Unrestrained Cats Research Articles

Comparative Assessment of Heart Rate Variability Obtained via Ambulatory ECG and Polar Heart Rate Monitors in Healthy Cats: A Pilot Study.

Chronic exposure to stressful environments can negatively impact cats' health and welfare, affecting behavioral, autonomic, endocrine, and immune function, as with cats in shelters. Low-stress handling practices likely improve shelter cat welfare, but data supporting improved outcomes remain limited. Cardiac activity, particularly heart rate variability (HRV), is an indicator of stress and emotional state in humans and non-human animals, tracking important body functions associated with stress responsiveness, environmental adaptability, mental, and physical health. HRV studies in cats are limited, involving mainly anesthetized or restrained cats. This pilot study tested the feasibility of obtaining HRV data from unrestrained cats, using a commercially available cardiac monitoring system (Polar H10 with chest strap), compared with data from a traditional ambulatory electrocardiogram. Simultaneous data for the two systems were obtained for five adult cats. Overall, the Polar H10 monitor assessments of HRV were lower than the true HRV assessment by ambulatory ECG, except for SDNN. Correlation between the two systems was weak. Possible reasons for the lack of agreement between the two methods are discussed. At this time, our results do not support the use of Polar H10 heart rate monitors for studies of HRV in cats.

Differential Gating of Thalamocortical Signals by Reticular Nucleus of Thalamus during Locomotion

The thalamic reticular nucleus (RE) provides inhibition to the dorsal thalamus, and forms a crucial interface between thalamocortical and corticothalamic signals. Whereas there has been significant interest in the role of the RE in organizing thalamocortical signaling, information on the activity of the RE in the awake animal is scant. Here we investigated the activity of neurons within the "motor" compartment of the RE in the awake, unrestrained cat during simple locomotion on a flat surface and complex locomotion along a horizontal ladder that required visual control of stepping. The activity of 88% of neurons in this region was modulated during locomotion. Neurons with receptive fields on the shoulder were located dorsally in the nucleus and had regular discharges; during locomotion they had relatively low activity and modest magnitudes of stride-related modulation, and their group activity was distributed over the stride. In contrast, neurons with receptive fields on the wrist/paw were located more ventrally, often discharged sleep-type bursts during locomotion, were very active and profoundly modulated, and their group activity was concentrated in the swing and end of stance. Seventy-five percent of RE neurons had different activity during the two locomotion tasks. We conclude that during locomotion the RE differentially gates thalamocortical signals transmitted during different phases of the stride, in relation to different parts of the limb, and the type of locomotion task.

An improved valve device for the continuous measurement of arterial blood pressure in the conscious unrestrained cat.

An improved valve device for the continuous measurement of arterial blood pressure in the conscious unrestrained cat.

Behavioral Response and Transmitter Release During Atonia Elicited by Medial Medullary Stimulation

Activation of the medial medulla is responsible for rapid eye movement (REM) sleep atonia and cataplexy. Dysfunction can cause REM sleep behavior disorder and other motor pathologies. Here we report the behavioral effects of stimulation of the nucleus gigantocellularis (NGC) and nucleus magnocellularis (NMC) in unrestrained cats. In waking, 62% of the medial medullary stimulation sites suppressed muscle tone. In contrast, stimulation at all sites, including sites where stimulation produced no change or increased muscle tone in waking, produced decreased muscle tone during slow-wave sleep. In the decerebrate cat electrical stimulation of the NGC increased glycine and decreased norepinephrine (NE) release in the lumbar ventral horn, with no change in γ-aminobutyric acid (GABA) or serotonin (5-HT) release. Stimulation of the NMC increased both glycine and GABA release and also decreased both NE and 5-HT release in the ventral horn. Glutamate levels in the ventral horn were not changed by either NGC or NMC stimulation. We conclude that NGC and NMC play neurochemically distinct but synergistic roles in the modulation of motor activity across the sleep-wake cycle via a combination of increased release of glycine and GABA and decreased release of 5-HT and NE. Stimulation of the medial medulla that elicited muscle tone suppression also triggered rapid eye movements, but never produced the phasic twitches that characterize REM sleep, indicating that the twitching and rapid eye movement generators of REM sleep have separate brain stem substrates.

