Department of Physiology, Hokkaido University School of Medicine, Sapporo 060-8638, Japan; and Department of Physiology and Biophysics and Regional Primate Research Center, University of Washington, Seattle, Washington 98195
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ABSTRACT |
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Fukushima, Kikuro,
Junko Fukushima,
Chris
R. S. Kaneko, and
Albert F. Fuchs.
Vertical Purkinje Cells of the Monkey Floccular Lobe:
Simple-Spike Activity During Pursuit and Passive Whole Body Rotation.
J. Neurophysiol. 82: 787-803, 1999.
To understand how the simian floccular lobe is involved in
vertical smooth pursuit eye movements and the vertical vestibuloocular reflex (VOR), we examined simple-spike activity of 70 Purkinje (P)
cells during pursuit eye movements and passive whole body rotation.
Fifty-eight cells responded during vertical and 12 during horizontal
pursuit. We classified P cells as vertical gaze velocity (V) if
their modulation occurred for movements of both the eye (during
vertical pursuit) and head (during pitch VOR suppression) with the
modulation during one less than twice that of the other and was less
during the target-fixed-in-space condition (pitch VOR X1) than during
pitch VOR suppression. V
P cells constituted only a minority of
vertical P cells (19%). Other vertical P cells that responded during
pitch VOR suppression were classified as vertical eye and head velocity
(V
/
) P cells (48%), regardless of the synergy of their
response direction during smooth pursuit and VOR suppression. Vertical
P cells that did not respond during pitch VOR suppression but did
respond during rotation in vertical planes other than pitch were
classified as off-pitch V
/
P cells (33%). The mean
eye-velocity and eye-position sensitivities of the three types of
vertical P cells were similar. One-third (2/7 V
, 2/11
V
/
, 6/13 off-pitch V
/
), in addition, showed eye position sensitivity during saccade-free fixations. Maximal vestibular activation directions (MADs) were examined during VOR suppression by applying vertical whole body rotation with the monkeys
oriented in different vertical planes. The MADs for V
P cells
and V
/
P cells with eye and vestibular sensitivity in the
same direction were distributed near the pitch plane, suggesting convergence of bilateral anterior canal inputs. In contrast, MADs of
off-pitch V
/
P cells and V
/
P cells with
oppositely directed eye and vestibular sensitivity were shifted toward
the roll plane, suggesting convergence of anterior and posterior canal inputs of the same side. Unlike horizontal
P cells, the
modulation of many V
and V
/
P cells when the
target was fixed in space (pitch VOR X1) was not well predicted by the
linear addition of their modulations during vertical pursuit and pitch
VOR suppression. These results indicate that the populations of
vertical and horizontal eye-movement P cells in the floccular lobe have
markedly different discharge properties and therefore may be involved
in different kinds of processing of vestibular-oculomotor interactions.
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INTRODUCTION |
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With the development of the high acuity fovea in
primates, smooth-pursuit eye movement has evolved to track small moving
objects. During head movement, the smooth pursuit system does not work independently but interacts with the vestibular system to match the
velocity of the eyes in space (i.e., gaze) with target velocity (Robinson 1981). Single-cell recording and lesion
studies have implicated the cerebellar flocculus and ventral
paraflocculus (which we refer to as the floccular lobe)
(Krauzlis and Lisberger 1996
) in the control of gaze
velocity. This control has been revealed both during horizontal
pursuit-vestibular interactions (Lisberger and Fuchs
1978
) and optokinetic eye movement (ocular following), which
shares many pathways with smooth pursuit (Kawano et al. 1992
,
1994
; Shidara and Kawano 1993
).
Gaze velocity () Purkinje (P) cells constitute the majority of
horizontal P cells in the simian floccular lobe (Miles et al.
1980
; Stone and Lisberger 1990
). They respond
during smooth pursuit and pursuit-vestibular interactions but little,
if at all, during the vestibuloocular reflex (VOR) when gaze is stable (Krauzlis and Lisberger 1996
; Lisberger and Fuchs
1978
; Lisberger et al. 1994
; Miles and
Fuller 1975
; Miles et al. 1980
; Shidara and Kawano 1993
; Stone and Lisberger 1990
).
Removal of the floccular lobe results in an impairment of slow gaze
movement during visual-vestibular interactions in many animal species
and substantial impairment of smooth pursuit in monkeys. The horizontal
VOR itself is not consistently affected (Flandrin et al.
1983
; Hassul et al. 1976
; Honrubia et al.
1982
; Ito et al. 1982
; Zee et al.
1981
).
The simian floccular lobe contains another class of P cells that
respond during downward smooth pursuit and ocular following (Krauzlis and Lisberger 1996; Miles et al.
1980
; Shidara and Kawano 1993
; Stone and
Lisberger 1990
). Although their activity has not been recorded
during vestibular stimulation or visual-vestibular interactions, lesion
studies have implicated them in the control of slow vertical eye
movements. Chemical inactivation of the floccular lobe results in
impairment of suppression of the vertical VOR but, as in the yaw plane,
does not effect the VOR itself in monkeys tested while lying on their
sides (Zhang et al. 1995
). Inconsistent with this
observation, many vertical P cells in the floccular lobe in alert cats
tested in the upright position, respond not only during
visual-vestibular interactions but also during the vertical VOR itself
(Fukushima et al. 1993
, 1996a
). The majority respond to
upward pitch rotation when the vertical VOR drives the eyes downward
and to downward eye movements induced by optokinetic stimuli. Their
firing is in-phase with eye velocity and these cells therefore were
classified as vertical eye-velocity (V
) P cells. They also
respond during vertical whole body oscillations in several planes and
therefore have a vestibular sensitivity. Although they respond best
during vertical eye movements, their preferred directions for
vestibular activation (i.e., maximal activation directions)
(Baker et al. 1984
) lay near the posterior canal (not
the pitch) plane. Finally, chemical inactivation of the floccular areas
where up-pitch P cells were recorded affected the downward VOR
exclusively in cats tested in the upright position (Fukushima et
al. 1993
, 1996a
,b
). Therefore several lines of evidence in
different species implicate the floccular lobe in the control of slow
vertical eye movements generated during smooth pursuit and vestibular stimulation.
