Department of Neurobiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
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ABSTRACT |
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Hartings, Jed A., Simona Temereanca, and Daniel J. Simons. High Responsiveness and Direction Sensitivity of Neurons in the Rat Thalamic Reticular Nucleus to Vibrissa Deflections. J. Neurophysiol. 83: 2791-2801, 2000. The thalamic reticular nucleus (Rt) is strategically positioned to integrate descending and ascending signals in the control of sensorimotor and other thalamocortical activity. Its prominent role in the generation of sleep spindles notwithstanding, relatively little is known of Rt function in regulating interactions with the sensory environment. We recorded and compared the responses of individual Rt and thalamocortical neurons in the ventroposterior medial (VPm) nucleus of the rat to controlled deflections of mystacial vibrissae. Transient Rt responses to the onset (ON) and offset (OFF) of vibrissa deflection are larger and longer in duration than those of VPm and of all other populations studied in the whisker/barrel pathway. Magnitudes of ON and OFF responses in Rt were negatively correlated with immediately preceding activities, suggesting a contribution of low-threshold T-type Ca2+ channels. Rt neurons also respond with high tonic firing rates during sustained vibrissa deflections. By comparison, VPm neurons are less likely to respond tonically and are more likely to exhibit tonic suppression. Rt and VPm populations are similar to each other, however, in that they retain properties of directional sensitivity established in primary afferent neurons. In both populations neurons are selective for deflection angle and exhibit directional consistency, responding best to a particular direction of movement regardless of the starting position of the vibrissal hair. These findings suggest a role for Rt in the processing of detailed sensory information. Temporally, Rt may function to limit the duration of stimulus-evoked VPm responses and to focus them on rapid vibrissa perturbations. Moreover, by regulating the baseline activity of VPm neurons, Rt may indirectly enhance the response selectivity of layer IV barrel neurons to synchronous VPm firing.
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INTRODUCTION |
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The thalamic reticular (Rt) nucleus, comprised of
inhibitory projection neurons, is an integrative center for control
over sensory and other signals en route to the cerebral cortex
(Scheibel and Scheibel 1966; for review see
Guillery et al. 1998
). Combining both ascending
thalamocortical and descending corticothalamic inputs with intrinsic
nonlinearities, Rt neurons can modulate and gate the activity of
thalamocortical neurons in a state- and behavior-dependent fashion.
During slow-wave sleep, Rt generates "spindle" oscillations which
are widely synchronous throughout thalamocortical circuits and
presumably disrupt refined spatiotemporal processing of sensory signals
(Steriade et al. 1987
; for review see Steriade et
al. 1993
). During wakefulness, Rt activity varies according to
an animal's level of vigilance (Steriade et al. 1986
) and has been suggested to be involved in high-level functions such as
attention (Crick 1984
; Yingling and Skinner
1976
). Indeed, lesions of Rt disrupt orienting responses to
sensory stimulation (Friedberg and Ross 1993
) and
abolish attentional effects of cueing on reaction times (Weese
et al. 1999
).
Anatomic and physiological findings suggest that Rt may also play a
more specific role in processing ascending sensory signals. Rt is
subdivided into sectors according to physiological modality and
anatomic connectivity with specific relay nuclei (Jones
1975; see also Jones 1985
), and within each
sector there are topographic maps of sensory space (Montero et
al. 1977
; Shosaku et al. 1984
; see
Guillery et al. 1998
). In behaving animals the visual
and somatosensory sectors showed selective increases in
c-fos expression depending on the modality of sensory
exploration (Montero 1997
, 1999
). In anesthetized
preparations, Rt neurons exhibit peripherally drivable responses which
resemble, but differ from, those of their afferent thalamocortical
neurons (visual: Dubin and Cleland 1977
; Ulrich
et al. 1991
; Wrobel and Tarnecki 1984
;
somatosensory: Shosaku 1985
; Sumitomo and Iwama
1987
). Characterizing the different sensory-evoked responses of
Rt versus relay neurons is a critical step in determining how Rt
affects sensory processing during different behavioral or attentional states.
Most in vivo single-unit studies of the somatosensory sector of Rt have
been conducted in the rat vibrissa system. In the ventroposterior
medial (VPm) nucleus neuronal aggregates, called "barreloids,"
correspond in a one-to-one fashion with individual vibrissae on the
contralateral face. Feedback inhibition from Rt is the only source of
inhibition onto barreloid neurons, as the rodent VPm is virtually
devoid of inhibitory interneurons (Barbaresi et al.
