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Effects of subthalamic lesions on peripheral and central arousal thresholds in cats
EXPERIMENTAL
NEUROLOGY
Effects and D.F. Department
26, 1@%119.
of Subthalamic
Lesions
Central
Arousal
Thresholds
LINDSLEY.
R.J.
of Physiology, Los Received
(1970)
Azrgmt
BARTON,AKD
Uhersity Angeles, 25;
on Peripheral
R.J.
in Cats ATKINS'
of Southern Califorrtia Cali,fowia 90033
Revisio+t
received
September
Medical
School,
22, 1969
Bilateral lesions of the subthalamic region of the diencephalon cause animals to become less responsive and even inattentive to sensory stimuli. To understand more about subthalamic involvement in attention and vigilance cats were implanted with electrodes in order to compare peripheral (sciatic nerve stimulation) and central (midbrain reticular formation stimulation) EEG and behavioral arousal thresholds. After making bilateral subthalamic lesions five of the six animals showed increased sciatic EEG and behavioral arousal thresholds. This increase in peripheral-arousal thresholds was not simply a reflection of increased central-arousal thresholds, as four of these five had reticular-formation EEG and behavioral arousal thresholds which were either less than or the same as before the lesions were made. The latency for EEG arousal was two to four times longer than before the lesion in some threshold trials. Durations of EEG and behavioral arousal were observed to be shorter than before lesions, which was consistent with the animals’ lack of sustained attention to sensory stimuli. There also appeared to be more slow-wave EEG activity following subthalamic lesions, which was perhaps related to the animals’ waxing and waning state of vigilance with increased periods of inattention and drowsiness. Gross behavioral observation indicated that the animals were hypophagic, hypokinetic, and hyporesponsive. We suggest that subthalamic involvement in problems of attention and vigilance may be related to interruption of interconnections of limbic and and neocortical areas with subcortical regions, interconnections which are important in initiating and sustaining attention to sensory stimuli. Introduction
Numerous experiments have indicated that the subthalamic region of the diencephalon plays an important role in attention and vigilance (l-3, 8, 9). Bilateral
lesions
of the subthalamus
have been observed
to cause animals
to
becomeless responsive and even inattentive to sensory stimuli (1, 2). The mechanismsof such subthalamic involvement, and particularly the relationship of the subthalamus to the midbrain reticular formation (RF), remain 1 The research was supported by Grant NB 07865 from National Institutes of Health. The authors gratefully acknowledge the histological assistance of Mrs. Judith Beckman and the technical assistance of Miss Marjorie J. Sherwood. 109
110
LINDSLEY,
BARTON,
AND
ATIiINS
unknown. Since the subthalamus lies just anterior to the midbrain RF, one questions whether the effects of subthalamic lesions are due to’ interruption of ascending pathways mediating arousal or descending influences maintaining RF responsiveness to sensory input. If it is an ascending pathway, it is necessary to admit that there is another pathway supplementing it, because in chronic experiments animals can always be reawakened from the sleep produced by cooling of the subthalamus (8, 9). -4dey and Lindsley (1) showed in acute experiments that bilateral subthalamic lesions reduced midbrain RF evoked potentials and the excitability of single neurons to peripheral stimuli, though brief tetanic stimulation of the RF was still followed by cortical EEG arousal, suggesting that descending influences passing through the subthalamus were important in maintaining the availability of the RF to sensory input. The study of Feldman and Waller (4) is also relevant to the problem of initiation of attention. They found that animals with lesions previously placed in the posterior hypothalamus could not be aroused behaviorally, though midbrain tetanization still elicited cortical EEG arousal. The present study was undertaken to compare thresholds for RF-induced and peripherally-elicited EEG and behavioral arousal before and after subthalamic lesions, and thus to learn more about the role of the subthalamus in vigilance and attention to peripheral input. Methods
Six cats were prepared for arousal esperiments by implanting under anesthesiastimulating electrodes in the midbrain RF [two bipolar, concentric electrodes-one at anterior-posterior plane A3 and the other at A0 according to the atlas of Snider and Niemer (lo)]. Stimulating electrodes were also placed around each sciatic nerve by means of a cuff described below, and electrodes for making lesions were implanted in the subthalamus (one pair on each side of the brain at A9 with electrodes 2 mm apart). EEG recording electrodes (bone screws) were placed over the right association-visual and left sensorimotor cortical areas. The sciatic electrode consisted of a plastic cuff containing the stimulating leads, which were in contact with the nerve but insulated from the surrounding muscular tissue by a layer of dental cement ; the whole electrode assembly was connected to a plug on the animal’s head by means of subcutaneous,insulated wires running along the back of the animal. The RF stimulating leads were two, 32gauge wires cemented to and extending 1 mm beyond a strut and insulated except for the last 0.5 mm. The lesion-producing electrodes were stainless steel wires 0.5 mm in diameter and insulated except for the last 1 mm. Histological determination of the electrode placements and of the extent of the subthalamic lesions was performed for each animal. After two weeks of recovery, the animals were run once a day in a be-
AROUSAL
THRESHOLDS
111
havioral, sound-attenuated box, and EEG and behavioral arousal thresholds were determined and allowed to stabilize over 1-2 weeks. The EEG was monitored on a Grass Model 7 polygraph. Next the animals were anesthetized with sodium pentobarbital, and 3 mA of anodal current from a d-c lesion maker was passed for 3 min through each electrode of one pair of subthalamic leads. Thresholds were determined, and then a lesion was made in the other subthalamus ; again thresholds were obtained for 2 weeks after recovery from the effects of the bilateral lesion. ThreshoId determinations on any particular day were only begun when the animal was drowsy and showing slow-wave and spindle activity. Central-arousal thresholds were obtained with l- to 6- set trains of lOO-cycie/sec, 1-msec-duration. monophasic square-wave pulses from a Grass SD-S stimulator. Peripheral-arousal thresholds were determined with trains of lo-cycle/set, 1-msec-duration, square-wave stimuli applied to the sciatic leads. The EEG-arousal threshold was considered to be that voltage that converted the synchronized to a desynchronized EEG trace. The behavioralarousal threshold was that voltage which first caused the animal to show signs of orienting to the stimulation. The animals never evinced any painful reactions, only orienting responses at threshold voltages for EEG and behavioral arousal; voltages greater than threshold were not used. Also the animals never showed any aversive reactions towards or agitation in the behavioral box and, in fact, ate and drowsed readily. Results
Lesions of the subthalamic area of the diencephalon in the six animals in this study (Table 1) primarily involved the subthalamic nucleus, zona incerta, and the fields of Fore1 and extended beyond these areas to include parts of the dorsal hypothalamus. These subthalamic lesions were similar to those shown in the paper of Adey and Lindsley ( 1) but were less extensive, as they were made by means of chronically-implanted electrodes. In LC-1 lesions were placed bilaterally in the subthalamus in a one-stage operation, whereas in the other five animals (LC-6 to LC-10) the lesions were made first on one side and later on the other side of the brain to ensure better postoperative recovery. In general unilateral lesions of the subthalamus produced some alteration of behavior, but bilateral lesions were necessary for the best and most enduring results. Histological examination confirmed that the margin of the subthalamic lesions and the RF-stimulating electrodes were separated by approximately 5 mm. The behavioral changes seen in these animals with subthalamic lesions were qualitatively similar to those described by Adey and Lindsley (1) but were less severe, as they were made through immovable, chronically-implanted electrodes. The three cardinal symptoms observed were hypophagia.
