Library

Notes: Abstract The primary function of the vestibuloocular reflex (VOR) is to maintain the stability of retinal images during head movements. This function is expressed through a complex array of dynamic and adaptive characteristics whose essential physiological basis is a disynaptic arc. We present a model of normal VOR function using a simple neural network architecture constrained by the physiological and anatomical characteristics of this disynaptic reflex arc. When tuned using a method of global optimization, this network is capable of exhibiting the broadband response characteristics observed in behavioral tests of VOR function. Examination of the internal units in the network show that this performance is achieved by rediscovering the solution to VOR processing first proposed by Skavenski and Robinson (1973). Type I units at the intermediate level of the network possess activation characteristics associated with either pure position or pure velocity. When the network is made more complex either through adding more pairs of internal units or an additional level of units, the characteristic division of unit activation properties into position and velocity types remains unchanged. Although simple in nature, the results of our simulations reinforce the validity of bottom-up approaches to modeling of neutral function. In addition, the architecture of the network is consistent with current ideas on the characteristics and site of adaptation of the reflex and should be compatible with current theories regarding learning rules for synaptic modification during VOR adaptation.

Notes: Abstract The vestibulo-ocular reflex (VOR), which stabilizes the eyes in space during head movements, can undergo adaptive modification to maintain retinal stability in response to natural or experimental challenges. A number of models and neural sites have been proposed to account for this adaptation but these do not fully explain how the nervous system can detect and correct errors in both gain and phase of the VOR. This paper presents a general error correction algorithm based on the multiplicative combination of three signals (retinal slip velocity, head position, head velocity) directly relevant to processing of the VOR. The algorithm is highly specific, requiring the combination of particular sets of signals to achieve compensation. It is robust, with essentially perfect compensation observed for all gain (0.25X–4.0X) and phase (-180°–+180°) errors tested. Output of the model closely resembles behavioral data from both gain and phase adaptation experiments in a variety of species. Imposing physiological constraints (no negative activation levels or changes in the sign of unit weights) does not alter the effectiveness of the algorithm. These results suggest that the mechanisms implemented in our model correspond to those implemented in the brain of the behaving organism. Predictions concerning the nature of the adaptive process are specific enough to permit experimental verification using electrophysiological techniques. In addition, the model provides a strategy for adaptive control of any first order mechanical system.

Notes: Abstract.  We present a controls systems model of horizontal-plane head movements during perturbations of the trunk, which for the first time interfaces a model of the human head with neural feedback controllers representing the vestibulocollic (VCR) and the cervicocollic (CCR) reflexes. This model is homeomorphic such that model structure and parameters are drawn directly from anthropomorphic, biomechanical and physiological studies. Using control theory we analyzed the system model in the time and frequency domains, simulating neck movement responses to input perturbations of the trunk. Without reflex control, the head and neck system produced a second-order underdamped response with a 5.2 dB resonant peak at 2.1 Hz. Adding the CCR component to the system dampened the response by approximately 7%. Adding the VCR component dampened head oscillations by 75%. The VCR also improved low-frequency compensation by increasing the gain and phase lag, creating a phase minimum at 0.1 Hz and a phase peak at 1.1 Hz. Combining all three components (mechanics, VCR and CCR) linearly in the head and neck system reduced the amplitude of the resonant peak to 1.1 dB and increased the resonant frequency to 2.9 Hz. The closed loop results closely fit human data, and explain quantitatively the characteristic phase peak often observed.

Notes: Summary Responses of motoneurons supplying muscles of the forelimbs, hindlimbs, back, and neck to stimulation of the medial pontomedullary reticular formation were studied with intracellular recording in cere-bellectomized cats under chloralose anesthesia. Stimulation of the midline or of a reticular region consisting of nucleus reticularis (n.r.) pontis caudalis and the dorsorostral part of n.r. gigantocellularis produced monosynaptic excitation of ipsilateral motoneurons supplying axial muscles and flexor and extensor muscles in both proximal and distal parts of the limbs. This widespread excitation appears to have been produced by rapidly conducting medial reticulospinal fibers. Stimulation of a second region consisting of n.r. ventralis and the ventrocaudal part of n. r. gigantocellularis produced monosynaptic excitation of ipsilateral neck and back motoneurons but only longer latency, apparently multisynaptic excitation of limb motoneurons. Collision tests indicated that this monosynaptic excitation did not involve fibers descending along the midline. It therefore appears to have been produced by lateral reticulospinal fibers. Reticular stimulation also produced short latency, monosynaptic inhibition of neck motoneurons, long latency, apparently polysynaptic inhibition of limb motoneurons and intermediate latency inhibition of back motoneurons. The latencies and properties of inhibitory responses of back motoneurons indicated that they were produced either disynaptically by fast fibers or monosynaptically by slower fibers. The data indicate that the medial pontomedullary reticular formation can be divided into a number of different zones each with a distinct pattern of connections with somatic motoneurons. These include the dorsorostrally located medial reticulospinal projection area, from which direct excitation of a wide variety of motoneurons can be evoked, the ventrocaudally located lateral reticulospinal projection area from which direct excitation of neck and back and direct inhibition of neck motoneurons can be evoked and the dorsal strip of n.r. gigantocellularis which has direct excitatory and inhibitory actions only on neck motoneurons.