1986; Harris and Hendrickson 1987
), and ablation of Rt alters the response properties of VPm neurons in a fashion similar to direct application of GABA receptor antagonists (Lee et al. 1994a
,b
). Like their afferents from VPm, Rt neurons have spatially focused receptive fields that are typically dominated by a
single vibrissa (Shosaku 1985
; Sumitomo and Iwama
1987
). Accordingly, anatomic studies have demonstrated that
axons of VPm neurons can have restricted collateral branching in Rt
(Harris 1987
) and that, conversely, individual Rt
neurons project mainly to specific subnuclear or single barreloid
domains (Pinault et al. 1995
; see also Pinault
and Deschenes 1998
). These reciprocal connections likely exist
between neuronal modules corresponding to the same vibrissa, as
physiological studies employing cross-correlation techniques have
provided evidence for functional connectivity between VPm and Rt
neurons only when they have the same vibrissa receptive field center
(Shosaku 1986
).
To study the role of Rt in processing low-threshold tactile
information, we recorded the responses of Rt neurons to the same controlled vibrissa deflections used to characterize other populations in the ascending lemniscal system. In previous studies we found that
approximately 75% of primary afferent (N. V.) neurons respond tonically to sustained vibrissa deflections in a preferred direction and are directionally well-tuned; the remainder respond only phasically and are less direction selective (Lichtenstein et al.
1990). In VPm a large percentage of neurons also exhibit high
direction selectivity. However, unlike N. V. neurons, only 37% of
VPm cells are tonically responsive, and many exhibit tonic suppression
at particular directions (Shosaku 1985
; Simons
and Carvell 1989
). Here, we report that Rt neurons discharge at
high firing rates during both transient and maintained vibrissa
deflections, and they do so in a manner that is dependent on stimulus
direction. These findings are consistent with the view that Rt
contributes to the processing of specific aspects of tactile stimuli.
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METHODS |
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Surgical procedures and recordings
Adult Sprague-Dawley rats weighing 250-300 g were prepared for
electrophysiological study using methods described previously in detail
(Simons and Carvell 1989). Halothane anesthesia was used
during surgical procedures. A steel post was fixed to the skull with
dental acrylic to hold the animal's head, and a craniectomy was made
at stereotaxic coordinates overlying the recording location in the
thalamus (VPm: 2.0-4.5 posterior, 1.5-4.0 lateral to bregma; Rt:
1.5-3.5 posterior, 2.5-4.5 lateral to bregma; Paxinos and Watson 1982
; Shosaku et al. 1984
). After surgery
halothane was discontinued, the animal was immobilized by pancuronium
bromide, artificially respired through a tracheal cannula, warmed by a servo-controlled heating blanket, and maintained in a lightly narcotized state by a steady infusion of fentanyl (Sublimaze, Jansen
Pharmaceuticals; ~10
µg · kg
1 · h
1).
The animal's condition was assessed by continuously monitoring electroencephalogram, femoral arterial blood pressure, tracheal airway
pressure, and pupillary reflexes.
Extracellular single-unit recordings were made in VPm or Rt with 10 M stainless steel microelectrodes (Frederick Haer, Brunswick, ME).
VPm and Rt were reliably distinguished during recording sessions on the
basis of their medial and lateral locations, respectively, relative to
the ventroposterior lateral nucleus, and by different burst patterns
during spontaneous firing (Domich et al. 1986
); Rt
neurons display longer bursts characterized by more spikes and an
accelerando-decelerando pattern. At the end of recording sessions
animals were deeply anesthetized with sodium pentobarbital (Nembutal)
and perfused transcardially. Brains were sectioned in the coronal plane
and stained with thionin to confirm the location of electrode tracks
through the appropriate nucleus.
Vibrissa stimulation and data analysis
Hand-held probes were used to identify the vibrissa evoking the
strongest response from an isolated unit, i.e., the principal whisker.
A piezoelectric mechanical stimulator was then attached to this whisker
10 mm from the face (Simons 1983). We did not record
responses to movements of adjacent whiskers or attempt any systematic
study of receptive field size. The stimulation protocol, which we have
used in previous studies, consisted of a ramp-and-hold deflection of
the vibrissa from its resting position. Deflections were 1 mm in
amplitude (~5.7°), with onset and offset velocities of 125 mm/s and
a plateau duration of 200 ms. Deflections were applied in eight
randomly interleaved directions spanning 360° in 45° increments and
were repeated for 10 trials, for a total of 80 stimuli. Peristimulus
time histograms (PSTHs) for each of the eight directions were computed
and displayed on-line. All units which exhibited even minimal
responsiveness to vibrissa deflections were included in our analyses.
