ISSN:0006-8977    

DOI:10.1159/000116569

出版商:S. Karger AG    

年代:1988

数据来源: Karger

ISSN:0006-8977    

DOI:10.1159/000116570

出版商:S. Karger AG    

年代:1988

数据来源: Karger

摘要:

Electroreceptors, distributed over the body surface of weakly electric fish, code the local amplitude and phase, or timing of zerocrossing, of the animal's electric signals. These signals are generated by rhythmic discharges of the electric organ and form a dipole-like field around the animal. This field is perturbed by interference with electric fields of other fish as well as by the appearance of objects electrically different from water. The spatial and temporal structure of such perturbations can be interpreted as the electric image of interfering fields and moving objects. This strategy of assessing the environment is called 'electrolocation', a form of 'seeing' with the body surface. Electric images are analyzed in somatotopically ordered strata of neurons within the central nervous system. Primary electrosensory afferents project to somatotopically ordered layers of higher-order neurons in the electrosensory lateral line lobe (ELL) of the hindbrain. Phase and amplitude information are processed in separate layers of the ELL. The phase of the signal in a given region of the body surface is coded by the timing of spikes of spherical cells marking the zerocrossings of the electric signal. This phase information is relayed to lamina 6 of the torus semicircularis of the midbrain. Rises and falls in local amplitude are coded by the activity of different pyramidal cell types, E- and I-units, which project to various laminae of the torus above and below lamina 6. The somatotopic organization of the torus allows for computations of spatial patterns in electrosensory information. Within lamina 6, differences in the phase of signals from different parts of the body surface are computed. Differential-phase information is then relayed to deeper laminae of the torus and remains in topographic register with amplitude information. This organization allows for joint evaluation of spatially related patterns of amplitude and phase modulations on the animal's body surface within local neuronal circuits of the torus. A topographic projection of the torus relays amplitude and differential-phase information to the optic tectum where a further joint evaluation of amplitude and phase serves to control behavioral responses. The control of a particular behavioral performance, the 'jamming avoidance response', is of a distributed nature in that the representations of individual sites on the body surface contribute cumulatively to shift the electric organ pacemaker frequency. Due to the simplicity of this behavior, no further mapping of information is required at the level of the motor output other than a functional separation of neurons which either lower or raise the pacemaker frequency. This system appears void of so-called 'pontifical' neurons that are individually instrumental for behavioral decisions. Instead, individual neurons appear to be of little importance and, in some cases, even provide ambiguous, if not erronous information. As a result, monitoring of individual neurons reveals little about the stimulus situation outside, and only the mass action of large assemblies of such neurons can bring about correct behavioral responses.

ISSN:0006-8977    

DOI:10.1159/000116571

出版商:S. Karger AG    

年代:1988

数据来源: Karger

摘要:

The vagal lobe of goldfish and some carps is a laminated, specialized lobe of the midmedulla containing both primary sensory terminals and primary motor neurons. Both the sensory and motor components are represented in the lobe in a matching, orotopic fashion, i.e. the oral cavity is mapped across the surface of the lobe. Anatomical tracing studies reveal that the circuitry exists for a point-to-point reflex system in which the superficial sensory layers are mapped directly onto the underlying motor layer. The utility of this relatively direct sensorimotor coupling appears to be in terms of sorting food within the mouth according to its gustatory properties. The direct coupling between the mapped sensory layer and the similarly mapped motor layer may be a useful model in which to study the evolutionary development of less tightly coupled sensorimotor systems.

ISSN:0006-8977    

DOI:10.1159/000116572

出版商:S. Karger AG    

年代:1988

数据来源: Karger

摘要:

The vestibulo-ocular reflex is a compensatory reflex that results in eye movements that are 180° out of phase with movements of the head but that match head velocity. Because of these reflex eye movements that are equal, but opposite to head movement, the viewed object remains on the fovea of the retina during head movement, thus resulting in visual acuity that is not degraded by visual image slip on the retina. This reflex is compensatory over a large spectrum of head movements in any plane of space. This is accomplished by a spatial and temporal transformation of the input from the vestibular semicircular canals to the motoneurons that innervate the extraocular muscles. The reflex is a three-neuron arc. The middle leg of the reflex is accomplished by secondary vestibular neurons whose axons branch to innervate more than one extraocular muscle. These secondary neurons thus program an eye movement rather than the contraction of a single extraocular muscle. These programmed eye movements that match the plane of the particular semicircular canal that is the input to the reflex constitute the spatial transformation. Primary vestibular afferents innervating the semicircular canals have a broad range of response dynamics that either lead, lag, or are in phase with head velocity. The predominant vestibular primary afferent input to the middle leg of the reflex, the same secondary neurons as mentioned above, is parcellated so that afferents more in phase with head velocity predominate. Because the eyeball presents mainly a viscous and elastic impedance to movement (rather than an inertial load), an input in phase with head velocity should produce a matching eye velocity output. The temporal transformation, matching head and eye velocity, is thus accomplished via a kind of impedance-matching of incoming signals to the 'motor impedance' of the ocular globe.

ISSN:0006-8977    

DOI:10.1159/000116573

出版商:S. Karger AG    

年代:1988

数据来源: Karger

摘要:

This article reviews some recent findings on the character of the neuronal organization lying between the optic tectum and motor pattern-generating circuitry in the case of orienting behaviors. It focuses on frogs but notes parallels to existing work on saccade control in mammals and suggests some additional ones for further exploration. In general, the map-like function of orienting does not appear to be subserved by a comparable map-like organization. It is argued that the current conceptual vocabulary for describing interface organization (sensory map, motor map, pattern-generating circuitry) is inadequate and that some additional concepts (activity-gated divergence, intermediate spatial representation) are necessary. Finally, some questions are raised about the appropriateness of the term 'motor map’.

ISSN:0006-8977    

DOI:10.1159/000116574

出版商:S. Karger AG    

年代:1988

数据来源: Karger

摘要:

This article reviews some recent findings on the character of the neuronal organization lying between the optic tectum and motor pattern-generating circuitry in the case of orienting behaviors. It focuses on frogs but notes parallels to existing work on saccade control in mammals and suggests some additional ones for further exploration. In general, the map-like function of orienting does not appear to be subserved by a comparable map-like organization. It is argued that the current conceptual vocabulary for describing interface organization (sensory map, motor map, pattern-generating circuitry) is inadequate and that some additional concepts (activity-gated divergence, intermediate spatial representation) are necessary. Finally, some questions are raised about the appropriateness of the term 'motor map’.

ISSN:0006-8977    

DOI:10.1159/000316031

出版商:S. Karger AG    

年代:1988

数据来源: Karger

摘要:

The sudden onset of a novel or behaviorally significant stimulus usually triggers responses that orient the eyes, external ears, head and/or body toward the source of the stimulus. As a consequence, the reception of additional signals originating from the source and the sensory guidance of appropriate limb and body movements are facilitated. Converging lines of evidence, derived from anatomical, electrophysiological and lesion experiments, indicate that the superior colliculus is an important part of the neural substrate responsible for the generation of orienting responses. This paper briefly reviews the functional organization of the mammalian superior colliculus and discusses possible linkages between the sensory and motor maps observed in this structure. The hypothesis is advanced that the sensory maps are organized in motor (not sensory) coordinates and that the maps of sensory space are dynamic, shifting with relative movements of the eyes, head and body.

ISSN:0006-8977    

DOI:10.1159/000116575

出版商:S. Karger AG    

年代:1988

数据来源: Karger