17
Hearing Research, 53 (1991) 17-27 0 1991 Elsevier Science Publishers B.V. 03785955/91/$03.50
HEARES
01545
Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons Dennis P. Phillips lq2 and Sarah A. Sark 1 Departments
of ’ Psychology and ’ Otolmyngologv, (Received
28 August
Dalhousie University, Halifax,
1990; accepted
14 November
Nova Scotia, Canada.
1990)
In the anesthetized cat. some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIS) in these responses. The ISIS were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length. were very regular. Except at threshold levels of stimulation, mean ISIS were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIS suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.
Auditory
cortex;
Single neuron;
Transient
response;
Spike timing;
Introduction Almost all studies of the stimulus-response relations of auditory cortical neurons have quantified unit responses using two measures: the total number of spike discharges evoked by a fixed number of stimulus presentations, and the mean (and standard deviation) of the first-spike latent periods. The former measure provides an indication of the strength of the excitatory drive to the neuron (Phillips, 1990), while the latter provides an indication of the precision with which the stimulus event is timed (Phillips and Hall, 1990). The two measures have jointly been used to describe, and in some cases explain, the sensitivity of cortical neurons to a variety of parameters of tonal and other stimulation (Brugge et al., 1969; Brugge and Merzenich, 1973; Phillips, 1990; Phillips et al., 1989).
Correspondence to: D.P. Phillips, Department of Psychology, Dalhousie University, Halifax. Nova Scotia, Canada B3H 451. PAX: (902) 494-6585
Inter-spike interval
Particularly in awake animals (Brugge and Merzenich, 1973; Kitzes et al., 1980) but also in anesthetized preparations (Phillips et al., 1989), cortical auditory neurons may respond to transient sounds with a brief train of spike discharges. The extent to which the properties of these secondary spike responses (e.g., their number and inter-spike intervals) are dependent on stimulus variables is almost completely unknown. The point is significant because even if the time structure of the spike train does not reflect that of the stimulus (c.f., phase-locked spikes to stimulus periodicities). the train itself might contain information about the stimulus. This is because the internal structure of the spike train may vary even though the total spike number and first-spike latency may be stable or saturated. In this respect, early studies of cochlear nucleus neurons (Goldberg and Greenwood, 1966; Pfeiffer and Kiang, 1965) revealed that the mean inter-spike intervals of responses to long tones showed orderly changes with variations in stimulus amplitude, and other studies have used inter-spike interval behavior as a basis for neuro-
nal classification (Rhode and Kettner. 1987; Rhode and Smith, 1986a,b; Young et al., 1988). There has been no previous description of the inter-spike intervals of those cortical neurons which discharge bursts of spikes in response to signal onset. In on-going studies of the anesthetized cat’s auditory cortex, especially the primary field, we have acquired detailed data on neurons which respond to tonal or noise signals with bursts of spike discharges. The purposes of this report are to describe the regularity of spike timing within these bursts, and to examine the extent to which stimulus conditions influence the timing of spikes in them. Methods The data come from 20 neurons in 4 adult cats prepared for acute recording sessions under barbiturate anesthesia. Detailed descriptions of the surgical, stimulating and recording procedures used in this laboratory have been described elsewhere (P~llips et al., 1989). Briefly, the cats were anesthetized (sodium pentobarbital, 35 “g/kg, ip) and prepared for acute recording studies. Surgical anesthesia was maintained throughout the recording session by supplemental doses administered intravenously. The right pinna was resected, and a stimulating system, which incorporated a calibrated probe-microphone assembly, was sealed into the transected right auditory meatus. The left auditory meatus was deliberately collapsed. A small hole was drilled in the skull overlying the posterior half of the crown of the left middle ectosylvian gyrus. The dura mater was left intact, and the extracellular recording electrode was advanced through it. Further movement of the electrode was by means of a remotely controlled, programmable stepping motor. Stimuli were 25-ms duration (including 2.5-ms rise-decay time) tone pulses with the carrier frequency set at each neuron’s characteristic frequency (CF), and delivered to the contralateral (right) ear. The CFs of all neurons in this report were between 10.5 and 21.5 kHz. Stimulus levels are expressed in dB sound pressure level (SPL: dB re 20 PPa). Data collected were usually spike count-versus-intensity functions (“intensity profiles’, 50 to 70 dB range in 5 or 10 dB steps) for
tones presented with repetition rates ranging from 1 to 10 Hz (in 1 Hz steps). Responses to each stimulus condition were based on 100 stimulus presentations. Spike waveforms were monitored continuously, and data were collected only from unequivocally isolated single units. Spike response and stimulus event times were digitized at 100 kHz and stored on-line (IBM PC-XT). Most of the units in this report were studied for 1 to 4 h. These stimuli evoked transient responses, discharged briskly in response to tone onset. The responses contained from 1 to 5 spikes, depending on the stimulus conditions. Spontaneous discharge rates were low ( < l/s), so the responses were salient and permitted statistical descriptions which were not contaminated by random (noise) spike activity. With the exception that statistical characterization of any given inter-spike interval (i.e., lst, 2nd, etc.) was restricted to responses containing at least 10 intervals, the following analyses were performed on the responses to each stimulus condition: (1) The total number of spikes evoked by 100 trials; (2) The mean and standard deviation (SD) of the first spike latent period; (3) The number of 1st. 2nd. 3rd, and 4th spikes in the response; (4) The mean (and SD) of the lst, 2nd and 3rd inter-spike intervals; (5) The total number of intervals, including the rare cases of 4th or 5th intervals; (6) The grand mean (and SD) of the intervals. Results Visu~~iz~tio~ of spike timing reguhity Analysis of the inter-spike intervals in the spike burst responses almost always revealed that the means of the intervals were larger than their SDS, and often by a factor of two to three, suggesting that the timing of secondary spikes was indeed quite regular. Nevertheless, the peri-stimulus time histogram (PSTH) of the response to any given stimulus condition only occasionally showed clear evidence of periodicities in spike timing. This was particularly true of responses to low amplitude signals, where some neurons discharge maximally (nonmonotonic units: P~llips (1990)). We reasoned that any response periodicities might be obscured in the PSTH by the jitter (SD) in the timing of the first spike, which at threshold levels
19
