Journal of Neuroscience Methods, 1 (1979) 159--164
159
© Elsevier/North-Holland Biomedical Press
PHOTOGRAPHIC ANALYSIS OF RELATION BETWEEN UNIT ACTIVITY AND MOVEMENT
JEROME M. SIEGEL, STEPHEN M. BREEDLOVE and DENNIS J. McGINTY Neurophysiology Research, V.A. Medical Center, Sepulveda, Calif. 91343; Department of Psychiatry and Brain Research Institute, UCLA School of Medicine, and Department of Psychology, University of California, Los Angeles, Calif. 90024 (U.S.A.)
(Received November 24th, 1978) (Accepted March 2nd 1979)
A technique for the cinematographic analysis of relationships between unit activity and movement is described. The method permits mapping of the topographic and temporal relationships between unit activity and movement in the unrestrained animal. We have used this technique in the cat to document relationships between unit activity in the pontine reticular formation and movement.
INTRODUCTION Observation of the behavioral correlates of unit discharge provides a powerful insight into the functional roles of brain areas. As the application of new techniques that allow the recording of unit activity in behaving animals becomes more widespread, a more quantitative description of observed relationships will be needed to d o c u m e n t observations and differentiate between the unit activity--behavior relations of different brain areas. The analysis of relationships between neuronal unit activity and movem e n t patterns has generally been restricted to animals trained to make specific movements such as bar presses (e.g. Burton and Onoda, 1977; DeLong, 1973) or to angular movements of eye and h e a d ( e . g . Keller and Robinson, 1972; Collewijn, 1977) and locomotor (Orlovsky, 1972) movements, which can be readily quantified. Since only one c o m p o n e n t of behavior is measured in such procedures, results cannot provide a complete picture of the relationship of unit discharge to natural movements and may therefore be misleading. The understanding of the functioning of neural units in sensory systems has been greatly facilitated by the ability to map receptive fields. However, a similar 'mapping' technique is not available for studies of unit activity in m o t o r systems. Photography has long been used in the analysis of the temporal interrelations of movement patterns in unrestrained animals. The present report describes the adaptation of cinematographic techniques to the
160 study of relations between unit activity and behavior. We have used these techniques in freely moving cats to map the m o v e m e n t relations of neurons in the gigantocellular nucleus of the pons, an area in which the activity of most units is related to specific movements (Siegel and McGinty, 1977}. METHODS AND RESULTS Procedures for the long-term recording of single units in the brain stem of the unrestrained cat have been ddveloped in our laboratory and r e p o r t e d in detail elsewhere (Harper and McGinty, 1973). Briefly, bundles of six 32 pm insulated microwires protruding 5 m m from a 24-gauge support cannula are attached to a mechanical microdrive consisting of concentric cannulae controlled by a miniature machine screw. Each microdrive can hold one or two bundles of microwires. Two microdrives are normally implanted in each cat. After recovery from surgery the cat is placed in a recording chamber and microwires are scanned for discriminable unit discharge. Microwire bundles are advanced in 50 pm steps until one or m ore isolated units with signal-tonoise ratios exceeding 3/1 and initially negative spikes are found. Signals are amplified with high impedance preamplifiers and conditioned with high pass 300 Hz filters to prevent interference f r om cable m o v e m e n t artifact. Unit signals are led to a window discriminator (Neurofeedback Instruments) which produces a pulse o u t p u t for each unit spike whose amplitude falls between two experimenter set voltage levels. The pulse o u t p u t can be used to drive polygraph pens, an audio monitor, and an electronic counter. With this technique, individual cells m a y be studied for periods ranging from several hours to m a n y days, depending on the region studied. Cats are placed in a shielded, 58 X 60 X 85 cm chamber with a glass front d o o r (see Fig. 1). A mirror, m o u n t e d at 45 ° o f f vertical at the t op of the chamber, allows simultaneous photographing of two views of the cat. Two digital LED counters are placed at the side of the chamber. The top c o u n t e r is incremented by each unit discharge via the pulse o u t p u t of a window discriminator. The b o t t o m c ount e r displays the current reading on a binary coded decimal time code (Biotronic Designs CGD200) with i nt ercount intervals o f 1 sec. Since this same time code is recorded