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Biomedical subjects

M A Meredith

Publications and source records attributed to M A Meredith.

34 records · Page 2Linked to original sources
PubMed1992

Visual, auditory and somatosensory convergence in output neurons of the cat superior colliculus: multisensory properties of the tecto-reticulo-spinal projection.

A select population of superior colliculus (SC) neurons receives and integrates information from the visual, auditory and somatosensory systems. Determining which SC neurons comprise this population and where they send their multisensory messages is important in understanding the functional impact of the SC on attentive and orientation behavior. One of the major routes by which the SC influences these behaviors is the tecto-reticulo-spinal tract, a descending pathway that plays an integral role in the orientation of the eyes, ears and head. Of the 182 tecto-reticulo-spinal neurons (TRSNs) encountered in the present study, almost all (94%) responded to sensory stimuli and the overwhelming majority (84%) were multisensory. The present results demonstrate that the TRSN serves as an important link among the different sensory systems and provides a substrate through which they may gain access to the circuitry mediating orientation behavior.

Acoustic Stimulation↗
PubMed1991

Somatotopic component of the multisensory map in the deep laminae of the cat superior colliculus.

The topographic organization of the somatosensory representation in the deep layers of the cat superior colliculus was reexamined using methods previously used to examine the visuotopy in these layers. This technique identified the distribution of neurons in the superior colliculus that represent a designated region of the body surface (i.e., a dermal image), as well as assessed the differential distribution of deep layer neurons representing different body regions (e.g., face, forelimb, hindlimb, etc.). When the area of densest representation within a dermal image was considered, a well-ordered somatotopy was evident that was similar to the one previously described (Stein et al., '76: J. Neurophysiol. 39:401-419). Each region of the body surface, however, was represented within a surprisingly broad area of the deep layers, which often had considerable overlap with the representations of adjacent body regions. This organization was similar to that of the deep layer visuotopy and emphasizes that the representation of a peripheral stimulus is accomplished by the simultaneous activation of a large population of deep layer neurons. Furthermore, an examination of the convergence patterns on somatosensory-responsive neurons demonstrated that the somatotopy was formed primarily by multisensory neurons. These data indicate that the somatosensory representation is best considered as a component of a comprehensive multisensory functional unit that plays a critical role in effecting behavioral responses to a wide variety of stimuli.

Animals↗
PubMed1990

The visuotopic component of the multisensory map in the deep laminae of the cat superior colliculus.

A well-defined map of visual space is located in the deep laminae of the cat superior colliculus. The horizontal meridian is oriented rostral-caudal, while the vertical meridian is oriented perpendicular to it in the rostral third of the structure. This map represents the entire contralateral visual field and extends approximately 40 degrees into ipsilateral visual space. Although the deep-laminae visuotopy is similar to that found in the superficial laminae of the same structure, the topographic register among these maps is most secure rostrally but becomes increasingly poorer at more caudal and lateral locations. The combination of 2 features distinguish the deep-layer visual representation from that found in the superficial laminae and in geniculocortical systems: (1) the constituent visual receptive fields are very large (mean diameter, 66.9 degrees), and (2) the majority (greater than 70%) of the neurons composing it receive nonvisual inputs. Because the visual receptive fields of visual-multisensory neurons are significantly larger than those of neighboring neurons that respond only to visual stimuli, far more visual-multisensory neurons are activated by any given visual stimulus. These data, when coupled with those from previous studies, suggest that, from a functional perspective, deep-laminae visual neurons form one component of an integrated multisensory map, and that their topographic organization is essential for the normal dynamics of multisensory integration.

Animals↗
PubMed1989

Auditory cortical projection from the anterior ectosylvian sulcus (Field AES) to the superior colliculus in the cat: an anatomical and electrophysiological study.

Sensory activity in the deep layers of the superior colliculus (SC) is strongly influenced by descending cortical inputs. Elimination (permanent or reversible) of specific regions of visual or somatosensory cortex, known to have direct access to the SC, abolishes or dramatically reduces SC responses to stimuli from those modalities. While many SC neurons are also responsive to auditory cues, the origin of auditory corticotectal connections is not clear at present and their affect on activity in the SC is unknown. Therefore, the present study was undertaken to determine the origin, organization, and functional characteristics of auditory corticotectal projections. Of the auditory cortices (AI; AII; Fields A, P, and VP), only the auditory subregion of the banks of the anterior ectosylvian sulcus (Field AES) showed a robust anatomical projection to the SC. These data were confirmed physiologically: auditory neurons in Field AES projected to the SC and auditory SC neurons responded to stimulation of the Field AES. However, neither anatomical nor physiological techniques revealed a clear topographic relationship between the Field AES and the SC but suggested instead a diffuse and extremely divergent/convergent projection. Stimulation and cryoblockade of Field AES demonstrated the excitatory nature of this corticotectal pathway, whose influence was most evident on SC responses to stimuli of reduced intensity. Given the short latency of this ear-cortex-SC circuit and its excitatory influence on unimodal as well as on multisensory auditory neurons, it seems likely that Field AES plays a significant role in facilitating SC responses to auditory stimuli.

