Search Results

The differential effects of voluntary head movements (horizontal oscillations about the z axis) and varying head positions on postural sway are investigated in this chapter. The methods of the experiments are presented here. Ten subjects took part in the experiments, where their head movements were recorded using a head-fixed angular accelerometer. The main finding of the experiment is that the differential effects of varying head positions were surprisingly small. Moreover, the body sway does not significantly increase with head rotation and that indicated the precise reevaluation of head sway with respect to the head position relative to the trunk.

The objective of this study is to evaluate the remaining range of head movement after surgery and to account for the behavior of the intervertebral levels beyond the site of arthrodesis. There is an accurate reduction of the fracture and/or dislocation. Moreover, compensatory motion can take place on either side of the site of operation. It is concluded that any surgical intervertebral fusion jeopardizes the range of motion of the cervical spine.

This chapter presents results concerning AH's head and eye movements and the consequences of these movements for her visual location perception. It shows that AH often moved her head and eyes in the wrong direction when attempting to orient toward a visual stimulus. It then reports a far more surprising result: AH's misperceptions of object location often remained stable across head and eye movements. For this latter result, the chapter offers a speculative interpretation concerning the processes that generate high-level visual location representations. Finally, it discusses the implications of AH's performance for issues concerning the levels of the visual system implicated in conscious visual experience.

Some of the functional properties of different otolith-ocular reflexes are discussed in this chapter, such as the linear vestibulo-ocular reflexes, which are activated by linear acceleration. The latter part of the chapter focuses on the interaction of otolith and canal signals during rotatory head movements. Stabilization of the line of sight and maintenance of spatial orientation requires appropriate transformation of sensory inputs of different modalities to oculomotor output. Rapid information about head movements and orientation in space is provided by the vestibular organs. The utricular and saccular otolith organs detect linear accelerations of the head, and hence convey information about translatory head movements as well as about head orientation relative to gravity, while the semicircular canals, activated by head angular accelerations, provide afferent information. Otolith signals interact with semicircular canal signals in at least two important ways involving the velocity storage system. First, otolith signals are processed to complement the canal-ocular reflexes at frequencies of head rotations below 0.1 Hz. Second, static otolith input interacts through the velocity storage integrator with head velocity signals to provide a spatial reference about head angular velocity. The otolith organs provide complementary signals that are used to detect the direction of the head rotation in space, i.e. relative to gravity, as well as eccentricity of rotation during fast head movements. Dynamic otolith signals can initiate appropriate transformations of canal-driven head velocity signals to account for eccentricity of ocular rotation and target distance.

The vestibular nuclei receive vestibular-related information from canal and otolith afferents as well as from the vestibulocerebellum. In addition and in contrast to other sensory systems, early vestibular processing is highly convergent and strongly multimodal. Notably, the vestibular nuclei receive substantial visual, somatosensory, and oculomotor inputs. Multimodal interactions, which also occur elsewhere in the central vestibular pathways, are vital for spatial perception and for gaze and postural stabilization. Because oculomotor signals play an important role in shaping the information encoded centrally, knowledge of how the brain controls eye movements is crucial for understanding vestibular processing. This chapter provides an overview of oculomotor processing. Topics discussed include an overview and classification of eye movement types, ocular structure and functional implications, gaze redirection, gaze stabilization, and interactions between eye and head movements.

This chapter begins by examining the connections between remnant movement, scrambling, and restructuring. It introduces a number of phenomena and concepts essential for the description of the syntactic structure of West Germanic as well as for the understanding of the discussion of restructuring infinitives. It discusses the essential properties of and the relevant restrictions on remnant movement. It also talks about the interaction between remnant movement and head movement. It argues that remnant categories created by head movement cannot undergo further movement and show how this restriction can be derived from Attract Closest as well. It demonstrates that the original account by Den Besten and Webelhuth is flawed. It argues that remnant VPs in German are created by licensing movement of VP-internal material into dedicated licensing positions in the lower middle field. It outlines the core ideas and concepts that were adopted and indicates the account of individual phenomena.

This book has explored head movement based on the central idea of minimal phases, and has argued that it cannot and should not be eliminated from narrow syntax, therefore implying a reconsideration of recent suggestions to replace certain cases of head movement with remnant movement and/or PF-movement. This concluding chapter reexamines Noam Chomsky’s objections to head movement, and considers the Extension Condition, which Chomsky claims is violated by the derived structure of head movement. The book has argued that its effects derive from Edge Features, in which case it is satisfied by some cases of head movement, but not by other types of head movement (those triggered by Agree and defectivity). This is similar to what can be observed with XP-movement: EF triggers A0-movement but not A-movement; the former thus obeys the Extension Condition, while the latter does not.

This chapter explores how head movement relates to the general theory of movement, focusing on “excorporation” and the Head Movement Constraint (HMC). Whereas excorporation is permitted in head movement only under highly constrained conditions, the HMC is nonexistent. As in all other cases of movement, head movement is subject to the locality conditions imposed by the Phase Impenetrability Condition and the general ban on interveners that is essential to the locality condition on Agree. The chapter also considers the possibility of head movement triggered by Edge Features (also known as “A0” head movement), and argues that such cases may exist, as “head topicalization/focalization” and as “wh-head movement.”

This chapter discusses phenomena that have been appealed to in work advocating ‘head‐to‐head movement’. It is argued that such a mechanism further weakens the restrictiveness of syntax, despite introducing further tracking-devices. On the basis of a discussion of English and French, an alternative analysis of ‘movement to Infl’ and ‘movement to Comp’ is offered. The latter is provided with the non‐movement lexical account already appealed to in the discussion of interrogatives in Chapter 4. Analysis of the former invokes differences between languages in how much argument incorporation they permit in the lexicon, and this is illustrated from a range of languages. French is interpreted as involving incorporation of pronominal complements that may be coreferential with an adjunct, where English has a complement. This accounts for various differences between the languages in adverb position.