112
LINDSLEY,
BARTON,
TABLE CENTRAL
vs PERIPHERAL
AROUSAL
AND
ATKINS
1 AFTER
SUBTHALAMIC
RF (A3) threshold (volts! EEG LC-1 LC-6 LC-7 LC-8 LC-9 LC-10
*0.9 4.7 3.5 2.0 4.4 2.5
-3 + -3 -+ + +
*1.2 + 6.1 + 5.0 + 2.7 -+ 6.0-3.7 4.2 -+
0
Sciatic threshold (volts)
Behavioral 1.8 4.8 1.9 1.5 3.0 1.7
LESION
2.6 6.0 3.0 2.1 2.5
EEC 0.8 7.0 2.0 *lo.5 1.1 1.0
+ -+ -+ + -+ +
Behavioral 4.8 18 3.0 1.6 1.7 1.3
1.0 9.6 2.9 *15 2.1 1.4
+ -+ + + --f -+
4.8 23 7.7 2.6 2.6 1.9
* Threshold voltages for peripheral (sciatic nerve stimulation) and central (midbrain RF stimulation at anterior-posterior level A3 )EEG and behavioral arousal for all of the six animals in the study. The number to the left of each arrow is the threshold voltage before lesion; the one to the right is the threshold after bilateral subthalamic lesions. Asterisks indicate the thresholds of the two deviant animals.
hypokineticity, and hyporesponsiveness. Most of the animals had to be force-fed milk and food, but showedgradual return of spontaneous feeding. Spontaneous movement was greatly reduced, and many of the animals displayed bizarre postures for extended periods of time. They were markedly lessresponsive than normal cats to various stimuli including the presence of other animals; their state of vigilance would wax and wane with long periods of inattentiveness and drowsiness. In five of the six animals EEG arousal thresholds to sciatic nerve stimulation increased after the bilateral subthalamic lesions were made, despite the finding that RF arousal thresholds either decreased or remained the samein four of the five cats. Figure 1A shows that 10 cycles/set stimulation of the sciatic nerve (period of stimulation delimited by the two arrows) produced cortical EEG arousal at a threshold of 2 volts. After recovery from bilateral subthalamic lesions 5 volts applied to the same sciatic electrode was required to produce a comparable EEG arousal (Fig. 1B) ; sciatic
stimulation
at 3 volts for almost
5 set (two
times
longer
than neces-
sary for arousal before placement of subthalamic lesions) produced little or no arousal (Fig. 1B). One hundred cycles/set stimulation of the midbrain RF (anterior-posterior level A3) at 4 volts led to cortical EEG arousal in the same animal before subthalamic lesions (Fig. 2A), but after recovery from bilateral lesions the threshold had decreased to 2 volts (Fig. 2B). Thus in animal LC-7, RF arousal (EEG) thresholds decreasedbut sciatic arousal thresholds increased. Figures 3 and 4 illustrate similar results for animal LC-9. Stimulation of the sciatic nerve at an amplitude of 1 volt caused cortical EEG arousal
AROUSAL
113
THRESHOLDS
A (2V. -AROUSAL
(3V,-
NO AROUSAL )
I (5v. -
)
I AROUSAL)
FIG. 1. Effect of bilateral subthalamic lesions and sciatic arousal (EEG) thresholds, LC-7. A. Sciatic arousal threshold in the animal before lesion is about 2 volts. In all figures involving sciatic arousal stimulation occurs between the two arrows and consists of 10 cycle/set, 1 msec duration, square-wave pulses. B. Sciatic arousal threshold after bilateral subthalamic lesions is increased; 3 volts produces little or no arousal, 5 volts required to produce a comparable EEG arousal, In this and all other figures the upper of each pair of EEG traces is a bipolar recording over the left sensorimotor cortex and the lower trace is from the right association-visual cortex; the horizontal calibration is 1 set and the vertical 50~~.