Notes: Summary The specificity of adaptation of vestibuloocular reflex direction was examined by exposing cats to combined pitch vestibular rotation and horizontal optokinetic motion at 0.25 Hz, while alternating body position between lying on the left side and lying on the right. The direction of optokinetic motion relative to head motion was reversed when the cat's body posture was changed so that, for example, if head upward rotation was coupled to leftward visual world motion when the cat was lying on its left side, then head upward rotation was coupled to rightward visual world motion when the cat was on its right side. Body position and optokinetic motion direction were changed every 10 min for a total of 2 h of adaptation on each side. Horizontal and vertical electrooculographic recordings were made during pitch rotations in darkness before and after adaptation. Saccades were removed from the records and vestibulo-ocular reflex gain was measured in the direction of optokinetic motion. In every case, the adaptation procedure produced a directional change in the vestibulo-ocular reflex specific to the posture during measurement and appropriate to reduce the retinal image motion caused by the combined vestibular and optokinetic stimuli. That is, adaptive horizontal eye movements measured on the two sides were in opposite directions for the same direction of head motion. This specificity suggests that adaptation of vestibulo-ocular reflex direction involves specific neural pathways that are controlled by body orientation signals which most likely arise from the otolith organs.

Notes: Abstract Electromyographic activity of dorsal neck muscles and neck torques was recorded to study vestibulocollic, cervicocollic, and combined reflexes in alert and decerebrate cats during rotations of the whole body, the body except for the head, and the head but not the rest of the body. Cats were rotated about many axes that lay in the frontal, sagittal, and horizontal planes using sinusoidal 0.25-Hz waveforms or sum-of-sinusoid waveforms. Robust electromyographic responses were recorded from six muscles, with response directionality that in most cases did not show strong dependence on the reflex tested or on other factors including exact neck angle, stimulus amplitude from 5° to 60°, and intact versus decerebrate state. Based on the strength of responses to rotations about all the tested axes, neck muscles could be characterized by maximal activation direction vectors representing the axis and direction of rotation in threedimensional space that was most excitatory during reflex responses. Responses to rotations about axes that lay in a coordinate plane were predicted by a cosine function of the angle between the axis under test and the maximally excitatory axis in the plane. All muscles were excited by the nose down phase of pitch rotation and by yaw and roll away from the side on which the muscle lay. Biventer cervicis was best activated by rotations with axes near nose-down pitch, and its axis of maximal activation also had small, approximately equal components of yaw and roll toward the contralateral side. Complexus was best excited by rotations with axes nearest roll, but with large components along all three axes. Occipitoscapularis was best excited by rotations about axes near pitch, but with a moderately large contralateral yaw component and a smaller but significant contralateral roll component. Splenius was best excited by rotations with a large component of contralateral yaw, considerable nose-down pitch, and a smaller component of contralateral roll. Rectus major was best excited by rotations near nose-down pitch, but with a substantial contralateral yaw component and smaller contralateral roll component. Obliquus inferior was best excited by rotations with a large component of contralateral yaw, but with considerable contralateral roll and nose-down pitch components. All muscles responded as though they received convergent input from all three semicircular canals. Vestibulocollic and combined reflex responses in alert cats and vestibulocollic, cervicocollic, and combined responses in decerebrate cats appeared to have the same directionality, as evidenced by insignificant shifts in maximal activation vectors. Cervicocollic responses in alert cats were inconsistent and often absent, but appeared upon decerebration, suggesting that higher centers suppress the cervicocollic reflex in intact animals. Decerebration and partial cerebellectomy had no significant effect on maximal activation directions, although electromyographic response magnitudes increased after each. The results suggest that common circuits or strategies are used by neck stretch and vestibular-neck reflexes. The reflex excitation directions do not match the mechanical actions of the neck muscles but agree fairly well with previously published predictions of a mathematical model of neck motor control.

Notes: Summary Human subjects attempted to modify their vestibuloocular reflex (VOR) in the dark by fixating imagined targets while experiencing predictable (SIN) sinusoidal (0.01–2.5 Hz) and unpredictable (SSN) sum of sines rotational stimuli (0.02–1.9 Hz). Modification was attempted under 2 instructional sets: VOR enhancement, ie tracking an imaginary earth-fixed target; VOR suppression, ie fixation of a chair fixed target. When compared to gain characteristics exhibited during the relax state with the same stimuli, subjects were able to alter VOR gain under both experimental conditions, raising it during the enhance paradigm and lowering it during the suppress paradigm. While ability to suppress the VOR was dependent on stimulus frequency, decreasing as frequency of rotation increased, subjects were equally able to modify their responses to the unpredictable and the predictable stimuli. Response phase did not change and was maintained close to 180 deg, regardless of instructional set, predictability, or frequency of stimulation for frequencies greater than 0.1 Hz. At frequencies below 0.1 Hz, a phase lead developed that was similar for all paradigms and rotational stimuli. In contrast, when subjects attempted to pursue visual targets that matched closely the velocities and frequencies of the chair rotation during predictable (SIN) and unpredictable stimulation (SSN), success was dependent on predictability of the stimulus. SSN target motion caused a significant decrease in pursuit velocity as compared to results using SIN target motion. Phase characteristics for both types of stimuli were similar, demonstrating a slight lead at lower frequencies and lagging as frequency of target oscillation increased. The results suggest that voluntary modulation of the VOR is not mediated by a neural control mechanism that is based on prediction. In addition, pursuit does not appear to contribute significantly to ability to cancel VOR. Instead, VOR modulation may be a cognitive event that involves use of a mechanism that produces simple parametric gain changes.

