Biomedical subjects

D L Tomko

Publications and source records attributed to D L Tomko.

At least 19 recordsLinked to original sources

PubMed1999

Adaptive plasticity in the naso-occipital linear vestibulo-ocular reflex.

The linear vestibulo-ocular reflex (LVOR) during motion along the naso-occipital (NO) axis is governed by eye position and viewing distance. These influences are necessary for the LVOR to maintain stable foveal images during head translation. The response to NO translation must be large when eye position is eccentric from the axis of head motion (i.e., during lateral gaze) and must diminish as eye position approaches straight-ahead, eventually reaching zero when the eye is aligned with the NO axis of motion (the "null point"). As eye position crosses to the opposite side, the LVOR response must reappear, but in the opposite direction, and must grow in magnitude as eccentricity increases. To determine whether the NO-LVOR is subject to adaptive plastic mechanisms, squirrel monkeys were conditioned during NO translation while they binocularly viewed a rich visual field through parallel base-right or base-left wedge prisms. This optical method effectively shifted the visual world 9 degrees leftward or rightward, respectively, thus inducing a mismatch between vision and the NO-LVOR during head movements. To restore compensatory function, the relationship between LVOR sensitivity and horizontal eye position must shift by 9 degrees in the same direction as the visual image shift, effectively shifting the null point. After 2 h of adaptive conditioning, all monkeys exhibited an adaptive shift in the appropriate direction by an average of 3.0 degrees (range 0.7-5.0 degrees), corresponding to 33% of the geometrically required adaptation.

Adaptation, Physiological↗

PubMed1998

Effect of gravity on vestibular neural development.

The timing, molecular basis, and morphophysiological and behavioral consequences of the interaction between external environment and the internal genetic pool that shapes the nervous system over a lifetime remain important questions in basic neuroscientific research. Space station offers the opportunity to study this interaction over several life cycles in a variety of organisms. This short review considers past work in altered gravity, particularly on the vestibular system, as the basis for proposing future research on space station, and discusses the equipment necessary to achieve goals. It is stressed that, in keeping with the international investment being made in this research endeavor, both the questions asked and the technologies to be developed should be bold. Advantage must be taken of this unique research environment to expand the frontiers of neuroscience.

Aging↗

PubMed1997

Effect of environmental enrichment devices on behaviors of single- and group-housed squirrel monkeys (Saimiri sciureus).

Squirrel monkeys display an interest in novel places, habituate to new situations, and spend most of their daily activity in the wild in large groups engaging in feeding behaviors over a broad area. Captivity limits these behaviors and consequently may disrupt normal social organizations. In captivity, squirrel monkeys may exhibit stereotypical behaviors that are believed to indicate decreased psychologic well-being. When a monkey's behavior can be made to approach that seen in the wild, and stereotypical behaviors are minimal, it is assumed that psychologic well-being is adequate. Environmental enrichment devices have been used to address the Animal Welfare Act requirement that psychologic well-being of captive nonhuman primates be considered. The purpose of the study reported here was to examine whether various environmental enrichment devices improve the psychologic well-being of captive squirrel monkeys. In the study, we used behavioral observation to quantify the effectiveness of several environmental enrichment devices for reducing stereotypical behaviors in squirrel monkeys housed alone or in groups. Analysis of our results revealed that the environmental enrichment devices did not affect the expression of normal or stereotypical behaviors, but that the type of housing did.

Animal Welfare↗

PubMed1993

Three dimensional eye movements of squirrel monkeys following postrotatory tilt.

Three-dimensional squirrel monkey eye movements were recorded during and immediately following rotation around an earth-vertical yaw axis (160 degrees/s steady state, 100 degrees/s2 acceleration and deceleration). To study interactions between the horizontal angular vestibulo-ocular reflex (VOR) and head orientation, postrotatory VOR alignment was changed relative to gravity by tilting the head out of the horizontal plane (pitch or roll tilt between 15 degrees and 90 degrees) immediately after cessation of motion. Results showed that in addition to post rotatory horizontal nystagmus, vertical nystagmus followed tilts to the left or right (roll), and torsional nystagmus followed forward or backward (pitch) tilts. When the time course and spatial orientation of eye velocity were considered in three dimensions, the axis of eye rotation always shifted toward alignment with gravity, and the postrotatory horizontal VOR decay was accelerated by the tilts. These phenomena may reflect a neural process that resolves the sensory conflict induced by this postrotatory tilt paradigm.

Animals↗

PubMed1992

Linear vestibuloocular reflex during motion along axes between nasooccipital and interaural.