(Fig. 3A), but after placement of bilateral subthalamic lesions 1 volt stimulation led only to slight EEG arousal, though the stimulation lasted almost four times longer (Fig. 3B). Only when the voltage was raised to 2 volts did an arousal occur which was comparable to that seenbefore subthalamic lesions were made (Fig. 3B). The latency of the EEG-arousal response was about 3 set for the 2-volt level (Fig. 3B), whereas before placement of
114
t
1
-
FIG. 2. Effect of bilateral subthalamic lesions on midbrain reticular formation (RF) arousal (EEG) thresholds, LC-7. A. RF arousal threshold in the animal before lesion is about 4 volts. In all figures involving RF arousal stimulation occurs between the two arrows and consists of 100 cycle/set, 1 msec duration, square-wave pulses. B. RF arousal threshold after bilateral subthalamic lesions is about 2 volts. Calibrations as in Fig. 1.
subthalamic lesionsthe latency was lessthan 1 set (Fig. 3A). Again the increasein the sciatic arousal threshold for LC-9 existed despite a decreasein the RF arousal threshold. In Fig. 4A stimulation of the midbrain RF (A3 level) at 4 volts produced EEG arousal, but after bilateral subthalamic lesions the threshold was 3 volts (Fig. 4B). The latency for EEG arousal was increased over values obtained before lesion (4 set in Fig. 4B as compared to 1 set latency in Fig. 4A). This increase in latency for EEG arousal was not seen prior to making subthalamic lesions ; it was observed to occur only on some of the arousal trials after bilateral subthalamic lesions in this animal. Latency increases were also seen in the other animals but less
frequently. Figures 3 and 4 indicate that latency increasesoccurred to both peripheral (Fig. 3) and central-induced (Fig. 4) EEG arousal. The interpretation of this increased latency of arousal seenafter subthalamic lesions is not clear, but it may be related to the gross behavioral observation of
AROUSAL
(Iv. Y
AROiJS’AL )
( Iv. +
SLIGHT AROUSAL
(2v.
+
115
THRESHOLDS
)
AROUSAL)
FIG. 3. Effect of bilateral subthalamic lesions on sciatic arousal (EEG) thresholds, LC-9. A. Sciatic arousal threshold in the animal before lesion is about 1 volt. B. Sciatic arousal threshold after bilateral subthalamic lesions is increased. 1 volt stimulation for a period almost four times that in Fig. 3A produced only slight arousaf ; 2 volts was required to produce a comparable EEG arousal. Note that the latency for EEG arousal in the animal before lesion was less than 1 set (Fig. 3A) while following subthalamic lesions it was about 3 set (Fig. 3B). Calibrations as in Fig. 1.
waxing and waning of the animals’ state of vigiiance with increased periods of inattention. Table 1 indicates the threshold voltages for peripheral (sciatic nerve stimulation) and central (midbrain RF stimulation at anterior-posterior level A3) EEG and behavioral arousal for the six animals. Though five of the six animals had RF thresholds which either decreasedor remained the same, four of these five showed increased sciatic-arousal thresholds. The EEG and behavioral-arousal thresholds for the two deviant animals are indicated by asterisks; there are no explanations for the results of these two animals. To obtain stable threshold values the animals were accustomed to
116
LINDSLEY,
BARTON,
AND
ATKINS
A (4X-+
AROUSAL)
(3V-AROUSAL)
I
I
-
FIG. 4. Effect of bilateral subthalamic lesions on RF arousal (EEG) thresholds, LC-9. A. RF arousal threshold in the animal before lesion is about 4 volts. B. RF arousal threshold following bilateral subthalamic lesions is about 3 volts. Note that the latency for EEG arousal in the animal before lesion is less than 1 set and after subthalamic lesions has increased to about 4 sec. Calibrations as in Fig. 1.
the testing situation by feeding them in the behavioral box every day and then leaving them there to sleep for extended periods. To get the animals drowsy once testing had begun, they were fed just before each experimental session.The values in Table 1 represent the average of about 1-4 weeks of stable threshold voltages. Arousal thresholds from the other pair of RF-stimulating electrodes (anterior-posterior level AO; not shown in Table 1) and from the other sciatic nerve, though differing somewhat in magnitude from the values showu in Table 1, all changed in the same direction after subthalamic lesions. Thresholds obtained after making unilateral subthalamic lesions were either intermediate in value between prelesion and bilateral-lesion thresholds or were similar to bilateral-lesion thresholds. Behavioral-arousal thresholds were always greater than EEG-arousal thresholds (Table l), but they were also more difficult to quantify. Uehavioral arousal in this study was considered to have occurred when the animal first showed obvious signs of arousing from the drowsy state or orienting to the stimulus. There were a number of occasions following bilateral subthalamic lesions when the animals showed hind-leg contractions to sciatic nerve
AROUSAL
THRESHOLDS
117
stimulation but no EEG or behavioral arousal. Though durations of EEG and behavioral arousal were not measured systematically, they were observed to be considerably shorter after making subthalamic lesions than before. These shortened periods of arousal, following threshold stimulation in the behavioral box, paralleled gross behavioral observations of the animals’ lack of sustained attention to stimuli in the general laboratory environment and their waxing and waning state of vigilance. Though no attempt was made to quantify the amount of slow, rhythmic activity in the on-going EEG, animals with bilateral subthalamic lesions showed more slow-wave EEG activity than they did before lesion. This increase in synchronized EEG activity may have been related to the tendency of the animals to show a waxing and waning vigilant state with increased periods of inattention and drowsiness. They did, however, show periods of desynchronized cortical EEG activity. Discussion