Notes: Summary 1. Responses of neck motoneurons to stimulation of the interstitial nucleus of Cajal (INC) were recorded intracellularly in cats under chloralose anesthesia. When stimuli were applied within or close to the INC, short latency, monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in many neck motoneurons. Such EPSPs were not evoked by stimulating mesencephalic regions outside the INC. 2. Stimulation of the ipsilateral INC produced monosynaptic EPSPs consistently in biventer cervicis-complexus (BCC) motoneurons, while such EPSPs were observed in about two thirds of the splenius (SP) motoneurons and half of the trapezius (TR) motoneurons tested. Stimulation of the contralateral INC produced weak monosynaptic EPSPs in about half the BCC motoneurons and in a few SP and TR motoneurons. All types of motoneurons also received longer latency, apparently polysynaptic, PSPs from both INCs. In BCC and TR motoneurons these were mainly EPSPs, in SP, mixed excitatory and inhibitory PSPs. 3. Monosynaptic EPSPs evoked by INC stimulation were not eliminated by acute and chronic parasagittal and transverse lesions placed to interrupt the bifurcating axons of all vestibulospinal and many reticulospinal neurons. No significant collision was observed between EPSPs evoked by INC and vestibular or reticular stimulation. The EPSPs evoked by stimulation of the INC therefore appear to have been produced by activation of interstitiospinal neurons rather than by an axon reflex mechanism. 4. The properties of a number of interstitiospinal neurons were observed while recording extracellularly from the mesencephalon to map the location of the INC. One third of the interstitiospinal neurons activated antidromically from the C4 segment could also be activated antidromically from L1. These lumbar-projecting neurons had conduction velocities ranging from 15–123 m/s. Several interstitiospinal neurons sending axons to the ventral horn of the neck segments were identified and two of these were found to be branching neurons that projected both to the neck and to lower levels of the spinal cord.

Notes: Summary Neurons in the caudal portions of the medial and descending vestibular nuclei and in vestibular cell group f that project to the cervical or lumbar spinal cord were located by antidromic spinal stimulation. These caudal Vestibulospinal tract (CVST) neurons have a median conduction velocity of 12 m/sec, which is well below the conduction velocities of typical lateral or medial Vestibulospinal tract (LVST, MVST) axons. The descending fiber trajectories of CVST neurons, determined by comparing thresholds for activation of each neuron from six points in the spinal white matter, were remarkably diverse. Unlike LVST and MVST axons, which are located in the ipsilateral ventral funiculi, CVST axons can be found in both the ventral and dorsolateral funiculi on both sides of the spinal cord. The CVST system is thus both anatomically and physiologically different from the LVST and MVST.

Notes: Summary Extracellular microelectrodes were used to record the activity of reticulospinal neurons within the medial ponto-medullary reticular formation in the cat. In one series of experiments reticulospinal neurons were activated from electrodes in the ventro-medial reticulospinal tract (RSTm) and in the ipsiand contralateral lateral reticulospinal tracts (RSTi, RSTC) at spinal levels C1–2, C4, Th1 and L1. RSTm neurons were found primarily in n.r. pontis caudalis and the rostro-dorsal part of n.r. gigantocellularis. 71% of these neurons projected as far as the lumbar spinal cord. RSTi neurons projecting to C4 and beyond were clustered in the caudo-ventral part of n.r. gigantocellularis, but those RSTi neurons projecting to the first three cervical segments were located more rostro-dorsally. In all, 63% of the RSTi neurons projected to the lumbar spinal cord. RSTc neurons, which comprised only 5% of the reticulospinal population, were found throughout n.r. gigantocellularis. RSTm neurons had a median conduction velocity of 101 m/sec whereas RSTi and RSTc had median conduction velocities on the order of 70 m/sec. In a second series of experiments microstimulation was used to activate branches of reticulospinal neurons within the gray matter of the cervical enlargement. Twenty-two of thirty-three neurons found to project to the cervical ventral horn were branching neurons that also sent axons to the lumbar spinal cord. Thus much of the reticulospinal activity reaching the cervical enlargement also acts at one or more other spinal levels. Detailed investigation of the course of reticulospinal axons within the cervical gray matter indicated that a single axon may traverse wide areas of the ventral horn including regions on both sides of the spinal cord.