Linear vestibuloocular reflexes (LVORs) stabilize retinal images by producing eye movements to compensate for linear head motion. LVOR response characteristics depend upon gaze relative to the motion axis and binocular fixation distance. LVOR sensitivity during NO-axis motion increases as gaze eccentricity relative to the motion axis increases and as binocular fixation distance decreases. To fixate targets during forward head motion and rightward gaze, eyes must move to the right, but when looking left, the eyes must move to the left. In this study, LVORs were measured (binocular search coils) during 5.0 Hz horizontal motion along axes between and including NO and IA. This reorients head and otolith inputs relative to linear motion. We found that LVORs follow the same kinematics regardless of eye position in the head or head orientation relative to motion. Eye position information must be quickly and accurately integrated with otolith inputs to determine eye position (gaze) relative to linear head motion in space. The LVOR provides a behaviorally useful reflex for maintaining ocular fixation on visual targets during translation along any axis.

Animals↗

PubMed1991

Eye movement responses to linear head motion in the squirrel monkey. I. Basic characteristics.

1. The purpose of this study was to quantify the response characteristics of eye movements produced by linear head oscillations in the dark (the linear vestibuloocular reflex, or LVOR). Horizontal, vertical, and torsional eye movements were measured in adult squirrel monkeys by the use of a dual scleral search-coil technique during linear oscillations (0.5, 1.5, and 5.0 Hz, 0.36 g peak acceleration) along the animals' interaural (IA), dorsoventral (DV), and nasooccipital (NO) axes. 2. Two LVOR responses, horizontal eye movements during IA-axis translation and vertical eye movements during DV-axis motion, were in a compensatory direction for head translation. Response amplitudes increased as frequency increased, whereas phase typically showed a lead. 3. Two other LVORs, torsional responses during IA-axis translation (all frequencies) and vertical responses during NO-axis oscillations (0.5 Hz), behaved differently. These two LVORs cannot be functionally compensatory for head translation because they degrade fixation on targets, and therefore image stability, by rotating the eyes off target (NO-vertical) or torting the eyes relative to the visual world (IA-torsional). Responses to NO-axis motion at frequencies greater than 0.5 Hz depended on initial eye position and fixation distance and are described in the companion paper. 4. The effect of head orientation on the LVOR was assessed by testing four head positions in 90 degrees steps around the axis of head motion for each of the three axes of translation. This was done, first, to determine whether the LVORs are responses to the "swinging vector" of gravitoinertial force during linear head motion or to head translation; and second, to quantify potential effects of static head (otolith) orientation on the LVORs. Results showed no systematic effects of head orientation on LVOR responses in the frequency bandwidth studied. This indicates that the LVORs are dependent on the direction of linear motion relative to the head (and otolith organs) but not on the swinging vector of gravitoinertial force, and that the LVORs are uninfluenced by static orientation of the head and reloading of the otoliths.

Animals↗

PubMed1991

Eye movement responses to linear head motion in the squirrel monkey. II. Visual-vestibular interactions and kinematic considerations.

1. Horizontal, vertical, and torsional eye movements were recorded (search coil technique) from five squirrel monkeys during horizontal linear oscillations at 0.5, 1.5, and 5.0 Hz, 0.36 g peak acceleration. Monkeys were positioned to produce linear motion in their nasooccipital (NO), interaural (IA), and dorsoventral (DV) axes. Responses of the linear vestibuloocular reflex (LVOR) were recorded in darkness and in the light with the subjects viewing a head-fixed field 22 or 9.2 cm from the eye. The latter condition provided a measure of "visual suppression" of the LVOR (VSLVOR). Responses were also recorded while monkeys viewed earth-fixed targets, which allowed visual enhancement of the LVOR (VLVOR). Vergence angle was recorded in two monkeys to assess directly the point of binocular fixation in space during linear motion. 2. Two LVOR response types, vertical responses during 0.5-Hz NO-axis translation (NO-vertical) and torsional responses at all frequencies during IA-axis oscillation (IA-torsional) could not be compensatory reflexes for head translation because they either move the eye off target (NO-vertical) or tort the eye relative to the visual world (IA-torsional), thereby degrading visual image stability. 3. Other response types are considered compensatory because they help maintain ocular fixation in space during linear head translation. These include horizontal responses to IA-axis motion (IA-horizontal), vertical responses to DV-axis translation (DV-vertical), and both horizontal and vertical responses to NO-axis oscillation (1.5 and 5 Hz). Observations focus on responses to 5-Hz oscillations, in which visual inputs are essentially ineffective in modifying the LVOR. 4. The kinematics of perfect ocular compensation during head translation indicate that the ideal ocular response is governed by the motion of the eye relative to target position. Relevant variables include target distance, which is crucial for all axes of motion, and target eccentricity, which is important only for head motion roughly parallel to the target (NO-axis translation). Findings are compatible with predictions based on ideal kinematics. However, it is the point of binocular fixation in space, not actual target position, that governs LVOR behavior. 5. The IA-horizontal and DV-vertical LVOR is in response to head motion roughly orthogonal to the line of sight. Responses under all stimulus conditions (LVOR, VSLVOR, and VLVOR) behaved similarly at 5 Hz, and were modulated linearly with vergence [in meter angles (MA), the reciprocal of binocular fixation distance].(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Spatial orientation of VOR to combined vestibular stimuli in squirrel monkeys.

