The Parietal Lobe

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• Publisher: Elsevier
• Year: 2018
• Print ISBN: 9780444636225
• eBook ISBN: 9780444636249
• Topic: Medicine
• Subtopic: Neurology

The Parietal Lobe, Volume 151, the latest release from the Handbook of Clinical Neurology series, provides a foundation on the neuroanatomy, neurophysiology and clinical neurology/neuropsychology of the parietal lobe that is not only applicable to both basic researchers and clinicians, but also to students and specialists who are interested in learning more about disorders brought on by damage or dysfunction. Topics encompass the evolution, anatomy, connections, and neurophysiology, the major neurological and neuropsychological deficits and syndromes caused by damage, the potential for improvement via transcranial stimulation, and the role of the parietal in the cerebral networks for perception and action.- Provides a broad overview of the neuroanatomy, neurophysiology and clinical neurology of this region of the cortex- Offers additional insights regarding the role of the parietal in the cerebral networks for perception and action- Addresses the most frequent complications associated with damage, including somatosensory, perceptual, language, and memory, deficits, pain, optic ataxia, spatial neglect, apraxia, and more- Edited work with chapters authored by global leaders in the field- Presents the broadest, most expert coverage available

Section II

Neurologic and neuropsychologic deficits after parietal lobe damage

Chapter 9

Somatosensory deficits

Carsten M. Klingner^1,2,*; Otto W. Witte^{1 1} Hans-Berger Department of Neurology, Jena University Hospital, Jena, Germany
² Biomagnetic Center, Hans-Berger Department of Neurology, Jena University Hospital, Jena, Germany
^* Correspondence to: PD Dr. Carsten M. Klingner, Hans-Berger Department of Neurology, Jena University Hospital, Erlanger Allee 101, 07747 Jena, Germany. Tel: + 49-3641-9323559, Fax: + 49-3641-9323402 email address: [email protected]

Abstract

The analysis and interpretation of somatosensory information are performed by a complex network of brain areas located mainly in the parietal cortex. Somatosensory deficits are therefore a common impairment following lesions of the parietal lobe. This chapter summarizes the clinical presentation, examination, prognosis, and therapy of sensory deficits, along with current knowledge about the anatomy and function of the somatosensory system. We start by reviewing how somatosensory signals are transmitted to and processed by the parietal lobe, along with the anatomic and functional features of the somatosensory system. In this context, we highlight the importance of the thalamus for processing somatosensory information in the parietal lobe. We discuss typical patterns of somatosensory deficits, their clinical examination, and how they can be differentiated through a careful neurologic examination that allows the investigator to deduce the location and size of the underlying lesion. In the context of adaption and rehabilitation of somatosensory functions, we delineate the importance of somatosensory information for motor performance and the prognostic evaluation of somatosensory deficits. Finally, we review current rehabilitation approaches for directing cortical reorganization in the appropriate direction and highlight some challenging questions that are unexplored in the field.

Keywords

somatosensory; thalamus; parietal lobe; pathways; deficits; rehabilitation

Introduction

Perception and interpretation of somatosensory information are general requirements in human life. Although the impression of touch is considered as one of the five traditional senses, this impression is formed from a multitude of different somatosensory modalities, including touch, pressure, vibration, temperature, pain, and skin stretch. Information for each of these modalities is transmitted from the receptors via sensory nerves that ascend through the spinal tracts and thalamus to the parietal lobe.

The processing of somatosensory information is performed in a complex interplay between different parietal cortical regions and the thalamus (Klingner et al., 2015b). A lesion at any stage of the forwarding or interpretation of somatosensory information alters the processing in the parietal cortex, leading to somatosensory deficits. While impairments in the early stages of somatosensory transmission also change the interpretation at later stages of the cortical processing (Caselli, 1991), in this chapter we will discuss impairments at the thalamic and parietal lobe levels of the processing hierarchy. It is important to note that somatosensory information is mostly interpreted in conjunction with information from other sensory modalities. Hence, the somatosensory network is closely interlinked with all other sensory networks. Particularly, visual information can modify the somatosensory experience by altering the context in which somatosensory information is interpreted (Schaefer et al., 2006; Torta et al., 2015;Van der Biest et al., 2016). Likewise, impairments in other brain systems can alter the associative interpretation of somatosensory information (Mundinano and Martinez-Millan, 2010).

