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The Journal of Neuroscience, September 1, 2002, 22(17):7766-7773
Activation of a Cortical Column by a Thalamocortical Impulse
Harvey A.
Swadlow1,
Alexander G.
Gusev1, 2, and
Tatiana
Bezdudnaya1, 3
1 Department of Psychology, The University of
Connecticut, Storrs, Connecticut 06269, 2 Institute of
Higher Nervous Activity and Neurophysiology, Russian Academy of Medical
Sciences, Moscow 117865, Russia, and 3 A. B. Kogan
Research Institute for Neurocybernetics, Rostov-on-Don 344090, Russia
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ABSTRACT |
Thalamocortical (TC) impulses potently influence the sensory
neocortex, but the functional impact of individual TC neurons throughout the layers of the cortex has proved difficult to assess. Here we examine, in awake rabbits, the vertical distribution of monosynaptic currents generated in a somatosensory cortical
"barrel" column by spontaneous impulses of single, topographically
aligned TC neurons. We show that closely neighboring TC neurons
generate widely differing patterns of monosynaptic activation within
layers 4 and 6 of their aligned column. Moreover, synaptic currents
generated by TC impulses with long preceding interspike intervals are
greatly enhanced in both of these layers. The degree of this
enhancement differs reliably among neighboring TC neurons but, for a
given neuron, is very similar in layers 4 and 6. Our results indicate that in the awake state, TC synapses throughout the depth of the cortex
serve as powerful filters of sensory information that reflect individual characteristics of their parent TC neuron.
Key words:
thalamocortical synapse; somatosensory cortex; cortical
column; barrel; current source-density analysis; synaptic
depression
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INTRODUCTION |
All sensory information but
olfaction is conveyed to the neocortex by action potentials of
thalamocortical (TC) neurons. The cortex is powerfully influenced by
these impulses, but the functional impact of individual TC neurons
throughout the layers of the cortex has proved difficult to assess.
Morphological reconstructions of single TC axons provide crucial
information about the spatial distribution of TC synapses (Humphrey et
al., 1985 ; Jensen and Killackey, 1987). However, because central
synapses vary widely in both efficacy and dynamics (Gil et al., 1997,
1999), morphological analyses can only hint at functional issues.
Physiological studies have examined the impact of TC neurons on
cortical target neurons (Reid and Alonso, 1995; Swadlow, 1995; Alonso
et al., 2001; Miller et al., 2001), but only a few postsynaptic neurons
have been studied with any single TC neuron, and these are usually in a
single cortical layer. Here, our aim is to gain a more comprehensive
view of the impact of single TC neurons throughout the depths of a
cortical column. We do this by combining current source-density (CSD)
analysis (Haberly and Shepherd, 1973; Freeman and Nicholson, 1975) with extracellular spike-triggered averaging (Swadlow and Gusev, 2000) to
assess, in fully awake rabbits, the monosynaptic currents generated within a somatosensory cortical (S1) "barrel" column (Woolsey and
Van Der Loos, 1970) by spontaneous impulses of individual, topographically aligned TC neurons. These measures reveal that closely
neighboring TC neurons that project to the same cortical barrel often
exert a remarkably different impact on cortical layers 4 and 6. Moreover, we show that monosynaptic currents generated by TC impulses
with long preceding interspike intervals are greatly (and similarly)
enhanced in both of these cortical layers, but the degree of
enhancement in neighboring TC neurons differs reliably. Our results
indicate that in the awake state, TC synapses throughout the depth of
the cortex serve as powerful filters of sensory information that
reflect individual characteristics of their parent TC neuron.
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MATERIALS AND METHODS |
Recordings were obtained from S1 cortical barrel columns and
from their topographically aligned thalamic "barreloids" in two female adult Dutch belted rabbits. Initial surgery was performed under
anesthesia using aseptic procedures. Subsequent recordings were
obtained from awake subjects using procedures approved by the
Institutional Animal Care and Use Committee at the University of
Connecticut. Methods to ensure the comfort of our subjects have been
described previously (Swadlow, 1995). Rabbits were held snugly within a
stocking and placed on a foam pad. A steel bar on the head was fastened
to a restraining device in a manner that minimized stress on the neck.