The interaction of angular and linear stimuli produces a complex alignment of spatial orientation and the VOR. This phenomenon was studied by measuring three dimensional eye movements in 6 squirrel monkeys during centrifugation in the dark. The axis of eye rotation was always aligned with gravity and with the spinal axis of the upright monkeys. The erect monkeys were oriented such that they were either facing toward the direction of motion or were facing away from the motion. Angular velocity trapezoids were utilized as the motion stimuli with a ramp acceleration of 10 degrees/s2 to a constant velocity of 200 degrees/s. This yields a final centripetal acceleration of 1 g. The orientation of centripetal acceleration dramatically altered the VOR by changing the axis of eye rotation, the peak value of slow phase eye velocity, and the time constant of per-rotary decay. The axis of eye rotation always tended to align with gravito-inertial force, the peak value of slow phase eye velocity was greater when the monkey faced the motion than when it faced away from the motion, and the time constant of decay was smaller when the monkey faced the motion than when it faced away from the motion. These findings were statistically significant (p less than 0.05) and were consistent across all monkeys. The data also indicate that the VOR may be separated into two reflexes, a linear reflex and a rotational reflex. The linear reflex decays as the axis of eye rotation aligns with gravito-inertial force (GIF). These results indicate that GIF is resolved into two components: one representing an internal estimate of linear acceleration and one representing an internal estimate of gravity.

Acceleration↗

PubMed1988

Influence of gravity on cat vertical vestibulo-ocular reflex.

The vertical vestibulo-ocular reflex (VOR) was recorded in cats using electro-oculography during sinusoidal angular pitch. Peak stimulus velocity was 50%/s over a frequency range from 0.01 to 4.0 Hz. To test the effect of gravity on the vertical VOR, the animal was pitched while sitting upright or lying on its side. Upright pitch changed the cat's orientation relative to gravity, while on-side pitch did not. The cumulative slow component position of the eye during on-side pitch was less symmetric than during upright pitch. Over the mid-frequency range (0.1 to 1.0 Hz), the average gain of the vertical VOR was 14.5% higher during upright pitch than during on-side pitch. At low frequencies (less than 0.05 Hz) changing head position relative to gravity raised the vertical VOR gain and kept the reflex in phase with stimulus velocity. These results indicate that gravity-sensitive mechanisms make the vertical VOR more compensatory.

Animals↗

PubMed1988

Yaw direction neurons in the cat inferior olive.

1. Single units that responded to yaw rotation were recorded extracellularly in the caudal inferior olive (IO) of barbiturate-anesthetized cats. Of 276 neurons, 55 responded reliably to yaw, and extensive quantitative data were recorded from 25. 2. No yaw-sensitive IO neuron responded to somatosensory or auditory stimuli but two responded, though unreliably, to flash. 3. Yaw-sensitive IO cells fired at low (1-4 spikes/s), irregular rates during one direction of rotation. Though cells responded reliably during yaw, firing rates varied considerably from cycle to cycle. Rotation speed and acceleration were not represented in any cell's firing rate. 4. Eighty five percent (47/55) of yaw-sensitive cells fired during contralateral rotation, 9% (5/55) during ipsilateral rotation, and 6% (3/55) fired from late in the ipsilateral phase of a sinusoidal oscillation to the middle of the contralateral phase. 5. Responses were tested to 0.1-Hz sinusoidal yaw oscillations with a range of peak angular velocities (1-200 degrees/s). Thresholds were not sharp because of the cycle to cycle variability in response rates but were estimated using averaged responses. The peak rate of the most sensitive cell was driven to criterion (2 SD above spontaneous rate) by an oscillation with a peak velocity of 1 degrees/s. Other cells reached criterion between 5 and 50 degrees/s. 6. Sinusoidal oscillation at all frequencies tested (0.01-0.5 Hz) elicited approximately the same firing rates. Even at 0.01 Hz cells responded well. Responses lagged acceleration by approximately 25 degrees at 0.01 Hz and shifted to later parts of the cycle as frequency increased so that firing lagged acceleration by approximately 200 degrees at 0.5 Hz. 7. Histological reconstruction showed that yaw-sensitive neurons were recorded in olivary subnucleus beta (N beta), the dorsal cap of Kooy (DC), the posterior medial region of the medial accessory division of the inferior olive (MAO), and in the medial-lateral center of the caudal MAO. 8. Yaw-sensitive neurons in the inferior olive provide a signal to the cerebellum that indicates the direction of passive rotation over a wide range of velocity and acceleration. The signal from individual neurons does not reliably encode either rotation velocity or acceleration. Yaw-sensitive IO neurons are therefore unlike other central vestibular neurons but are similar to somatosensory IO cells which signal the presence, but not the intensity of a stimulus.