on the polygraph and on magnetic tape, it serves as a reference point that allows one to correlate unit discharge--movement relations with EEG variables. It also permits the experimenter to compare changes in the reading of the unit c o u n t e r with the magnetic tape o f the unit recording, and to determine retrospectively if noise has triggered the counter. The chamber has a 'discriminative stimulus' light visible to b o t h the cat and the camera. This light can be used to label behavioral contingencies, such as the operant r e i n f o r c e m e n t of increased rates o f unit discharge with hypothalamic stimulation. This r e i n f o r c e m e n t procedure was used to increase discharge rates during the filming sessions used in the present study and will be r eport ed on in detail elsewhere. The chamber has an LED indicator light which can be used to visually label short
161
L
discriminative used during stimulus reinforcement sessions
50mark cm cal
l!i~i ~,
m rror
:~
cable
glass door unit counter
displays
1061 ,i .oo e
)
LED
indicator
Super 8m camera
Fig. 1. Arrangementof equipmentused for filmingunit discharge--movementrelations. duration events such as discrete a u d i t o r y stimuli or pulsed brain stimulation. T h i r t y cm rulers placed on the rear wall, floor and door of the cage are used t o calibrate m o v e m e n t measurements. The cat, unit counter, time code and indicator lights are simultaneously filmed by a Super 8 m m 'existing light' camera placed 1 m f r om t he cage d o o r and run at 18 or 36 frames/sec. Films are analyzed on a Bolex V180 Duo editor. This editor has a flat, wide screen and bright image, which facilitate data analysis. For each analysis, we locate a m i ni m um of 10 instances in which an isolated unit discharge has occurred, by observing the unit counter. To be considered isolated there must be at least 5 frames w i t h o u t unit activity preceding the discharge. Measurements of m o v e m e n t are made as follows. (1) Each frame is centered on the screen. The relative location of a selected b o d y landmark is measured and p l o t t e d on graph paper. In the cells presented here the cats' nares served as a landmark for mapping head movements. (2) The n e x t frame is then viewed and the relative location of the b o d y part again plotted. (3) A vector between t he t w o pl ot t ed points indicates the m ovem ent . (4) By plotting the vectors f r om the same starting point for each pair of frames, a figure th at describes m o v e m e n t in the interframe interval before the first unit discharge in an isolated burst, and a second one to describe m o v e m e n t
162
/ /
B
C
Fig. 2. A. Upper portion: vectors describing the movement of the cat's head during the film frames spanning the 56 msec period before the onset of unit activity. Calibration mark for A, B and C is 10 mm. Lower portion: vectors describing the movement of the cat's head during the film frames spanning 56 msec period in which the unit began to discharge. Note the nearly random distribution of vectors in the upper portion of the figure, and the preponderance of movements toward the upper left-hand quadrant accompanying the onset of unit activity in the lower portion of the figure. B: same as A. Note the shift in movement to upper right-hand quandrant during onset of unit activity. C: same as A and B. Note the shift in movement in the left-hand quadrants during the onset of unit activity.
w h e n t h e unit begins discharging can be c o n s t r u c t e d . (5) S e p a r a t e sets o f v e c t o r d i a g r a m s f o r each level o f r a t e i n c r e m e n t can be p l o t t e d , i.e. o n e set o f p l o t s f o r cases in w h i c h t h e u n i t discharges o n c e b e t w e e n a d j a c e n t frames, a n o t h e r set f o r cases in w h i c h it discharges twice, etc. (6) M o v e m e n t s in each i n t e r f r a m e interval f o l l o w i n g or p r e c e d i n g isolated u n i t discharge can be p l o t t e d . In this w a y o n e can d e t e r m i n e at w h a t t i m e t h e m a x i m u m m o v e m e n t e x c u r s i o n s o c c u r relative t o u n i t discharge. (7) A c o n t i n u o u s p l o t o f t h e m o v e m e n t s b e f o r e , during a n d a f t e r individual u n i t discharges c a n be d r a w n b y p u t t i n g t h e b e g i n n i n g o f each i n t e r f r a m e v e c t o r at t h e end o f t h e p r e v i o u s v e c t o r . This allows o n e t o d e t e r m i n e t h e r e l a t i o n s h i p o f u n i t discharge t o c o m p l e x o n g o i n g m o v e m e n t p a t t e r n s . P l o t t i n g m a y be d o n e in t w o or t h r e e d i m e n s i o n s . T h e p r e s e n t r e p o r t used t w o d i m e n s i o n a l plots. Unlike magnetic techniques of recording head