Thalamic microelectrodes were constructed of quartz-insulated,
platinum-tungsten filaments (Reitboeck, 1983) with a diameter of 40 µm. Filaments were pulled to a taper and sharpened to a fine tip. A
concentric array of seven such electrodes (spaced at ~160 µm) was
chronically implanted within the ventrobasal (VB) thalamus after
receptive field mapping. Electrodes were guided within stainless steel
tubes (160 µm outer diameter) and controlled by seven miniature
microdrives. We could often record from the same ventrobasal thalamic
barreloid (which represents a single vibrissa) with two to three
microelectrodes. All of the TC neurons studied had receptive fields
that were clearly dominated by a single vibrissa.
Cortical recordings were obtained using 16 channel silicone probes
(University of Michigan Center for Neural Communication Technology, Ann
Arbor, MI). Probe sites were separated vertically by 100 µm
and had impedances of 1-2 M . Topographic alignment of thalamic and
cortical recording sites was achieved by receptive field mapping and
was confirmed by spike-triggered averages of cortical field potentials
elicited by the TC neurons (Swadlow and Gusev, 2000). After alignment
and mapping of the barrel, the 16 channel probe was placed into the
estimated center of the barrel (based on maximal responses from the
principle vibrissa and minimal responses from surrounding vibrissas).
We were careful to ensure that the probe was aligned normal to the
cortical surface so sites remained within the same barrel column in
layers 4 and 6. The probe was lowered until deeper channels registered
long-latency antidromic spikes to microstimulation via thalamic
microelectrodes [indicative of layer 6 (Swadlow, 1989)] and was
cemented into place. Probes retained good recording characteristics and
stable impedances for 24-48 hr, allowing two to three lengthy
recording sessions, during which several topographically aligned TC
neurons were usually studied.
Each cortical probe site was filtered at 3 Hz to 1.9 kHz (one-half
amplitude) and sampled continuously at 5 kHz. Spike data from TC
neurons and field potential data were acquired using a "Plexon"
data acquisition system (Dallas, TX). Spike-triggered averages of
cortical field activity were generated from the spontaneous activity of
TC neurons. For the "control" record, we used all spikes in the
data set except those occurring within 5 msec of another spike. We
eliminated spikes with short interspike intervals to avoid generating
compound averages from multiple, high-frequency spikes. When examining
the effects of the preceding interval on spike-triggered potentials, we
analyzed only TC spikes that were: (1) preceded by interspike intervals
of a given value and (2) not followed by another spike within 5 msec.
The one-dimensional CSD was calculated from the second spatial
derivative of the field potential profile using the general methods of
Freeman and Nicholson (1975):
( 2 /z2) = [(z + n z)  2(z) + (z nz)]/(nz)2,
where is the field potential, z is the coordinate
perpendicular to the layers, z is the sampling interval
(100 µm in the present study), and nz is the
differentiation grid (n = 2 in the present study). The
differentiation grid n = 2 is equivalent to spatial smoothing and allows reduction of high spatial frequency noise. To
obtain upper and lower boundary sites, we used an extrapolation method
that assumes no additional decay in the field potential above and below
the uppermost and lowermost recording sites, respectively (Vaknin et
al., 1988). This was justified because our field potentials were close
to zero at the extreme recording sites, and no potential reversals are
expected above or below these sites. In CSD traces, current sinks are
indicated by downward deflections and sources by upward deflections. To
facilitate visualization of CSD profiles, we generated color image
plots using linear interpolation along the depth axis.
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RESULTS |
In each of six barrel columns, we examined the vertical
distribution of field potentials and currents that were generated by
the spontaneous spikes of two to six aligned TC neurons.
The extracellular fields and currents generated by TC impulses
Figure 1A
illustrates a case in which two microelectrodes recorded from six TC
neurons of a single thalamic barreloid. Three TC neurons were studied
on each microelectrode (160 µm horizontal separation), and the total
vertical distance between recordings made on each electrode was <80
microns. The cortical probe is near the center of the aligned barrel
column. A CSD trace from layer 4 is shown at the right (Fig.
1A, trace 8 from N1, below). Figure
1B (left side, N1) shows responses
generated within the aligned column by one TC neuron (Fig.