The finding in the cat that the majority of vertical floccular P cells
discharge with eye rather than gaze movement contrasts with data on the
monkey where the majority of horizontal floccular P cells discharge
with gaze rather than eye movement. To determine whether these
observations indicate a fundamental difference in the role of the
floccular lobe during vertical and horizontal pursuit and
pursuit-vestibular interactions or simply reflect a species difference,
we recorded simple-spike activity of vertical P cells in trained
monkeys. Preferred directions during vestibular rotation (i.e.,
vestibular direction tuning) of these cells also were examined by
applying vertical whole body rotation in several planes while the
monkeys were required to suppress the VOR. Some of these results have
been presented in abstract form (Fukushima et al.
1996d).
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METHODS |
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General procedures
A total of five male macaques (3 Macaca fuscata and 2 M. mulatta) were used. Experiments on the three Japanese macaques (M. fuscata) were performed at Hokkaido University School of Medicine, Sapporo, Japan, and experiments on the two rhesus macaques (M. mulatta) were performed in the Regional Primate Research Center, University of Washington, Seattle, WA. The first author participated in all of the experiments, and he digitized and analyzed all of the data with the same computer programs. All experiments were performed in strict compliance with the Guide for the Care and Use of Laboratory Animals (DHEW Publication NIH85-23, 1985) and recommendations from the Institute of Laboratory Animal Resources and the American Association for Accreditation of Laboratory Animal Care International. Specific protocols were approved by the Institutional Animal Care and Use Committee at the University of Washington (ACC 2602-01) and by the Animal Care and Use Committee of Hokkaido University School of Medicine (00421 and 9290).
Our methods for animal preparation and training are described in detail
elsewhere (Fuchs et al. 1994; Fukushima et al.
1996c
) and therefore are summarized here only briefly. The
monkeys were prepared surgically under aseptic conditions with a pair
of head holders or head stabilization lugs, and a scleral search coil was implanted on the right eye to record horizontal and vertical eye
movement (Fuchs and Robinson 1966
; Judge et al.
1980
). The monkeys sat in a restraining chair with their heads
held firmly in the stereotaxic plane. The chair was fixed to a
turntable with three rotational degrees of freedom under computer
control. A visual target was created by projecting a laser spot
(~0.2-0.5° diam) onto a screen in front of the monkeys. The
monkeys were trained to track the spot for an apple sauce (for rhesus
macaques) or apple juice (for Japanese macaques) reward during a
variety of target and vestibular stimulus conditions. Vestibular
stimulation was applied by rotating the primate chair either about a
vertical axis (yaw stimulation) or an interaural axis (pitch
stimulation). After the animals were trained, recording chambers were
installed over holes cut in the skull at AP 0 and L ± 10 to allow
single-cell recording in both floccular lobes.
Recording procedures and behavioral paradigms
All stimuli were applied sinusoidally. As our search stimulus,
the target was moved either obliquely or vertically (at 0.5 Hz ± 5 or ± 10°) in association with vestibular rotation (0.25 Hz ± 10 or 0.5 Hz ± 5 or 10°) in the pitch plane. The
floccular lobe was identified physiologically by its characteristic
saccade-related burst discharge (e.g., Noda and Suzuki
1979). Unit activity was recorded extracellularly with tungsten
microelectrodes. P cells were identified by the existence of complex
spikes as illustrated in Fig.
1B (
). Once responding
cells were encountered, we assigned the monkeys each of three standard
tasks (Fig. 1A) that used various combinations of whole
body rotation and target movement at 0.5 Hz (±5 or ±10°). Pursuit
responses were tested in two directions (vertical and horizontal) to
determine the preferred direction for activation of each neuron (Fig.
1A, top). Because the great majority of neurons showed
either a horizontal or vertical referred smooth pursuit direction,
chair rotation was applied in either the horizontal (yaw) or vertical
(pitch) plane, respectively.
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To dissociate eye movement in the orbit from that in space (i.e., gaze), we employed two tracking conditions (Fig. 1A). In the first, the monkeys tracked a target that moved in space with the same amplitude and phase as the chair and in the same plane. This condition requires the monkeys to suppress the VOR so that the eyes remain virtually motionless in the orbit and gaze therefore moves with the chair (VOR suppression, 2nd row). In the second condition, the target stayed stationary in space during chair rotation, which requires perfect VOR and no gaze movement (target fixed in space, VOR X1, 3rd row).
To assess possible eye-position sensitivity, we required stationary animals to fixate a target placed at various target eccentricities along the vertical or horizontal meridian.
To examine the vestibular direction tuning of vertical P cells, we
determined the maximum activation directions (MADs) induced by vertical
rotation, according to the method described by Baker et al.
(1984). The horizontal orientation of the animal was selected by positioning the chair at different angles (see Figs.