A novel system for recording from single neurons in unrestrained animals

To observe neural activity in animals engaged in natural behavior, it is often desirable to minimize or eliminate restraint of the animal. We have developed a simple system for recording from single units in unrestrained cats. An implant with multiple guide tubes and a tiny microdrive is placed inside the recording chamber. An indwelling Pt–Ir microelectrode is advanced incrementally during recording sessions that occur over a period of weeks or months. Electrodes can be easily replaced. We obtain excellent recording stability, and also have been able to sample extensively from a region of cortex or brain stem in a single animal. The essential electronics have been miniaturized and sewn into a light-weight walking jacket, so that we can collect data from a cat who is not connected to any fixed equipment.

Development of a pressure nociceptive threshold testing device for evaluation of analgesics in cats

A pressure analgesiometric device was developed for unrestrained cats. Eleven cats were studied. Stimulation was via three rounded pins within a bracelet on the forearm. The pins were advanced by manual bladder inflation. Bladder pressure was measured using a strain gauge pressure transducer. The threshold was recorded at the behavioural end point. Thresholds were measured at 5 and 15 min intervals for 2–4 h, after removal/replacement of the cuff, for 120 min after SC butorphanol (0.4 mg/kg), and with mild skin inflammation at the testing site. Data were analysed using ANOVA. Pressure thresholds in untreated cats were around 150 mmHg. The minimum interval for testing was established as 15 min. Data were reproducible over 4 h and beyond 24 h. Thresholds in 5 cats increased ( P < 0.05) above baseline for 45 min after butorphanol with a maximum increase of 270 ± 182 mmHg at 10 min. Thresholds decreased with inflammation. The method appears suitable for feline analgesia investigations.

Activity of the Motor Cortex During Scratching

In awake cats sitting with the head restrained, scratching was evoked using stimulation of the ear. Cats scratched the shoulder area, consistently failing to reach the ear. Kinematics of the hind limb movements and the activity of ankle muscles, however, were similar to those reported earlier in unrestrained cats. The activity of single neurons in the hind limb representation of the motor cortex, including pyramidal tract neurons (PTNs), was examined. During the protraction stage of the scratch response, the activity in 35% of the neurons increased and in 50% decreased compared with rest. During the rhythmic stage, the motor cortex population activity was approximately two times higher compared with rest, because the activity of 53% of neurons increased and that of 33% decreased in this stage. The activity of 61% of neurons was modulated in the scratching rhythm. The average depth of frequency modulation was 12.1 +/- 5.3%, similar to that reported earlier for locomotion. The phases of activity of different neurons were approximately evenly distributed over the scratch cycle. There was no simple correlation between resting receptive field properties and the activity of neurons during the scratch response. We conclude that the motor cortex participates in both the protraction and the rhythmic stages of the scratch response.

Preoptic hypothalamic warming suppresses laryngeal dilator activity during sleep.

Upper airway dilator activity during sleep appears to be diminished under conditions of enhanced sleep propensity, such as after sleep deprivation, leading to worsening of obstructive sleep apnea (OSA). Non-rapid eye movement (NREM) sleep propensity originates in sleep-active neurons of the preoptic area (POA) of the hypothalamus and is facilitated by activation of POA warm-sensitive neurons (WSNs). We hypothesized that activation of WSNs by local POA warming would inhibit activity of the posterior cricoarytenoid (PCA) muscle, an airway dilator, during NREM sleep. In chronically prepared unrestrained cats, the PCA exhibited inspiratory bursts in approximate synchrony with inspiratory diaphragmatic activity during waking, NREM, and REM. Integrated inspiratory PCA activity (IA), peak activity (PA), and the lead time (LT) of the onset of inspiratory activity in PCA relative to diaphragm were significantly reduced in NREM sleep and further reduced during REM sleep compared with waking. Mild bilateral local POA warming (0.5-1.2 degrees C) significantly reduced IA, PA, and LT during NREM sleep compared with a prewarming NREM baseline. In some animals, effects of POA warming on PCA activity were found during waking or REM. Because POA WSN activity is increased during spontaneous NREM sleep and regulates sleep propensity, we hypothesize that this activation contributes to reduction of airway dilator activity in patients with OSA.

A thermal threshold testing device for evaluation of analgesics in cats

A thermal analgesiometric device was developed for unrestrained cats. Heat was provided by an electrical element potted together with a temperature sensor in thermally conductive epoxy in a 5gm probe. This was attached to an elasticated band round the cat's thorax with an inflated bladder maintaining constant pressure between probe and skin. A safety cut-off was set at 60°C. End point was a skin flick, turning, or jumping. Threshold temperatures in untreated cats were around 40°C and repeatable to 4°C with 5, 10 or 15 minutes between tests. Threshold temperature was stable in tests at 15 minutes intervals without false positives or negatives. Tests repeated at weekly intervals were repeatable to within 4°C. Treatment with the opioid analgesic pethidine increased the threshold temperatures 10·2 (6·7)°C 45 minutes after treatment. The device was well tolerated for at least 24 hours and the analgesic effect of an opioid was detected. The system appears suitable for use in investigations into analgesic pharmacology in cats.