The present results indicate that after subthalamic lesions, EEG and behavioral arousal thresholds to sciatic nerve stimulation are increased despite decreasedor unchanged reticular formation arousal thresholds. This observation of increased sciatic-arousal thresholds appears consistent with the data of Adey and Lindsley (1) showing that RF unit activity and evoked potentials to sciatic stimulation were much reduced in animals with subthalamic lesions. Though it is difficult to rule out changes in peripheral resistance at the sciatic nerve-electrode junction, becauseconstant current stimulation was not used, there is evidence which argues against this interpretation. Recovery from implantation of brain and sciatic electrodes and initial threshold testing lasted some 4 weeks or more before subthalamic lesions were made without indication of major changes in threshold voltage. The increase in the sciatic arousal threshold in five of the six animals was associated specifically in time with lesions of the subthalamus. Also after placement of subthalamic lesions, sciatic stimulation at prelesion threshold voltage levels sometimesresulted in marked hindleg contractions but no discernible EEG or behavioral arousal. These observations tend to rule out explanations of increased thresholds basedon changesin peripheral resistance. Furthermore this increase in sciatic-arousal thresholds correlated with gross behavioral observation of the reduction in the animals’ response to other types of stimuli administered in the general laboratory environment, including the presence of and the threatening gestures of other cats. The animals with subthalamic lesions were either much lessresponsive than before lesion or totally unresponsive and inattentive to the surrounding environment. This increase in peripheral-arousal thresholds is not simply a reflection of increased central-arousal thresholds, as RF-induced EEG and
118
LINDSLEY,
BARTON
AND
ATKINS
behavioral arousal occurred at voltages either less than or the same as in the animal before subthalamic lesions. This latter finding confirms the work of several groups (1, 3, 4, 8, 9) which shows that brief tetanic stimulation of the midbrain RF after bilateral subthalamic or posterior hypothalamic coagulation was still able to produce EEG arousal. The values in Table 1 indicate that there was a difference in threshold between EEG and behavioral arousal; in all cases behavioral arousal OCcurred at a higher threshold. Since the first signs of behavioral arousal are difficult to detect, behavioral arousal thresholds in this study were considered to be those voltages which produced quite obvious manifestations of the orienting response. Consequently, the data from Table 1 do not support as clear-cut a dissociation between EEG and behavioral arousal as that shown by Feldman and Waller (4) in animals with posterior hypothalamic lesions. In fact, animals with bilateral subthalamic lesions, unlike those of Feldman and Waller with posterior hypothalamic lesions, could be aroused both electroencephalographically and behaviorally. The behavioral changes observed after subthalamic lesions were similar to those described in a previous report ( 1) , except that they were less severe, because the lesions were made by passing current through immovable, chronically-implanted electrodes and, therefore, the subthalamic lesions were smaller. Interpretation of the increased latency for EEG arousal seen in Figs. 3 and 4 is difficult. Latency increases were not seen on arousal trials given before subthalamic lesions. In the animal whose EEG recordings are shown in Figs. 3 and 4, latency increases occurred only on some of the arousal trials; they were observed less frequently in other animals. Though there were no obvious behavioral changes associated with this increased latency, it is suggested that such increases might be caused by the waxing and waning of the state of vigilance with increased periods of inattention. Also there is no explanation for the decreased RF-arousal thresholds seen in four of the six animals (Table 1) , though it may be related to the finding of Adey and Lindsley (1) that in animals with subthalamic lesions there was a loss of phasic inhibition occurring briefly after a peripheral influx. The basic finding in the present study, i.e., sciatic-arousal thresholds are increased after bilateral subthalamic lesions though RF-arousal thresholds are decreased or unchanged, lends further support to the concept of Adey and Lindsley (1, 2, 5-7) that certain tonic descending facilitatory influences passing through the subthalamus are important in maintaining the availability of the reticular formation to sensory input. The problem of sensory inattention seen both in this study and in other work involving subthalamic lesions (1-3, 8, 9) has also been observed in animals with posterior hypothalamic lesions (4) and lateral midbrain lesions ( 11, 12), and with superior colliculus lesions (13) it is manifested as visual inattention. Thus
AROUSAL
THRESHOLDS
119
it appears that deficits in attention can arise from lesions over a considerable extent of the brain stem and diencephalon and may be related to interruption of interconnections of limbic and neocortical areas with subcortical regions. References
W. R., and D. F. LINDSLEY. 1959. On the role of subthalamic areas in the maintenance of brain-stem reticular excitability. Exptl. Newel. 1: 407-426. ADEY, W. R., D. 0. WALTER, and D. F. LINDSLEY. 1962. Subthalamic lesionseffects on learned behavior and correlated hippocampal and subcortical slowware activity. Arch. Neural. 6 : 194207. DENAVIT-SAVBIE, M. 1968. “Zone Subthalamique Intervenant dans le Comportement de Veille et de Sommeil. &de des Afferences Sensorielles Qui L’Activent.” Thesis submitted to Faculty of Sciences of Paris, France. pp. 158. FELDMAN, S. M., and H. J. WALLER. 1962. Dissociation of electrocortical activation and behavioral arousal. Natwc 196 : 1320-1322. LINDSLEY, D. F., and W. R. Anw. 1961. Availability of peripheral input to the midbrain reticular formation. Exptl. Neural. 4 : 358-376. LINDSLEY, D. F., T. H. MORTON, and T. ZAROODSY. 1967. Effects of subthalamic stimulation on sensory-evoked potentials in the reticular formation and cortex. Exptl. Nerrrol. 17: 439450. LINDSLEY, D. F., T. ZAXOODNY, and T. H. MORTON. 1967. Effects of subthalamic lesions on sensory-evoked potentials in the reticular formation and sensorimotor cortex. Exptl. Nerwol. 17: 210-220. NAQVET, R., M. DENAVIT, and D. ALBE-FESSARD. 1966. Comparison entre lc Gle du subthalamus et celui des differentes structures bulbom&encPphaliques dans le maintien de la vigilance. Electroencepiaalog. Clkt. Newophysiol. 20 : 149-164. NAQUET, R., M. DENAVIT, J. LANOIR and D. ALBE-FESSARD. 1965. 4lterations transitores ou definitives de zones diencephaliques chez le chat. Leurs effets sur l’activit.5 Clectrique corticale et le sommeil, pp. 107-131. In “Aspects AnatomoFonctionnels de la Physiologie du Sommeil.” M. Jouvet [Ed.] Centre National de la Recherche Scientifique, Paris, France. SNIDER, R. S., and W. T. NIEMER. 1961. “A Stereotaxic Atlas of the Cat Brain.” Univ. of Chicago Press, Chicago, Illinois. SPRAGUE, J. M., W. W. CHAMBERS, and E. STELLAR. 1961. Attentive, affective and adaptive behavior in the cat. Sensory deprivation of the forebrain by lesions in the brain stem results in striking behavioral abnormalities. Science 133: 165-173. SPRAGUE, J. M., M. LEVITT, K. ROBSON, C. N. Lru, E. N. STELLAR, and W. W. CHAMBERS. 1963. A neuroanatomical and behavioral analysis of the syndromes resulting from midbrain lemniscal and reticular lesions in the cat. Arch. Ital. Biol. 101: 225-295. SPRAGUE, J. M., and T. H. MEIKLE, JR. 1965. The role of the superior collicnlus in visually guided behavior. Expfl. Neztrol. 11: 115-146.
