Ординатура / Офтальмология / Английские материалы / Imaging of Orbital and Visual Pathway Pathology_Muller-Forell_2005

Fig. 2.37. Axial T1-weighted MRI of the optic nerve, demonstrating the tortuous course of the left as a normal variant

2.7

Intracranial Visual Pathway

2.7.1

Optic Tract (Figs. 2.38, 2.39)

Dorsolaterally of the chiasm, the optic visual pathway continues backwards, forming the optic tracts. They cross posterior to the anterior perforated substance and anterior to the lateral tuber cinereum and the lateral perforated substance, respectively (Duvernoy 1998). More dorsolaterally, the optic tracts lie above the uncus and surround the crus cerebri. The main part of the tracts ends in the lateral geniculate nuclei (Figs. 2.43, 2.44). Small divisions of the tracts terminate in the superior colliculus, in the pretectal area, in nuclei of the accessory optic tract, and in the hypothalamic suprachiasmatic nucleus (Nieuwenhuys et al. 1988). The brachium colliculi superiores, which courses below the lateral pulvinar of the thalamus between the lateral geniculate nucleus and the colliculus superior, also contains others fibers that connect the lateral geniculate nucleus with the colliculus superior. On high resolution MRI, the lateral geniculate nucleus and the brachium colliculi superioris (Fig. 2.44) can be depicted (Nieuwenhuys et al. 1988; Horton et al. 1990; Duvernoy 1998).

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2.7.2

Optic Radiation (Figs. 2.45, 2.46)

About half of the fibers that originate in the lateral geniculate nucleus and project to the occipital visual cortex run directly backward lateral to the more superior aspect of the lateral ventricle within the inferior parietal lobe. The other half of the fibers describes a large forward directed loop (Meyer’s loop) within the temporal lobe above and lateral to the anterior temporal horn of the lateral ventricle. Then the fibers curve backward lateral to the temporal and occipital horn of the lateral ventricle. The fibers of the optic radiation are separated by the interposed tapetum from the lateral wall of the ventricle. The tapetum is a thin layer composed of fibers from the forceps major of the callosal radiation. Tapetum and optic radiation together form a thin caudo-cranially directed plate of fibers, termed the sagittal stratum. On T2-weighted MRI, it is possible to identify and trace systems of parallel coursing, compact, myelinated fibers as the optic radiation, which are embedded in more loosely arranged white matter (Curnes et al. 1988). On MRI in most cases, it is not possible to subdivide the sagittal stratum and differentiate the optic radiation and the tapetum. Only under special conditions, if there is edema or a gliosis, can they be separated on T2-weighted MRI (Kitajima et al. 1996). Behind the sagittal stratum, the myelinated fibers of the optic radiation bend medially to project on the visual cortical field and diverge in a fan-like manner, no longer depicted on MRI as the fibers are looser than the optic radiation.

Fig. 2.45.a Axial T2-weighted view of the brain at the level of the internal capsule. b Corresponding diagram: 12.6 = optic radiation, 13.10 = thalamus, 13.18 = frontal horns of the ventricles, 13.20 = foramen of Monro, 13.21 = third ventricle, 15.3 = vein of Galen

a

13.20 13.20

13.21

13.1013.10

b

a

13.313.3

b

Fig. 2.46.a Axial PD-weighted view of the optic radiation some millimeters above Fig. 2.45. b Corresponding diagram: 12.6 = optic radiation, 13.3 = occipital lobe, 13.7 = (posterior knee of the ) corpus callosum, 13.14 = fornix

contralateral visual field shows its representation below the horizontal meridian. The peripheral visual field is projected to the anterior part of this ellipsoid map where the calcarine fissure joins the parietooccipital fissure. The striate cortex around and lateral to the occipital pole contains the representation of the central visual field, especially of the fovea. An expanded area of the visual cortex is attributed to the central visual field, and a relatively small part is occupied by the peripheral visual field representation. This anatomic arrangement is called cortical magnification of the central vision (Holmes 1945). Correlative studies of the past few years, since functional MRI has been able to localize occipital lesions with attributed visual field defects in an anatomical way, have led to a postulated refinement of the classic retinotopic Holmes map. It seems that the abovementioned so-called cortical magnification of the central visual field was underestimated by Holmes (Horton and Hoyt 1991b; Wong and Shape 1999). A revised map of the retinotopic representation of the human visual striate cortex is shown in Figs. 2.49, 2.50 (Horton and Hoyt 1991a). According to the literature, high resolution MRI is able to identify in vivo the specific intracortical myeloarchitecture of the striate cortex that differs from the extrastriate cortex (Clark et al. 1992). This can be demonstrated by comparison of the striate and precentral cortex obtained from specimens (Fig. 2.51), where the myeloarchitecture shows a more distinct visibility than on in vivo MRI.