Behavioral correlates of the activity of serotonergic and non-serotonergic neurons in caudal raphe nuclei.

We investigated the behavioral correlates of the activity of serotonergic and non-serotonergic neurons in the nucleus raphe pallidus (NRP) and nucleus raphe obscurus (NRO) of unanesthetized and unrestrained cats. The animals were implanted with electrodes for recording single unit activity, parietal oscillographic activity, and splenius, digastric and masseter electromyographic activities. They were tested along the waking-sleep cycle, during sensory stimulation and during drinking behavior. The discharge of the serotonergic neurons decreased progressively from quiet waking to slow wave sleep and to fast wave sleep. Ten different patterns of relative discharge across the three states were observed for the non-serotonergic neurons. Several non-serotonergic neurons showed cyclic discharge fluctuations related to respiration during one, two or all three states. While serotonergic neurons were usually unresponsive to the sensory stimuli used, many non-serotonergic neurons responded to these stimuli. Several non-serotonergic neurons showed a phasic relationship with splenius muscle activity during auditory stimulation. One serotonergic neuron showed a tonic relationship with digastric muscle activity during drinking behavior. A few non-serotonergic neurons exhibited a tonic relationship with digastric and/or masseter muscle activity during this behavior. Many non-serotonergic neurons exhibited a phasic relationship with these muscle activities, also during this behavior. These results suggest that the serotonergic neurons in the NRP and NRO constitute a relatively homogeneous population from a functional point of view, while the non-serotonergic neurons form groups with considerable functional specificity. The data support the idea that the NRP and NRO are implicated in the control of somatic motor output.

Neurochemical relationships in the action of opioid analgesics on cerebral cortex.

The experiments with unrestrained cats showed that opioid analgesics in near-analgesic doses decreased the amplitude of the primary test response recorded in the second sensorimotor zone of the cerebral cortex in 20-150-msec interval between the stimuli applied to thalamocortical fibers radiating from thalamic n. VPL. Application of test substances affecting certain neurotransmitter processes and microionophoretic application of drugs and neurotransmitters to cortical neurons showed that the inhibitory effect of opioid analgesics on cerebral cortex is probably realized through GABA-, serotonin-, beta-adreno-, and cholinergic structures. The excitatory amino acids are also involved into this process.

Velocity and position guided orienting in unrestrained cats

Velocity and position guided orienting in unrestrained cats

Optical imaging of the ventral medullary surface across sleep-wake states.

We hypothesized that spontaneous activity declines over widespread areas of the cat ventral medullary surface (VMS) during rapid eye movement (REM) sleep. We assessed neural and hemodynamic activity, measured as changes in reflected 660- and 560-nm wavelength light, from the VMS during sleep and waking states in five adult, unrestrained cats and in two control cats. Relative to quiet sleep, overall activity declined, and variability, assessed by standard deviation, increased by 25% during REM sleep. Variability in activity during waking also increased by 45% over quiet sleep, but mean activity was unchanged. REM sleep onset was preceded by a reduction in the hemodynamic signal from 5 to 60 s before neural activity decline. The activity decline during REM sleep, previously noted in the goat rostral VMS, extends to intermediate VMS areas of the cat and differs from most neural sites, such as the cortex, hippocampus, and thalamus, which increase activity during REM sleep. The activity decline during REM sleep has the potential to modify VMS responsiveness to baroreceptor and chemoreceptor challenges during the REM state.

Responses in the motor cortex time-locked to the sensory stimuli conditioning target-reaching in the cat

Single unit activity in the forelimb area of the sensorimotor cortex (area 4γ) was investigated in unrestrained cats during target-reaching with the contralateral limb to a morsel of food. An acoustic stimulus (tone) was used as a cue for movement initiation. A total of 159 cells which modulated their activity in relation to the task were investigated and the responses were analysed with respect to the onset of the stimulus and the movement, respectively. Eighteen per cent of the recorded cells showed responses to the conditional signal with a latency ranging from 20 to 100 ms. The response pattern of these neurones always had a second component coupled to the movement. Correlation between timing of neuronal responses and movement onset revealed that the short latency responses were strongly time-locked to the sensory cue but not to the movement. A Go/No-go discrimination task in which the animal was required to initiate or withhold the movement depending on the frequency of the tone showed that the short latency responses were selective to the cue relevant for initiation of the movement and were substantially decreased or absent when a No-go cue was presented. It was also shown that these neurones did not respond to an indifferent cue, but acquired short-latency responses when it was repeatedly rewarded. The results indicate that some motor cortical neurones are recruited in the early stages of sensory-to-motor transformation related to the processing of the conditional signal. It is suggested that these cells may be involved in identifying a cue as relevant for initiation of a goal-directed movement.