1. ADEY, 2.
3.
4. 5.
6.
7.
8.
9.
10. 11.
12.
13.
NEUROLOGY
Effects and D.F. Department
26, 1@%119.
of Subthalamic
Lesions
Central
Arousal
Thresholds
LINDSLEY.
R.J.
of Physiology, Los Received
(1970)
Azrgmt
BARTON,AKD
Uhersity Angeles, 25;
on Peripheral
R.J.
in Cats ATKINS'
of Southern Califorrtia Cali,fowia 90033
Revisio+t
received
September
Medical
School,
22, 1969
Bilateral lesions of the subthalamic region of the diencephalon cause animals to become less responsive and even inattentive to sensory stimuli. To understand more about subthalamic involvement in attention and vigilance cats were implanted with electrodes in order to compare peripheral (sciatic nerve stimulation) and central (midbrain reticular formation stimulation) EEG and behavioral arousal thresholds. After making bilateral subthalamic lesions five of the six animals showed increased sciatic EEG and behavioral arousal thresholds. This increase in peripheral-arousal thresholds was not simply a reflection of increased central-arousal thresholds, as four of these five had reticular-formation EEG and behavioral arousal thresholds which were either less than or the same as before the lesions were made. The latency for EEG arousal was two to four times longer than before the lesion in some threshold trials. Durations of EEG and behavioral arousal were observed to be shorter than before lesions, which was consistent with the animals’ lack of sustained attention to sensory stimuli. There also appeared to be more slow-wave EEG activity following subthalamic lesions, which was perhaps related to the animals’ waxing and waning state of vigilance with increased periods of inattention and drowsiness. Gross behavioral observation indicated that the animals were hypophagic, hypokinetic, and hyporesponsive. We suggest that subthalamic involvement in problems of attention and vigilance may be related to interruption of interconnections of limbic and and neocortical areas with subcortical regions, interconnections which are important in initiating and sustaining attention to sensory stimuli. Introduction
Numerous experiments have indicated that the subthalamic region of the diencephalon plays an important role in attention and vigilance (l-3, 8, 9). Bilateral
lesions
of the subthalamus
have been observed
to cause animals
to
becomeless responsive and even inattentive to sensory stimuli (1, 2). The mechanismsof such subthalamic involvement, and particularly the relationship of the subthalamus to the midbrain reticular formation (RF), remain 1 The research was supported by Grant NB 07865 from National Institutes of Health. The authors gratefully acknowledge the histological assistance of Mrs. Judith Beckman and the technical assistance of Miss Marjorie J. Sherwood. 109
110
LINDSLEY,
BARTON,
AND
ATIiINS
unknown. Since the subthalamus lies just anterior to the midbrain RF, one questions whether the effects of subthalamic lesions are due to’ interruption of ascending pathways mediating arousal or descending influences maintaining RF responsiveness to sensory input. If it is an ascending pathway, it is necessary to admit that there is another pathway supplementing it, because in chronic experiments animals can always be reawakened from the sleep produced by cooling of the subthalamus (8, 9). -4dey and Lindsley (1) showed in acute experiments that bilateral subthalamic lesions reduced midbrain RF evoked potentials and the excitability of single neurons to peripheral stimuli, though brief tetanic stimulation of the RF was still followed by cortical EEG arousal, suggesting that descending influences passing through the subthalamus were important in maintaining the availability of the RF to sensory input. The study of Feldman and Waller (4) is also relevant to the problem of initiation of attention. They found that animals with lesions previously placed in the posterior hypothalamus could not be aroused behaviorally, though midbrain tetanization still elicited cortical EEG arousal. The present study was undertaken to compare thresholds for RF-induced and peripherally-elicited EEG and behavioral arousal before and after subthalamic lesions, and thus to learn more about the role of the subthalamus in vigilance and attention to peripheral input. Methods
Six cats were prepared for arousal esperiments by implanting under anesthesiastimulating electrodes in the midbrain RF [two bipolar, concentric electrodes-one at anterior-posterior plane A3 and the other at A0 according to the atlas of Snider and Niemer (lo)]. Stimulating electrodes were also placed around each sciatic nerve by means of a cuff described below, and electrodes for making lesions were implanted in the subthalamus (one pair on each side of the brain at A9 with electrodes 2 mm apart). EEG recording electrodes (bone screws) were placed over the right association-visual and left sensorimotor cortical areas. The sciatic electrode consisted of a plastic cuff containing the stimulating leads, which were in contact with the nerve but insulated from the surrounding muscular tissue by a layer of dental cement ; the whole electrode assembly was connected to a plug on the animal’s head by means of subcutaneous,insulated wires running along the back of the animal. The RF stimulating leads were two, 32gauge wires cemented to and extending 1 mm beyond a strut and insulated except for the last 0.5 mm. The lesion-producing electrodes were stainless steel wires 0.5 mm in diameter and insulated except for the last 1 mm. Histological determination of the electrode placements and of the extent of the subthalamic lesions was performed for each animal. After two weeks of recovery, the animals were run once a day in a be-
AROUSAL
THRESHOLDS
111
havioral, sound-attenuated box, and EEG and behavioral arousal thresholds were determined and allowed to stabilize over 1-2 weeks. The EEG was monitored on a Grass Model 7 polygraph. Next the animals were anesthetized with sodium pentobarbital, and 3 mA of anodal current from a d-c lesion maker was passed for 3 min through each electrode of one pair of subthalamic leads. Thresholds were determined, and then a lesion was made in the other subthalamus ; again thresholds were obtained for 2 weeks after recovery from the effects of the bilateral lesion. ThreshoId determinations on any particular day were only begun when the animal was drowsy and showing slow-wave and spindle activity. Central-arousal thresholds were obtained with l- to 6- set trains of lOO-cycie/sec, 1-msec-duration. monophasic square-wave pulses from a Grass SD-S stimulator. Peripheral-arousal thresholds were determined with trains of lo-cycle/set, 1-msec-duration, square-wave stimuli applied to the sciatic leads. The EEG-arousal threshold was considered to be that voltage that converted the synchronized to a desynchronized EEG trace. The behavioralarousal threshold was that voltage which first caused the animal to show signs of orienting to the stimulation. The animals never evinced any painful reactions, only orienting responses at threshold voltages for EEG and behavioral arousal; voltages greater than threshold were not used. Also the animals never showed any aversive reactions towards or agitation in the behavioral box and, in fact, ate and drowsed readily. Results
Lesions of the subthalamic area of the diencephalon in the six animals in this study (Table 1) primarily involved the subthalamic nucleus, zona incerta, and the fields of Fore1 and extended beyond these areas to include parts of the dorsal hypothalamus. These subthalamic lesions were similar to those shown in the paper of Adey and Lindsley ( 1) but were less extensive, as they were made by means of chronically-implanted electrodes. In LC-1 lesions were placed bilaterally in the subthalamus in a one-stage operation, whereas in the other five animals (LC-6 to LC-10) the lesions were made first on one side and later on the other side of the brain to ensure better postoperative recovery. In general unilateral lesions of the subthalamus produced some alteration of behavior, but bilateral lesions were necessary for the best and most enduring results. Histological examination confirmed that the margin of the subthalamic lesions and the RF-stimulating electrodes were separated by approximately 5 mm. The behavioral changes seen in these animals with subthalamic lesions were qualitatively similar to those described by Adey and Lindsley (1) but were less severe, as they were made through immovable, chronically-implanted electrodes. The three cardinal symptoms observed were hypophagia.