9A and 10A). With the animal pointed
straight ahead (i.e., 0°), vertical rotation about a horizontal
interaural axis stimulated all four vertical canals. The animal then
was oscillated around the same earth-horizontal axis while it was
oriented in several different horizontal directions. At least five, and
typically seven, horizontal orientations were tested to determine MADs
for individual cell responses. For example, if the monkey was oriented
to 45° toward the animal's right, the vertical rotation would now
maximally stimulate the left anterior and right posterior canals and
not stimulate the other pair at all (see Figs. 9A and
10A). If, in addition, the monkey was required to fixate
a target spot that rotated with it, the vertical VOR would be
suppressed and unit modulation was attributed to a vestibular
sensitivity. The MAD was the horizontal orientation that produced the
maximum modulation in firing rate.
Data analysis
The data were analyzed off-line. Isolated single cell activity
was digitized together with eye-, chair-, and target-position signals
at 303 or 500 Hz using a 16-bit A/D board (Fuchs et al. 1994; Fukushima et al. 1995
). These signals were
differentiated by software to obtain velocity. Gaze velocity was
calculated as the sum of eye velocity and chair velocity. Saccades were
marked with a cursor on eye- and gaze-velocity traces and removed using an interactive computer program (Fukushima et al. 1995
).
Those occasional bursts or pauses in cell discharge associated with saccades were marked manually and removed from the analysis. Rasters and histograms were constructed by averaging between 10 and 30 cycles.
Each cycle was divided into 64 equal bins together with averaged
velocity. Marked bursts or pauses in discharge did not contribute to
the histograms although they are shown in the cycle rasters.
A sine function was fit to all velocity traces and to the cycle
histograms of cell discharge exclusive of the bins with zero spike rate
by means of a least-squares error algorithm. Responses that had a
harmonic distortion (HD) of >50% or a signal-to-noise ratio (S/N) of
<1.0 were discarded. S/N was defined as the ratio of the amplitude of
the fitted fundamental frequency to the root mean square amplitude of
the third through eighth harmonics. HD was taken as the ratio of the
amplitude of the second harmonic to that of the fundamental. The phase
shift of the peak in the fitted-function relative to upward eye
velocity or upward stimulus-velocity was calculated as a difference in
degrees. Sensitivity was calculated as the peak amplitude of the
fundamental component fitted to the cycle histogram divided by the peak
amplitude of stimulus velocity (i.e., target velocity for pursuit and
chair velocity for other tasks during vestibular rotation, Fig.
1A). A sensitivity 0.10 spikes/s per °/s was taken as
significant modulation. Eye or gaze gain was calculated by dividing
peak eye or gaze velocity by stimulus velocity.
We calculated eye position sensitivity (k) during sinusoidal
smooth pursuit using the equation {k = [Au cos
(p)]/Ae} of Chubb and Fuchs (1982), where
Au was the amplitude of the modulation in cell activity, Ae was the
amplitude of the eye movement, and p was the phase lead of
the modulation in cell activity with respect to eye position.
Eye-position sensitivity also was examined for the periods when monkeys
fixated the stationary target for >1 s. Discharge rates of individual
cells were plotted against the corresponding horizontal or vertical eye
position, and a linear regression analysis was performed to examine
eye-position sensitivity.
We also calculated eye-velocity sensitivity (r) during
smooth pursuit using the equation r = [Au
sin(p)]/(2pi f Ae) of Chubb and Fuchs
(1982), where Au, Ae, and p are the same as for the previous equation and f is the frequency of the stimulus.
The MAD was calculated from responses to sinusoidal vertical rotation
at different horizontal orientations while the monkeys performed the
VOR suppression task (see Figs. 9A and 10A). We
used the convention of Baker et al. (1984), who plotted
sensitivity and phase (re: chair position) of neural responses against
horizontal orientation relative to the recording side; sensitivity
values were plotted as positive when the phase values lagged chair
position and as negative when they led. Horizontal orientations were
plotted as positive for those toward the side contralateral to the
recording site and as negative for those toward the side ipsilateral to the recording. Responses were considered to be evoked primarily by
canal inputs when their modulation varied sinusoidally with the
orientation of the rotation plane but phase was mostly constant except
when rotation was near the null orientation where there was no
modulation (see Figs. 9 and 10). The null orientation was calculated
from response reversal by fitting a least-squares error sinusoid to
each sensitivity curve (Baker et al. 1984
). The amount of error (in degrees) in fitting a sine curve also was estimated by
calculating the mean square root of errors divided by the square root
of the number of horizontal orientation angles examined. The null
orientation was used to estimate the MAD by assuming that the latter
should be perpendicular to the former (Blanks et al.
1978
; Estes et al. 1975
). Paired or unpaired
Student's t-tests were used for statistical analysis.
Histological procedures
Near the conclusion of the recording period in each
monkey, the sites of vertical P cell activity were marked by iron
deposits produced by passing positive current (10-15 µA for 60-100
s; 800-1,200 µ Coulombs) or by electrolytic lesions produced by
passing negative current through the microelectrode (20 µA for
30 s). After floccular recording was completed, the three Japanese
monkeys were anesthetized deeply by pentobarbital sodium (50 mg/kg ip)
and perfused by physiological saline followed by 3.5% formalin for
reconstruction of electrolytic lesions and by both formalin and 2%
ferrocyanide (Suzuki and Azuma 1976) for iron marking.
After histological fixation, transverse sections of the brain stem and
cerebellum were cut at 100-µm thickness on a freezing microtome in
the plane of recording electrode tracks with the aid of two pins left
during perfusion. The sections were then stained using the
Klüver-Barrera method (Klüver and Barrera 1953
). The flocculus and ventral paraflocculus were identified according to the description of Gerrits and Voogd
(1989)
, and the locations of recording sites were verified. The
two rhesus monkeys are still being used for other experiments. However,
because of our extensive experience with the characteristic discharge properties of P cells of the floccular lobe (e.g., Lisberger and Fuchs 1978
), we are certain that recordings in these monkeys
also were from the floccular lobe.