and eye movement, our photographic p r o c e d u r e can r e c o r d b o t h angular a n d linear t r a n s l a t i o n s o f a n y b o d y part. T h e c o m p l e t e v e c t o r analysis p r o c e d u r e , including filming a n d v e c t o r plotting, t a k e s a p p r o x i m a t e l y 12 h f o r each unit. Significance o f changes in vect o r d i s t r i b u t i o n can b e assessed b y a p p r o p r i a t e statistical tests. Fig. 2 p r e s e n t s p l o t s o f h e a d m o v e m e n t s r e l a t e d t o u n i t discharge in t h r e e units r e c o r d e d in t h e p o n t i n e n u c l e u s gigantocellularis. T h e s e cells w e r e histologically localized b e t w e e e n P 1.5 a n d P 5.5 a n d L 1.5 a n d L 2.0 ( B e r m a n , 1968). N o t e t h e r a n d o m a r r a n g e m e n t o f m o v e m e n t v e c t o r s d u r i n g t h e interf r a m e intervals prior t o t h e s t a r t o f u n i t discharge. During t h e film f r a m e s
163
spanning the interval in which unit discharge begins, the distribution of vectors shifts. Our previous work, which employed visual and EMG observations to describe the behavioral correlates of pontine gigantocellularis unit discharge, found that activity in these cells was related to specific movements (Siegel and McGinty, 1977; Siegel et al., 1977). The most c o m m o n relation was to head movements. The preliminary data presented here indicate that these movements begin within 56 msec of unit discharge and that the movements have a high degree of directional consistency. Within a 56 msec interval more than one movement can be executed. Therefore the scatter observed in Fig. 2 could result either from other movements within the interval obscuring a very tight coupling of unit discharge to a highly specific movement, or from the units' relation to a wide range of movements concentrated in one direction. Higher speed filming could resolve such questions. DISCUSSION
This procedure provides an objective m e t h o d of mapping a unit's m o t o r correlates. The temporal relation of unit firing to behavior, and the consistency of this relation, can be documented with great precision w i t h o u t using restraint or movement restriction. This technique can be applied to describe unit activity--movement relations in unrestrained animals during a wide range of natural behaviors. Correlative data cannot itself prove causal relationships. Thus a unit discharge that precedes motion of an identified body part need not be directly in the pathway mediating that movement. Rather, the unit activity and movement may both be temporally related to other aspects of the complex sequence of sensory and m o t o r events surrounding the observed movement. For example, unit discharge may relate to increased tone in axial musculature, alterations in reflex excitability, activity in cutaneous muscles, etc. The photographic technique is able to detect unit discharge relations with a wide variety of movements and is therefore less likely to miss underlying relationships than restricted movement paradigms that measure only the movement of a single b o d y part. However, results from both of these techniques must be correlated with behavioral, physiological and anatomical findings in order to determine the mechanisms underlying observed relationships. ACKNOWLEDGEMENTS
Supported by the Medical Research Service of the Veterans Administration and National Institutes of Health Research Grant NS14610.
164 REFERENCES Berman, A.L. (1968) The Brain Stem of the Cat, University of Wisconsin Press, Madison, p. 175. Burton, J.E. and Onoda, N. (1977) Interpositus neuron discharge in relation to a voluntary movement, Brain Res., 121: 167--172. Collewijn, H. (1977) Gaze in freely moving subjects. In R. Baker and A. Berthoz (Eds.), Control of Gaze by Brain Stem Neurons, Developments in Neuroscience, Vol. 1, Elsevier/North-Holland Biomedical Press, Amsterdam, pp. 13--22. DeLong, M.R. (1973) Putamen: activit:~ of single units during slow and rapid arm movements, Science, 179: 1240--1242. Harper, R.M. and McGinty, D.J. (1973) A technique for recording single neurons from unrestrained animals. In M.I. Phillips (Ed.), Brain Unit Activity during Behavior, Thomas, Springfield, Ill., pp. 80--104. Keller, E.L. and Robinson, D.A. (1972) Abducens unit behavior in the monkey during vergence movements, Vision Res., 12 : 369--382. Orlovsky, G.N. (1972) Activity of vestibulospinal neurons during locomotion, Brain Res., 46: 85--98. Siegel, J.M. and McGinty, D.J. (1977) Pontine reticular formation neurons: relationship of discharge to motor activity, Science, 196: 678--680. Siegel, J.M., McGinty, D.J. and Breedlove, S.M. (1977) Sleep and waking activity of pontine gigantocellular field neurons, Exp. Neurol., 56: 553--573.