112
LINDSLEY,
BARTON,
TABLE CENTRAL
vs PERIPHERAL
AROUSAL
AND
ATKINS
1 AFTER
SUBTHALAMIC
RF (A3) threshold (volts! EEG LC-1 LC-6 LC-7 LC-8 LC-9 LC-10
*0.9 4.7 3.5 2.0 4.4 2.5
-3 + -3 -+ + +
*1.2 + 6.1 + 5.0 + 2.7 -+ 6.0-3.7 4.2 -+
0
Sciatic threshold (volts)
Behavioral 1.8 4.8 1.9 1.5 3.0 1.7
LESION
2.6 6.0 3.0 2.1 2.5
EEC 0.8 7.0 2.0 *lo.5 1.1 1.0
+ -+ -+ + -+ +
Behavioral 4.8 18 3.0 1.6 1.7 1.3
1.0 9.6 2.9 *15 2.1 1.4
+ -+ + + --f -+
4.8 23 7.7 2.6 2.6 1.9
* Threshold voltages for peripheral (sciatic nerve stimulation) and central (midbrain RF stimulation at anterior-posterior level A3 )EEG and behavioral arousal for all of the six animals in the study. The number to the left of each arrow is the threshold voltage before lesion; the one to the right is the threshold after bilateral subthalamic lesions. Asterisks indicate the thresholds of the two deviant animals.
hypokineticity, and hyporesponsiveness. Most of the animals had to be force-fed milk and food, but showedgradual return of spontaneous feeding. Spontaneous movement was greatly reduced, and many of the animals displayed bizarre postures for extended periods of time. They were markedly lessresponsive than normal cats to various stimuli including the presence of other animals; their state of vigilance would wax and wane with long periods of inattentiveness and drowsiness. In five of the six animals EEG arousal thresholds to sciatic nerve stimulation increased after the bilateral subthalamic lesions were made, despite the finding that RF arousal thresholds either decreased or remained the samein four of the five cats. Figure 1A shows that 10 cycles/set stimulation of the sciatic nerve (period of stimulation delimited by the two arrows) produced cortical EEG arousal at a threshold of 2 volts. After recovery from bilateral subthalamic lesions 5 volts applied to the same sciatic electrode was required to produce a comparable EEG arousal (Fig. 1B) ; sciatic
stimulation
at 3 volts for almost
5 set (two
times
longer
than neces-
sary for arousal before placement of subthalamic lesions) produced little or no arousal (Fig. 1B). One hundred cycles/set stimulation of the midbrain RF (anterior-posterior level A3) at 4 volts led to cortical EEG arousal in the same animal before subthalamic lesions (Fig. 2A), but after recovery from bilateral lesions the threshold had decreased to 2 volts (Fig. 2B). Thus in animal LC-7, RF arousal (EEG) thresholds decreasedbut sciatic arousal thresholds increased. Figures 3 and 4 illustrate similar results for animal LC-9. Stimulation of the sciatic nerve at an amplitude of 1 volt caused cortical EEG arousal
AROUSAL
113
THRESHOLDS
A (2V. -AROUSAL
(3V,-
NO AROUSAL )
I (5v. -
)
I AROUSAL)
FIG. 1. Effect of bilateral subthalamic lesions and sciatic arousal (EEG) thresholds, LC-7. A. Sciatic arousal threshold in the animal before lesion is about 2 volts. In all figures involving sciatic arousal stimulation occurs between the two arrows and consists of 10 cycle/set, 1 msec duration, square-wave pulses. B. Sciatic arousal threshold after bilateral subthalamic lesions is increased; 3 volts produces little or no arousal, 5 volts required to produce a comparable EEG arousal, In this and all other figures the upper of each pair of EEG traces is a bipolar recording over the left sensorimotor cortex and the lower trace is from the right association-visual cortex; the horizontal calibration is 1 set and the vertical 50~~.