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RESULTS |
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In the floccular lobes of five monkeys, we examined the
discharge characteristics of 102 cells that responded to vertical or
horizontal smooth pursuit and/or chair rotation. Of these, 70 cells
were identified as P cells by the existence of complex spikes (e.g.,
Fig. 1B, ). Of the 70, 58 cells responded during vertical
pursuit and/or vertical whole-body rotation; the remaining 12 responded
to horizontal pursuit and horizontal rotation. The small number of
cells with horizontal sensitivities does not reflect the true
percentage of horizontal P cells because we documented them only
occasionally as a basis for comparison with vertical P cells. Among the
32 of 102 cells in which complex spikes were not discernable, 25 responded to vertical rotation and vertical pursuit; the remaining 7 responded mainly to horizontal rotation and horizontal pursuit.
Of the 58 vertical P cells, 30 were recorded in the Japanese macaques and 28 in the rhesus macaques. Because the response properties of cells recorded in the two types of macaque were similar, we considered the cells as one population and their data were combined. All 58 cells were recorded during vertical smooth pursuit and VOR suppression in the pitch plane. For 54 of these, the target-fixed-in-space condition during pitch rotation (pitch VOR X1, Fig. 1A) also was examined. Overall mean (± SD) eye-movement gains were 0.80 ± 0.16 during vertical pursuit, 0.17 ± 0.09 during pitch VOR suppression, and 1.03 ± 0.13 during the target-fixed-in-space condition (pitch X1).
We identified P cells as "gaze velocity" if they met the following
criteria, which characterized the horizontal gaze velocity cells of
Lisberger and Fuchs (1978): modulation occurred for
movements of the eye (during smooth pursuit) and the head (during VOR
suppression) in the same direction, modulation during pursuit was less
than twice the modulation during suppression or vice versa, and
modulation during the target-fixed-in-space condition (VOR X1) was less
than that during VOR suppression. Based on these criteria, 11 of the 58 (~19%) were classified as vertical gaze velocity (V
) P cells.
The remaining P cells were separated into two further types based on
whether they responded during pitch VOR suppression. Twenty-eight of
the 58 (~48%) responded during pitch VOR suppression, and we called
them vertical eye and head velocity (V/
) P cells (cf.
Scudder and Fuchs 1992
). Of the 28, 12 preferred smooth
pursuit and head movement (during VOR suppression) for eye and head
movements in the same direction but did not satisfy all of the criteria for gaze velocity cells. Sixteen had oppositely directed eye and head-movement sensitivity. Four of the 58 that were not examined during
the target-fixed-in-space condition (pitch X1) responded during VOR
suppression in pitch; however, they did not meet criteria #1, so they
were classified as V
/
P cells.
The remaining 19 of the 58 (~33%) did not respond during pitch
suppression. However, when eight cells of this type were tested during
the suppression condition in a rotation plane other than pitch (see
Fig. 10), seven had head-movement modulation. So we call these cells
off-pitch V/
P cells.
We now will examine the behavior of representative neurons of each of the three cell types in some detail.
Vertical gaze velocity (V) P cells
The P cell shown in Fig. 2 was
deeply modulated for downward smooth pursuit (A) and
downward head rotation during suppression of the pitch VOR
(B). Both responses were essentially in phase with eye or
head velocity, consistent with the lack of eye position sensitivity of
this cell during fixation of a stationary target at different vertical
eye positions (Fig. 2F). During the target-fixed-in-space condition (pitch X1) in which gaze was largely stable (Fig.
2C, ), the modulation was much less than during
either smooth pursuit (Fig. 2A) or VOR suppression (Fig.
2B). This neuron showed essentially no modulation during
either horizontal smooth pursuit (Fig. 2D) or suppression of
the VOR during yaw rotation (Fig. 2E).
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Figure 3A compares the responses of all 11 neurons in this group during the three task conditions at 0.5 Hz (A, 1, 2, and 4); the responses are displayed in polar coordinates with sensitivity plotted as a radius and the phase as an angle (re:stimulus velocity). Most of these neurons (8/11) had downward ON directions for pursuit and VOR suppression, whereas three had upward ON directions. The average sensitivity is 0.91 ±0.56 (SD) spikes/s per °/s for vertical smooth pursuit and 0.99 ± 0.59 (SD) spikes/s per °/s for pitch VOR suppression. Although 9 of the 11 were modulated when gaze was roughly constant in the pitch X1 condition (A4, mean sensitivity = 0.49 ± 0.42 SD spikes/s per °/s, n = 11), their values scattered around zero except for one outlying cell (Fig. 3A4). Gaze gain during recording of these cells ranged from 0 to 0.23 with the mean of 0.10 ± 0.08.
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We next examined whether the modulation due to vertical pursuit cancels that due to head rotation (assessed during pitch VOR suppression) during the pitch × 1 condition when the eyes and head move in opposite directions and gaze movement is nearly zero. Figure 3A3 plots predicted modulation of individual cells during the VOR (pitch X1) in polar coordinates by adding spike histograms of each cell during vertical pursuit and pitch VOR suppression. Their values scattered around zero (Fig. 3A3), suggesting that their response as a whole is consistent with a gaze velocity response.