Action Potentials↗

PubMed1987

Cat vestibular neurons that exhibit different responses to active and passive yaw head rotations.

Neurons in the vestibular nuclei were recorded in alert cats during voluntary yaw rotations of the head and during the same rotations delivered with a turntable driven from a record of previous voluntary movements. During both voluntary and passive rotations 35% (6/17) of neurons tested responded at higher rates or for a larger part of the movement during voluntary movements than during the same rotations delivered with the turntable. Neck sensory input was evaluated separately in many of these cells and can account qualitatively for the extra firing present during active movement.

Animals↗

PubMed1987

Gain and phase of cat vertical eye movements generated by sinusoidal pitch rotations with and without head tilt.

Vertical EOGs were recorded in cats during sinusoidal head pitch from 0.01 to 4.0 Hz with peak velocities of 50 degrees X s-1. The purpose of the experiments was to determine whether dynamic response properties of the vertical vestibulo-ocular reflex (VOR) elicited by pitch with the animal lying on its side (on-side pitch) differ from those resulting from normal (upright) pitch. During on-side pitch (not changing head position with respect to gravity), the vertical VOR gain was 13.5% less than during upright pitch. Thus, the vertical VOR was more compensatory than during on-side pitch. Phase did not differ between the two conditions. The results indicate the importance of interactions between otolith and vertical canal stimulation for vertical eye movement control. The results imply that in micro-gravity, where head tilt does not lead to otolith stimulation as regards gravity, vertical head and eye movements may not be controlled appropriately, leading to vestibular-visual conflicts at the same time that horizontal eye movement controls are normal.

Animals↗

PubMed1987

An analysis of asymmetries in cat vertical eye movements generated by sinusoidal pitch.

Asymmetries in the fast and slow components of nystagmus in the cat vertical vestibulo-ocular reflex (VVOR) were analyzed. Sinusoidal pitch stimuli were used in two experimental conditions, one with the animals on their sides and the other with the animals upright. The half-periods of upward and downward slow component position were generally not of equal duration in the on-side condition. Such was not the case for upright pitch where the slow component was symmetric. In addition, the number of fast components in the two directions was not equal with downward-directed predominating regardless of pitch condition. These two results led to the conclusions that gravity plays an essential role in the normal VVOR and fast component asymmetries may be inherent in the reflex.

Animals↗

PubMed1985

Responses to head tilt in cat central vestibular neurons. II. Frequency dependence of neural response vectors.

The responses of central vestibular neurons in the decerebrate cat subjected to whole-body tilt were examined as a function both of stimulus orientation (with respect to the cat's head) and frequency, with the aim of understanding the neural processing responsible for producing the observed response patterns. Responses to whole-body tilt were recorded from vestibular neurons in and around the lateral vestibular nucleus (LVN). By plugging all six semicircular canals, the otolith contribution was studied in isolation. For each neuron, a response vector was defined as having three components: orientation, gain, and phase. These components were examined using sinusoidal stimulus frequencies of 0.01 to 2 Hz. The orientation component of the neural response vector does not vary as a function of stimulus frequency. Thus response dynamics previously described with a fixed (roll) axis cannot be explained by changes in the angle between the response vector orientation and a fixed stimulus axis. Two major classes of neural responses were observed. One class had a phase lead at low frequencies and gain that showed a modest increase with frequency. It could be described by a model that included a fractional s exponent operator. These response dynamics resemble that of otolith afferents, suggesting that these neurons may be acting as simple relays. The other major response class was characterized by a large gain increase and a phase lag of as much as 180 degrees as frequency increased; such response dynamics have been previously observed in otolith-evoked neck and forelimb reflexes. A more complex model, consisting of a parallel excitatory and high-pass-filtered inhibitory limb, was necessary to describe these responses. The orientation component of the response vector of most of the neurons whose dynamics were best described by the parallel pathway model pointed toward the contralateral side, implying they would be excited by side-up tilt (at low frequencies). Most other neurons had ipsilateral vectors.