This chapter reviews current ideas on the neural control of eye and head movements, with special emphasis on the role of the superior colliculus. The problem of how gaze shifts are controlled has too many aspects to be grasped in a single model, and it is therefore not yet possible to develop a model that does justice to the complexity of the neural control system and which can account for its rich repertoire of capabilities. A more realistic approach is to concentrate on a few well-studied aspects of system behavior that can be isolated as challenges for a much more limited endeavour. The models discussed here concentrate on the role and the neural implementation of internal feedback, the transformation from topographically to temporally coded signals (and vice versa), the neural implementation of Listing's law, and the collicular role in the co-ordination of combined eye–head movements. Although each of these models concentrates on a particular set of aspects of the total problem and ignores others, taken together they can serve to illustrate how the signal processing at various levels in the saccadic control system can be viewed from different angles, and to clarify the issues surrounding the collicular role in the control of gaze. Before these models can be discussed, we will have to review earlier relevant models.

Although human motor performance appears to be remarkably flexible and easy, the underlying neuronal operations are only vaguely understood, even for well-studied eye, head, and arm movements. The same flexibility of the human motor system, which provides animals and man with a large repertoire of motor behavior such as running, eating, and mating, is also one of the obstacles in understanding the basic principles that underlie their motor behavior. Normal motor behavior really needs the complex organization of muscles and joints, and one can certainly not make the general claim that the motor system is redundant. Given the flexibility and the large number of degrees of freedom in the motor system, one might wonder what happens for simple motor tasks such as grasping for a nearby ball. A ball in the same position can be grasped in various ways by different combinations of joint angles in the wrist, elbow, and shoulder. Is the same motor task realized by movements that are randomly chosen from the available repertoire of movements which might do, or is there a consistent reproducible pattern of motor behavior? If the latter is the case, can the constraints that are imposed in order to reproduce the same motor behavior every time when the same motor task is repeated, be understood? This chapter addresses these questions, aiming to give a review of some recent and important experimental data on this topic and to provide a theoretical framework on how to interpret this data.

This chapter touches upon some issues concerning the coordination between visual information and motor action, and is divided into three parts. The first one deals with certain aspects of the interaction between motor processes and perception. Although this interaction has attracted the interest of both psychologists and physiologists for more than a century, unambiguous experimental data have not been abundant. The chapter illustrates a new approach to the study of this topic, which has emerged in recent years. The second part of the chapter considers the role that cognitive representations may have in establishing the interface between perceptual inputs and motor outputs, and the third part discusses the problem of the appropriate dimensionality of visuomotor control space. The issues considered in the last two sections are discussed within the context of one specific perceptuo-motor performance, namely visuomanual pursuit tracking. With the exception of touch and kinaesthesis, all perceptual systems feed on sensory data originating from organs located in the head. Moreover, in all mammals, at least one major exteroceptive sense, vision, is highly directional. As a consequence, head and eye movements are in most cases required to bring into sharp focus a specific source of information and to stabilize it against relative displacements with respect to the retina. Head and eye movements also provide the basis for active exploration of the environment. Exploratory actions generally entail sequences of movements, to each of which is associated an information intake.

Historically, the research on head movement has emphasized the central mechanisms ruling the neck motoneurons activity. In the last decade, however, new knowledge about neck muscles has led to an increasing awareness that muscle properties also affect how these motoneurons must be recruited to achieve the needed head movements. Individual muscles differ in their cross-sectional areas and pulling directions, and they also contain different proportions of fast and slow fiber types and have highly specialized patterns of fiber architecture and motor unit distribution. All these must be taken into consideration when trying to comprehend the neural control of the head movement.

This chapter only deals with the immediate premotor neuronal organization of this coordination at the level of the brain stem. It is known that the activity of the superior colliculus contributes to the initial saccade and the head movement. The afferent collicular neurons of the crossed tectoreticulospinal pathways activate a network of neurons in the brain stem that produces the premotor neuronal signals. It is well established that the signals that are carried by descending tectoreticulospinal neurons (TRSNs) are mainly phasic bursts. It is discussed that their firing rate is related to either eye velocity or dynamic motor error signals.

This chapter clarifies that “Listing's Law”, which governs the rotations of voluntary eye movements, also governs specific types of voluntary head movements. Listing's law for the eye states that there is a “displacement plane” associated with the orientation assumed by the eye, and there is exactly one eye position p in which the displacement plane is orthogonal to the line of sight. The analogy of Listing's law for the head states that there is an associated displacement plane DPh such that the head assumes only those positions from the reference position by rotating about the axis lying on DPh. This chapter also presents the methods of the experiments performed on seven human subjects where their three-dimensional head position and velocity vectors were measured using Robinson's magnetic field-search coil technique. The primary finding of this study is the Listing's Law for the head.

This paper explains the results of Zangemeister et al. The experimental and modeling results of Hannaford et al. were also reassessed with respect to normally fast and very fast time-optimal movements. More importantly, this study employs mathematical and manipulation analysis of the model, specifically the threefold approach of sensitivity analysis to gain valuable insights about the pathologic features of clinical neurologic deficits. The modification of an existing model is also presented and briefly discussed in this chapter. This chapter concludes that applying powerful mathematical tools such as threefold sensitivity analysis to analytic models is helpful in explaining and treating disorders of motor control.