Acoustic Stimulation↗
PubMed1989

Does an extraocular proprioceptive signal reach the superior colliculus?

1. The primary functions of the superior colliculus (SC) are thought to include both the spatial localization of sensory stimuli and the initiation of an orienting response. It has been hypothesized that, in cat, both of these SC functions may be influenced by feedback from the extraocular muscles. The present investigation was initiated to determine which SC cells receive this extraocular muscle feedback and how this feedback influences the discharge properties of SC cells and their ability to integrate input from other sensory modalities. These questions were addressed in cats prepared with various anesthetic agents. 2. During the course of these experiments it became apparent that responses of SC cells to extraocular muscle stimulation could be elicited only under very specific conditions, and these observations questioned the existence of functional extraocular inputs to SC cells. 3. Rotating the eye or stretching the extraocular muscles was never found to be effective in activating SC cells unless the drug chloralose was used in the experimental preparation. In these chloralose-anesthetized animals, responses to eye rotation or muscle stretch were long and variable in latency, the discharge did not reflect the metrics of the stimulus, and the velocity and amplitude thresholds of these cells usually exceeded the cat's oculomotor range. Responses usually consisted of one to three impulses and, in appropriate conditions, could be inhibited by responses to visual or auditory stimuli. 4. The origin of this SC response to eye rotation/extraocular-muscle stretch could not be localized to the extraocular muscles. Responses to passive stretch of extraocular muscles were not eliminated by anesthetizing the muscles with injections of lidocaine. Active contraction of the extraocular muscles induced by electrical stimulation of the oculomotor nerve was never observed to evoke SC responses. Furthermore, transection of the muscle nerves, which isolated the extraocular-muscle receptors from the CNS, did not affect the response initiated by stretching the extraocular muscles. However, in the absence of intact muscle nerves, pulling the periorbital tissue elicited responses very much like those produced by stretching the muscles in the intact preparation, suggesting the periorbita as the source of responses in intact preparations as well. 5. These data are not consistent with the hypothesis that feedback from extraocular-muscle receptors influences the activity of SC cells.

Action Potentials↗
PubMed1988

Neurons and behavior: the same rules of multisensory integration apply.

Combinations of different sensory cues (e.g. auditory and visual) that are coincident in space enhance the responses of multisensory superior colliculus neurons, while the responses of these same neurons are depressed if the stimuli are separated in space. Using a behavioral paradigm modeled after that used in physiological studies, the present experiments demonstrate that the rules governing multisensory integration at the level of the single neuron also predict the responses to these stimuli in the intact behaving animal.

Acoustic Stimulation↗
PubMed1987

Determinants of multisensory integration in superior colliculus neurons. I. Temporal factors.

One of the most impressive features of the central nervous system is its ability to process information from a variety of stimuli to produce an integrated, comprehensive representation of the external world. In the present study, the temporal disparity among combinations of different sensory stimuli was shown to be a critical factor influencing the integration of multisensory stimuli by superior colliculus neurons. Several temporal principles that govern multisensory integration were revealed: (1) maximal levels of response enhancement were generated by overlapping the peak discharge periods evoked by each modality; (2) the magnitude of this enhancement decayed monotonically to zero as the peak discharge periods became progressively more temporally disparate; (3) with further increases in temporal disparity, the same stimulus combinations that previously produced enhancement could often produce depression; and (4) these kinds of interactions could frequently be predicted from the discharge trains initiated by each stimulus alone. Since multisensory superior colliculus neurons project to premotor areas of the brain stem and spinal cord that control the orientation of the receptor organs (eyes, pinnae, head), they are believed to influence attentive and orientation behaviors. Therefore, it is likely that the temporal relationships of different environmental stimuli that control the activity of these neurons are also a powerful determinant of superior colliculus-mediated attentive and orientation behaviors.