159
© Elsevier/North-Holland Biomedical Press
PHOTOGRAPHIC ANALYSIS OF RELATION BETWEEN UNIT ACTIVITY AND MOVEMENT
JEROME M. SIEGEL, STEPHEN M. BREEDLOVE and DENNIS J. McGINTY Neurophysiology Research, V.A. Medical Center, Sepulveda, Calif. 91343; Department of Psychiatry and Brain Research Institute, UCLA School of Medicine, and Department of Psychology, University of California, Los Angeles, Calif. 90024 (U.S.A.)
(Received November 24th, 1978) (Accepted March 2nd 1979)
A technique for the cinematographic analysis of relationships between unit activity and movement is described. The method permits mapping of the topographic and temporal relationships between unit activity and movement in the unrestrained animal. We have used this technique in the cat to document relationships between unit activity in the pontine reticular formation and movement.
INTRODUCTION Observation of the behavioral correlates of unit discharge provides a powerful insight into the functional roles of brain areas. As the application of new techniques that allow the recording of unit activity in behaving animals becomes more widespread, a more quantitative description of observed relationships will be needed to d o c u m e n t observations and differentiate between the unit activity--behavior relations of different brain areas. The analysis of relationships between neuronal unit activity and movem e n t patterns has generally been restricted to animals trained to make specific movements such as bar presses (e.g. Burton and Onoda, 1977; DeLong, 1973) or to angular movements of eye and h e a d ( e . g . Keller and Robinson, 1972; Collewijn, 1977) and locomotor (Orlovsky, 1972) movements, which can be readily quantified. Since only one c o m p o n e n t of behavior is measured in such procedures, results cannot provide a complete picture of the relationship of unit discharge to natural movements and may therefore be misleading. The understanding of the functioning of neural units in sensory systems has been greatly facilitated by the ability to map receptive fields. However, a similar 'mapping' technique is not available for studies of unit activity in m o t o r systems. Photography has long been used in the analysis of the temporal interrelations of movement patterns in unrestrained animals. The present report describes the adaptation of cinematographic techniques to the
160 study of relations between unit activity and behavior. We have used these techniques in freely moving cats to map the m o v e m e n t relations of neurons in the gigantocellular nucleus of the pons, an area in which the activity of most units is related to specific movements (Siegel and McGinty, 1977}. METHODS AND RESULTS Procedures for the long-term recording of single units in the brain stem of the unrestrained cat have been ddveloped in our laboratory and r e p o r t e d in detail elsewhere (Harper and McGinty, 1973). Briefly, bundles of six 32 pm insulated microwires protruding 5 m m from a 24-gauge support cannula are attached to a mechanical microdrive consisting of concentric cannulae controlled by a miniature machine screw. Each microdrive can hold one or two bundles of microwires. Two microdrives are normally implanted in each cat. After recovery from surgery the cat is placed in a recording chamber and microwires are scanned for discriminable unit discharge. Microwire bundles are advanced in 50 pm steps until one or m ore isolated units with signal-tonoise ratios exceeding 3/1 and initially negative spikes are found. Signals are amplified with high impedance preamplifiers and conditioned with high pass 300 Hz filters to prevent interference f r om cable m o v e m e n t artifact. Unit signals are led to a window discriminator (Neurofeedback Instruments) which produces a pulse o u t p u t for each unit spike whose amplitude falls between two experimenter set voltage levels. The pulse o u t p u t can be used to drive polygraph pens, an audio monitor, and an electronic counter. With this technique, individual cells m a y be studied for periods ranging from several hours to m a n y days, depending on the region studied. Cats are placed in a shielded, 58 X 60 X 85 cm chamber with a glass front d o o r (see Fig. 1). A mirror, m o u n t e d at 45 ° o f f vertical at the t op of the chamber, allows simultaneous photographing of two views of the cat. Two digital LED counters are placed at the side of the chamber. The top c o u n t e r is incremented by each unit discharge via the pulse o u t p u t of a window discriminator. The b o t t o m c ount e r displays the current reading on a binary coded decimal time code (Biotronic Designs CGD200) with i nt ercount intervals o f 1 sec. Since this same time code is recorded on the polygraph and on magnetic