A time/amplitude window discriminator (BAK Electronics) and a digital storage oscilloscope were used to isolate single units. Sequential spike event times were recorded with 100-µs resolution and analyzed with custom-designed programs on a DEC LSI 11/73. Responses to the onset and offset of vibrissa deflections in each direction were computed as the mean number of spikes recorded for all 10 trials during 20-ms epochs after the onset and offset of vibrissa deflections (ON and OFF responses, respectively). This time period was chosen on the basis of the duration of evoked responses of VPm neurons (see PSTHs in Fig. 7). For purposes of direct comparison, 20-ms periods were also used for Rt data unless noted otherwise. Responses to the stimulus plateau, when the vibrissa was maintained in a deflected state, were measured during a 100-ms period beginning 75 ms after stimulus onset (see Fig. 1). Spontaneous activity was measured during a 100-ms period preceding the deflection. Maximally effective directions for ON, OFF, and plateau responses were defined as those which evoked the most activity during the corresponding time epochs.
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Data analyses and statistical tests were performed on an IBM PC using
Microsoft Excel/Visual Basic and the statistics package from SPSS, Inc.
Mann-Whitney U tests were used to compare distributions. The
slowly or rapidly adapting nature of each unit's response was assessed
by using a procedure employed previously in a study of N. V. neurons (Lichtenstein et al. 1990). Two-tailed
t-tests with 95% confidence limits were used to compare
spontaneous firing rates with those evoked during the stimulus plateau.
A cell was classified as tonically responding if the plateau response
at the maximally effective direction was significantly greater than spontaneous firing (P < 0.025). Units with
significantly less activity at the minimally effective direction were
characterized as "inhibited."
We assessed each unit's sensitivity to vibrissa movements in different
directions using three measures. Tuning ratios were computed as the
response magnitude evoked at the maximally effective direction divided
by the response averaged over all eight directions (Kyriazi et
al. 1996); a higher value denotes greater selectivity. In a
second analysis each unit was classified into one of eight tuning
categories based on the number of deflection angles (0-7) that evoked
responses statistically smaller than those of the maximally effective
direction (P < 0.05, one-tail; see Simons and
Carvell 1989
); category 7 represents the most selective
responses. Tuning ratios and tuning categories were calculated
separately for ON, OFF, and plateau responses,
because neurons in the trigeminal ganglion and in central populations
often have different preferred directions for the different response phases.
A third measure, used previously to analyze responses of N. V. neurons (Lichtenstein et al. 1990), assessed the degree
to which a unit responded to a particular stimulus direction regardless of the vibrissa's initial position relative to rest. This property, termed directional consistency, was quantified by computing a linear
correlation coefficient for mean ON and OFF
response magnitudes evoked at each stimulus angle. Cells that fire most
vigorously to stimulus onset in one direction and to stimulus offset
after initial movements in the opposite direction exhibit negatively correlated ON and OFF responses. A unit was
classified as "directionally consistent" if the correlation
coefficient was significantly negative (P < 0.05, one-tail).
Each of the three measures captures related but different aspects of a neuron's directional properties. It is not clear which, if any of these measures, represents the most salient aspect of directional processing in the vibrissa system. Here, we use these measures as quantitatively descriptive probes for comparing response specificity between VPm and Rt neurons.
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RESULTS |
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We recorded and analyzed the responses of 62 VPm and 85 Rt neurons
discharging to vibrissa deflections in a total of 13 animals. These
neurons constituted the majority of neurons encountered in
vibrissa-responsive regions in our recordings; no attempts were made to
activate unresponsive units by other modalities of stimulation. The
response characteristics of VPm neurons reported here are comparable to
populations previously studied under the same physiological and
stimulus conditions (Kyriazi et al. 1994; Simons
and Carvell 1989
).
Response profiles
The most prominent features of Rt neuronal activity are the large magnitude and long duration of stimulus-evoked responses, compared with those of VPm neurons. This is illustrated in the population PSTHs of Fig. 1, A and B, which show the accumulated responses of all neurons to all deflection angles. Both VPm and Rt responses to the onset and offset of vibrissa deflection (ON and OFF responses, respectively) peak within a few milliseconds, the rise of the Rt response being delayed relative to VPm by approximately 1.3 ms. VPm activity, however, begins to decay within 5 ms, whereas Rt neurons continue to respond for approximately 20 ms before decaying more slowly to a steady firing rate (see also Fig. 7). To quantify these differences in the temporal dispersion of the response, we calculated the time in the 20-ms response period of the 50th percentile spike for each cell. The population means and standard deviations for VPm and Rt were 7.2 ± 6.6 and 14.3 ± 4.2 ms, respectively, for the ON response (P < 0.001), and 10.0 ± 4.9 and 12.9 ± 4.2 ms for the OFF response (P < 0.001).