(Fig. 3A), but after placement of bilateral subthalamic lesions 1 volt stimulation led only to slight EEG arousal, though the stimulation lasted almost four times longer (Fig. 3B). Only when the voltage was raised to 2 volts did an arousal occur which was comparable to that seenbefore subthalamic lesions were made (Fig. 3B). The latency of the EEG-arousal response was about 3 set for the 2-volt level (Fig. 3B), whereas before placement of
114
t
1
-
FIG. 2. Effect of bilateral subthalamic lesions on midbrain reticular formation (RF) arousal (EEG) thresholds, LC-7. A. RF arousal threshold in the animal before lesion is about 4 volts. In all figures involving RF arousal stimulation occurs between the two arrows and consists of 100 cycle/set, 1 msec duration, square-wave pulses. B. RF arousal threshold after bilateral subthalamic lesions is about 2 volts. Calibrations as in Fig. 1.
subthalamic lesionsthe latency was lessthan 1 set (Fig. 3A). Again the increasein the sciatic arousal threshold for LC-9 existed despite a decreasein the RF arousal threshold. In Fig. 4A stimulation of the midbrain RF (A3 level) at 4 volts produced EEG arousal, but after bilateral subthalamic lesions the threshold was 3 volts (Fig. 4B). The latency for EEG arousal was increased over values obtained before lesion (4 set in Fig. 4B as compared to 1 set latency in Fig. 4A). This increase in latency for EEG arousal was not seen prior to making subthalamic lesions ; it was observed to occur only on some of the arousal trials after bilateral subthalamic lesions in this animal. Latency increases were also seen in the other animals but less
frequently. Figures 3 and 4 indicate that latency increasesoccurred to both peripheral (Fig. 3) and central-induced (Fig. 4) EEG arousal. The interpretation of this increased latency of arousal seenafter subthalamic lesions is not clear, but it may be related to the gross behavioral observation of
AROUSAL
(Iv. Y
AROiJS’AL )
( Iv. +
SLIGHT AROUSAL
(2v.
+
115
THRESHOLDS
)
AROUSAL)
FIG. 3. Effect of bilateral subthalamic lesions on sciatic arousal (EEG) thresholds, LC-9. A. Sciatic arousal threshold in the animal before lesion is about 1 volt. B. Sciatic arousal threshold after bilateral subthalamic lesions is increased. 1 volt stimulation for a period almost four times that in Fig. 3A produced only slight arousaf ; 2 volts was required to produce a comparable EEG arousal. Note that the latency for EEG arousal in the animal before lesion was less than 1 set (Fig. 3A) while following subthalamic lesions it was about 3 set (Fig. 3B). Calibrations as in Fig. 1.
waxing and waning of the animals’ state of vigiiance with increased periods of inattention. Table 1 indicates the threshold voltages for peripheral (sciatic nerve stimulation) and central (midbrain RF stimulation at anterior-posterior level A3) EEG and behavioral arousal for the six animals. Though five of the six animals had RF thresholds which either decreasedor remained the same, four of these five showed increased sciatic-arousal thresholds. The EEG and behavioral-arousal thresholds for the two deviant animals are indicated by asterisks; there are no explanations for the results of these two animals. To obtain stable threshold values the animals were accustomed to
116
LINDSLEY,
BARTON,
AND
ATKINS
A (4X-+
AROUSAL)
(3V-AROUSAL)
I
I
-
FIG. 4. Effect of bilateral subthalamic lesions on RF arousal (EEG) thresholds, LC-9. A. RF arousal threshold in the animal before lesion is about 4 volts. B. RF arousal threshold following bilateral subthalamic lesions is about 3 volts. Note that the latency for EEG arousal in the animal before lesion is less than 1 set and after subthalamic lesions has increased to about 4 sec. Calibrations as in Fig. 1.
the testing situation by feeding them in the behavioral box every day and then leaving them there to sleep for extended periods. To get the animals drowsy once testing had begun, they were fed just before each experimental session.The values in Table 1 represent the average of about 1-4 weeks of stable threshold voltages. Arousal thresholds from the other pair of RF-stimulating electrodes (anterior-posterior level AO; not shown in Table 1) and from the other sciatic nerve, though differing somewhat in magnitude from the values showu in Table 1, all changed in the same direction after subthalamic lesions. Thresholds obtained after making unilateral subthalamic lesions were either intermediate in value between prelesion and bilateral-lesion thresholds or were similar to bilateral-lesion thresholds. Behavioral-arousal thresholds were always greater than EEG-arousal thresholds (Table l), but they were also more difficult to quantify. Uehavioral arousal in this study was considered to have occurred when the animal first showed obvious signs of arousing from the drowsy state or orienting to the stimulus. There were a number of occasions following bilateral subthalamic lesions when the animals showed hind-leg contractions to sciatic nerve
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stimulation but no EEG or behavioral arousal. Though durations of EEG and behavioral arousal were not measured systematically, they were observed to be considerably shorter after making subthalamic lesions than before. These shortened periods of arousal, following threshold stimulation in the behavioral box, paralleled gross behavioral observations of the animals’ lack of sustained attention to stimuli in the general laboratory environment and their waxing and waning state of vigilance. Though no attempt was made to quantify the amount of slow, rhythmic activity in the on-going EEG, animals with bilateral subthalamic lesions showed more slow-wave EEG activity than they did before lesion. This increase in synchronized EEG activity may have been related to the tendency of the animals to show a waxing and waning vigilant state with increased periods of inattention and drowsiness. They did, however, show periods of desynchronized cortical EEG activity. Discussion