tape, it serves as a reference point that allows one to correlate unit discharge--movement relations with EEG variables. It also permits the experimenter to compare changes in the reading of the unit c o u n t e r with the magnetic tape o f the unit recording, and to determine retrospectively if noise has triggered the counter. The chamber has a 'discriminative stimulus' light visible to b o t h the cat and the camera. This light can be used to label behavioral contingencies, such as the operant r e i n f o r c e m e n t of increased rates o f unit discharge with hypothalamic stimulation. This r e i n f o r c e m e n t procedure was used to increase discharge rates during the filming sessions used in the present study and will be r eport ed on in detail elsewhere. The chamber has an LED indicator light which can be used to visually label short
161
L
discriminative used during stimulus reinforcement sessions
50mark cm cal
l!i~i ~,
m rror
:~
cable
glass door unit counter
displays
1061 ,i .oo e
)
LED
indicator
Super 8m camera
Fig. 1. Arrangementof equipmentused for filmingunit discharge--movementrelations. duration events such as discrete a u d i t o r y stimuli or pulsed brain stimulation. T h i r t y cm rulers placed on the rear wall, floor and door of the cage are used t o calibrate m o v e m e n t measurements. The cat, unit counter, time code and indicator lights are simultaneously filmed by a Super 8 m m 'existing light' camera placed 1 m f r om t he cage d o o r and run at 18 or 36 frames/sec. Films are analyzed on a Bolex V180 Duo editor. This editor has a flat, wide screen and bright image, which facilitate data analysis. For each analysis, we locate a m i ni m um of 10 instances in which an isolated unit discharge has occurred, by observing the unit counter. To be considered isolated there must be at least 5 frames w i t h o u t unit activity preceding the discharge. Measurements of m o v e m e n t are made as follows. (1) Each frame is centered on the screen. The relative location of a selected b o d y landmark is measured and p l o t t e d on graph paper. In the cells presented here the cats' nares served as a landmark for mapping head movements. (2) The n e x t frame is then viewed and the relative location of the b o d y part again plotted. (3) A vector between t he t w o pl ot t ed points indicates the m ovem ent . (4) By plotting the vectors f r om the same starting point for each pair of frames, a figure th at describes m o v e m e n t in the interframe interval before the first unit discharge in an isolated burst, and a second one to describe m o v e m e n t
162
/ /
B
C
Fig. 2. A. Upper portion: vectors describing the movement of the cat's head during the film frames spanning the 56 msec period before the onset of unit activity. Calibration mark for A, B and C is 10 mm. Lower portion: vectors describing the movement of the cat's head during the film frames spanning 56 msec period in which the unit began to discharge. Note the nearly random distribution of vectors in the upper portion of the figure, and the preponderance of movements toward the upper left-hand quadrant accompanying the onset of unit activity in the lower portion of the figure. B: same as A. Note the shift in movement to upper right-hand quandrant during onset of unit activity. C: same as A and B. Note the shift in movement in the left-hand quadrants during the onset of unit activity.
w h e n t h e unit begins discharging can be c o n s t r u c t e d . (5) S e p a r a t e sets o f v e c t o r d i a g r a m s f o r each level o f r a t e i n c r e m e n t can be p l o t t e d , i.e. o n e set o f p l o t s f o r cases in w h i c h t h e u n i t discharges o n c e b e t w e e n a d j a c e n t frames, a n o t h e r set f o r cases in w h i c h it discharges twice, etc. (6) M o v e m e n t s in each i n t e r f r a m e interval f o l l o w i n g or p r e c e d i n g isolated u n i t discharge can be p l o t t e d . In this w a y o n e can d e t e r m i n e at w h a t t i m e t h e m a x i m u m m o v e m e n t e x c u r s i o n s o c c u r relative t o u n i t discharge. (7) A c o n t i n u o u s p l o t o f t h e m o v e m e n t s b e f o r e , during a n d a f t e r individual u n i t discharges c a n be d r a w n b y p u t t i n g t h e b e g i n n i n g o f each i n t e r f r a m e v e c t o r at t h e end o f t h e p r e v i o u s v e c t o r . This allows o n e t o d e t e r m i n e t h e r e l a t i o n s h i p o f u n i t discharge t o c o m p l e x o n g o i n g m o v e m e n t p a t t e r n s . P l o t t i n g m a y be d o n e in t w o or t h r e e d i m e n s i o n s . T h e p r e s e n t r e p o r t used t w o d i m e n s i o n a l plots. Unlike magnetic techniques of recording head and eye movement, our