The greater responsiveness of the Rt population is evident in all measures of unit activity. Table 1 lists the mean responses of VPm and Rt neurons averaged over all deflection angles as well as mean responses to each neuron's maximally effective angle. For each response measure, and for spontaneous activity, Rt activity is greater than VPm; in each case the difference is highly significant, even after correcting response magnitudes for differences in spontaneous activities (P < 0.001). Figure 2 shows the distributions of these measures for the two populations, illustrating that differences in mean values reflect trends throughout the populations, rather than outlying responses of a few cells. As shown in Fig. 3, the differences in response profiles are readily observable even in the responses of individual VPm and Rt neurons. The mean ON and OFF responses of the VPm neuron consist of 1.19 and 1.35 spikes, respectively, which are precisely time-locked to the stimulus, yielding narrow peaks in PSTHs. The Rt neuron, on the other hand, responds to stimulus onsets and offsets with 4.49 and 2.25 spikes, respectively, which in both cases are more broadly distributed over tens of milliseconds.
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Tonic versus phasic response characteristics
A particularly striking difference between VPm and Rt responses occurs during the stimulus plateau (i.e., maintained vibrissa deflection). In Rt mean plateau activity averaged over all deflection angles (49.7 spikes/s) is more than twice the mean spontaneous activity (22.4 spikes/s), whereas VPm activity is nearly equivalent during these periods (plateau: 14.9 spikes/s; spontaneous: 13.2 spikes/s; see Fig. 1, A and B). A similar relationship holds for VPm and Rt responses to directions evoking the greatest plateau activity (Fig. 1, C and D).
We quantified differences in Rt and VPm plateau activity by classifying
individual neurons as tonically or phasically responsive. Units were
considered "tonic" if their plateau activity at the maximally
effective plateau direction was significantly elevated above
spontaneous levels; all others were categorized as "phasic." Within
Rt, 75.3% of the units responded tonically compared with only 45.1%
of VPm units (2 test, P < 0.001, Fig. 4A). Even among
the subpopulations of tonic Rt and VPm neurons, the former displayed
greater plateau activity in terms of both absolute spike counts and
relative to spontaneous levels (Table 2).
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At the least effective angle for evoking plateau responses, many
neurons fire at lower than spontaneous rates (see Fig. 1E). We classified units as putatively inhibited if their plateau activity at the minimally effective direction was significantly less than spontaneous firing. Figure 4B shows that a greater
proportion of VPm neurons are inhibited during vibrissa deflections
than Rt neurons (59.7 versus 21.2%, 2 test,
P < 0.001).
Data were examined to determine whether neurons classified as phasic
were more likely to be inhibited in at least one direction than neurons
classified as tonic. In VPm, 71% (24/34) of phasic units were
inhibited compared with 46% (13/28) of tonic cells (2, P = 0.054). The same
relationship was observed in Rt (phasic: 48% inhibited; tonic: 13%
inhibited; P < 0.001). Similarly, in both VPm and Rt,
tonic cells fire more spikes at the least effective plateau direction
than phasic cells; conversely, inhibited cells have smaller maximum
plateau responses than noninhibited cells (Table 2). Together, these
analyses indicate that inhibition, if present, influences a neuron's
plateau responses in all directions, rather than in nonpreferred
directions only. As such, tonic responsiveness in the thalamus is not
simply determined by a neuron's excitatory inputs, but is also
influenced by inhibitory mechanisms.
Direction sensitivity
As demonstrated previously, many VPm neurons, like N. V. neurons, respond differently depending on the direction of vibrissa movement (Simons and Carvell 1989). Polar plots of
single neuron responses, such as those shown in Fig.
5, reveal that Rt neurons also exhibit
directionally sensitive firing for the ON, OFF,
and plateau response periods. Note, however, as in Fig. 5, that the maximally effective directions for evoking ON,
OFF, and plateau responses often differ for the same
neuron. This is true for primary afferent neurons as well
(Lichtenstein et al. 1990
), especially in the case of
ON and OFF responses (see below). To compare
directional tuning in VPm and Rt, tuning ratios were computed
separately for ON, OFF, and plateau responses
by dividing each neuron's maximal angle response by its mean response
to all deflection angles. By these measures, ON,
OFF, and plateau responses of the VPm population are
significantly better tuned than those of Rt neurons (P < 0.001). The ON response tuning of the sampled VPm and Rt
populations is directly compared in the normalized polar plots of Fig.
