The motivation for this thesis came from the intriguing idea that we continuously restructure our brain through everyday learning. How can this highly complex, highly adaptive “learning device” change and reorganize itself all the time while keeping the illusion that we are constantly “ourselves”? The question is, whether learning has the power to trigger functional and structural changes in the brain. Several levels of thinking are involved in an interdisciplinary way. Thus, on a psychological level, 3 major topics enter this work: learning, memory and preconscious or pre-attentive perception and processing of information. Pre-attentive perception means that the subjects' attention and awareness is not mirrored in the neuronal response at a great deal. Learning is involved in this study as an improving discrimination of fine frequency and word duration differences; the latter was examined in a group of native and non-native speakers. Memory is referred to as sensory memory, a short-time memory trace that is established through the repetition of the same “standard” stimulus. In the auditory modality this has been termed “echoic memory”. A long, repetitive training engraves deep “traces” into the memory. The lifelong training of one’s native language results in a very fast and highly automated long-term memory access. On a neurophysiological level the main topics are plasticity and the reorganization of the underlying representational brain areas. Plastic changes on a molecular, synaptic and neuronal level and reorganization of cortical “maps” have been demonstrated abundantly in animal studies. On a physical level the measured magnetic fields and the calculation of the source parameters of their underlying neural generators are discussed in the light of the neurophysiological and psychological phenomena. Therefore, the aim of this dissertation thesis was, to transfer the insights of animal plasticity research onto the human brain and to draw a connection line between discrimination learning and the underlying neurophysiological changes. In a second step, these effects of discrimination learning are tested on speech perception.
Table of Contents
1 Introduction
1.1 Overview
1.2 Delimitation of the topic
2 Theoretical framework
2.1 Cortical Plasticity
2.1.1 Basic principles of cortical plasticity
2.1.1.1 Synaptic plasticity - the weighting of synaptic strength
2.1.1.2 Long-term potentiation
2.1.1.3 Long-term depression
2.1.1.4 Axonal sprouting
2.1.1.5 Neurogenesis in enriched environments
2.1.2 From synapses to representational maps
2.1.3 Reorganization of cortical maps by deprivation
2.1.4 Reorganization after lesions in sensory areas
2.1.5 Experience-dependent plasticity
2.1.6 Reorganization of cortical maps after training
2.1.7 Reorganization of cortical maps in experienced learners
2.2 From cell assemblies to spatiotemporal patterns
2.3 Perceptual discrimination learning
2.3.1 Discrimination learning
2.3.2 Perceptual learning
2.4 Auditory sensory memory
2.5 Conclusions
3 Methodology
3.1 Basic Principles of MEG
3.2 Event Related Potentials and Fields
3.2.1 Dipole source model
3.2.2 The sensitivity of AERs for change
3.2.3 The Mismatch Negativity
3.2.3.1 The MMN change detection mechanism
3.2.3.2 Sources of MMN
3.2.3.3 Differential Impact of Native Language on the MMN
3.3 Conclusions
4 Experimental work
4.1 Experiment 1: Plastic changes as a result of frequency discrimination learning
4.1.1 Theoretical framework
4.1.2 Materials and Methods
4.1.2.1 Subjects
4.1.2.2 Discrimination Training
4.1.2.3 Discrimination Test
4.1.2.4 MEG Measurements
4.1.3 Results
4.1.3.1 Psychophysics
4.1.3.2 The Mismatch Field
4.1.3.3 The N1m responses
4.1.4 Discussion
4.1.5 Conclusions
4.2 Experiment 2: Plasticity due to discrimination learning of non-native mora-timing
4.2.1 Theoretical framework
4.2.2 Materials and methods
4.2.2.1 Subjects
4.2.2.2 Stimuli
4.2.2.3 Categorization test
4.2.2.4 Discrimination test and training
4.2.2.5 MEG recordings
4.2.3 Results
4.2.3.1 Behavioral results
4.2.3.2 Evoked Neuromagnetic Responses
4.2.4 Discussion
4.3 Experiment 3: Short-term plasticity in native speakers
4.3.1 Methods
4.3.2 Materials and methods
4.3.2.1 Subjects
4.3.2.2 Stimuli
4.3.2.3 Discrimination test and training
4.3.2.4 MEG measurements
4.3.3 Results
Behavioral results
4.3.3.2 Evoked Responses
4.3.4 Discussion
5 General Discussion
5.1 Considerations of methodological limitations
5.2 Final remarks on the ends of plasticity
5.3 Summary
Research Goals and Objectives
This dissertation investigates whether intensive discrimination learning triggers functional and structural plastic changes in the human auditory cortex, specifically examining how such learning modulates neuromagnetic brain responses in both native and non-native speakers.