The role of the pedunculopontine tegmental nucleus in relation to conditioned motor performance in the cat. I. Context-dependent and reinforcement-related single unit activity.

The activity of the pedunculopontine tegmental nucleus (PPTg) neurons was recorded in three unrestrained cats operantly conditioned to perform a lever-release movement. The movement had to be initiated either rapidly after a (click) stimulus in a simple reaction-time paradigm or had to be delayed after the same stimulus in trials identified by a tone cue. Successful trials were rewarded by a food pellet. A total of 107 neurons were recorded with microelectrodes. Brief spike neurons (mean duration: 0.7 ms) and broad spike neurons (mean duration: 2 ms) presumed to be cholinergic were detected. Of the 73 neurons localized in the PPTg area, 53 had brief spikes and 20 broad spikes. Changes in activity most commonly occurred very early after the stimulus or during the reinforcement process. Most neurons with brief spikes exhibited very early excitation after the stimulus and reinforcement-related activity. These neurons had a mean activity of 23.7 impulses/s in the period preceding the stimulus. The onset of activation after the stimulus had a latency of 8.6+/-6.9 ms (mean+/-SD), with a range of 4-35 ms. In trials where the movement had to be delayed after the stimulus, the early activation disappeared or was considerably reduced, showing that it was context-dependent. A small proportion of neurons with brief spikes initially decreased activity after the stimulus, but with a latency >9 ms. All the neurons with broad spikes, except one, had reinforcement-related activity. Half of them showed exclusively reinforcement-related activity, the other half also early activation after the stimulus. These neurons were about half as active in the period preceding the stimulus occurrence than the neurons with brief spikes. The early context-dependent activation is discussed in relation to the excitatory projection of PPTg neurons on the subthalamic nucleus. The reinforcement-related activity, preferentially evidenced in broad spike neurons presumed to be cholinergic, is speculated to be associated with cholinergic projection of PPTg neurons to the dopaminergic neurons of the substantia nigra. Finally, the role of PPTg in the ongoing control of motor performance and reinforcement processes is discussed in relation to the basal ganglia circuitry.

Corticoreticular pathways in the cat. I. Projection patterns and collaterization.

This paper summarizes and compares the projection patterns and the receptive fields of cortical neurons in areas 4 and 6 that project to the pontomedullary reticular formation (PMRF). A total of 326 neurons were recorded in area 4 and 129 in area 6 in four awake, unrestrained cats that were chronically implanted with arrays of electrodes in the PMRF and the pyramidal tract (PT). In area 4, 47% of the neurons projected to the caudal PT but not to the PMRF (PTNs); 19% were activated only from the PMRF [corticoreticular neurons (CRNs)], whereas 27% were activated from both the PT and the PMRF (PTN/CRNs). More PTN/CRNs conducted at velocities >20 m/s (82%) than did CRNs (23%). In area 6, only 19% of the neurons were identified as PTNs, 12% were PTN/CRNs and 31% were CRNs; a further 38% could not be activated from either structure. Collateral branches within the PMRF conducted at maximum velocities of 20 m/s (average = 6.5 m/s). No significant differences in the conduction velocities of the collateral branches were found either between fast and slow PTNs or between area 4 and area 6 neurons. A large proportion of neurons in area 4 (85/173, 49%) were activated by passive manipulation of the more distal, contralateral forelimb, with approximately equal numbers being classed as PTNs, PTN/CRNs and CRNs. Most neurons in area 6 for which a receptive field could be found were excited by lightly touching or tapping the face and neck; a receptive field could not be determined for 39% of the area 6 neurons compared with only 5% of those in area 4. Finally, there was evidence that neurons in quite widespread areas of the pericruciate cortex, including both areas 4 and 6 projected onto similar, restricted regions of the PMRF. The fact that the cortical projection from area 4 to the PMRF includes a high percentage of fast PTNs with a receptive field on the distal forelimb is consistent with the view that this projection may serve to integrate movement and the dynamic postural adjustments that accompany them. The fact that the cortical projection from area 6 to the PMRF is primarily from slow PTNs with receptive fields on the face, neck and back is consistent with a role for this cortical area in adjusting the general posture of the animal on which movements are superimposed.