6. Population data from 83 N. V. neurons are also presented for comparison (Shoykhet and Simons,
unpublished data). At each sequential stage of processing directional
tuning becomes broader.
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The tuning ratio used here, like those commonly employed in other
systems (e.g., width at half-height of tuning curve), employs normalization and hence is a relative measure of neuronal activity at
different directions. In terms of absolute spike counts, however, we
find that the difference between the maximum angle response and the
response averaged over all eight angles is greater in Rt (1.54 ± 0.85 spikes) than VPm (0.87 ± 0.50 spikes; P < 0.001). The same relationship holds for the OFF and plateau
responses (Table 1). The smaller tuning ratios (Fig. 6 polar plot) for Rt neurons thus reflect their greater mean responsiveness (denominator of tuning ratio) rather than more reliable discrimination between directions. For example, a difference of 2.0 spikes/stimulus between the response at the preferred angle and the average for all eight angles yields a higher tuning ratio for a neuron whose average response
is 2.0 (4.0/2.0 = 2.0) than for a neuron whose average response is
6.0 (8.0/6.0 = 1.33). We therefore also assessed directional tuning by using a measure based on the mean and variance of the spike
counts. Each unit was classified into one of eight tuning categories
based on the number of deflection angles (0-7) that evoked responses
statistically smaller than those of the maximally effective direction
(P < 0.05, one-tail t-test; see
Simons and Carvell 1989); category 7 represents the most
selective responses. Figure 6 shows that the distribution of cells
across the eight categories is similar for VPm and Rt populations
(
2, P's > 0.1). For example,
37 and 46% of VPm and Rt neurons, respectively, have significantly
greater ON responses in the maximally effective direction
than in five or more other directions.
A third measure of a unit's directional properties is directional
consistency. Directionally consistent cells discharge most vigorously
to the onset of vibrissa deflection in one direction and to stimulus
offset after initial movements in the opposite direction (Fig.
5A). The proportions of directionally consistent cells in
VPm and Rt are similar (25.8% versus 30.6%;
2, P = 0.53). Moreover, in
both populations directionally consistent cells have higher
ON and OFF tuning indices than cells that are not directionally consistent (P < 0.05); the same
relationship is observed in primary afferent neurons
(Lichtenstein et al. 1990
; see DISCUSSION).
However, unlike the primary afferent population tonic and phasic
neurons in VPm are equally likely to display directional consistency
[tonic: 50.0% (8/16); phasic: 43.5% (20/46), P = 0.65]. The same is true for Rt [tonic: 71.2% (42/59); phasic: 84.6%
(22/26), P = 0.18].
We also examined whether inhibited and noninhibited neurons differ with respect to directional tuning ratios. VPm cells with inhibited plateau responses are better tuned for the plateau response than cells that are not inhibited (P < 0.001). Although the mean plateau activity is significantly lower for inhibited cells, the preferred angle response is also smaller (Table 2; P < 0.01). The difference in absolute spike count between maximum and mean plateau activities is therefore the same for inhibited (14.9 ± 12.0 spikes/s) and noninhibited (13.4 ± 9.5 spikes/s) cells (P = 0.60). As explained above, the higher tuning ratios of inhibited versus noninhibited cells thus reflect their lower overall (mean) discharge rates at all directions. There are no significant differences in ON or OFF tuning ratios between inhibited and noninhibited VPm neurons, and their response magnitudes are equivalent. In Rt, on the other hand, inhibited cells are better tuned for ON, OFF, and plateau responses (P < 0.05). Again, the higher tuning ratios are due to a significantly lower mean responsiveness of inhibited cells (P < 0.05), rather than a greater difference in the maximum and mean number of evoked spikes. Inhibited and noninhibited cells did not differ with respect to tuning categories in either VPm or Rt.
ON and OFF response transformations
VPm and Rt neurons differ substantially with respect to the
relative magnitudes and temporal profiles of their ON and
OFF responses. We computed OFF:ON
response ratios for VPm and Rt neurons (Table 1) and found that Rt
neurons, like cortical and N. V. populations (Kyriazi et
al. 1994), have significantly smaller ratios (i.e., relatively
smaller OFF responses) than VPm neurons (P = 0.007 for 20-ms Rt window, P < 0.001 for 38-ms window).
