H . Straka et al. Brain Research 880 2000 70 –83
73
vestibular nuclear complex after stimulation of the VIIIth nerve or of one of its canal nerve branches. To minimize
the variability of results we averaged the amplitudes recorded in a given depth track separately for each
stimulated canal nerve, compared these averaged values between different parasagittal depth tracks and normalized
the averaged values according to the largest values evoked by a given canal nerve branch in a particular track see Fig.
3. This procedure facilitated a comparison of the results following stimulation of different semicircular canal nerves
or of the VIIIth nerve and a comparison of data from different experiments. From the analysis of data obtained
in parasagittal depth tracks we obtained evidence for a differential spatial distribution of the largest evoked field
potential amplitudes following stimulation of different semicircular canal nerves Fig. 3. In later experiments we
estimated the maximal response amplitude of N or N in
1
each series of depth tracks for a given nerve branch and normalized all other responses evoked by the same nerve
branch. In order to include depth information in our analysis field potentials are continuous functions and
recording tracks were relatively close to each other in space isopotential surface plots were calculated through
linear interpolation between maximal response amplitudes Stanford graphics, 3-D visions, Torrance, CA. Four
groups of relative response amplitudes up to 25, 50, 75 or 100 were represented by different intensities of
gray tones.
Statistical analyses were performed with the aid of
Fig. 2. Field potentials in the vestibular nuclear complex evoked by
commercially available computer software INSTAT;
stimulation of the VIIIth nerve or its semicircular canal nerve branches on
Graphpad, San Diego, CA. Statistical differences in
the ipsilateral side. A–D Pre- N and postsynaptic N field potential
1
latencies, amplitudes and areas were calculated according
components evoked by stimulation of the VIIIth nerve A, the horizontal
to the Wilcoxon signed rank test test for paired parame-
canal HC nerve B, the anterior canal AC nerve C or the posterior
ters. Graphical presentations were performed with the aid
canal PC nerve branch D. Each of the responses in A–D were recorded at the same site. Dashed lines indicate baseline and arrow head
of commercially available computer software Origin,
the onset of stimulus. Calibration bars in B apply also for C and D. Each
Microcal Software, Northampton, MA; Designer, Microg-
record represents an average of 20 responses.
rafx, Richardson, TX. differed between nerve branches and recording sites.
3. Results Horizontal canal HC nerve evoked field potentials were
characterized by a negativity Fig. 2B that lasted longer 3.1. Field potentials following stimulation of the VIIIth
than that of the anterior canal AC, posterior canal PC nerve or of individual semicircular canal nerve
or VIIIth nerve evoked field potentials Fig. 2A,C,D. branches
. Here, areas instead of peak amplitudes of evoked N field
1
potentials were compared Table 2. This comparison Canal nerve evoked presynaptic N and postsynaptic N
indicated that HC nerve evoked postsynaptic field po-
1
field potential components recorded at the standard record- tentials were characterized by a significantly P0.0001
ing site had significantly longer onset latencies and smaller larger relative area than AC, PC or VIIIth nerve evoked
amplitudes than the corresponding VIIIth nerve evoked field potentials recorded at the same site Table 2.
potentials Fig. 2; Tables 1 and 2. This difference in onset latency was expected, since the site of canal nerve
3.2. Topography of vestibular field potential components stimulation was further away from the recording site than
the site of VIIIth nerve stimulation. N and N components In a parasagittal plane, VIIIth nerve or semicircular canal
1
were separated by a synaptic delay of 1.8–1.9 ms at a bath nerve evoked field potentials were recorded in dorso-
temperature of 148C as in an earlier study [33]. The shapes ventrally oriented tracks at a laterality of zero mm see Fig.
of the canal nerve evoked N field potentials Fig. 2B–D 1B. The most rostral recording site was at 20.7 mm and
1
74 H
Table 1 Parameters of presynaptic N field potential components evoked by electrical stimulation of the VIIIth nerve or of an individual semicircular canal nerve on
a
the ipsilateral side at the standard recording site VIIIth nerve
Horizontal semicircular canal Anterior semicircular canal
Posterior semicircular canal Latency
0.960.2 ms 1.760.6 ms
1.860.4 ms 1.760.4 ms
Amplitude at 43T 5026202 mV
161684 mV 109672 mV
147662 mV
a
n529 for all instances. P0.0001, significance of difference with respect to values evoked by VIIIth nerve stimulation Wilcoxon signed-rank test.