Vertical eye and head velocity (V/
) P cells
The P-cell shown in Fig. 4 was
modulated during downward smooth pursuit (A) and the peak
discharge occurring between the peaks in eye velocity, indicating that
the cell had both eye velocity and position sensitivity. Indeed, when
the animal looked at fixed positions, firing rate was weakly related to
eye position (Fig. 4D). Although this cell also was
modulated during downward head rotation during suppression of the pitch
VOR (Fig. 4B), its modulation was less than half of its
modulation during vertical pursuit (Fig. 4A). Moreover, its
modulation during the target-fixed-in-space condition in which gaze was
roughly constant (pitch x1, Fig. 4C, ) was larger
than the modulation during pitch VOR suppression (Fig. 4B).
This neuron showed essentially no modulation during either horizontal
smooth pursuit (Fig. 4E) or suppression of the VOR during
yaw rotation (Fig. 4F).
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Figure 3B plots responses of V/
P cells during
the three diagnostic task conditions at 0.5 Hz. (B, 1, 2, and 4). Their average response sensitivity is 0.75 ± 0.51 spikes/s per °/s for vertical smooth pursuit and 0.52 ±0.31
spikes/s per °/s for pitch VOR suppression. Twenty of the 24 tested
were modulated when gaze was roughly constant in the pitch X1 condition
(Fig. 3B4, mean sensitivity = 0.67 ± 0.51 SD
spikes/s per °/s). Their predicted modulations during the pitch VOR
(X1), obtained by adding spike histograms of individual cells during
vertical pursuit and pitch suppression, more widely scattered than
those for V
P cells (Fig. 3, B3 vs. A3).
Off-pitch V/
P cells
In contrast to the cells in Figs. 2 and 4, the P cell illustrated in Fig. 5 responded to vertical smooth pursuit and the target-fixed-in-space (pitch X1) condition with similar magnitude (Fig. 5, A and C), but there was no modulation (Fig. 5B) during VOR suppression in the pitch plane. Slightly more than half of such cells (11/19) had downward ON directions during pursuit; the remaining 8 had upward ON directions. The peak discharge of this cell during smooth pursuit and the target-fixed-in-space (X1) condition lay between peak eye and target/head velocity (Fig. 5, A and C), indicating that the cell had both eye velocity and position sensitivity. Indeed, when the animal looked at fixed positions, firing rate was related to eye position (Fig. 5D). This neuron did not respond during either horizontal smooth pursuit or yaw suppression (Fig. 5, E and F). Although cells of this type did not respond during pitch VOR suppression, all but one of the eight tested were modulated when the chair was oscillated in other vertical planes (see Fig. 10). This vestibular sensitivity and its preferred plane of modulation is examined later.
|
Figure 3C plots responses of off-pitch
V/
P cells during the three diagnostic task conditions at
0.5 Hz (C, 1, 2, and 4). The average sensitivity
for vertical smooth pursuit is 1.15 ±0.79 spikes/s [per] deg/s. All
but one of the 18 responded during the target-fixed-in-space (x1)
condition with the mean sensitivity of 0.99 ±0.0.59 SD spikes/s[per]
deg/s (n = 19). Their predicted modulations during the
VOR (pitch X1) obtained by adding spike histograms of individual cells
during vertical pursuit and pitch VOR suppression were scattered much
more widely than those of V
P cells (Fig. 3, A3 vs.
C3).
Comparison of discharge characteristics of the three cell types
EYE MOVEMENT SENSITIVITY.
During smooth pursuit, the phase of the modulation of some
vertical P cells (e.g., Fig. 5A) lay intermediate to eye
position and velocity, suggesting that their discharge was sensitive to both. To examine P-cell activity related to smooth pursuit further, we
calculated the phase shifts of individual P cells relative to upward
eye velocity (Fig. 6, A1, B1,
and C1). Compared with V P cells (Fig.
6A1), V
/
P (Fig. 6B1), and
off-pitch V
/
P cells (Fig. 6C1) showed a wide
range of phases. We calculated eye-velocity and eye-position
sensitivities during sinusoidal vertical pursuit by decomposing the
modulation into separate components related to eye velocity and
position (see METHODS) (Chubb and Fuchs
1982
). The distribution of eye-velocity sensitivities for the
three cell types is similar (Fig. 5, A2, B2, and
C2) with means ± SD of 0.24 ± 0.23, 0.21 ± 0.16, and 0.35 ± 0.38 spikes/s per °/s, for V
,
V
/
,) and off-pitch V
/
P cells, respectively.
|
ADDITION OF EYE- AND HEAD-MOVEMENT SENSITIVITIES.
We asked whether the eye- and head-movement sensitivities accounted for
the entire behavior of vertical P cells during the condition when gaze
was stable in space (pitch X1). To do this, we compared actual cell
modulation during the target-fixed-in-space (pitch X1) condition with
the predicted modulation calculated by adding averaged histograms
during pitch VOR suppression and vertical pursuit. In Fig.
7, this comparison is shown for a
V P cell (A) and for V
/
P cells with
eye- and head-velocity sensitivity in the same direction
(B), or the opposite direction (C). For the
majority of vertical P cells, the modulation during the
target-fixed-in-space condition (actual pitch X1) was not well
predicted from the linear addition of the pursuit and suppression conditions (predicted pitch X1, Fig. 7, A-C). For example,
the actual modulation of the cell in Fig. 7A is 180° out
of phase with the predicted response (predicted pitch X1, Fig.
7A), whereas the cell in Fig. 7B actually was
unmodulated (actual pitch X1) although a fairly robust modulation was
predicted (predicted pitch X1, Fig. 7B).
