscillations. To test the hypothesis, we investigated SWR in vitro and in vivo in the mouse, using intracellular recordings of currents in CA1 pyramidal cells referenced to the local field potential. Expanding current hypotheses on SWR generation, we found powerful, well ripple-locked and spatially pervasive but CA1-local excitatory inputs, indicative of presynaptic assemblies of CA1 principal neurons. Combining a novel peeling reconstruction algorithm for synaptic currents with recordings at different holding potentials, we could for the first time unravel individual synaptic contributions during ripples. Analysis of the strikingly precise timing of currents demonstrated that inhibition aligns its phase to excitation over the course of a ripple. We carried on the dissection of ripples to the theoretical domain by incorporating the effect of inhibition into a mean field model of sequence replay. Using this model, we inquired what are the neuronal assembly size and inhibitory feedback strength that maximize the capacity of a hippocampal network to store memories, so that those memories can be successfully retrieved during ripple episodes. We found that a linearly coupled inhibitory population indeed helps increase storage capacity by dynamically stabilizing replay in an oscillatory manner for lower assembly sizes than in absence of inhibition. The findings about the temporal structure of neuronal activation during ripples complement our experimental observations. Collectively, they offer new insights on the physiology and function of sharp-wave ripples, paving the way for an integrated, continuous-time model of large networks of sparsely connected neurons that replay activity sequences concomitant to transient ensemble oscillations.