can be in excess of 1 to 2 ms (Phillips and Hall, 1990). We therefore constructed new response histograms in which the timing of the 2nd, 3rd and later spikes was measured from the time of the first spike, rather than from tone onset. These ‘normalized’ response histograms (NRHs) showed more clearly the regularity of spike timing. Fig. 1 shows the PSTH and the NRH of the responses of three neurons to stimuli of SPLs and repetition rates which evoked responses of maximal spike count. The bin width of all the histograms is 200 ps. These neurons were selected to illustrate the range of response regularity seen in this study. The neuron shown in Fig. l(A,B) was the most vigorously discharging unit in the sample; that in Fig. l(C,D) was among the least regular responders, and Fig. l(E,F) shows data for a neuron with a more typical spike rate, but high regularity. The panels on the left are conventional PSTHs, and, with the possible exception of that in Fig. lC, they show little indication of regularity in spike timing. The right panels are NRHs of the same responses. These contain 98 to 100 spikes fewer than do the corresponding PSTHs: the missing spikes are the first ones from each of the 100 trials, and from whose time of occurrence the later spike times were measured. Each of the NRHs shows an uneven but regular distribution of spike response times. The second spike of the burst response, which appears as the first peak in the NRH, occurs with a mean ‘latency’ of 1.45 (Fig. 1B) to 2.53 ms (Fig. 1D) after the first spike. The first mean inter-spike interval was never less than 1.4 ms. The next peak of the NRH, which contains almost exclusively 3rd spikes, occurs after a similar interval following the second spike. Interestingly, inspection of individual spike trains sometimes revealed that if a second spike did not occur with an IS1 very close to the mode, then it was delayed until the modal time of the next spike in the train. Later peaks in the NRH often were lower, and reflected both fewer spike numbers, and poorer spike timing. Thus, whether a neuron discharged as many as 4 or 5 spikes per stimulus trial (Fig. lA,B) or as few as two spikes/trial, second and later spikes tended to occur at preferred intervals. The descriptive statistics on these responses are
of interest. These data are provided in the insets of each panel. Note that for the neurons in the top- and bottom-most panels, the SDS of the inter-spike intervals were much smaller than those of the first-spike latencies in the same responses. The temporal precision in the relative timing of the second and third spikes of the trains was thus superior to precision with which the first spike marked the initiation of the responses. In the other neuron, the IS1 SDS were comparable to, or larger than, that of the first spike latency. Nevertheless, for most cells, removal of the jitter provided by the first-spike latent period helped to reveal the regularity in the timing of the subsequent spikes. Table 1 presents summary data on the length of ISIS, as calculated from the raw spike times, for responses of maximal spike count. The population mean 1st 2nd, and 3rd ISIS were typically close to 2 ms, and across the 20 neurons in the sample, the range of ISIS was very small. There were no mean intervals shorter than 1.1 ms, and only very rarely were ISIS in individual spike trains less than 1.0 ms. There were no individual ISIS less than 0.9 ms. Equally of interest is that, in general, coefficients of variation (CV: the ratio of the interval SD to the interval mean) for these responses were close to 0.5. They ranged from as small as 0.16 (indicating high timing precision) to as great as 0.86 (indicating more temporally scattered spike discharges). These population data confirm the impression provided by Fig. 1: the temporal distribution of spikes in the burst response is markedly non-random, and the intervals can be almost as short as neuronal biophysics permit. Stimulus dependence of inter-spike intervals In contrast to the marked effects of tone level (and repetition rate) on spike counts, the ISIS were largely invariant across broad ranges of both of those variables. For the sake of brevity, we shall present data only on the effects of tone level. Fig. 2 presents data for two neurons to illustrate the effect on IS1 of the amplitude parameter. These two neurons were selected for presentation because they represent cases drawn from those with the poorest IS1 regularity (left panels) and those with the greatest regularity (right panels). The neuron whose data are presented in the left panels
20
2c
-I
ID: RR408 CF = 10.55 kHz 20 dB, 1 Hz = 468 spikes Mean = 19.57 ms sd. = 1.01 ms
A
IB
N = 368 spikes 1st mean = 1.45 1 st s.d. = 0.36 ms 2ndmean=lS8ms 2nd sd. = 0.59 ms
L
30
20 10,
-
10
0
15
ID: RR303 CF = 20.82 kHz 62 dB, 1 Hz N = 252 spikes Mean = 13.81 ms s.d. = 1.33 ms
N = 152 spikes lstmean=2.53ms 1st s.d. = 1.91 ms 2nd mean = 2.64 ms 2nd sd. = 1.28 ms 10
5
0
ID: RR207 CF = 17.74 kHz 80 dB, 2 Hz N = 240 spikes Mean = 18.73 ms s.d. = 1.07 ms
10
20
30
N = 142 spikes lstmean=1.81 ms 1st sd. = 0.29 ms 2nd mean = 1.87 ms 2nd s.d. = 0.47 ms
I
10
40
TIM;OAFTER;ONE ONSET (ms)
0
30
TIhk”AFTEF? i&ST SPIKE (ms)
21
had a saturating, monotonic intensity profile. The data in the right panels of Fig. 2 come from a neuron with a nonmonotonic intensity profile. Regularity of spike timing was not related in any obvious way to intensity profile shape. The upper panels (Fig. 2A,D) plot the number of lst, 2nd, 3rd, and 4th spikes as a function of the SPL of a CF tone delivered at l/s. In both neurons, thresholds for the excitation of a second (or later) spike increased with the serial number of the spike in the response. For the monotonic neuron, only the first spike achieved perfect entrainment (i.e., lo@ spikes/100 trials). The later spikes saturated at progressively lower rates. For the nonmonotonic neuron, each of the first to fourth spikes exceeded 90% of the maximum-possible response rate, and they each did so at 20 dB SPL. Note that the neuron expresses its sensitivity to high-amplitude tones in the number of spikes in the burst response, but not in the number of effective trials. Thus, the number of first spikes remains constant at lOO/lOO trials, while the number of subsequent spikes in the burst response falls precipitously (Fig. 2D). We shall return to this point later in this report. The middle panels of Fig. 2 show the effect of tone amplitude on the mean length of the lst, 2nd and 3rd ISIS in the same responses. Except for signal levels very close to threshold, ISIS were relatively constant. For the nonmonotonic cell (Fig. 2E), the ISIS were stable over an SPL range up to 70 dB wide, and over which the number of 3rd and 4th spikes fell from maximum to zero. For the monotonic cell (Fig. 2B), ISIS were longer and more variable, but they had reached minima at tone levels lower than those which saturated spike numbers. Note that there is a further consequence of this ISI stability. Since the ISIS were relatively uninfluenced by tone SPL, they were also uncorrelated with response spike rate, and
were only weakly influenced by first spike latency. Not shown in Fig. 2 are scatter-plots of mean interval against absolute spike rate and first-spike latency that were made for each neuron in the sample, using thedata combined across all effective stimulus conditions. The distribution of data points in both kinds of plots took the form of a nearly horizontal band. The only outlying points were for threshold responses, where the mean ISIS were sometimes slightly lengthened. A broader impression of the relative insensitivity of the ISIS to stimulating condition is provided in the bottom panels of Fig. 2. Here, we have plotted the SD of each IS1 as a function of its mean, and have combined data across all effective SPL-by-repetition-rate stimulus conjunctions. There were 29 such stimulus conditions, covering a 40 dB range of SPLs, for the monotonic cell (Fig. 2C), and there were 68 stimulus conditions spanning a 75 dB range for the nonmonotonic one (Fig. 2F). Two features of these data are apparent. One is that the interval SD was proportional to the interval mean. For 17 cells for which there were sufficient data, we estimated the linear regression between the SD and the mean of the total interval data. Many of these lines had slopes close to 1.0, but the range extended from 0.75 to 3.14 (mean = 1.36). A second feature of the data in the scattergrams is that the absolute variation in mean IS1 (and for each ISI, i.e., 1st. 2nd, 3rd) was quite small. Thus, for the monotonic neuron (Fig. 2C), the majority of ISIS were between 2.0 and 4.5 ms in length, and for the nonmonotonic cell (Fig. 2F). most of the ISIS were between 1.3 and 2.3 ms in length.