Figure 7 shows ON and OFF PSTHs at high-temporal resolution. The Rt ON response has a rapidly rising early peak, similar to the VPm ON response, followed by a second phase of large magnitude, which develops and decays more slowly. In contrast, the Rt OFF response consists of a single peak which begins to decay earlier than its ON response counterpart. These profiles suggest that there are two components of Rt responses, and that their differential engagement might account for the smaller OFF:ON ratios of Rt neurons relative to those in VPm. This is indeed the case. We computed OFF:ON ratios separately for early and late portions (see bars in Fig. 7) of the responses. Ratios for the early components of the Rt responses are comparable to those in VPm (P = 0.81, M-W U test), but are larger than ratios for the late Rt components (early: 0.925; late: 0.646; P < 0.001).
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Based on a previous study of VPm neurons (Kyriazi et al.
1994), we hypothesized that the early component of the Rt
ON and OFF responses is mediated by fast,
ion-gated glutamate receptors, whereas the late component reflects
activation of voltage-gated T-type Ca2+
(IT) channels (Huguenard and
Prince 1992
). IT channels
require hyperpolarization to deinactivate (i.e., open inactivation
gates) and hence to conduct regenerative depolarizing current on
subsequent suprathreshold depolarization. If the early response is
mediated by voltage-independent ion-gated channels and the late
component by IT, the magnitude of the
early response should be directly proportional to the level of
immediately preceding activity, and that of the late response should be
inversely related. We therefore examined how the magnitudes of the
early and late components of Rt responses correlate with the presence
or absence of a spike in each 1-ms bin preceding the evoked response.
For each millisecond, a correlation coefficient was computed based on
10 trials × 8 directions × 85 neurons. Figure
8B shows that the late
component of the OFF response is negatively correlated with
activity in each of the 24 consecutive bins preceding the beginning of
the OFF response. By contrast, the early component is
positively correlated with the same preceding activity (Fig.
8C). To determine more precisely the time course of these
relationships, the total spike count over the 25 ms preceding the
OFF response was correlated with the presence or absence of
a spike in each 1-ms bin of the response (Fig. 8D).
Consistent with the preceding analyses, correlations are positive
during the first 10 ms of the OFF response but become negative after ~15 ms. The same analyses were applied to the
ON response, and similar results were obtained (data not
shown). Specifically, the early component of the ON
response was positively correlated with preceding (spontaneous)
activity, whereas the late component was negatively correlated.
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DISCUSSION |
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In this study we employed the same ramp-and-hold vibrissa deflections to study the response characteristics of Rt neurons as were used previously to characterize neuronal signaling in N. V., thalamocortical, and cortical neurons. There are three main results presented here as follows: 1) Transient responses of Rt neurons are larger in magnitude and longer in duration than those of thalamocortical and other populations previously studied, 2) High firing rates emerge in Rt neurons during sustained vibrissa deflection, in the absence of proportionately high activity in the VPm population, and 3) Rt neurons exhibit directional sensitivities consistent with their receiving specific afferent inputs. These results demonstrate a modification/transformation of response characteristics in Rt neurons relative to those at other stages of the sensory pathway and implicate Rt functionally in the processing of sensory information.
ON and OFF responses
The magnitudes of transient responses in Rt are by far the largest
of any vibrissa-responsive population studied thus far under the same
conditions (compare mean ON response for all angles: Rt = 5.69 spikes, N. V. neurons = 2.15, cortical layer
IV regular-spike units = 0.92, layer IV fast-spike units = 1.70), including those of the VPm neurons which are the principal
source of Rt afferent drive (mean ON = 1.27)
(Kyriazi et al. 1994). The peak firing rates of VPm and
Rt populations are similar; rather, it is the long duration of the Rt
response which accounts for its larger magnitude (Fig. 7). The analyses
presented in Fig. 8, demonstrating a negative correlation between this
later component of Rt responses and activity preceding the response,
strongly suggest that it is partially mediated by
IT (see Bal and McCormick
1993
; Contreras et al. 1992
, 1993
;
Sumitomo et al. 1989
). This interpretation is based on
the presumption that extracellularly recorded spike activity reflects
the extent of a unit's membrane polarization. When a cell is
relatively hyperpolarized (e.g., in the absence of a stimulus),
IT channels become deinactivated and
can conduct large, regenerative currents on subsequent stimulus-evoked
depolarization (e.g., ON response). When relatively
depolarized (e.g., during the stimulus plateau), a greater proportion
of IT channels are inactivated and
less depolarizing current can be elicited by a subsequent
depolarization of equal amplitude (e.g., OFF response). If
the stimulus plateau is hyperpolarizing, as for VPm neurons, the
OFF response is enhanced (Kyriazi et al.