In clinical practice, the testing of somatosensory functions is one of the most important, but also most difficult, aspects of the neurologic examination and requires a detailed knowledge of the anatomy and functions of the somatosensory system. The multitude of somatosensory modalities and the complexity of their transmission and processing lead to very complex, but nevertheless, remarkably differentiable patterns of impairment. Therefore, the exact investigation of somatosensory deficits in the clinical examination provides important information to determine the anatomic location of the lesion causing the deficits and avoid unnecessary diagnostic tests (e.g., magnetic resonance imaging (MRI)/computed tomography from multiple body regions). Somatosensory deficits are a major complaint in most peripheral lesions, and somatosensory deficits due to central brain lesions have prevalence rates between 65% and 100% (Carey et al., 1993; Kim and Choi-Kwon, 1996; Rand et al., 2001; Connell et al., 2008). This variability is caused by differences in the definitions and assessment of somatosensory deficits. Outside of scientific studies, impaired sensory function is often underdiagnosed. Even in patients who were diagnosed with a pure motor stroke identified by a neurologic screening examination, sensory dysfunction was found in 88% of cases (Kim and Choi-Kwon, 1996).

The outstanding importance of somatosensory deficits in neurologic examinations, and their high prevalence, is accompanied by relatively minor importance in the neurologic rehabilitation of somatosensory deficits, as compared to motor deficits. However, impaired somatosensory input, and particularly proprioceptive information, was shown to severely impair motor performance, especially in goal-directed movement, grasping (Gentilucci et al., 1994), reaching (Gordon et al., 1995), balance (Sainburg et al., 1995), and locomotion (Dietz, 2002). Somatosensory deficits can severely exacerbate functional deficits, even in patients with good motor function (Doyle et al., 2010). Likewise, the ability to acquire new motor skills during rehabilitation depends heavily on the functioning of the somatosensory system (Vidoni and Boyd, 2009).

The purpose of this chapter is to provide a comprehensive overview of knowledge about somatosensory deficits along with the underlying anatomic and functional features of the somatosensory system.

We further summarize the clinical presentations, examination, differential diagnoses, and therapy of sensory syndromes. Herein, impairments and lesions located in the parietal lobe will be of particular importance. To integrate all findings within a theoretic framework, we briefly review the theory of cortical responses and predictive coding (Friston, 2005), and use this theory to explain the available experimental data.

Somatosensory processing in the parietal lobe

Principles of somatosensory learning and inference by the brain

This section considers the current knowledge about the processing of somatosensory information. Before we review the anatomic and functional properties of the somatosensory system, we will briefly discuss the broader framework of what the brain attempts to achieve.

The overall goal of somatosensory processing is to infer the cause of a stimulus. In this context, the cause is any process in the world that generates the somatosensory data, which can be another person who touches us, or the movements of our fingers that touch the keyboard while we are writing these lines. We are able to obtain a conscious impression about specific features of a somatosensory stimulus, e.g., “the fabric is silky soft and pleasant to touch.” However, from an evolutionary perspective, the ability to sense the different somatosensory properties of an object is mainly used to improve the inference about the object itself. It is important to note that the ability to infer the correct cause of a stimulus requires the availability of a representation of the world and a representation of the process that caused the stimulus. Consider, for example, a light pressure on our back. The brain has to embed this new information into the existing representation of our surrounding (“Is someone standing behind us and are they touching us, or are we in a wood and have taken a step back?”). Often the information delivered by a simple somatosensory stimulus is not sufficient to infer its cause.

Current theories about how the brain infers the cause of a stimulus suggest that the brain has learned a generative model of our world (Friston, 2005). This includes representations of processes that generate specific somatosensory stimuli. To infer the cause of a stimulus, the brain tries to invert its generative model. However, the combinatorial explosion of how stochastic generative models can generate input patterns leads to a “many-to-one” relationship between causes and inputs. This indeterminacy renders the problem of inferring causes from sensory information ill posed, in that there is no uique solution. Because of the “many-to-one” relationship and the often insufficient sensory data, making inferences about the possible causes requires an internal representation of the probability of the specific causes given the data. Mathematically, this process can be described with empiric Bayes and hierarchic models that provide an account for many aspects of cortical organization and responses (Friston, 2005; Friston and Kiebel, 2009). In recent years, this theory of brain responses has bec...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Handbook of Clinical Neurology 3rd Series
• Foreword
• Preface
• Contributors
• Section I: Neuroanatomy and neurophysiology of the parietal lobe
• Section II: Neurologic and neuropsychologic deficits after parietal lobe damage
• Section III: The parietal lobe and brain networks for action and perception
• Index