Correlations between activity of pallidal neurons and motor parameters

The relationships between the activity of 87 pallidal neurons and parameters characterizing motor performance were analysed quantitatively in six unrestrained cats performing a forelimb flexion movement controlled by a reaction-time paradigm. Three motor variables were considered: the static force exerted when the trigger stimulus occurred, the latency of the force change initiating the lever-release movement and the velocity of the force change. For all the cats, the latency of the change in force was the prevalent parameter in determining the movement onset occurrence. The single-unit activity was quantified on a trial-by-trial basis by measuring the mean firing rate in windows of fixed duration. Both normal (total) and partial linear correlation coefficients were used. Partial correlation coefficients were calculated between the mean firing rate during selected windows and each of the motor parameters. A total of 17% and 9% of all partial correlation coefficients tested ( n = 1566) was found to be significantly different from zero, respectively with p 0.05 and p 0.01. The mean percentages of explained variation, measured by the square of the correlation coefficient, were 17% ( p 0.05) and 21% ( p 0.01). The 46 pallidal neurons presenting an increase in discharge after the trigger stimulus could be classified in homogeneous subgroups with respect to the signs of the correlations. Representations of the motor programme underlying reaction-time performance evidenced at the pallidal level further substantiate a specific involvement of the basal ganglia in the latency of motor triggering.

Activity of neurons of the subthalamic nucleus in relation to motor performance in the cat.

The activity of subthalamic nucleus neurons related to motor performance was studied in three unrestrained cats operantly conditioned to perform a lever-release movement. The movement was initiated either rapidly after the trigger stimulus (a brief sound) in a simple reaction-time paradigm or after a delay in trials identified by a tone cue. These paradigms were randomly presented. The activity of 171 neurons was recorded in the contralateral and in the ipsilateral subthalamic nucleus, with respect to the performing limb. The mean spontaneous activity of cells in the ipsilateral side (18.5 +/- 13.8 imp/s, mean +/- SD) was higher than that in the contralateral side (8.5 +/- 8.1 imp/s). A total of 145 cells (85%) presented significant changes in activity in relation to the lever-release movement (task-related cells). The remaining 26 cells were either related to other events of the task (n = 15; lever-press or reinforcement occurrence) or not related at all to the task performance (n = 11). The majority of changes of activity of task-related cells were initial increases in discharge, which started on average, 127 ms before movement onset and lasted several hundreds of milliseconds. These increases in discharge were more frequent in the contralateral side (75 of 80 task-related cells, 94%) than in the ipsilateral side (43 of 65 task-related cells, 66%). The changes in activity, either increases or decreases, occurred early after the trigger stimulus, since 62% of them had a latency of less than 100 ms. Although the mean latency of initial increases was rather similar in both sides (97 ms contralateral versus 104 ms ipsilateral), the contralateral side was characterized by a high proportion of very early responses (less than 20 ms). For most neurons, the early changes in activity described above were absent after the trigger stimulus in the delayed condition. For certain neurons, the changes in activity prior to movement were different in reaction-time condition and in delayed condition, showing that the pattern of activity preceding movement might depend on the temporal requirements for motor initiation. The results suggest that a significant proportion of subthalamic cells are involved in the preparation and the initiation phases of the lever-release movement studied, although other hypotheses (e.g. stimulus-related responses) cannot be definitely ruled out. The timings and patterns of the changes in activity observed in the subthalamic nucleus in the present study, and in the pallidal complex previously, cannot be explained easily by the classical scheme where the external pallidum inhibits the subthalamic nucleus. The results suggest rather that the subthalamic nucleus, driven by a yet-to-be-determined excitatory input, exerts an excitatory influence on the pallidum and plays a crucial role in the control of the basal ganglia output neurons.