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COMPARISON WITH HORIZONTAL GAZE VELOCITY P CELLS.
The sensitivity of horizontal P cells during pursuit is well
correlated with their sensitivity during VOR suppression
(Lisberger and Fuchs 1978
). Although a significant
correlation was observed between the two sensitivities for V
P
cells (Fig. 8A), there was no significant correlation (r = 0.24, P > 0.1) between the two sensitivities for
V
/
P cells (Fig. 8B).
|
Vestibular direction tuning of three cell types
DIRECTION SELECTIVITY.
We examined the direction selectivity of our cell types during
VOR suppression (n = 41, 6 V, 11 V
/
, 12 off-pitch V
/
, and 12 H P cells). The
majority of vertical P cells responded for either yaw or pitch
rotations but not both (27/41 = ~66%). The remaining cells
(n = 11, 3/6 V
, 8/11 V
/
, 0/12
off-pitch V
/
and 3/12 H P) responded to both (Fig. 8,
F and G). In particular, for eight of these
vertical P cells, the modulation during yaw suppression was >0.5 times
that during pitch suppression; one H P cell also was equally sensitive
during yaw and pitch suppression. These results suggest a convergence
of vertical and horizontal canal inputs on these cells (see
DISCUSSION). During smooth pursuit (n = 41), a majority of P cells (25/41 = 61%) also responded to either
vertical or horizontal pursuit. A minority (n = 16, 3/6 V
, 6/13 V
/
, 3/12 off-pitch V
/
, and
4/12 H P) responded to both; six of these (6/41=~15%), had almost
equal sensitivities to both (not shown), suggesting that their
preferred directions were oblique.
MAD ANALYSIS.
To examine the direction tuning of the vestibular response, we
calculated MADs for seven V, seven V
/
, and eight
off-pitch V
/
P cells. Figure
9 shows examples of V
(B and C) and V
/
P cells
(D and E). During vertical chair rotation, their
activity depended on the horizontal orientation of the head (Fig. 9,
top, cartoons). Modulation was almost maximal during pitch
suppression (0°, C1 and E1) and almost zero
(E3, and E5) or reversed (C3 vs. C5) during roll rotation (ipsilateral or contralateral
90°, C3, C5, E3, and E5). In Fig. 9,
F and G, the sensitivity and phase of the
modulation of these two cells are plotted against horizontal orientation relative to the recording side. The phase (
) remained fairly constant near either +90 or
90° except for abrupt shifts when sensitivity was ~0. The sensitivity curves (
) are fit well with a least-squares sinusoid. The peak response calculated from the
least-squares sinusoid yielded MADs for these two cells of +14 and
3° (Fig. 9, F and G) with errors for the fit
sine functions (see METHODS) of 4.8 and 1.5°,
respectively.
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|
Horizontal and
P cells
Nine of 12 horizontal P cells in this study were identified
as gaze velocity (H) P cells. All H
P cells increased
their activity during ipsilateral pursuit. Figure 8H shows
that their sensitivity during horizontal pursuit is well correlated
with their sensitivity during yaw VOR suppression. Although we tested only a small number of H
P cells (Fig. 8H,
), the
correlation is significant with a slope near one. For the remaining
three cells, sensitivities during yaw VOR suppression were only 31, 45, and 58% of the target-fixed-in-space (yaw X1) condition. Because their
response phases in the yaw X1 condition were near eye velocity, we call
these cells
P cells (Lisberger 1996
). Two of the
three
P cells exhibited an increased activity during
ipsilateral pursuit and one during contralateral pursuit.
Incompletely identified cells
Among the 25 cells with vertical smooth pursuit sensitivity in
which complex spikes were not discernable, 5 were classified as
V and 13 as V
/
. The remaining seven did not
respond at all during pitch VOR suppression. However, two of the seven
responded during vertical rotation in the roll plane, suggesting that
they had a vestibular sensitivity like that of off-pitch
V
/
P cells. Of seven cells with horizontal smooth pursuit
sensitivity in which complex spikes were not discernable, four were
and three were eye-movement cells. Thus our incompletely tested
vertical floccular cells seemed to fall into the three cell types of
our characterized population.
Recording location
An example of an iron deposit left after recording a
V/
P cell with downward smooth pursuit sensitivity is
shown in Fig. 11D. Actual
recording tracks found in histological sections in this monkey are
superimposed in three rostral (Fig. 11A) to caudal (C) sections. Recording tracks in two other Japanese monkeys
were found in similar locations. The tracks were found in both the flocculus and ventral paraflocculus and immediately adjacent dorsal paraflocculus (cf. Noda and Mikami 1986
). Of the 30 vertical P cells, which we recorded in the Japanese monkeys, only 3 were recorded in the dorsal paraflocculus. We often observed all three types of vertical P cells on the same tracks, suggesting that the three
groups of P cells are intermingled in the floccular lobe. The two
rhesus monkeys have not yet provided histology (see METHODS).
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DISCUSSION |
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This study showed that the populations of vertical and horizontal eye-movement P cells in the floccular lobe have markedly different discharge properties.