1A, N1). Spike-triggered field potentials and CSD
analysis of these field potentials are shown. Responses during the
first few milliseconds after the TC spike consist of two components:
(1) an initial biphasic or triphasic "axon-terminal" component
(Swadlow and Gusev, 2000) (latency of <1 msec), and (2) a longer
latency (usually 1.2-1.8 msec), negative-going potential. We conclude
(see Discussion) that this latter component reflects a current sink
resulting from monosynaptic TC excitation. For this neuron, the
axon-terminal potential had a latency of 0.81 msec and was clearly
isolated from the dominant postsynaptic effect, a current sink, with an
onset latency of 1.58 msec.
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Figure 1.
A, An illustrative case. Two
independent microelectrodes in a thalamic barreloid are shown. During
two recording sessions, three TC neurons were studied via each
electrode (N1-N6). The cortical probe is in the aligned barrel, with
deeper sites within layer 6. An example of CSD traces showing the
axon-terminal potential and the postsynaptic current sink
(arrows) and the synaptic delay (time between the onset
of these two potentials) is shown on the right. The
vertical dashed line indicates the time of the TC spike.
B, N1 and N2 show spike-triggered field and CSD profiles
generated by two TC neurons (N1 and N2 in A) recorded
simultaneously via one of the microelectrodes. The vertical
dashed lines indicate the time of the TC spike. Colorized CSDs
are shown below (color intensities reflect source and sink amplitudes).
Bottom right, CSD profile generated by an air puff (200 presentations, note the long time base). Horizontal
arrows in colorized CSDs here and in Figure 2 denote sites
known to be within layer 6 (see Materials and Methods). Gains
for N1 and N2 are identical and are 20 times higher than those used for
the air-puff stimulus.
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The laminar distribution of postsynaptic current sinks generated by
TC impulses
The major postsynaptic current sink generated by the above TC
neuron was centered in layer 4. However, three of six TC neurons projecting to this barrel generated significant sinks in layer 6. N2
(Fig. 1) shows profiles of one such neuron (Fig. 1A,
N2). This neuron was recorded simultaneously and on the same
microelectrode as the TC neuron shown in N1. Thus, these two TC neurons
were close neighbors and projected to the same barrel column but
generated very different patterns of current within the column under
identical recording conditions. The major postsynaptic response extends deeper in N2 (~100 µm) than in N1. Moreover, there is a substantial sink deep within the cortex, in layer 6. Figure 1B
(right) shows the CSD profile generated to stimulation of
the principle vibrissa (gain is 1/20 that used in N1 and N2). Note the
strong sink in the middle of the cortex, which we assume to be centered
within layer 4. A weaker current sink is seen in layer 6 (Fig.
1B, arrows, see Materials and Methods).
Figure 2 shows CSD profiles for all six
TC neurons that projected to the column shown in Figure
1A. N1 and N2 show the same CSD profiles (but
compressed) shown in Figure 1, and N3-N6 show the profiles generated
by the four other TC neurons. Of the 24 TC neurons that we studied, 14 generated postsynaptic current sinks that were limited to layer 4 (as
in Fig. 2, N1, N4, N6). Nine of the remaining TC neurons showed strong
current sinks in layer 4 and weaker (seven cases, e.g., N2 and N3) or
similarly strong (two cases) sinks in layer 6. In only one case (Fig.
2, N5) was a postsynaptic current sink restricted to layer 6.
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Figure 2.
CSD profiles for six TC neurons projecting to a
single column (same as in Fig. 1A). N1-N3 were
recorded on one microelectrode, and N4-N6 were recorded on the second
electrode. The profile generated by the air puff is shown for
reference. Gain (and color) settings for all TC profiles are identical
and are 20 times higher than those used for the air-puff stimulus. The
vertical dashed lines indicate the time of the TC action
potential.
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Dynamics of postsynaptic currents generated by a TC impulse
The postsynaptic impact of a TC impulse depended strongly on the
duration of the preceding interspike interval. Figure
3A shows CSD profiles of a TC
neuron that generated powerful sinks in both layer 4 and layer 6 of the
aligned barrel. The control profile is shown on the left.