1994
). The long, late component of Rt responses may also be
mediated in part by N-methyl-D-aspartate (NMDA)
receptors, which have been identified histochemically in Rt neurons
(Liu 1997
). Although NMDA receptors are also found on
VPm neurons, the expression of their currents may be masked by the
greater levels of Rt inhibition onto these neurons than onto other Rt neurons.
Functionally, high stimulus-evoked activity of Rt neurons may be
necessary for adequate summation of IPSPs to terminate VPm responses
and limit them to a small number of precisely timed spikes. Kim
and McCormick (1998) have shown that there is increasing facilitation of IPSPs in thalamocortical neurons for at least three
consecutive spikes at 100-500 Hz and that the peak amplitude of
compound IPSPs increases linearly with the frequency of presynaptic spikes. Thus ~6 Rt spikes over 30 ms (200 Hz) would powerfully inhibit VPm responses and prevent subsequent spikes that are not as
closely time-locked to stimulus onset. This feedback inhibition would
enhance the temporal specificity of thalamocortical signaling and,
therefore the information carried by individual spikes. Indeed, Lee et al. (1994a
,b
) found that eliminating Rt
influences on VPm resulted in an increase in VPm response magnitude and
duration. Even if, under the influence of Rt, individual VPm neurons
fire only single spikes per stimulus, synchronous firing within the population can serve as an efficient, rapidly transmitted signal to
which cortical circuits are particularly sensitive (Pinto et al.
2000
).
We found that the mean OFF:ON ratio is smaller
for Rt than for VPm and that this transformation can be accounted for
in part by a diminished late component in the Rt OFF
response. This diminution is likely due to the inactivation of
IT channels during periods of elevated
activity preceding stimulus offset. Previously, Kyriazi et al.
(1994) used the same correlation analysis presented here to
suggest the involvement of IT in
enhancing VPm OFF responses. Decreased VPm activity during
the stimulus plateau (i.e., greater hyperpolarization due to
Rt-mediated inhibition) deinactivates IT channels, rendering them available
to enhance VPm responses to stimulus offset. The converse influences of
IT on Rt and VPm OFF
responses depend on the balance of activity in the thalamic circuit
during stimulus plateaus.
Tonic responses
The mean plateau activity averaged over all deflection angles for
Rt is elevated 27.3 spikes/s (122%) above its spontaneous levels. By
contrast, VPm activity is only 1.7 spikes/s (13%) greater than
spontaneous activity. How can such elevated firing rates emerge in Rt
in the absence of significant drive from VPm? One explanation,
consistent with the larger ON and OFF responses
in Rt, is that there is high gain at VPm-Rt synapses. This could be
due to the presence of amplifying ion- and voltage-gated currents (e.g., NMDA and IT) and also to the
high-input resistances of Rt neurons (McCormick and Prince
1986). In addition, Ca2+ influx through
IT channels during the ON
response may activate Ca2+-dependent cation
currents, which contribute to tonic activity after burst discharges
(Bal and McCormick 1993
). A second factor may be a high
degree of convergence of VPm synapses onto single Rt neurons. Although
there are an estimated 250-300 VPm neurons with the same receptive
field center (i.e., within a barreloid; Land et al.
1995
), there are only 10-30 corresponding Rt neurons (Shosaku 1986
). Because ~76% of VPm neurons
collateralize within Rt (Harris 1987
) and 80% of VPm
neurons show physiological connections with Rt neurons in
cross-correlation studies (Shosaku 1986
), there is
likely to be a many-to-one relation in connections from VPm to Rt
cells. A third mechanism underlying large tonic Rt responses may be a
disproportionately large number of inputs to Rt from tonic versus
phasic VPm neurons. Finally, Rt neurons are less likely to inhibit each
other than they are to inhibit VPm neurons (Fig. 4B); this
allows Rt neurons to be tonically active and thereby inhibit the
plateau activity of VPm neurons.