The present results indicate that after subthalamic lesions, EEG and behavioral arousal thresholds to sciatic nerve stimulation are increased despite decreasedor unchanged reticular formation arousal thresholds. This observation of increased sciatic-arousal thresholds appears consistent with the data of Adey and Lindsley (1) showing that RF unit activity and evoked potentials to sciatic stimulation were much reduced in animals with subthalamic lesions. Though it is difficult to rule out changes in peripheral resistance at the sciatic nerve-electrode junction, becauseconstant current stimulation was not used, there is evidence which argues against this interpretation. Recovery from implantation of brain and sciatic electrodes and initial threshold testing lasted some 4 weeks or more before subthalamic lesions were made without indication of major changes in threshold voltage. The increase in the sciatic arousal threshold in five of the six animals was associated specifically in time with lesions of the subthalamus. Also after placement of subthalamic lesions, sciatic stimulation at prelesion threshold voltage levels sometimesresulted in marked hindleg contractions but no discernible EEG or behavioral arousal. These observations tend to rule out explanations of increased thresholds basedon changesin peripheral resistance. Furthermore this increase in sciatic-arousal thresholds correlated with gross behavioral observation of the reduction in the animals’ response to other types of stimuli administered in the general laboratory environment, including the presence of and the threatening gestures of other cats. The animals with subthalamic lesions were either much lessresponsive than before lesion or totally unresponsive and inattentive to the surrounding environment. This increase in peripheral-arousal thresholds is not simply a reflection of increased central-arousal thresholds, as RF-induced EEG and
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ATKINS
behavioral arousal occurred at voltages either less than or the same as in the animal before subthalamic lesions. This latter finding confirms the work of several groups (1, 3, 4, 8, 9) which shows that brief tetanic stimulation of the midbrain RF after bilateral subthalamic or posterior hypothalamic coagulation was still able to produce EEG arousal. The values in Table 1 indicate that there was a difference in threshold between EEG and behavioral arousal; in all cases behavioral arousal OCcurred at a higher threshold. Since the first signs of behavioral arousal are difficult to detect, behavioral arousal thresholds in this study were considered to be those voltages which produced quite obvious manifestations of the orienting response. Consequently, the data from Table 1 do not support as clear-cut a dissociation between EEG and behavioral arousal as that shown by Feldman and Waller (4) in animals with posterior hypothalamic lesions. In fact, animals with bilateral subthalamic lesions, unlike those of Feldman and Waller with posterior hypothalamic lesions, could be aroused both electroencephalographically and behaviorally. The behavioral changes observed after subthalamic lesions were similar to those described in a previous report ( 1) , except that they were less severe, because the lesions were made by passing current through immovable, chronically-implanted electrodes and, therefore, the subthalamic lesions were smaller. Interpretation of the increased latency for EEG arousal seen in Figs. 3 and 4 is difficult. Latency increases were not seen on arousal trials given before subthalamic lesions. In the animal whose EEG recordings are shown in Figs. 3 and 4, latency increases occurred only on some of the arousal trials; they were observed less frequently in other animals. Though there were no obvious behavioral changes associated with this increased latency, it is suggested that such increases might be caused by the waxing and waning of the state of vigilance with increased periods of inattention. Also there is no explanation for the decreased RF-arousal thresholds seen in four of the six animals (Table 1) , though it may be related to the finding of Adey and Lindsley (1) that in animals with subthalamic lesions there was a loss of phasic inhibition occurring briefly after a peripheral influx. The basic finding in the present study, i.e., sciatic-arousal thresholds are increased after bilateral subthalamic lesions though RF-arousal thresholds are decreased or unchanged, lends further support to the concept of Adey and Lindsley (1, 2, 5-7) that certain tonic descending facilitatory influences passing through the subthalamus are important in maintaining the availability of the reticular formation to sensory input. The problem of sensory inattention seen both in this study and in other work involving subthalamic lesions (1-3, 8, 9) has also been observed in animals with posterior hypothalamic lesions (4) and lateral midbrain lesions ( 11, 12), and with superior colliculus lesions (13) it is manifested as visual inattention. Thus
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it appears that deficits in attention can arise from lesions over a considerable extent of the brain stem and diencephalon and may be related to interruption of interconnections of limbic and neocortical areas with subcortical regions. References
W. R., and D. F. LINDSLEY. 1959. On the role of subthalamic areas in the maintenance of brain-stem reticular excitability. Exptl. Newel. 1: 407-426. ADEY, W. R., D. 0. WALTER, and D. F. LINDSLEY. 1962. Subthalamic lesionseffects on learned behavior and correlated hippocampal and subcortical slowware activity. Arch. Neural. 6 : 194207. DENAVIT-SAVBIE, M. 1968. “Zone Subthalamique Intervenant dans le Comportement de Veille et de Sommeil. &de des Afferences Sensorielles Qui L’Activent.” Thesis submitted to Faculty of Sciences of Paris, France. pp. 158. FELDMAN, S. M., and H. J. WALLER. 1962. Dissociation of electrocortical activation and behavioral arousal. Natwc 196 : 1320-1322. LINDSLEY, D. F., and W. R. Anw. 1961. Availability of peripheral input to the midbrain reticular formation. Exptl. Neural. 4 : 358-376. LINDSLEY, D. F., T. H. MORTON, and T. ZAROODSY. 1967. Effects of subthalamic stimulation on sensory-evoked potentials in the reticular formation and cortex. Exptl. Nerrrol. 17: 439450. LINDSLEY, D. F., T. ZAXOODNY, and T. H. MORTON. 1967. Effects of subthalamic lesions on sensory-evoked potentials in the reticular formation and sensorimotor cortex. Exptl. Nerwol. 17: 210-220. NAQVET, R., M. DENAVIT, and D. ALBE-FESSARD. 1966. Comparison entre lc Gle du subthalamus et celui des differentes structures bulbom&encPphaliques dans le maintien de la vigilance. Electroencepiaalog. Clkt. Newophysiol. 20 : 149-164. NAQUET, R., M. DENAVIT, J. LANOIR and D. ALBE-FESSARD. 1965. 4lterations transitores ou definitives de zones diencephaliques chez le chat. Leurs effets sur l’activit.5 Clectrique corticale et le sommeil, pp. 107-131. In “Aspects AnatomoFonctionnels de la Physiologie du Sommeil.” M. Jouvet [Ed.] Centre National de la Recherche Scientifique, Paris, France. SNIDER, R. S., and W. T. NIEMER. 1961. “A Stereotaxic Atlas of the Cat Brain.” Univ. of Chicago Press, Chicago, Illinois. SPRAGUE, J. M., W. W. CHAMBERS, and E. STELLAR. 1961. Attentive, affective and adaptive behavior in the cat. Sensory deprivation of the forebrain by lesions in the brain stem results in striking behavioral abnormalities. Science 133: 165-173. SPRAGUE, J. M., M. LEVITT, K. ROBSON, C. N. Lru, E. N. STELLAR, and W. W. CHAMBERS. 1963. A neuroanatomical and behavioral analysis of the syndromes resulting from midbrain lemniscal and reticular lesions in the cat. Arch. Ital. Biol. 101: 225-295. SPRAGUE, J. M., and T. H. MEIKLE, JR. 1965. The role of the superior collicnlus in visually guided behavior. Expfl. Neztrol. 11: 115-146.
1. ADEY, 2.
3.
4. 5.
6.
7.
8.
9.
10. 11.
12.
13.