the most caudal site was at 1.5 mm with respect to the maxima evoked by stimulation of the AC nerve were
caudal end of the entry of the VIIIth nerve see Fig. 1D. located rostral with respect to the entry of the VIIIth nerve
The stimulus intensity was 4-times threshold 43T of the 0 mm; Fig. 5A,B. In the medio-lateral direction the
N component recorded at the standard recording site. maximal amplitudes of the N and N components were
1 1
The spatial distribution of the normalized average observed in the intermediate and lateral part of the
amplitudes of N and of N components evoked at a given vestibular nuclear complex. Only very small field potential
1
stimulation site was not uniform along the rostro-caudal amplitudes were recorded in the medial vestibular nucleus.
search area but exhibited peaks that differed in their In frontal plane recordings the rostro-caudal location and
location Fig. 3. Stimulation of each of the different nerve the depth of the local maxima following stimulation of
branches evoked potentials that peaked at a different but each of the three different canal nerves were again similar
for this nerve branch characteristic site in the vestibular for N and for N components Figs. 6 and 7. The spatial
1
nuclear complex Fig. 3A–D for the N and Fig. 3E–H for distribution of these potentials was consistent with those
the N components. N
and N response components
observed in the parasagittal and in the oblique medio-
1 1
evoked by a given nerve branch had rather similar spatial lateral planes Figs. 4 and 5. The depth of the local
distributions along the rostro-caudal extent of the vestibu- maxima of the N
as of the N components ranged
1
lar nuclear complex. between 0.2 and 0.6 mm. Differences in depth of the local
To include depth information, the data shown in Fig. 3 maxima were observed between the rostral plane i.e. at
are presented in Fig. 4 as isopotential surface plots. The 20.4 mm AC nerve, Figs. 6B
and 7B and the two
1 1
largest relative amplitudes 75–100 of the N and N caudal planes i.e. at 0.4 and 0.7 mm HC nerve, Figs.
1
field potential components following stimulation of an 6A ,A and 7A ,A ; PC nerve, Figs. 6C ,C and 7C ,C .
2 3
2 3
2 3
2 3
individual semicircular canal nerve were recorded at a Differences in the laterality of the local maxima between
depth between 0.4 and 0.6 mm from the top of the N and N components were observed in the frontal planes
1
brainstem at a laterality of 0 mm. This location corre- following stimulation of all three semicircular canal
sponds to the anatomical center of the vestibular nuclear nerves. Independent of the stimulated semicircular canal
complex [22–24]. Differences in the spatial distribution of nerve the maximal amplitudes of the N components were
the peak amplitudes of N and N field potentials evoked located more laterally Fig. 6 than the corresponding N
1 1
by different canal nerves were now more apparent. components Fig. 7.
Semicircular canal nerve evoked N and N
field
1
potential components recorded in an oblique medio-lateral 3.3. Location of the maxima of semicircular canal nerve
plane see dashed line 2 in Fig. 1A and F and evoked by evoked N components in the vestibular nuclei
1
stimulation of a given nerve branch had rather similar topographies, but exhibited different spatial distributions
The maximum of the HC nerve evoked N component
1
for different canal nerves. The rostro-caudal extent of the representing the monosynaptic excitation of 28 HC neurons
largest relative amplitudes 75–100 of the N and N was located in the lateral vestibular nucleus and more
1
field potential components Fig. 5A,B were rather similar caudally in the descending vestibular nucleus Figs. 8A
2,3
to those observed in the parasagittal plane Fig. 4A,B. and 9A,D. The maximum of the AC nerve evoked N
1
The maxima of the field potentials evoked by the HC nerve component was observed in the central part of the superior
or by the PC nerve were located caudal whereas the vestibular nucleus Figs. 8B and 9B,E. After PC nerve
1
Table 2 Parameters of N field potentials evoked by electrical stimulation of the VIIIth nerve or of an individual semicircular canal nerve on the ipsilateral side at
1 a
the standard recording site VIIIth nerve
Horizontal semicircular canal Anterior semicircular canal
Posterior semicircular canal Latency
2.660.4 ms 3.660.7 ms
3.760.6 ms 3.560.6 ms
Amplitude at 43T 10316379 mV
2696153 mV 2086105 mV
3006247 mV Normalized area
13.164.4 mV3ms 22.068.1 mV3ms
11.864.3 mV3ms 11.464.2 mV3ms
a
n529 for all instances. P0.0001, significance of difference with respect to values evoked by VIIIth nerve stimulation Wilcoxon signed-rank test.