Specificity of connections within VPm-Rt circuit
In a study employing cross-correlation analyses of VPm and Rt
neuronal activity, Shosaku (1986) found that cells
exhibited functional interactions only if they responded best to the
same vibrissa. Extending this finding of connectional specificity, we
report that a substantial proportion of Rt neurons exhibit directionally selective responses (Figs. 5 and 6). This result demonstrates precision in connectivity from VPm to Rt, inasmuch as
nonspecific convergence of VPm cells having different, or no, directional preferences onto individual Rt neurons would render them
nondirectional. Like N. V. and VPm neurons, many Rt neurons respond selectively to vibrissa movement in a particular direction regardless of the starting point of the movement, a property termed directional consistency. This property, too, would require specific inputs to Rt neurons from directionally consistent VPm cells having the
same directional preference. These results demonstrate that VPm axons
synapse onto Rt cells in a manner that is defined by specific
parameters of sensory activation.
Our findings contrast with results of Shosaku (1985),
who, relying principally on manual vibrissa stimulation, reported that Rt neurons are not directionally selective. According to his model, nondirectional Rt cells mediate both cross- and iso-directional inhibition in VPm (Shosaku et al. 1989
). Indeed, the
present findings suggest that nondirectional inhibition from Rt could
enhance tuning of plateau activity in VPm by simply raising the
threshold for responses. On the other hand, Lee et al.
(1994a
,b
) reported that directional tuning of VPm responses was
unaffected by eliminating Rt-mediated inhibition. Interpretation of
those findings is complicated somewhat by the use of a response measure
that combined discharges to deflection onsets and offsets, which, as
described, often have opposite directional tuning. Although Rt may not
play an active role in imparting directional tuning to VPm neurons, our
finding that many Rt cells are directionally well-tuned nevertheless
raises the possibility that Rt-mediated inhibition could affect the
firing of VPM neurons during sequential whisker movements in opposite directions, as occur, for example, during transitions between whisker
retraction and protraction. These influences may be better revealed
experimentally by sequentially deflecting the same whisker in different directions.
Rt control of thalamocortical circuit activity
Strong feedforward engagement of cortical inhibitory neurons by
thalamocortical afferents sets a brief "window of opportunity" for
temporally synchronous thalamic inputs to elicit a response in
excitatory neurons (Pinto et al. 1996). Because of the
circuit's nonlinear properties, the layer IV "barrel" acts to
increase response differentials between preferred (synchronous) and
nonpreferred (temporally dispersed) thalamic input signals.
Importantly, the strength of this tuning effect (i.e., the size of the
window) is modulated by thalamic background activity, which tonically activates inhibitory barrel neurons more so than excitatory ones (Brumberg et al. 1996
; Swadlow 1995
,
1998
). Thus by regulating ongoing activity in VPm, Rt neurons
could control the balance of excitation and inhibition, and hence
response tuning, in the cortex.
High evoked firing rates emerge in Rt neurons during maintained
deflections despite minimal increases in VPm activity. Because slowly
adapting N. V. units, which comprise 75% of the population, fire
at a mean rate of 70 spikes/s above their spontaneous activity during
stimulus plateaus (Lichtenstein et al. 1990), it is
likely that Rt acts to suppress corresponding increases in the VPm
population. This is supported by our finding that phasic VPm cells were
more likely to be inhibited at nonpreferred directions than tonic
cells. Through amplification mechanisms, Rt neurons are able to respond to small changes in baseline VPm firing and to subsequently counteract these changes by increasing or decreasing their own activity. The high
spontaneous and stimulus-evoked firing rates of Rt neurons would permit
them to do this over a broad range of stimulus conditions without
encountering floor or ceiling effects. Thus by controlling the
maintained activity of VPm cells on a long time scale (10s or 100s of
milliseconds), Rt may act as a gain control that keeps thalamocortical
activity in a range optimal for signal processing and transmission in
the cortical barrel. Depending on the state of the animal (see
Crick 1984
), this indirect influence of Rt inhibition
could act in concert with its enhancement of transient thalamocortical spiking on a shorter time scale by means of
deinactivation of IT channels.
Efficient signaling of rapid changes in vibrissa position is likely to
be important in enabling the fine vibrissae-based texture
discriminations of which rodents are capable (Carvell and Simons
1990
, 1995
).
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ACKNOWLEDGMENTS |
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We are grateful to H. T. Kyriazi for a thoughtful review of the manuscript and to M. Shoykhet for use of directional data on primary afferent neurons.
This work was supported by National Science Foundation Grant IBN9421380 and National Institute of Neurological Disorders and Stroke Grant NS-19950.
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FOOTNOTES |
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Address for reprint requests: D. J. Simons, Dept. of Neurobiology, E1440 Biomedical Science Tower, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261.
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 11 August 1999; accepted in final form 1 February 2000.
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REFERENCES |
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