Observations relevant to the genesis of nonmonotonic spike mount-v~-intensity profiles We mentioned above that, for nonmonotonic neurons, the number of spikes in the spike bursts
Fig. 1. &$ punefs: Conventional PSTHs of the responses of 3 neurons to 100 presentations of a CF tone pulse evoking maximal spike counts. Bin width is 200 ns. Text within the panels indicates unit ID number, unit CF, stimulus SPL and repetition rate, number of spikes in the histogram (N), mean latency to first spike, and standard deviation (SD) of first-spike latency. Right panels: NRHs for the same responses. Each histogram shows the event times of 2nd, 3rd and later spikes, as measured from the first-spike time in each trial. Bin width is 200 ps. The apparent presence of second spikes occurring within 0.8 ms of the first spike (panel D) is an artifact of the histogram binning. Text within each panel indicates the number of spikes in the histogram (N), mean and standard deviation of the 1st and 2nd inter-spike intervals. Description in text.
“
120
ID: RR303. CF = 20.82
.A
120I-
kHz
ID: RR408, CF = 10.55 kHz
D 100
-
-
-
-
-
c
10
20 30 40 50 60 TONE LEVEL (dB SPL)
70
0
80
20
80
100
40 60 80 TONE LEVEL (dB SPL)
100
TONE4:EVEL (ii 8
I-B
SPL)
E
7 6
0
10
20 30 40 50 60 TONE LEVEL (dB SPL)
70
80
0
20
F
x
.
x
1Sl
l
2nd 3rd
0
0
4 2 6 MEAN INTERVAL (ms)
8
Fig. 2. Data for neurons RR303 (left) and RR408 (right). A and D show the to tone level. B and E show the effect of tone level on the mean length of covariation of interval SD with interval mean for a wide array of stimulus plotted separately for 1st. 2nd and 3rd
0
2 4 MEAN INTERVAL (ms)
6
sensitivity of the numbers of 1st. 2nd. 3rd. and 4th spikes the 1st. 2nd and 3rd inter-spike intervals, C and F show conditions varying in SPL and repetition rate. Data are inter-spike intervals.
23
is reduced in responses to tone levels associated with the descending limb of the intensity profile, and that this is expressed first in a loss of fourth, then third, then second spikes (Fig. 2D). This could come about through the action of at least two mechanisms. First, it is possible that it is the shortest-latency (i.e., first) spikes which are lost, so that what would otherwise have been the second spike now becomes the first (etc.), with the result that there are fewer spikes in the train. Second, it might genuinely be the case that the late spikes in the train are those which are lost first by the inhibitory processes that shape the descending limb of the spike count function. Note that in plots of the kind in Fig. 2D, both hypotheses would express themselves as an apparent differential sensitivity of the late spikes to tone level. These two hypotheses can be differentiated by examination of the first-spike latent periods for responses to different suprathreshold tone levels. Especially for tones with rise-times as brief as those used here (2.5 ms). the first-spike latency function has a very shallow negative slope at suprathreshold stimulus amplitudes (e.g., Phillips, 1988b). The former hypothesis would predict that latent periods for responses to higher amplitude tones within this range might become longer than those for lower amplitude tones. This is because any modest first-spike latency shortening due to the latency-intensity relation could be offset by the loss of the first spike and the attendant addition of the first IS1 to the period between tone onset and response onset. In contrast, the second hypothesis would predict a normal first-spike latency function, because first-spike latency may be independent of whatever processes determine the number of later spikes in the train. Our data support the second hypothesis, and more detailed evidence on the responses depicted in Fig. 2D is presented in Fig. 3 to show why. Fig. 3 shows the NRHs for the responses of one nonmonotonic neuron to eight tone levels over a 65 dB range. The text within each panel indicates the stimulus level, the number of spikes in the histogram, the number of first spikes (which occur in bin zero, and are not shown), and the mean first spike latency. The number of 2nd (and later) spikes increases from 116 at 15 dB to 368 at 20 dB, and then declines to 68 spikes at 70 dB.
Across this range, the number of first spikes, which indicates the number of effective stimulus trials, is almost constant at 100. What is of most interest here are the responses at grossly suprathreshold levels, where the latency function has its shallowest slope, but where spike counts are still declining. For this neuron, that range is from 50 dB (Fig. 3E) to 70 dB (Fig. 3G). Over this range of signal levels, the number of spikes dropped by 116, yet the first-spike latency shortened by 0.64 ms. Since the first IS1 in this neuron was close to 1.5 ms, then loss of the first spikes would have added about 1.5 ms to the mean latent period. This did not occur. It might be argued that, in this neuron, the ‘uninhibited’ latency function could have had a slope in excess of 1.5 ms/lO dB, so that loss of the first spikes would have resulted in the first-spike latencies seen here. Note, however, that increments in stimulus level beyond 70 dB (to 80 dB: Fig. 3H, and 90 dB: not shown) shortened the first-spike latency to 12.87 and 12.59 ms respectively, but with no further reduction in spike count. The slope of the latency function between 70 and 90 dB (25 ps/dB) is thus quite close to that between 50 and 70 dB (32 ps/dB). The lack of any discontinuity in the latency function between SPL ranges that reduced spike rate, and those that did not, confirms that it is the last spikes in the bursts which are suppressed by inhibition to produce the descending limb of the nonmonotonic intensity function. Trends of this kind were common to all of the nonmonotonic cells in our sample (N = 8). although different spike timing behavior has been seen in some neurons described by Brugge et al (1969). The present observations may provide one reason for why many nonmonotonic neurons have normal first-spike latency-intensity functions over SPL ranges where spike rates are declining: Each stimulus event may evoke both an excitatory onset response and a longer-latency inhibitory one. At low tone levels, the inhibitory response may have too long a latent period to suppress any spikes. The inhibitory response latency may shorten more quickly with increasing SPL than does the excitatory one, so that the later spikes driven by the short-latency excitatory input are the first to drop out.
A
B
60.