Comparison of vertical and horizontal floccular P cells
Using behavioral paradigms that dissociate eye movement in the
orbit from eye movement in space (i.e., gaze) in the vertical plane, we
have shown that individual vertical P cells could be classified as
either V (19%, n = 11), V
/
(48%,
n = 28), or off-pitch V
/
P cells (33%,
n = 19). V
P cells constituted only a small
portion of the total vertical P-cell population with vertical pursuit
sensitivity. In contrast, but consistent with previous reports
(Lisberger 1996
; Lisberger and Fuchs
1978
; Miles et al. 1980
; Stone and
Lisberger 1990
), horizontal P cells that encoded gaze velocity
outnumbered (9/12) horizontal P cells with predominantly eye-movement
sensitivity (3/12) even in our small sample. In alert cats, V
P
cells also are more numerous than P cells that seem to encode vertical
gaze velocity (Fukushima et al. 1996a
) (80 vs. 20%,
n = 30). Only 3 of the 30 in the monkeys we have
histology for lay in the dorsal paraflocculus. Therefore the two cell
types we describe here have not been drawn from two separate anatomic locations.
During the target-fixed-in-space condition (VOR X1), vertical P cells
and H P cells not only preferred different
smooth pursuit directions but also exhibited qualitative differences in
their response. The great majority of vertical P cells (9/11 V
,
20/24 V
/
, 18/19 off-pitch V
/
P, 47/54 = 87%) were modulated during this condition when the mean VOR gain was
1.03 ± 0.13. In contrast, H
P cells exhibited virtually no
modulation under the target-fixed-in-space condition (yaw X1) in
monkeys with a VOR gain of 1.0 (Lisberger and Fuchs
1978
) (also our H
P cells). Moreover, unlike H
P
cells (Lisberger and Fuchs 1978
), the modulation of many
V
and V
/
P cells when the target was fixed in
space (pitch X1) was not well predicted by the linear addition of their
modulations during smooth pursuit and VOR suppression (Fig. 7).
Unlike horizontal P cells the responses of which were predominantly in
phase with gaze or eye velocity (e.g., Lisberger and Fuchs 1978; Miles
et al. 1980
; Stone and Lisberger 1990
), the responses of some
V
/
and off-pitch V
/
P cells were in phase
with eye position during smooth pursuit (Fig. 6, B1 and C1). Nevertheless, population eye-velocity and eye-position
sensitivities of the three types of vertical P cells calculated from
sinusoidal vertical pursuit were similar (Fig. 6, A, 2 and
3, B, 2 and 3, C, 2 and 3). In
contrast, the three types of vertical P cells behaved very differently
during pitch VOR suppression and the target-fixed-in-space condition
(pitch X1, Figs. 2, 4, 5). The difference among the three cell types in
the interaction conditions therefore can be attributed primarily to
vestibular (ipsilateral or contralateral anterior or posterior canal)
inputs to these cells, suggesting that the three types of vertical P
cells might constitute extremes of a continuum reflecting different
vestibular sensitivities. The difference in the vestibular direction
tuning of the three types of P cells using the MAD analysis supports this interpretation.
Vestibular direction tuning
Although eye-movement tuning has been tested during smooth pursuit
for monkey floccular P cells (Krauzlis and Lisberger
1996; Stone and Lisberger 1990
), there is no
previous data available for vestibular direction tuning, so we will
compare our data with those in the cat. The MAD distribution of
V
P cells was shifted considerably toward the pitch plane (Fig.
9H). This distribution is similar to the distribution of
V
/
P cells with eye- and head-velocity sensitivity in the
same direction (Fig. 9I, *). Because errors for calculating
MADs were <10°, the majority of the MADs for these P cells
apparently are shifted considerably toward the pitch plane away from
the MADs of the vertical rectus muscles (~25°) (Baker and
Peterson 1991
; Robinson 1982
). In contrast, none
of the off-pitch V
/
P cells and V
/
P cells
with oppositely directed eye- and head-movement sensitivity had MADs in
the quadrant near the pitch plane (Fig. 10H). Instead, they
were more aligned with the roll plane.
The MAD distribution of the three types of vertical P cells in
this study (Figs. 9, H and I, and 10H)
is different from the distribution found in previous studies on alert
cats (Fukushima et al. 1993) in two ways. First, in this
study, the MADs are shifted either more toward the pitch plane (Fig.
9H) or toward the roll plane particularly on the
contralateral side (Fig. 10H). The former distribution can
be attributed to convergence of excitatory inputs from the bilateral
anterior canals (Baker et al. 1984
; cf. King and
Leigh 1982
). In contrast, the latter distribution suggests convergent excitatory inputs from anterior and posterior canals on the
contralateral side (Baker et al. 1984
; Fukushima
et al. 1990
). Second, some MADs are distributed in the quadrant
of the ipsilateral posterior canal. Although previous studies in
decerebrate cats showed more widely distributed MADs for vertical P
cells (Powell et al. 1996
), P cells with MADs in the
ipsilateral posterior canal quadrant (Figs. 9I and
10H) rarely were observed, even in alert cats
(Fukushima et al. 1993
). A portion of the variability of
MADs in our study may be attributable to otolith influences on the
possibility of modulation of V
/
P cells (e.g., Fig. 9G, near 0°) (Baker et al. 1984
), but we
did not examine their contribution.
Convergence of multiple canal inputs to three types of vertical P cells
In addition to possible convergence of vertical canal and/or
otolith inputs on the three types of vertical P cells, the responses of
22% (9/41) of our P cells during pitch and yaw suppression (Fig. 8,
F and G) suggest convergence of inputs from the
vertical and horizontal canals. The P-cell response during yaw
suppression might have been contaminated from stimulation of the
vertical canals because in our experiments the head was stabilized in
the stereotaxic plane, where yaw rotation would have stimulated
vertical canals, which are tilted backward by ~21°
(Böhmer et al. 1985). However, if those P
cells had been activated by vertical canal inputs alone, their response
magnitudes during yaw suppression would have been only half of those
during pitch [i.e., cos(90-21 = 69)°/cos 45° = 0.36/0.71 = 0.5]. In fact, the responses of these vertical P
cells during yaw suppression were >0.5 times those during pitch
suppression; five responded with equal magnitude to pitch and yaw (Fig.