photographic p r o c e d u r e can r e c o r d b o t h angular a n d linear t r a n s l a t i o n s o f a n y b o d y part. T h e c o m p l e t e v e c t o r analysis p r o c e d u r e , including filming a n d v e c t o r plotting, t a k e s a p p r o x i m a t e l y 12 h f o r each unit. Significance o f changes in vect o r d i s t r i b u t i o n can b e assessed b y a p p r o p r i a t e statistical tests. Fig. 2 p r e s e n t s p l o t s o f h e a d m o v e m e n t s r e l a t e d t o u n i t discharge in t h r e e units r e c o r d e d in t h e p o n t i n e n u c l e u s gigantocellularis. T h e s e cells w e r e histologically localized b e t w e e e n P 1.5 a n d P 5.5 a n d L 1.5 a n d L 2.0 ( B e r m a n , 1968). N o t e t h e r a n d o m a r r a n g e m e n t o f m o v e m e n t v e c t o r s d u r i n g t h e interf r a m e intervals prior t o t h e s t a r t o f u n i t discharge. During t h e film f r a m e s
163
spanning the interval in which unit discharge begins, the distribution of vectors shifts. Our previous work, which employed visual and EMG observations to describe the behavioral correlates of pontine gigantocellularis unit discharge, found that activity in these cells was related to specific movements (Siegel and McGinty, 1977; Siegel et al., 1977). The most c o m m o n relation was to head movements. The preliminary data presented here indicate that these movements begin within 56 msec of unit discharge and that the movements have a high degree of directional consistency. Within a 56 msec interval more than one movement can be executed. Therefore the scatter observed in Fig. 2 could result either from other movements within the interval obscuring a very tight coupling of unit discharge to a highly specific movement, or from the units' relation to a wide range of movements concentrated in one direction. Higher speed filming could resolve such questions. DISCUSSION
This procedure provides an objective m e t h o d of mapping a unit's m o t o r correlates. The temporal relation of unit firing to behavior, and the consistency of this relation, can be documented with great precision w i t h o u t using restraint or movement restriction. This technique can be applied to describe unit activity--movement relations in unrestrained animals during a wide range of natural behaviors. Correlative data cannot itself prove causal relationships. Thus a unit discharge that precedes motion of an identified body part need not be directly in the pathway mediating that movement. Rather, the unit activity and movement may both be temporally related to other aspects of the complex sequence of sensory and m o t o r events surrounding the observed movement. For example, unit discharge may relate to increased tone in axial musculature, alterations in reflex excitability, activity in cutaneous muscles, etc. The photographic technique is able to detect unit discharge relations with a wide variety of movements and is therefore less likely to miss underlying relationships than restricted movement paradigms that measure only the movement of a single b o d y part. However, results from both of these techniques must be correlated with behavioral, physiological and anatomical findings in order to determine the mechanisms underlying observed relationships. ACKNOWLEDGEMENTS
Supported by the Medical Research Service of the Veterans Administration and National Institutes of Health Research Grant NS14610.
164 REFERENCES Berman, A.L. (1968) The Brain Stem of the Cat, University of Wisconsin Press, Madison, p. 175. Burton, J.E. and Onoda, N. (1977) Interpositus neuron discharge in relation to a voluntary movement, Brain Res., 121: 167--172. Collewijn, H. (1977) Gaze in freely moving subjects. In R. Baker and A. Berthoz (Eds.), Control of Gaze by Brain Stem Neurons, Developments in Neuroscience, Vol. 1, Elsevier/North-Holland Biomedical Press, Amsterdam, pp. 13--22. DeLong, M.R. (1973) Putamen: activit:~ of single units during slow and rapid arm movements, Science, 179: 1240--1242. Harper, R.M. and McGinty, D.J. (1973) A technique for recording single neurons from unrestrained animals. In M.I. Phillips (Ed.), Brain Unit Activity during Behavior, Thomas, Springfield, Ill., pp. 80--104. Keller, E.L. and Robinson, D.A. (1972) Abducens unit behavior in the monkey during vergence movements, Vision Res., 12 : 369--382. Orlovsky, G.N. (1972) Activity of vestibulospinal neurons during locomotion, Brain Res., 46: 85--98. Siegel, J.M. and McGinty, D.J. (1977) Pontine reticular formation neurons: relationship of discharge to motor activity, Science, 196: 678--680. Siegel, J.M., McGinty, D.J. and Breedlove, S.M. (1977) Sleep and waking activity of pontine gigantocellular field neurons, Exp. Neurol., 56: 553--573.