15dB
60-
20 dB
N=ll6spikes (+
40 -
N = 368 spikes
91 first spikes in bin 0) Mean = 28.44 ms
40 -
20 -
I 0
dA
5
I
15
10
100 firs! spikes in bin 0) Mean = t 9.57 ms
(+
20_~ 0
10
5
15
0 60 -
60
(+
100 first spikes in bin 0) Mean = 15.60 ms
0
40 -
5OdB N = 184 spikes (+ 100 first spikes in bin 0) Mean = 13.73 ms
5
10
15
r
E 60 ”
40 dB N = 248 spikes (+ 100 first spikes in bin 0) Mean = 14.27 ms
60 -
40 *
F 60 de N = 123 spikes (+ 100 first spikes in bin 0’ Mean
= t3.35
ms
20 -
0
10
5
15
H -
60
80 dB N = 70 spikes
100 first spikes in bin 0)
(+
40 -
(+ 100
first spikes in bin 0) Mean = 12.87 ms
Mean = 13.09 ms
0
5
10
15
0
5
10
15
TIME AFTER FIRST SPIKE (ms) Fig. 3. Detailed data on neuron RR408. Each panel shows the NRH for responses to 100 trials of a tone pulse whose amplitude is specified in each panel. Other text within the panels indicates the number of spikes in the histogram, the number of first spikes, and the mean latent period of the first spike.
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Mechanisms shaping spike burst responses The neurons in this sample discharged bursts of spikes locked in time to tone onset. The intervals between these spikes were often highly regular (Fig. l), and commonly had lengths between 1.5 and 3.5 ms (Table I). The neurons in the sample all had CFs in excess of 10 kHz, indicating that the periodicities in the response were not tied to the fine structure of the tones (cf. Kitzes et al., 1978), i.e., they were not phase-locked responses to the first few cycles of the sinusoidal signals. It is also unlikely that the spike regularity is a cortical expression of basilar membrane ‘ringing’ (i.e., decaying oscillations driven by tone onset). This is for two reasons. One is that although the tones were brief, they had rise-times of 2.5 ms, which are unlikely to generate significant such oscillations, let alone at the stimulus levels used here (e.g., Fig. 1B). The second is that at the level of cochlear output, periodicity in neural responses evoked by mechanical ringing is most marked in low-CF cells. where the frequency of the oscillating neural response matches the cell’s CF (Kiang et al., 1965).
TABLE
I
DESCRIPTIVE STATISTICS ON THE LENGTH OF MEAN ISIS SEEN IN RESPONSES OF 20 NEURONS TO STIMULI EVOKING MAXIMAL SPIKE COUNTS Population
measure
Mean of interval means (ms) Range of interval means (ms) Mean of interval s.d.s (ms) Range of interval s.d.s (ms) Mean of interval cvs Range of interval cvs Number of cells
First IS1
Second
IS1
Third IS1
2.34
2.26
2.02
1.45 to 3.68
1.58 to 3.20
1.10 to 2.65
1.14
1.09
1.12
0.29 to 2.33
0.47 to 1.82
0.67 to 1.80
0.46
0.47
0.57
0.16 to 0.79 20
0.25 to 0.73 14
0.44 to 0.86 9
Note that these stimulus conditions were not always optimal for spike time regularity. since 40% of the data come from nonmonotonic cells whose maximal discharge rates were achieved using low amplitude tones.
In contrast to data obtained from responses to long tones in the cochlear nuclear complex (Goldberg and Greenwood, 1966; Pfeiffer and Kiang, 1965), the inter-spike intervals in the transient response of cortical cells showed little dependence on stimulus variables, except in threshold responses, where the intervals lengthened slightly (Fig. 2). The very fact that the cortical responses are transient necessarily restricts the range of ISIS to be expected, but it need not impose either the regularity or the stimulus independence seen here. The significance of this relative insensitivity is that it was expressed over SPL ranges which could have profound effects on the number of spikes, especially in nonmonotonic cells. Accordingly, quite separate mechanisms must have controlled the number of spikes in the burst, and the relative spacing of the spikes in the responses. In monotonic cells, ISIS typically reached minimal lengths quite close to threshold. The further implication of this is that for both cell types, information about the SPL of the tones resided largely in the number of spikes, and not in their relative timing. The slopes of the SD/mean regression lines for ISI length (or first-spike latency) describe the rate at which the precision of spike timing becomes poorer with increases in interval length (or latent period). The slopes of interval SD/mean regression lines in the present data were close to, or greater than 1.0, which is in contrast to those for the first-spike latency (which are typically less than 0.8: Phillips and Hall, 1990). This suggests that different processes shape the timing of the first spike and the relative timing of the later ones. In this respect, the timing of the first spike is a sensitive function of many stimulus variables (Fig. 3; Brugge et al., 1969; Phillips, 1990; Phillips et al., 1989; Phillips and Hall, 1990) while the present study suggests that the relative timing of the later spikes is not. As such, stimulus conditions set both the time when the neuron’s response is initiated, and the number of spikes in the discharge, but the internal time structure of the spike train is controlled by other factors. The processes that determine the inter-spike intervals of cortical auditory neurons are not known. The afferent pathway to cortical neurons is extraordinarily convergent (Phillips, 1988a), which raises the possibility that the ISIS might be
26
shaped by factors similar in principle to those which produce comparably brief and regular ISIS in some chopper units of the cochlear nucleus (Rhode and Kettner, 1987; Rhode and Smith. 1986a,b: Rhode et al., 1983: Young et al., 1988). Another possible mechanism is an adapting, excitatory feedback circuit. Because of the brevity and regularity of the intervals seen in this study, such a circuit would probably have to contain only one or two synapses. A further possibility is some form of damped oscillatory mechanism. whose frequency is an intrinsic property of the neuron. but whose expression is determined by the duration of the excitatory drive on the cell. This hypothesis might eventually be testable in a brain slice preparation, where the intrinsic electrical properties of a cortical neuron can be partially isolated from the properties of the afferent pathway to it. Functional significance of spike burst responses A further question concerns the functional significance of this interval behavior, particularly since the intervals themselves appear to carry little information about the stimulus. One clue lies in the brief length of the intervals. It is becoming increasingly clear that cortical unit responses are often best driven by transient stimulus events (Phillips. 1988b. 1990). We recently showed that the precision with which the onset latency of cortical neurons times these events is comparable to that described for cochlear nerve fibers (Phillips and Hall, 1990). The present study shows that the burst responses are composed of spikes whose temporal separations are almost as short as biophysical limitations permit. This means that optimal stimuli evoke not only a larger number of spikes from a given neuron, but also that those spikes are sufficiently closely spaced as to provide a salient neural signal against a background of ongoing or spontaneous discharges whose ISIS are likely much longer. It might, therefore, be a mechanism which enhances the neural representation of transient events in the cortex. These observations might help us further understand the contribution of the auditory cortex to speech perception. It is clear that for the vast majority of cortical cells. steady-state temporal responses (time-locking to stimulus periodicities)