The center CSD profile was generated by the same TC neuron but was
limited to impulses with preceding interspike intervals of 250-500
msec. Note that the amplitude of the postsynaptic sinks in both layers
4 and 6 (Fig. 3A, traces magnified in insets) has
increased substantially. The amplitude of the axon-terminal potential,
however, is unchanged (Fig. 3A, arrows). The
right side of Figure 3A shows the CSD profile
generated by the initial spikes of TC "bursts." These impulses have
no preceding impulse for  100 msec but are followed by another TC
impulse at intervals of <4 msec. Note the enhanced current sink in
both cortical layers. Figure 3B shows, for this same TC
neuron, the relationship between the preceding interspike interval and
the magnitude of the axonal-terminal potential (left) and
postsynaptic current sinks (right) as a function of cortical
depth. For this latter measure (and for those presented in Fig.
4, below), we integrated the
value of the current sink occurring only during the initial 1 msec of
the postsynaptic response to avoid the possibility of including
disynaptic currents. The amplitude of the axon-terminal potential is
not affected by interspike interval, but the magnitude of the
postsynaptic responses in both layers increases with increasing interspike intervals.
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Figure 3.
A, CSD profiles for another TC
neuron. The control profile is shown on the left
(n = ~120,000 TC spikes), with an amplified view
(gain is 3.5 times greater) of the responses in layer 4 (top
horizontal arrows) and layer 6 (bottom horizontal
arrows). Oblique arrows indicate axon-terminal
responses. The colorized CSD profile is shown below.
Middle profiles were generated by TC spikes with
preceding interspike intervals of 250-500 msec (n = 2290). Profiles on the right were generated by initial
spikes of TC bursts (n = 2427). Gain settings and
color intensities for all CSDs are identical. The vertical dashed
lines indicate the time of the TC action potential.
B, The amplitude (peak-peak) of the
axon-terminal response (left) and the magnitude of the
initial 1 msec of the postsynaptic current sink (right)
were plotted at different CSD sites as a function of preceding
interspike interval. Control response and the response to the initial
spike in a TC burst are also plotted.
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Figure 4.
A, For 11 TC neurons, the magnitude
of postsynaptic current sinks generated in layer 4 by spikes with
various preceding interspike intervals. B, For these
same 11 neurons, the correlation between the response to TC spikes with
preceding intervals of 250-500 msec and with preceding interspike
intervals of >500 msec. C, The relationship for five TC
neurons between the magnitude of the response in layers 4 and 6 at
three preceding interspike intervals.
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Qualitatively similar results were obtained in each of 10 additional
cases. For each TC neuron, impulses with long preceding intervals
generated stronger postsynaptic responses than did control impulses.
However, the degree of enhancement varied considerably among different
TC neurons. Figure 4A shows, for each of these 11 TC
neurons, the magnitude of the postsynaptic sink generated in layer 4 by
impulses with various preceding interspike intervals. For each TC
neuron, the values generated are normalized against the value for
impulses with short interspike intervals (<100 msec). For some TC
neurons, current sinks generated by spikes with the longest preceding
intervals (>500 msec) were more than two times greater than those
generated at the shortest intervals. Other TC neurons showed much less
enhancement, and there was a strong correlation between the
enhancements generated at the two longest intervals (Fig.
4B, 250-500 vs >500 msec; r = +0.89; p = < 0.001). This shows that differences among
TC neurons in the amplitude of the enhanced response reflect a stable
feature of the TC neuron.
The above results show stable differences among TC neurons in the
dynamics of TC transmission. We subsequently asked whether, for a given
TC neuron, the magnitude of the enhancement seen in layer 4 was similar
to the magnitude seen in layer 6. The case illustrated in Figure 3
shows that postsynaptic current sinks generated in both layers 4 and 6 are similarly enhanced at longer interspike intervals. This was found
for each of the five TC neurons that we could adequately test. Figure
4C (filled diamonds) shows, for each of
these TC neurons, the postsynaptic enhancement seen in layer 4 and in
layer 6 at interspike intervals of >500 msec. There is a positive
correlation between the strength of the enhancement seen in these two
layers (r = 0.80). Similar positive relationships were
obtained for each of these five TC neurons at interspike intervals of
250-500 msec (r = 0.66) and at intervals of 100-250 msec (r = 0.83). The results of meta-analysis of these
separate correlations (Johnson and Eagly, 2000) are highly significant (p < 0.004). These results show that the
individual differences among TC neurons seen in the temporal dynamics
of TC transmission apply equally to layers 4 and 6. Moreover, the slope
of ~1 in the above correlations indicates that the magnitude of the
enhanced responses in these two layers is very similar.