8, F and G), indicating that their behavior cannot be explained by vertical canal input alone. Consequently we
conclude that some vertical P cells in the floccular lobe receive convergent inputs from the horizontal and vertical canals.
In alert and decerebrate cats, the existence of biphasic responses in
vertical P cells of the floccular lobe during pitch rotation previously
has been taken as evidence of convergent multiple vertical canal inputs
(Powell et al. 1994, 1996
). Nine V
/
P
cells during pitch VOR suppression and three V
/
P cells during roll VOR suppression exhibited responses with a second harmonic
1.0 (e.g., Fig. 10C1, see METHODS). This
distortion may have been the result of convergent input from the
vertical canals. It is also possible that the lack of modulation during
pitch VOR suppression in the off-pitch V
/
P cells may
have been due to convergence of opposing anterior and posterior canal inputs.
Vertical rotation in the roll plane should elicit counterrolling eye
movements, which may have contributed to unit modulation. Unfortunately, we were unable to record torsional eye movements in this
study. Although the functional significance of convergence of multiple,
even orthogonal, canal and otolith inputs onto single P cells is not
clear, such convergence may provide the opportunity to produce adaptive
plasticity in behavioral conditions such as cross axis VOR adaptation
(e.g., Fukushima et al. 1990, 1996d
; Schultheis
and Robinson 1981
).
Differential role of the floccular lobe in vertical and horizontal eye movements
To track a moving object with the head moving, gaze velocity
signals must be calculated to match the velocity of the eyes in space
to target velocity (Robinson 1981). Gaze movement, which can occur in any direction, might be driven by the omnidirectional signals represented in different cells in visual areas like the medial
superior temporal (MST) cortex (Kawano et al. 1984
;
Thier and Erickson 1992
) and the dorsolateral pontine
nucleus, which receives direct projections from the MST and projects
directly to the floccular lobe (Kawano et al. 1992
). To
drive ocular motoneurons, however, two stages of signal conversions are
necessary: omnidirectional gaze velocity signals must be sorted into
roughly horizontal and vertical components and such gaze velocity
components must be converted into oculomotor signals. In floccular P
cells, the first sorting has largely already taken place
(Krauzlis and Lisberger 1996
; Lisberger and Fuchs
1978
; Miles et al. 1980
; Shidara and Kawano 1993
; Stone and Lisberger 1990
) although
a small portion of cells with oblique preferred directions are present
(Krauzlis and Lisberger 1996
) (Fig. 8, F and
G).
The second stage of conversion may occur downstream of the floccular
lobe for the horizontal component because the dominant type of
horizontal floccular P cell is related to gaze velocity (Lisberger and Fuchs 1978 and others). However, our
results showing that V
P cells are in the minority may suggest
that the transformation of retinotopic visual signals into gaze
velocity signals may be generated upstream of the floccular lobe and
then converted to the eye-movement signals there. Thus vertical P cells
may play some role in this conversion. A possible source of gaze
velocity signals to the brain stem is the MST area (Kawano et
al. 1984
; Their and Erickson 1992
) and the
pursuit area of the frontal eye fields (Gottlieb et al.
1994
; Tanaka and Fukushima 1998
), where many
neurons carry gaze velocity signals during smooth pursuit and VOR
suppression in every direction (Fukushima 1997
;
Kawano et al. 1984
; Their and Erickson
1992
).
Our results showing that most vertical P cells are modulated during the
VOR (pitch × 1) apparently are inconsistent with those of
Zhang et al. (1995), who showed that chemical
inactivation of the floccular lobe does not effect the vertical VOR
itself. However, their monkeys were lying on their sides, and it is
unknown how a static ocular counterrolling reflex, which should have
been evoked by placing the animals on their sides, might have affected the vertical VOR. For example, it has been reported that the vertical VOR gain in alert cats tested with cats sitting normally is 14.5% higher even at 0.1-1.0 Hz than when they lie on their sides
(Tomko et al. 1988
). Although similar studies have not
been performed in monkeys, these results suggest that interactions of
gravity-sensitive signals and vertical canal signals are required for
the vertical VOR to work properly, even at the relatively higher
stimulus frequency of 0.5 Hz. Although further studies are needed to
examine whether the simian floccular lobe plays an essential role in
the vertical VOR itself in the normal upright position, it is clear
that the vertical VOR, which relies on the normal activity of four
vertical canals and otolith inputs, differs in a fundamental way from
the horizontal VOR, which relies solely on signals from two
semicircular canals (cf. Tomko et al. 1988
).
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ACKNOWLEDGMENTS |
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We thank Y. Kobayashi, T. Yasuda, S. Usher, and J. Balch for superb technical assistance, Dr. Sergei Kurkin for computer programs, Dr. Yasuo Suzuki for surgical procedures, Drs. Steve Wells and Masaki Tanaka for valuable comments, and K. Elias for the editorial acumen.
This research was supported by grants from Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Corporation, Japanese Ministry, Science and Culture (09268201, 09680806), Marna Cosmetics, and National Institutes of Health (RR-00166, EY-06558, and EY-00745).
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FOOTNOTES |
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Address for reprint requests: K. Fukushima, Dept. of Physiology, Hokkaido University School of Medicine, West 7, North 15, Sapporo 060-8638, Japan.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 8 September 1998; accepted in final form 29 April 1999.
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REFERENCES |
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