Soleus forces and soleus force potential during unrestrained cat locomotion

It has been suggested that the soleus muscle in the cat is maximally activated during slow walking (e.g. Walmsley et al.,.Am. J. Physiol. 41, 1203–1216, 1978); however, this opinion is not shared throughout the scientific community (e.g. Hodgson, J. Physiol. 337, 553–562, 1983). Whether or not the soleus is maximally activated for low-demand movements is critical for understanding the interaction and force-sharing of the soleus with the remaining ankle extensor muscles. The purpose of this study was to test experimentally whether the soleus muscle was maximally activated during low-demand, unrestrained movements. In order to achieve this purpose, the soleus was stimulated supramaximally at different instants of the step cycle during walking using a chronically implanted nerve cuff electrode. Forces of the soleus during normal step cycles and during step cycles following supramaximal nerve stimulation were measured using a standard E-shaped tendon force transducer. For slow (0.4 m s −1) and intermediate (0.8 m s −1) speeds of walking, soleus forces could be substantially increased through supramaximal nerve stimulation during the swing and mid-stance phases of the step cycle; for fast walking (1.2 m s −1), such increases in force could be produced only during the swing phase. For all speeds of walking, the soleus forces could not be increased in the early (i.e. from paw contact to about peak force occurrence) or the late (i.e. the last 50–100 ms) parts of the stance phase. Specifically, peak forces and the rate of increase in force immediately following paw contact could not be increased substantially through supramaximal nerve stimulations at the appropriate instant in time for any of the conditions tested here. The results of this study indicate that force production of the soleus is maximal for fast walking (1.2 m s −1) throughout the stance phase, and is maximal for large portions of the stance phase for walking at slow (0.4 m s −1) and intermediate (0.8 m s −1) speeds. These findings have direct implications for explaining the possible mechanism of force-sharing among the cat ankle extensor muscles, and the in vivo mechanical properties of the soleus.

Chapter 7 Firing characteristics of neurones in the superior colliculus and the pontomedullary reticular formation during orienting in unrestrained cats

Cats were trained to fixate a center LED or light spot for 1-10 s and to orient to a target (LED or light spot), which appeared simultaneous with the disappearance of the fixation light. Firing characteristics of neurones in the superior colliculus (SC) and the pontomedullary reticular formation were examined systematically in cats performing the above orienting task. In the head-free condition, a majority of the superficial neurones in the SC was activated when a target appeared in a particular area in the visual field (receptive field) as in head fixed animals. We identified a new group of neurones in superficial layers of the SC, superficial fixation neurones, which fired tonically when fixated on a target light with little activity at fixating a center fixation light. In the intermediate and deep layers of the SC, many neurones fired when directed to a particular area in the visual field (movement field). In addition to these neurones, we found new groups of neurones fired tonically in a particular phase of orienting. These neurones were divided into three principal types, target activated, target suppressed and center fixation activated neurones. Target activated neurones fired tonically during the fixation of a target, but not during fixation of a center spot. Target suppressed neurones were characterized by a pause during fixation of targets irrespective of their location. They had clear phasic and tonic components. The phasic component was characterized by a movement field resembling that seen in a typical intermediate layer neurone. Center fixation activated neurones discharged during fixation of the center spot and exhibited suppression during fixation of the target. Many neurones in the pontomedullary reticular formation fired during orienting. Four major types of neurones, phasic, phasic sustained, pause and tonic neurones, were differentiated on the basis of their firing patterns. Phasic units were characterized by brief phasic firing during orienting and could be divided into four subtypes. Long lead phasic neurones began to fire 50-100 ms prior to the onset of a gaze shift and stopped well before the end of the gaze shift and the total number of firing was best correlated with the total angle of head rotation. Short lead phasic neurones started to fire 10-20 ms prior to a gaze shift and were further subdivided into decrement and plateau types. The decrement type exhibited a brief burst just prior to gaze shift and stopped discharging prior to the end of the gaze shift, while the plateau type fired tonically during the gaze shift and stopped firing after the gaze shift. Total number of discharges of both type was related to maximal angular velocity. The fourth type, gaze neurone, began firing coincident with the onset of the gaze shift and stopped firing at the completion of the gaze shift and the total number of discharges was closely related to total angle of head rotation Phasic sustained (PS) neurones fired 50-100 ms before the onset of a gaze shift and their discharges continued after the end of the gaze shift and were divided into three subtypes: augmenting, pause and plateau. The augmenting type was characterized by a transient increase of firing during the gaze shift and a maintained slow rate of firing following the gaze shift. Total number of discharges related to head movement amplitude and the sustained activity was correlated with head position. The pause type was suppressed during the gaze shift and was tonically active during a head movement. The plateau type had a weak phasic component and discharged at a fairly constant rate. The pause and plateau types appeared to be related to head amplitude and head position, respectively. Tonic units were classified into several types. Target fixation neurones exhibited sustained firing when the cat was fixating a target, irrespective of its location, but did not show sustained firing when the animal fixated the fixation light. Fixation neurones fired tonical