are too poor to represent the time waveform of speech sounds on a temporal basis (Steinschneider et al., 1980). The onset latencies of cortical cells are sufficiently regular, however, to be demonstrably capable of indicating the timing of the phonetically important elements in speech sounds, while the spectra1 identity of those sound elements presumably resides in which neurons of the tonotopic array are activated (Steinschneider et al.. 1982: Phillips and Hall, 1990). A neural specialization which produces spike-bursts might generate a salient neural signal of the timing of these events. In this sense, the present data are compatible with the notion that the temporal representation of closely spaced sounds might be an important function of the cortex. Interestingly, there is evidence that patients with auditory cortical lesions have profound deficits in speech discrimination, but that what underlies the deficit is a failure in the discrimination of any sound whose identity lies in acoustic events whose durations or spacings are in the ms to tens-of-ms range (see Phillips and Farmer, 1990, for review). This range happens jointly to be the temporal grain of the phonetically-important speech elements, and encodable by the onset response timing of cortical neurons. That is, the lesions deprive the listener of the cortical representation of the timing of events with this grain, and this deprivation underlies the perceptual failure. It remains to query the extent to which our use of general anesthesia has influenced the spike timing properties of the cells in this sample. Certainly, spontaneous rates are higher in the alert animal, and this can serve to contaminate evoked ISIS with neural noise (Rhode and Kettner, 1987; Phillips and Hall, 1990). Whether genera1 anesthesia additionally alters the pattern of excitability of cortical neurons by affecting membrane properties is an issue that might best be addressed by empirical examination of the ISIS of cortical cells in the awake animal, or in a brain slice study. Acknowlegements
Thanks are due to S.E. Hall and to Dr. S.R. Shaw for helpful discussions of some of the issues raised here. C. Zinck wrote the data acquisition
27
and analysis software. This research was supported by NSERC equipment and operating grants, and by a Dalhousie Research Development award, to DPP. References Brugge, J.F.. Dubrovsky, N.A.. Aitkin, L.M. and Anderson, D.J. (1969) Sensitivity of single neurons in auditory cortex of cat to binaural tonal stimulation; effects of varying interaural time and intensity. J. Neurophysiol. 32. 10051024. Brugge, J.F. and Merzenich, M.M. (1973) Responses of neurons in auditory cortex of the macaque monkey to monaural and binaural stimulation. J. Neurophysiol. 36. 1138% 1158. Goldberg, J.M. and Greenwood. D.D. (1966) Response of neurons of the dorsal and posteroventral cochlear nuclei of the cat to acoustic stimuli of long duration. J. Neurophysiol. 29. 72-93. Kiang, N.Y-S.. Watanabe. T.. Thomas. E.C. and Clark, L.F. (1965) Discharge Patterns of Single Fibers in the Cat’s Auditory Nerve. MIT Research Monograph No. 35. Cambridge, MA, MIT Press. Kitzes, L.M., Gibson. M.M., Rose, J.E. and Hind, J.E. (1978) Initial discharge latency and threshold considerations for some neurons in cochlear nuclear complex of the cat. J. Neurophysiol. 41, 116551182. Kitzes. L.M.. Wrege. KS. and Cassady, J.M. (1980) Patterns of responses of cortical cells to binaural stimulation. J. Comp. Neural. 192. 455-472. Pfeiffer. R.R. and Kiang, N.Y-S. (1965) Spike discharge patterns of spontaneous and continuously stimulated activity in the cochlear nucleus of anesthetized cats. Biophys. J. 5, 301-316. Phillips, D.P. (1988a) Introduction to anatomy and physiology of the central auditory nervous system. In: A.F. Jahn and J.R. Santos-Sacchi (Eds.), Physiology of the Ear. New York, Raven Press. pp. 407-429.
Phillips, D.P. (1988b) Effect of tone-pulse rise time on rate-level functions of cat auditory cortex neurons: Excitatory and inhibitory processes shaping responses to tone onset. J. Neurophysiol. 59. 1524-1539. Phillips, D.P. (1990) Neural representation of sound amplitude in the auditory cortex: Effects of noise masking. Behav. Brain Res. 37, 197-214. Phillips, D.P. and Farmer, M.E. (1990) Acquired word deafness. and the temporal grain of sound representation in the primary auditory cortex. Behav. Brain Res. 40. 85-94. Phillips, D.P. and Hall, S.E. (1990) Response timing constraints on the cortical representation of sound time structure. J. Acoust. Sot. Am. 88, 1403-1411. Phillips, D.P., Hall, SE. and Hollett, J.L. (1989) Repetition rate and signal level effects on neuronal responses to brief tone pulses in cat auditory cortex. J. Acoust. Sot. Am. 85, 1524-1539. Rhode. W.S. and Kettner. R.E. (1987) Physiological study of neurons in the dorsal and posteroventral cochlear nucleus of the unanesthetized cat. J. Neurophysiol. 57. 414-442. Rhode, W.S., Oertel. D. and Smith, P.H. (1983) Physiological response properties of cells labeled intracellularly with horseradish peroxidase in cat ventral cochlear nucleus. J. Comp. Neural. 213, 448-463. Rhode. W.S. and Smith, P.H. (1986) Encoding timing and intensity in the ventral cochlear nucleus of the cat. J. Neurophysiol. 56. 261-286. Rhode. W.S. and Smith, P.H. (1986) Physiological studies on neurons in the dorsal cochlear nucleus of cat. J. Neurophysiol. 56. 287-307. Steinschneider. M., Arezzo, J. and Vaughan, H.G. Jr. (1980) Phase-locked responses to a human speech sound and lowfrequency tones in the monkey. Brain Res. 198, 75-84. Steinschneider. M., Arezzo, J. and Vaughan, H.G.Jr. (1982) Speech evoked activity in the auditory radiations and cortex of the awake monkey. Brain Res. 252. 353-365. Young, E.D.. Robert, J-M. and Shofner. W.P. (1988) Regularity and latency of units in ventral cochlear nucleus: Implications for unit classification and generation of response properties. J. Neurophysiol. 60, l-29.
Hearing Research, 53 (1991) 17-27 0 1991 Elsevier Science Publishers B.V. 03785955/91/$03.50
HEARES
01545
Separate mechanisms control spike numbers and inter-spike intervals in transient responses of cat auditory cortex neurons Dennis P. Phillips lq2 and Sarah A. Sark 1 Departments
of ’ Psychology and ’ Otolmyngologv, (Received
28 August
Dalhousie University, Halifax,
1990; accepted
14 November
Nova Scotia, Canada.