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DISCUSSION |
The extracellular currents under study
Central to this study is the negative field potential within
layers 4 and/or 6 that follows the axon-terminal potential. Because of
its latency and focal depth distribution and because it is (1)
reversibly blocked by AMPA/kainate antagonists and (2) sharply restricted in horizontal extent to a single S1 barrel (Swadlow and
Gusev, 2000), we conclude that this potential reflects an extracellular
current sink generated by monosynaptic TC activation. Thus, it serves
as an index of all monosynaptic excitatory currents generated by the
impulses of this single TC neuron. The delay between the onset of the
axon-terminal response and the onset of the postsynaptic current sink
(means = 0.72 and 0.68 msec in layers 4 and 6, respectively)
supports this interpretation. Although synaptic currents are generally
thought to be the main contributors to cortical field potentials
(Mitzdorf, 1985), local action potentials undoubtedly made some
contribution to the observed postsynaptic current sinks.
For several reasons, we think it highly unlikely that the observed
spike-triggered current profiles reflect significant contributions from
neighboring TC neurons that are firing in synchrony with the TC neuron
under study (cf. Alonso et al., 1996; Hughes et al., 2002): (1) Both
the presynaptic (axonal) and postsynaptic depth profiles generated by
closely neighboring TC neurons were often highly distinct (Figs. 1, 2).
If neighboring, synchronously firing TC neurons were contributing
significantly to the spike-triggered depth profiles, results from
different TC neurons would be more similar. (2) The early components of
the spike-triggered current profiles display very high temporal
precision. Thus, a recent analysis showed that the rise times
(10-90%) of the axon-terminal and monosynaptic potentials were only
0.23 and 0.68 msec, respectively (Swadlow and Gusev, 2000), and similar
results were obtained in the present study. In contrast, even
"precisely correlated firing" between dorsal LGN neurons (Alonso et
al., 1996) occurs at a longer time scale (±1 msec). Thus, if
synchronously firing TC neurons were contributing significantly to the
observed profiles, the time course of these very rapid events would
have been blurred considerably. (3) Finally, we have recorded from >50
pairs of VB neurons located within the same thalamic barreloid
(H. A. Swadlow and A. G. Gusev, unpublished observations) and
have observed only weakly synchronous activity (timescale of many milliseconds).
The laminar distribution of current sinks: relationship to TC
bouton distribution
Physiological studies based on electrical stimulation of the
thalamus have generally reported synaptic input to layers 4 and 6 (Bullier and Henry, 1979; Ferster and Lindstrom, 1983; Castro-Alamancos and Connors, 1996). However, synaptic responses generated by thalamic stimulation are difficult to interpret because of synaptic input mediated by the intracortical collaterals of antidromically activated corticothalamic neurons (cf. Ferster and Lindstrom, 1985).
Morphological studies of TC boutons in the somatosensory and visual
system show strong TC input to layer 4 and deep layer 3 and a lesser
input to layer 6 (Ferster and LeVay, 1978; Jensen and Killackey, 1987). Single-fiber labeling studies confirm this general result but show
unexplained heterogeneity in the laminar arborization pattern among
different TC axons. For example, Humphrey et al. (1985) found that
different dorsal LGN neurons distribute 1% to >20% of their boutons
within layer 6 but were "unable to relate this layer V1 variability
to variability in any other anatomical or physiological feature."
Similar variability among TC neurons in the layer 6 projection is seen
in the somatosensory barrel system (Jensen and Killackey, 1987; Pierret
et al., 2000). Consistent with these studies, our results showed layer
4 to be the dominant site of monosynaptic current sinks, with most TC
neurons generating sinks that were limited to this layer. The remaining
neurons showed current sinks of variable magnitude within layer 6. Thus, our results confirm a highly heterogeneous input to layer 6. In
addition, we show that even closely neighboring TC neurons can generate remarkably different patterns of current sinks within layers 4 and 6 (Fig. 1).