- Documentation of changes in cortical evoked responses induced by discrimination learning using Magnetoencephalography (MEG).
- Application of the Hebbian plasticity model to explain how synaptic weight changes lead to cortical reorganization.
- Comparison of neurophysiological responses in subjects undergoing frequency discrimination training versus speech feature discrimination (mora-timing).
- Evaluation of short-term versus long-term plastic changes in the auditory cortex.
- Assessment of the role of native language experience in shaping pre-attentive auditory discrimination processes as reflected by the Mismatch Negativity (MMN).
Excerpt from the Book
2.1.1 Basic principles of cortical plasticity
Cortical plasticity can be defined as the potential of the neocortex its function by strengthening or loosening synaptic connections. At the end of the nineteenth century, the psychologist William James suggested that learning might alter synaptic connectivity (James, 1890). Half a century later, Donald Hebb developed a model, which assumed that learning and memory are based on changes of synaptic efficacy (Hebb, 1949). The tenet of Hebb's theory was that simultaneously active adjacent pre- and postsynaptic neurons mutually contribute to a strengthening of their synaptic connections. Non-simultaneously active adjacent neurons do not strengthen their connections. “Cells that fire together, wire together”.
Learning occurs throughout life. In terms of plasticity, learning can be conceived as the induction of neural change through experience and memory as the storage and application of these changes. The mechanisms underlying these changes are briefly explained in the next section. First, a short overview over the basic mechanisms and forms of plasticity that occur on the pre- and postsynaptic interface is given (2.1.1.1-2.1.1.4). The special contribution of enriched environment to plastic changes in the brain is described in the next section (2.1.1.5).
Summary of Chapters
1 Introduction: Provides a general overview of the research purpose, the choice of MEG as a methodology, and defines cortical plasticity within the context of the thesis.
2 Theoretical framework: Outlines the mechanisms of cortical plasticity, including synaptic models, Hebbian learning, and the reorganization of representational maps based on animal and human research.
3 Methodology: Details the principles of Magnetoencephalography (MEG) and Event Related Potentials/Fields (ERP/ERF), specifically focusing on the Mismatch Negativity (MMN).
4 Experimental work: Presents three experiments investigating frequency discrimination and mora-timing learning, evaluating behavioral performance and neurophysiological correlates.
5 General Discussion: Synthesizes the experimental findings, discusses methodological limitations, and offers final remarks on the nature and limits of brain plasticity.
Keywords
Cortical Plasticity, Auditory Cortex, Magnetoencephalography (MEG), Mismatch Negativity (MMN), Discrimination Learning, Hebbian Learning, Synaptic Plasticity, Speech Perception, Mora-timing, Sensory Memory, Brain Reorganization, Perceptual Learning.
Frequently Asked Questions
What is the primary focus of this thesis?
This work explores the neurophysiological correlates of learning-induced plasticity in the human auditory cortex, examining whether intensive discrimination training can trigger measurable changes in the brain.
What are the central thematic fields?
The core themes include neuroplasticity, auditory perception, discrimination learning (both frequency and speech features), and memory formation at the synaptic and cortical levels.
What is the primary research goal?
The goal is to transfer insights from animal plasticity research to the human brain, specifically determining how discrimination training influences auditory evoked responses.
Which scientific methods are employed?
The study primarily utilizes Magnetoencephalography (MEG) to track neuromagnetic changes, complemented by psychophysical testing to assess behavioral discrimination performance.
What does the main body cover?
The main body details three distinct experiments: frequency discrimination training, the learning of non-native Japanese mora-timing in German subjects, and an investigation into short-term plasticity in native Japanese speakers.
What characterizes the work's core terminology?
Key concepts include the "Mismatch Negativity" (MMN) as a change detector, the Hebbian "cell assembly" model, and "reorganization" of cortical representational maps.
How do Japanese and German subjects differ in their perception of mora-timing?
Japanese speakers, having lifelong experience with the moraic structure, show higher sensitivity and earlier MMN responses compared to non-native German speakers, who exhibit significant learning gains after intensive training.
What does the study suggest about the "limits" of plasticity in adulthood?
The study suggests that while the adult brain remains malleable, the effects of short-term training in native speakers are less pronounced than in non-native learners, likely due to pre-existing, highly automated language-specific memory traces.
- Quote paper
- Hans Menning (Author), 2002, Plasticity in auditory cortex on the grounds of learning discrimination, Munich, GRIN Verlag, https://www.grin.com/document/33623