1990)
In the anesthetized cat. some cortical auditory neurons discharge a train of up to 5 spikes in response to the onset of a characteristic frequency tone pulse. This report provides the first description of the inter-spike intervals (ISIS) in these responses. The ISIS were typically close to 2.0 ms in length, and, as indexed by the standard deviation of the interval length. were very regular. Except at threshold levels of stimulation, mean ISIS were relatively insensitive to both tone amplitude and repetition rate. This was true even over ranges of those variables that exerted dramatic effects on spike numbers and first spike latency. These data suggest that the relative timing of discharges within the spike burst is controlled by a mechanism which is separable from that which determines the number of spikes in them. The brevity of the ISIS suggest that they may be a means of enhancing the salience of the transient response against a background of spontaneous discharges.
Auditory
cortex;
Single neuron;
Transient
response;
Spike timing;
Introduction Almost all studies of the stimulus-response relations of auditory cortical neurons have quantified unit responses using two measures: the total number of spike discharges evoked by a fixed number of stimulus presentations, and the mean (and standard deviation) of the first-spike latent periods. The former measure provides an indication of the strength of the excitatory drive to the neuron (Phillips, 1990), while the latter provides an indication of the precision with which the stimulus event is timed (Phillips and Hall, 1990). The two measures have jointly been used to describe, and in some cases explain, the sensitivity of cortical neurons to a variety of parameters of tonal and other stimulation (Brugge et al., 1969; Brugge and Merzenich, 1973; Phillips, 1990; Phillips et al., 1989).
Correspondence to: D.P. Phillips, Department of Psychology, Dalhousie University, Halifax. Nova Scotia, Canada B3H 451. PAX: (902) 494-6585
Inter-spike interval
Particularly in awake animals (Brugge and Merzenich, 1973; Kitzes et al., 1980) but also in anesthetized preparations (Phillips et al., 1989), cortical auditory neurons may respond to transient sounds with a brief train of spike discharges. The extent to which the properties of these secondary spike responses (e.g., their number and inter-spike intervals) are dependent on stimulus variables is almost completely unknown. The point is significant because even if the time structure of the spike train does not reflect that of the stimulus (c.f., phase-locked spikes to stimulus periodicities). the train itself might contain information about the stimulus. This is because the internal structure of the spike train may vary even though the total spike number and first-spike latency may be stable or saturated. In this respect, early studies of cochlear nucleus neurons (Goldberg and Greenwood, 1966; Pfeiffer and Kiang, 1965) revealed that the mean inter-spike intervals of responses to long tones showed orderly changes with variations in stimulus amplitude, and other studies have used inter-spike interval behavior as a basis for neuro-
nal classification (Rhode and Kettner. 1987; Rhode and Smith, 1986a,b; Young et al., 1988). There has been no previous description of the inter-spike intervals of those cortical neurons which discharge bursts of spikes in response to signal onset. In on-going studies of the anesthetized cat’s auditory cortex, especially the primary field, we have acquired detailed data on neurons which respond to tonal or noise signals with bursts of spike discharges. The purposes of this report are to describe the regularity of spike timing within these bursts, and to examine the extent to which stimulus conditions influence the timing of spikes in them. Methods The data come from 20 neurons in 4 adult cats prepared for acute recording sessions under barbiturate anesthesia. Detailed descriptions of the surgical, stimulating and recording procedures used in this laboratory have been described elsewhere (P~llips et al., 1989). Briefly, the cats were anesthetized (sodium pentobarbital, 35 “g/kg, ip) and prepared for acute recording studies. Surgical anesthesia was maintained throughout the recording session by supplemental doses administered intravenously. The right pinna was resected, and a stimulating system, which incorporated a calibrated probe-microphone assembly, was sealed into the transected right auditory meatus. The left auditory meatus was deliberately collapsed. A small hole was drilled in the skull overlying the posterior half of the crown of the left middle ectosylvian gyrus. The dura mater was left intact, and the extracellular recording electrode was advanced through it. Further movement of the electrode was by means of a remotely controlled, programmable stepping motor. Stimuli were 25-ms duration (including 2.5-ms rise-decay time) tone pulses with the carrier frequency set at each neuron’s characteristic frequency (CF), and delivered to the contralateral (right) ear. The CFs of all neurons in this report were between 10.5 and 21.5 kHz. Stimulus levels are expressed in dB sound pressure level (SPL: dB re 20 PPa). Data collected were usually spike count-versus-intensity functions (“intensity profiles’, 50 to 70 dB range in 5 or 10 dB steps) for
tones presented with repetition rates ranging from 1 to 10 Hz (in 1 Hz steps). Responses to each stimulus condition were based on 100 stimulus presentations. Spike waveforms were monitored continuously, and data were collected only from unequivocally isolated single units. Spike response and stimulus event times were digitized at 100 kHz and stored on-line (IBM PC-XT). Most of the units in this report were studied for 1 to 4 h. These stimuli evoked transient responses, discharged briskly in response to tone onset. The responses contained from 1 to 5 spikes, depending on the stimulus conditions. Spontaneous discharge rates were low ( < l/s), so the responses were salient and permitted statistical descriptions which were not contaminated by random (noise) spike activity. With the exception that statistical characterization of any given inter-spike interval (i.e., lst, 2nd, etc.) was restricted to responses containing at least 10 intervals, the following analyses were performed on the responses to each stimulus condition: (1) The total number of spikes evoked by 100 trials; (2) The mean and standard deviation (SD) of the first spike latent period; (3) The number of 1st. 2nd. 3rd, and 4th spikes in the response; (4) The mean (and SD) of the lst, 2nd and 3rd inter-spike intervals; (5) The total number of intervals, including the rare cases of 4th or 5th intervals; (6) The grand mean (and SD) of the intervals. Results Visu~~iz~tio~ of spike timing reguhity Analysis of the inter-spike intervals in the spike burst responses almost always revealed that the means of the intervals were larger than their SDS, and often by a factor of two to three, suggesting that the timing of secondary spikes was indeed quite regular. Nevertheless, the peri-stimulus time histogram (PSTH) of the response to any given stimulus condition only occasionally showed clear evidence of periodicities in spike timing. This was particularly true of responses to low amplitude signals, where some neurons discharge maximally (nonmonotonic units: P~llips (1990)). We reasoned that any response periodicities might be obscured in the PSTH by the jitter (SD) in the timing of the first spike, which at threshold levels
19