Acoustic Stimulation↗
PubMed1986

Spatial factors determine the activity of multisensory neurons in cat superior colliculus.

The responses of a neuron to stimuli from one sensory modality can be profoundly influenced by inputs from other sensory modalities. The present experiments demonstrate that the nature and the magnitude of these multisensory interactions depend on the positions of the stimuli in relation to their respective receptive fields. The spatial rules governing these interactions underscore the significance of the alignment of sensory maps in the brain.

Animals↗
PubMed1986

Contractile differences between muscle units in the medial rectus and lateral rectus muscles in the cat.

Conjugate eye movements in the horizontal plane are accomplished by the coactivation of the medial rectus (MR) muscle of one orbit and the lateral rectus (LR) muscle of the other. While control of these excursions has been thought to be effected by identical inputs to these muscles, recent studies have demonstrated that MR motoneurons receive different inputs than LR motoneurons. This raises the question of whether the character of the muscles they control are different. The present study evaluated the contractile properties of MR and LR muscle units in the cat. Based on the mechanical aspects of their contractile properties, only two physiological types of muscle units were identified within the MR and LR muscles: twitch and non-twitch muscle units. Twitch muscle units represented over 90% of the units sampled in each muscle. Significant differences in the rate-related and the tension-related contractile properties were demonstrated between MR and LR twitch muscle units. MR muscle units exhibited significantly faster twitch contractions than did LR units. The rate of stimulation at which MR units exhibited fused tetany was significantly higher than for LR units, although units from both muscles demonstrated similar rates of rise of tension at fusion. The rate of rise of tension was closely correlated to tension production (twitch and tetanus) in each muscle. However, MR muscle units demonstrated significantly weaker maximum tetanic tensions and lower tetanus-to-twitch ratios than LR units. These data indicate that while similar physiological types of muscle fibers are present within the MR and LR, MR muscle units are adapted for faster rate-related properties, whereas LR units are adapted for greater tetanic tensions. These distinctions between MR and LR muscle units, coupled with differences between the afferent inputs to their respective motoneurons, suggest that the preservation of conjugacy during horizontal gaze shifts may require a complex interaction of peripheral and central factors.

Animals↗
PubMed1986

Visual, auditory, and somatosensory convergence on cells in superior colliculus results in multisensory integration.

Convergence of inputs from different sensory modalities onto individual neurons is a phenomenon that occurs widely throughout the brain at many phyletic levels and appears to represent a basic neural mechanism by which an organism integrates complex environmental stimuli. In the present study, neurons in the superior colliculus (SC) were used as a model to examine how single neurons deal with simultaneous cues from different sensory modalities (e.g., visual, auditory, somatosensory). The functional result of multisensory convergence on an individual cell was determined by comparing the responses evoked from it by a combined-modality (multimodal) stimulus with those elicited by each (unimodal) component of that stimulus presented alone. Superior colliculus cells exhibited profound changes in their activity when individual sensory stimuli were combined. These "multisensory interactions" were found to be widespread among deep laminae cells and fell into one of two functional categories: response enhancement, characterized by a significant increase in the number of discharges evoked; and response depression, characterized by a significant decrease in the discharges elicited. Multisensory response interactions most often reflected a multiplicative, rather than summative, change in activity. Their absolute magnitude varied from cell to cell and, when stimulus conditions were altered, within the same cell. However, the percentage change of enhanced interactions was generally inversely related to the vigor of the responses that could be evoked by presenting each unimodal stimulus alone and suggest that the potential for response amplification was greatest when responses evoked by individual stimuli were weakest. The majority of cells exhibiting multi-sensory characteristics were demonstrated to have descending efferent projections and thus had access to premotor and motor areas of the brain stem and spinal cord involved in SC-mediated attentive and orientation behaviors. These data show that multisensory convergence provides the descending efferent cells of the SC with a dynamic response character. The responses of these cells and the SC-mediated behaviors that they underlie need not be immutably tied to the presence of any single stimulus, but can vary in response to the particular complex of stimuli present in the environment at any given moment.

Afferent Pathways↗
PubMed1985

Descending efferents from the superior colliculus relay integrated multisensory information.

By means of their efferent projections to motor and premotor structures, the cells in the deep superior colliculus are intimately involved in behaviors that control the orientation of the eyes, pinnae, and head. These same efferent cells receive multiple sensory inputs, thereby apparently enabling an animal to orient its receptor organs in response to a wide variety of cues. This sensory convergence also provides a system in which motor responses need not be immutably linked to individual stimuli but can vary in reaction to the multitude of stimuli present in the environment at any given moment.