H . Straka et al. Brain Research 880 2000 70 –83
75
Fig. 3. Normalized average amplitudes of the VIIIth nerve or semicircular canal nerve evoked N and N
field potential components recorded in
1
rostro-caudal depth tracks along the vestibular nuclear complex at a laterality of 0 mm. Symbols connected by dashed lines represent results from four different experiments, bold lines connect mean values of these data. HC, AC and PC stand for horizontal, anterior and posterior semicircular canal,
respectively. Zero in rostro-caudal extension is the caudal end of the entry of the VIIIth nerve in the brainstem.
stimulation the largest amplitudes were observed in the the descending vestibular nucleus Figs. 8 and 9. Interest-
dorsal part of the lateral vestibular nucleus and to a ingly, the N component was absent or was less than 25
1
somewhat lesser degree more caudally in the descending of the maximal amplitude Figs. 8 and 9 in the medial
vestibular nucleus Figs. 8C and 9C,F. Although, there
vestibular nucleus throughout its rostro-caudal extent.
2,3
were distinct areas with maximal amplitudes of the N
1
components following stimulation of each one of the three 3.4. Correlation between the location of identified
semicircular canal nerves areas of overlap were observed second-order canal neurons and the topography of canal
as well. However, these areas of overlap consisted pre- nerve evoked N field potentials
1
dominantly of areas in which the amplitudes of the N
1
components from at least one or two semicircular canal In an earlier study [34] we had investigated second-
nerves were submaximal. These areas included the su- order vestibular neurons intracellularly at a laterality
perior vestibular nucleus, the central and dorsal part of the between the top of the brainstem zero and about 0.2 mm
lateral vestibular nucleus and caudally the dorsal part of more medially between 0.5 mm rostral and 1.0 mm caudal
76 H
Fig. 4. Amplitude distribution plot of semicircular canal nerve evoked field potential components recorded in a parasagittal plane through the vestibular nuclear complex at a laterality of 0 mm. A,B Normalized amplitudes of semicircular canal nerve evoked N A and N B field potential components
1
from four experiments. HC, AC and PC stand for horizontal, anterior and posterior semicircular canal, respectively. Zero in rostro-caudal extension refers to the caudal end of the entry of the VIIIth nerve. Relative amplitude magnitudes are represented by gray tones.
to the caudal end of the VIIIth nerve in tracks that were Rather, the amplitudes of these potentials were prominent
separated by 0.1 mm. These neurons were identified by in distinct subregions of the vestibular nuclear complex.
their monosynaptic excitation from one of the three The spatial differences between the maxima of pre- and
ipsilateral semicircular canal nerves. Second-order canal postsynaptic components of field potentials evoked by
neurons were searched systematically in depth tracks that stimulation of a particular semicircular canal nerve were
were regularly spaced over the entire rostro-caudal exten- only minor when compared with the spatial differences
sion of the vestibular nuclear complex. The rostro-caudal between the maxima of field potentials evoked by different
and dorso-lateral location of these second-order semicircu- semicircular canal nerves. However, the overlap of these
lar canal neurons is shown in Fig. 10A–C. Regional potentials representing the afferent inputs from different
differences in the distribution of these second-order canal semicircular canals in the ipsilateral labyrinth was exten-
neurons are expressed by the histograms in Fig. 10D–F sive. Large field potentials following stimulation of each of
gray tone. Comparison of these distributions with those the three semicircular canal nerves were encountered in all
of the canal nerve evoked N field potential amplitudes
vestibular nuclei with the notable exception of the medial
1
dashed lines in Fig. 10D–F revealed compatible local vestibular nucleus.
maxima. Anatomical tracer studies with selective labeling of
individual vestibular nerve branches demonstrated in vari- ous species that the central projections of afferent fibers
4. Discussion from different semicircular canals, as from different otolith