2.7.4

Extrastriate Visual Association Cortex

2.7.3

Striate Cortex (Figs. 2.47–2.50)

The primary visual cortex, the striate cortex, is located in the wall along the calcarine fissures, seen on the medial surface of the occipital lobe (Figs. 2.47, 2.48). It should be emphasized that the striate cortex wraps around the occipital pole to the posterolateral aspect of the occipital lobe. In other words, one can imagine folding back the calcarine fissure laterally and smoothing out the normally gyrated striate cortex, leading to an artificially flattened ellipsoid map of the visual cortex that is about 80–40 mm (Horton and Hoyt 1991b). The folding along the calcarine fissure indicates the horizontal meridian. The lower contralateral visual field is represented above the horizontal meridian, whereas the upper

Except anteriorly, the primary visual cortex (V1 or area 17) is surrounded by the secondary visual area V2 (area 18), which in turn is surrounded by the tertiary visual area V3 (area 19 corresponds partially to V3). These areas are designated the circumstriatal visual association cortex. Whereas the upper and lower quadrants of the primary visual cortex are in continuation within the depth of the calcarine fissure, the extrastriate cortex is divided into separate upper and lower quadrants flanking above and below the intervening primary visual cortex. Each quadrant of the bisected secondary visual cortex runs on both sides of the primary visual cortex along the upper and lower vertical meridians which are shared by the primary and secondary cortex. On the outer border of the secondary cortex, the quadrants of the bisected tertiary cortex are arranged along the horizontal

meridian that is shared by the adjoining secondary and tertiary cortex (Fig. 2.52). This explains why a lesion of the extrastriate cortex that crosses the horizontal meridian between areas V2 and V3 will respect the horizontal meridian and result in a perfect quadrantanopia. In this described condition, a precise

anatomical location or extension of the lesion is not required. In the case of a quadrantanopia, this does not necessarily imply that a lesion lies within the extrastriate cortex; it can also be caused indirectly by compression by a bordering tumor (Van Essen et al. 1986; Horton and Hoyt 1991a).

Anatomy

Fig. 2.52. Schematic coronal section through right occipital lobe showing connections from V1 (stippled area) to V2/V3 (small triangles and striped, respectively). Projections pass between common retinotopic coordinates. In V1, the horizontal meridian is represented along the base of the calcarine sulcus. Fibers coursing in the white matter of the upper and lower calcarine banks serve the lower and upper visual quadrants, respectively. For simplicity, feedback pathways from V2/V3 to V1 and projections between V2 and V3 are omitted. The ventral V1 and V3 pathway is doubtful (Van Essen et al. 1986). (With permission of Horton and Hoyt 1991a)

References

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CONTENTS

3.1Introduction 61

3.2.1Receptive Fields 64

3.3Eye Movements 74

3.3.1The Oculomotor Nuclei and the Extraocular Muscles 74

3.3.2Supranuclear Synchronization of Eye Movements 84

3.3.4.3Vergence 98

3.3.4.4Fixation 98 References 98

3.1 Introduction

The spectacular symbiosis between the visual system, the vestibular system, and the optomotor system, each of which is a miracle on its own, is able to solve an abundant array of amazing tasks (Fig. 3.1). In particular, the connection between vision and the optomotor control is most intriguing, as the eye performsboththeinputandtheoutputsimultaneously. Altogether,it is not surprising that the implementation of these intricate neural networks encompasses almost

every part of the brain (Fig. 3.2). Moreover, these systems are probably amongst the best investigated so far; and enormous knowledge has been amassed over the past few decades employing a variety of techniques from single cell recordings, optical imaging, eye movement recordings, functional magnetic resonance imaging, to neuropsychological examinations. In addition,this research has clarified many fundamental principles of neural and neuronal processing, which subsequently have enriched other fields of brain research as well as computational neuroscience.

The optomotor system provides a magnificent windowintothenervoussystemforclinicians,aspeople suffering from even the smallest disruption soon are unable to pursue their regular routines. Armed with thorough neuroanatomical and neurophysiological knowledge of this system’s workings,a clinician will be able to diagnose and localize many conditions right at the bedside.

A systematic review bearing on the whole spectrum of normal and pathological visual,vestibular,and optomotor neural processing would be more than one’s lifetime work. Hence, this improperly short primer is very selective, fractional, and undeniably biased. An attempt was made to seduce the reader to dive further into this fascinating realm of neuro-ophthalmology by presenting a basic framework of functional neuroanatomy. The basic neuroscientist will find plenty in the works of Carpenter (1988), Zeki (1993), Milner and Goodale (1995), Wandell (1995), Zigmond et al. (1999), Kandel et al. (2000); the clinician, on the other hand, will find more answers in the seminal oeuvres (Baloh and Honrubia 1990; Grüsser and Landis 1991; Miller and Newman 1997; Huber and

Kömpf 1998; Leigh and Zee 1999).

3.2

Vision and Visual Perception

photoreceptors.A geometrically well-defined array of these highly specialized cells connects via an intrinsic local network to one single ganglion cell, thus implementing the first receptive field of the visual system. Consecutively, signals of the retinal ganglion cells are transmitted through their axons, which together form the optic nerve, to the optic chiasm in front of the pituitary stalk, where half of them are

cross routed: Axons from the temporal half of one retina join axons from the nasal part of the other retina in the optic tract. Axons from the temporal retina remain ipsilateral. Hence, visual objects in one hemifield, which are projected nasally onto the ipsilateral retina and temporally onto the contralateral retina, are processed in contralateral central visual centers with information coming from both retinae.