Animals↗

PubMed1984

Differences between cats in response properties of horizontal semicircular canal primary afferents.

The response properties of cat horizontal canal afferents (N = 81) were characterized by three parameters: their long time constants (tau), low frequency gain constants (G1), and middle frequency gain constants (Gm). An average value of each of these parameters was calculated for each of eight animals and comparisons were made across animals. There were significant differences between individual animals in their average values of tau and Gm. There was also a significant negative correlation between tau's and Gm's. An animal with a larger average tau tended to have a smaller average Gm. We also used anatomic data on membranous canal duct diameter from the literature to independently estimate the potential effect of interanimal anatomic variability on the predicted range of tau and Gm values in a population. We then compared the data from our 81 afferents with the predictions from the anatomic data.

Animals↗

PubMed1984

Responses to head tilt in cat central vestibular neurons. I. Direction of maximum sensitivity.

Responses to head tilt were recorded from vestibular neurons in and around the lateral vestibular nucleus (LVN) of the decerebrate cat. Each animal had all six semicircular canals rendered nonfunctional by a plugging procedure. Each cell was studied by slowly tilting the cat, using one or both of two paradigms. In the first method, sinusoidal tilts (0.05 or 0.1 Hz) were used to produce bidirectional stimuli in up to 12 pairs of directions, including left/right (roll tilt) and fore/aft (pitch). The second method imposed a constant 10 degree tilt; the direction of the tilt was rotated around the animal by an appropriate combination of roll and pitch motions. Neurons responded by maximally increasing their discharge frequency in a particular direction of head tilt from the horizontal. Each cell's response could be described by a vector in the animal's horizontal plane whose orientation is given by the direction of the most effective stimulus and whose length represents the neuron's maximal sensitivity to tilt. The two methods of stimulation yielded equivalent response vectors. Response vectors were obtained for 100 neurons. The distribution of vector directions for these vestibular neurons was not uniform; there was a conspicuous absence of neurons with fore/aft-directed vectors. The sensitivity of these cells (length of the response vector) ranged from 10 to 230 impulses X s-1 X g-1 (median 50). Neurons whose vectors lay in the ipsilateral half-plane (which would be excited by ear-down tilt) tended to be less sensitive than those with contralateral vectors. Neurons excited by ear-up tilt tended to be located ventrally in the LVN, while those excited by ear-down tilt were more evenly distributed. There was no other obvious correlation of vector orientation with the anatomical locus of the cell in the LVN. The directional selectivity of the responses of these neurons to head tilts are similar to those previously reported tin utricular afferents. The broad distribution of response vector orientations provides an appropriate substrate for vestibulospinal reflexes to a wide variety of head tilts.

Afferent Pathways↗

PubMed1981

Effect of body tilt on receptive field orientation of simple visual cortical neurons in unanesthetized cats.

The receptive field (RF) orientation of 53 simple visual cortical neurons was determined by recording the activity of single cells during presentation of stationary bars of light. An RF tuning curve was constructed for each cell by averaging the neural discharge resulting from the repeated presentation of a number of slit orientations. RF curves were then determined again, following a 45 degrees roll tilt of the entire head and body, and subsequently after the return of the animal to the original horizontal position. RF tuning curves were typical of what others have found to characterize simple cells, and were highly replicable on the return to the starting position. In 73% of the cells studied, the RF orientation after tilt remained unaltered relative to the head axis (+/- 15 degrees); in the remaining 27% of the cells RF orientations either under- or over-shot the retinal tilt by more than 15 degrees, and in some cases by as much as 45 degrees. These results support the hypothesis that the well documented vestibular inputs to visual cortex play a role in modifying the RF orientation selectively of visual cortical neurons, and suggest that such information may be an important neurophysiological substrate underlying visual spatial constancy mechanisms.

Animals↗

PubMed1981

Responses to head tilt in cat eighth nerve afferents.

Responses to head tilt were recorded from eighth nerve axons in barbiturate anesthetized cats. The maximally excitatory head tilt (polarization vector), a zero-force discharge rate, and tilt sensitivity were measured for each cell. In one population of afferents, the maximum discharge frequency was obtained by aligning the saccular plane with gravity. The response properties of these saccular afferents were compared with a second population arising from the utriculus. Both the resting discharge rate and the response sensitivity were lower for saccular than utricular afferents in the cat. The average resting discharge was about 20% lower and the sensitivity about 15% higher in the cat than in the squirrel monkey.

Afferent Pathways↗