can be in excess of 1 to 2 ms (Phillips and Hall, 1990). We therefore constructed new response histograms in which the timing of the 2nd, 3rd and later spikes was measured from the time of the first spike, rather than from tone onset. These ‘normalized’ response histograms (NRHs) showed more clearly the regularity of spike timing. Fig. 1 shows the PSTH and the NRH of the responses of three neurons to stimuli of SPLs and repetition rates which evoked responses of maximal spike count. The bin width of all the histograms is 200 ps. These neurons were selected to illustrate the range of response regularity seen in this study. The neuron shown in Fig. l(A,B) was the most vigorously discharging unit in the sample; that in Fig. l(C,D) was among the least regular responders, and Fig. l(E,F) shows data for a neuron with a more typical spike rate, but high regularity. The panels on the left are conventional PSTHs, and, with the possible exception of that in Fig. lC, they show little indication of regularity in spike timing. The right panels are NRHs of the same responses. These contain 98 to 100 spikes fewer than do the corresponding PSTHs: the missing spikes are the first ones from each of the 100 trials, and from whose time of occurrence the later spike times were measured. Each of the NRHs shows an uneven but regular distribution of spike response times. The second spike of the burst response, which appears as the first peak in the NRH, occurs with a mean ‘latency’ of 1.45 (Fig. 1B) to 2.53 ms (Fig. 1D) after the first spike. The first mean inter-spike interval was never less than 1.4 ms. The next peak of the NRH, which contains almost exclusively 3rd spikes, occurs after a similar interval following the second spike. Interestingly, inspection of individual spike trains sometimes revealed that if a second spike did not occur with an IS1 very close to the mode, then it was delayed until the modal time of the next spike in the train. Later peaks in the NRH often were lower, and reflected both fewer spike numbers, and poorer spike timing. Thus, whether a neuron discharged as many as 4 or 5 spikes per stimulus trial (Fig. lA,B) or as few as two spikes/trial, second and later spikes tended to occur at preferred intervals. The descriptive statistics on these responses are
of interest. These data are provided in the insets of each panel. Note that for the neurons in the top- and bottom-most panels, the SDS of the inter-spike intervals were much smaller than those of the first-spike latencies in the same responses. The temporal precision in the relative timing of the second and third spikes of the trains was thus superior to precision with which the first spike marked the initiation of the responses. In the other neuron, the IS1 SDS were comparable to, or larger than, that of the first spike latency. Nevertheless, for most cells, removal of the jitter provided by the first-spike latent period helped to reveal the regularity in the timing of the subsequent spikes. Table 1 presents summary data on the length of ISIS, as calculated from the raw spike times, for responses of maximal spike count. The population mean 1st 2nd, and 3rd ISIS were typically close to 2 ms, and across the 20 neurons in the sample, the range of ISIS was very small. There were no mean intervals shorter than 1.1 ms, and only very rarely were ISIS in individual spike trains less than 1.0 ms. There were no individual ISIS less than 0.9 ms. Equally of interest is that, in general, coefficients of variation (CV: the ratio of the interval SD to the interval mean) for these responses were close to 0.5. They ranged from as small as 0.16 (indicating high timing precision) to as great as 0.86 (indicating more temporally scattered spike discharges). These population data confirm the impression provided by Fig. 1: the temporal distribution of spikes in the burst response is markedly non-random, and the intervals can be almost as short as neuronal biophysics permit. Stimulus dependence of inter-spike intervals In contrast to the marked effects of tone level (and repetition rate) on spike counts, the ISIS were largely invariant across broad ranges of both of those variables. For the sake of brevity, we shall present data only on the effects of tone level. Fig. 2 presents data for two neurons to illustrate the effect on IS1 of the amplitude parameter. These two neurons were selected for presentation because they represent cases drawn from those with the poorest IS1 regularity (left panels) and those with the greatest regularity (right panels). The neuron whose data are presented in the left panels
20
2c
-I
ID: RR408 CF = 10.55 kHz 20 dB, 1 Hz = 468 spikes Mean = 19.57 ms sd. = 1.01 ms
A
IB
N = 368 spikes 1st mean = 1.45 1 st s.d. = 0.36 ms 2ndmean=lS8ms 2nd sd. = 0.59 ms
L
30
20 10,
-
10
0
15
ID: RR303 CF = 20.82 kHz 62 dB, 1 Hz N = 252 spikes Mean = 13.81 ms s.d. = 1.33 ms
N = 152 spikes lstmean=2.53ms 1st s.d. = 1.91 ms 2nd mean = 2.64 ms 2nd sd. = 1.28 ms 10
5
0
ID: RR207 CF = 17.74 kHz 80 dB, 2 Hz N = 240 spikes Mean = 18.73 ms s.d. = 1.07 ms
10
20
30
N = 142 spikes lstmean=1.81 ms 1st sd. = 0.29 ms 2nd mean = 1.87 ms 2nd s.d. = 0.47 ms
I
10
40
TIM;OAFTER;ONE ONSET (ms)
0
30
TIhk”AFTEF? i&ST SPIKE (ms)
21
had a saturating, monotonic intensity profile. The data in the right panels of Fig. 2 come from a neuron with a nonmonotonic intensity profile. Regularity of spike timing was not related in any obvious way to intensity profile shape. The upper panels (Fig. 2A,D) plot the number of lst, 2nd, 3rd, and 4th spikes as a function of the SPL of a CF tone delivered at l/s. In both neurons, thresholds for the excitation of a second (or later) spike increased with the serial number of the spike in the response. For the monotonic neuron, only the first spike achieved perfect entrainment (i.e., lo@ spikes/100 trials). The later spikes saturated at progressively lower rates. For the nonmonotonic neuron, each of the first to fourth spikes exceeded 90% of the maximum-possible response rate, and they each did so at 20 dB SPL. Note that the neuron expresses its sensitivity to high-amplitude tones in the number of spikes in the burst response, but not in the number of effective trials. Thus, the number of first spikes remains constant at lOO/lOO trials, while the number of subsequent spikes in the burst response falls precipitously (Fig. 2D). We shall return to this point later in this report. The middle panels of Fig. 2 show the effect of tone amplitude on the mean length of the lst, 2nd and 3rd ISIS in the same responses. Except for signal levels very close to threshold, ISIS were relatively constant. For the nonmonotonic cell (Fig. 2E), the ISIS were stable over an SPL range up to 70 dB wide, and over which the number of 3rd and 4th spikes fell from maximum to zero. For the monotonic cell (Fig. 2B), ISIS were longer and more variable, but they had reached minima at tone levels lower than those which saturated spike numbers. Note that there is a further consequence of this ISI stability. Since the ISIS were relatively uninfluenced by tone SPL, they were also uncorrelated with response spike rate, and
were only weakly influenced by first spike latency. Not shown in Fig. 2 are scatter-plots of mean interval against absolute spike rate and first-spike latency that were made for each neuron in the sample, using thedata combined across all effective stimulus conditions. The distribution of data points in both kinds of plots took the form of a nearly horizontal band. The only outlying points were for threshold responses, where the mean ISIS were sometimes slightly lengthened. A broader impression of the relative insensitivity of the ISIS to stimulating condition is provided in the bottom panels of Fig. 2. Here, we have plotted the SD of each IS1 as a function of its mean, and have combined data across all effective SPL-by-repetition-rate stimulus conjunctions. There were 29 such stimulus conditions, covering a 40 dB range of SPLs, for the monotonic cell (Fig. 2C), and there were 68 stimulus conditions spanning a 75 dB range for the nonmonotonic one (Fig. 2F). Two features of these data are apparent. One is that the interval SD was proportional to the interval mean. For 17 cells for which there were sufficient data, we estimated the linear regression between the SD and the mean of the total interval data. Many of these lines had slopes close to 1.0, but the range extended from 0.75 to 3.14 (mean = 1.36). A second feature of the data in the scattergrams is that the absolute variation in mean IS1 (and for each ISI, i.e., 1st. 2nd, 3rd) was quite small. Thus, for the monotonic neuron (Fig. 2C), the majority of ISIS were between 2.0 and 4.5 ms in length, and for the nonmonotonic cell (Fig. 2F). most of the ISIS were between 1.3 and 2.3 ms in length.