Animals↗
PubMed1983

Interactions among converging sensory inputs in the superior colliculus.

The responses of superior colliculus cells to a given sensory stimulus were influenced by the presence or absence of other sensory cues. By pooling sensory inputs, many superior colliculus cells seem to amplify the effects of subtle environmental cues in certain conditions, whereas in others, responses to normally effective stimuli can be blocked. The observations illustrate the dynamic, interactive nature of the multisensory inputs which characterize the deeper laminae of the superior colliculus.

Acoustic Stimulation↗
PubMed1981

Retractor bulbi muscle responses to oculomotor nerve and nucleus stimulation in the cat.

This study demonstrates the presence of retractor bulbi motoneurons within the oculomotor nucleus which activate muscle units within all 4 slips of the cat retractor bulbi muscle. These muscle units are mechanically different and physiologically separate from retractor bulbi muscle units innervated by the abducens nerve. The refractor bulbi muscle, then, is innervated by two separate pools of motoneurons whose axons are carried in two different cranial nerves. These observations of mechanical properties of retractor bulbi muscle suggest that the oculomotor retractor bulbi motor units may be activated during patterned eye movements.

Abducens Nerve↗
PubMed1981

The cardiac malformations. Double inlet left ventricle and corrected transposition explained as deviations in the normal development of the interventricular septum.

To examine the hypothesis that malpositions of cardiac ventricles could be explained by altered development of the interventricular septum, we studied hearts from the Johns Hopkins Hospital autopsy files with double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet ventricle both atrioventricular valves connect normally developed and positioned right and left atria to a posterior morphologic left ventricle. In hearts with corrected transposition the atria are normally positioned and the morphologic right ventricle is on the left and is continuous with the anteriorly positioned aorta. The morphologic left ventricle is on the right, connected to the posteriorly positioned pulmonary trunk. Normal ventricular septation may be understood as arising from the mechanics of a spiral fold in the primary heart tube produced by the left interventricular sulcus. The ventral limb of the spiral induces the muscular interventricular septum while the dorsal limb becomes a component of the crista supraventricularis. We propose that double inlet left ventricle and corrected transposition are the result of minor deviations in the position of the interventricular sulcus on the primary heart tube. Double inlet left ventricle may develop from the formation of a closed unspiraled ring around the interventricular canal. Corrected transposition may result from a left interventricular sulcus whose ventral limb gives rise to a left sided crista supraventricularis, which determines in part the right ventricular morphology of the left sided ventricle. The dorsal limb spirals toward th atrioventricular canal, gives rise to a malpositioned interventricular septum, and displaces the embryonic trabeculated right ventricle to the left. The concept presented accounts for the morphologic findings characteristic of double inlet left ventricle and corrected transposition.

Adolescent↗
PubMed1979

Role of the left interventricular sulcus in formation of interventricular septum and crista supraventricularis in normal human cardiogenesis.

Serial sections of normal human embryos were studied and three-dimensional images reconstructed to determine the early development of the interventricular septum. The position of the interventricular septum is determined in stage 9 of normal development by the formation of the left interventricular sulcus. As a result of unknown properties of the cells of the myocardial layer, the left interventricular sulcus persists while the right disappears, producing the initial lateral asymmetry of the primary heart tube. By stage 14, the left interventricular sulcus forms a spiral which is continuous with the developing interventricular septum. The dorsal limb of the spiral passes to the right between the atrioventricular canal and the origin of the outflow tract, and is lost in the wall of the trabeculated right ventricle. It appears that this dorsal limb of the spiral is the precursor of part of the cirsta supraventricularis. The midportion of the sulcus, the bulboventricular groove, becomes the so-called fibrous continuity between the aortic and mitral valves. The ventral limb of the spiral passes caudally in the anterior interventricular groove and then dorsally and cranially toward the dorsal cushion of the atrioventricular canal. The ventral limb of the spiral is continuous with the crest of the muscular interventricular septum, which develops by apposition of tissue from the expanding right and left ventricles. From stage 14 to stage 19, the muscular interventricular septum, the atrioventricular endocardial cushions, and the ventricular end of the spiral ridges of the outflow tract appose and fuse. Subsequent formation of the membranous interventricular septum completes the physical separation of the right and left ventricles.

Endocardium↗