Dynamics of thalamocortical transmission in layers 4 and 6: the
effects of interspike interval
The initial impulse of a thalamic burst generates an enhanced
response in putative inhibitory interneurons of S1 (Swadlow and Gusev,
2001). Thalamic bursts are preceded by long interspike intervals (>100
msec), and this interval is crucial for generating the enhanced
cortical response. The present analysis extends the above finding in
two significant ways: (1) These results are not limited to TC synapses
onto inhibitory interneurons. TC impulses with long preceding intervals
generated current sinks that were much stronger than those with short
preceding intervals. These sinks reflect the summed activity of all the
synapses made within the column by a given TC neuron. Because the great
majority of these synapses are onto dendritic spines (e.g., 83% in rat
visual cortex) (Peters and Feldman, 1976), we conclude that much of the enhanced current flow is attributable to activation (largely
subthreshold) of excitatory cortical neurons. (2) Enhanced cortical
activation occurs in layer 6 as well as in layer 4, and the magnitude
of the effect within these layers is similar. This finding is
significant, because TC impulses with long preceding interspike
intervals are more likely to activate corticothalamic "feedback"
neurons of layer 6, which receive substantial synaptic input from TC
terminals (White and Hersch, 1982).
TC synapses in vitro exhibit paired-pulse depression that
lasts for hundreds of milliseconds (Gil et al., 1997), and similar effects have been observed in vivo (Swadlow and Gusev, 2001;
Castro-Alamancos and Oldford, 2002; Chung et al., 2002). Because of
this, the high spontaneous activity of these neurons in intact adults
(10-20 Hz in the present study) should result in a chronic state of
synaptic depression. Our results are consistent with the notion that
long intervals preceding TC spikes allow recovery from such
activity-dependent depression, and that such spikes should evoke a
maximum EPSP in cortical targets (Ramcharan et al., 2000; Swadlow and
Gusev, 2001).
The different TC axons studied here showed enhancements of 35% to
>100% in the magnitude of the postsynaptic response seen at long
interspike intervals. The extent of this variability is consistent with
findings in vitro, where similar variability is seen in the
magnitude of paired-pulse depression observed at different TC synapses
[Gil et al. (1997), their Fig. 2C]. It is important to
know whether this variability is at the level of the specific synaptic
contact under study or at the level of the TC neuron. Our results show
that much of this variability can be globally related to the particular
TC neuron under study. Some TC neurons form synapses that are, on
average, more depressing than those of other TC neurons, and these
differences are maintained over a range of preceding interspike
intervals (Fig. 4A,B). Moreover, for a given TC
neuron, the dynamics seen in layers 4 and 6 are very similar but differ
consistently from the dynamics seen in other TC neurons.
Finally, these results show that TC synapses in layers 4 and 6 may
serve as significant filters of sensory information. Sensory information carried by impulses with long preceding interspike intervals, including the initial spikes of a thalamic burst, have a
special status: they generate stronger postsynaptic responses than
impulses with short preceding intervals. Because of this potent
cortical response in layers 4 and 6, these findings are consistent with
suggestions that thalamic bursts could serve as a "wake-up" call to
the cortex (Sherman and Guillery, 1996; Sherman, 2001). They are also
consistent with suggestions that a sensory "interval code" may be
decoded through mechanisms of synaptic depression or facilitation
(Gerstner et al., 1997; Goldman et al., 1999; Reich et al., 2000). Our
results show that sensory information encoded within the interval
distribution of TC impulses may be decoded by the amplitude of the
postsynaptic currents generated by those impulses. Thus, at the TC
synapse, an interval code may be transformed into an "amplitude
code," and the nature of this transform may differ considerably,
reflecting individual characteristics of the parent TC neuron. How (or
whether) the cortex uses this amplitude-modulated message in retrieving
encoded aspects of the environment is an open question.
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FOOTNOTES |
Received May 7, 2002; revised June 17, 2002; accepted June 21, 2002.
This work was supported by grants from the National Science Foundation
(IBN-0077694), the National Institute of Mental Health (MH-64024), and
the Russian Foundation of Basic Research (00-04-49163). Multichannel
probes were provided by the University of Michigan Center for Neural
Communication Technology (National Institutes of Health Grant
P41-RR09754). We thank J.-M. Alonso, K. Lukatela, and H. L. Read
for insightful suggestions and V. Serdyukov for expert hardware and
software development.
Correspondence should be addressed to Dr. H. A. Swadlow,
Department of Psychology (U-20), The University of Connecticut, Storrs, CT 06269. E-mail: Swadlow{at}psych.psy.uconn.edu.
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ABSTRACT
INTRODUCTION