Observations relevant to the genesis of nonmonotonic spike mount-v~-intensity profiles We mentioned above that, for nonmonotonic neurons, the number of spikes in the spike bursts
Fig. 1. &$ punefs: Conventional PSTHs of the responses of 3 neurons to 100 presentations of a CF tone pulse evoking maximal spike counts. Bin width is 200 ns. Text within the panels indicates unit ID number, unit CF, stimulus SPL and repetition rate, number of spikes in the histogram (N), mean latency to first spike, and standard deviation (SD) of first-spike latency. Right panels: NRHs for the same responses. Each histogram shows the event times of 2nd, 3rd and later spikes, as measured from the first-spike time in each trial. Bin width is 200 ps. The apparent presence of second spikes occurring within 0.8 ms of the first spike (panel D) is an artifact of the histogram binning. Text within each panel indicates the number of spikes in the histogram (N), mean and standard deviation of the 1st and 2nd inter-spike intervals. Description in text.
“
120
ID: RR303. CF = 20.82
.A
120I-
kHz
ID: RR408, CF = 10.55 kHz
D 100
-
-
-
-
-
c
10
20 30 40 50 60 TONE LEVEL (dB SPL)
70
0
80
20
80
100
40 60 80 TONE LEVEL (dB SPL)
100
TONE4:EVEL (ii 8
I-B
SPL)
E
7 6
0
10
20 30 40 50 60 TONE LEVEL (dB SPL)
70
80
0
20
F
x
.
x
1Sl
l
2nd 3rd
0
0
4 2 6 MEAN INTERVAL (ms)
8
Fig. 2. Data for neurons RR303 (left) and RR408 (right). A and D show the to tone level. B and E show the effect of tone level on the mean length of covariation of interval SD with interval mean for a wide array of stimulus plotted separately for 1st. 2nd and 3rd
0
2 4 MEAN INTERVAL (ms)
6
sensitivity of the numbers of 1st. 2nd. 3rd. and 4th spikes the 1st. 2nd and 3rd inter-spike intervals, C and F show conditions varying in SPL and repetition rate. Data are inter-spike intervals.
23
is reduced in responses to tone levels associated with the descending limb of the intensity profile, and that this is expressed first in a loss of fourth, then third, then second spikes (Fig. 2D). This could come about through the action of at least two mechanisms. First, it is possible that it is the shortest-latency (i.e., first) spikes which are lost, so that what would otherwise have been the second spike now becomes the first (etc.), with the result that there are fewer spikes in the train. Second, it might genuinely be the case that the late spikes in the train are those which are lost first by the inhibitory processes that shape the descending limb of the spike count function. Note that in plots of the kind in Fig. 2D, both hypotheses would express themselves as an apparent differential sensitivity of the late spikes to tone level. These two hypotheses can be differentiated by examination of the first-spike latent periods for responses to different suprathreshold tone levels. Especially for tones with rise-times as brief as those used here (2.5 ms). the first-spike latency function has a very shallow negative slope at suprathreshold stimulus amplitudes (e.g., Phillips, 1988b). The former hypothesis would predict that latent periods for responses to higher amplitude tones within this range might become longer than those for lower amplitude tones. This is because any modest first-spike latency shortening due to the latency-intensity relation could be offset by the loss of the first spike and the attendant addition of the first IS1 to the period between tone onset and response onset. In contrast, the second hypothesis would predict a normal first-spike latency function, because first-spike latency may be independent of whatever processes determine the number of later spikes in the train. Our data support the second hypothesis, and more detailed evidence on the responses depicted in Fig. 2D is presented in Fig. 3 to show why. Fig. 3 shows the NRHs for the responses of one nonmonotonic neuron to eight tone levels over a 65 dB range. The text within each panel indicates the stimulus level, the number of spikes in the histogram, the number of first spikes (which occur in bin zero, and are not shown), and the mean first spike latency. The number of 2nd (and later) spikes increases from 116 at 15 dB to 368 at 20 dB, and then declines to 68 spikes at 70 dB.
Across this range, the number of first spikes, which indicates the number of effective stimulus trials, is almost constant at 100. What is of most interest here are the responses at grossly suprathreshold levels, where the latency function has its shallowest slope, but where spike counts are still declining. For this neuron, that range is from 50 dB (Fig. 3E) to 70 dB (Fig. 3G). Over this range of signal levels, the number of spikes dropped by 116, yet the first-spike latency shortened by 0.64 ms. Since the first IS1 in this neuron was close to 1.5 ms, then loss of the first spikes would have added about 1.5 ms to the mean latent period. This did not occur. It might be argued that, in this neuron, the ‘uninhibited’ latency function could have had a slope in excess of 1.5 ms/lO dB, so that loss of the first spikes would have resulted in the first-spike latencies seen here. Note, however, that increments in stimulus level beyond 70 dB (to 80 dB: Fig. 3H, and 90 dB: not shown) shortened the first-spike latency to 12.87 and 12.59 ms respectively, but with no further reduction in spike count. The slope of the latency function between 70 and 90 dB (25 ps/dB) is thus quite close to that between 50 and 70 dB (32 ps/dB). The lack of any discontinuity in the latency function between SPL ranges that reduced spike rate, and those that did not, confirms that it is the last spikes in the bursts which are suppressed by inhibition to produce the descending limb of the nonmonotonic intensity function. Trends of this kind were common to all of the nonmonotonic cells in our sample (N = 8). although different spike timing behavior has been seen in some neurons described by Brugge et al (1969). The present observations may provide one reason for why many nonmonotonic neurons have normal first-spike latency-intensity functions over SPL ranges where spike rates are declining: Each stimulus event may evoke both an excitatory onset response and a longer-latency inhibitory one. At low tone levels, the inhibitory response may have too long a latent period to suppress any spikes. The inhibitory response latency may shorten more quickly with increasing SPL than does the excitatory one, so that the later spikes driven by the short-latency excitatory input are the first to drop out.
A
B
60.
15dB
60-
20 dB
N=ll6spikes (+
40 -
N = 368 spikes
91 first spikes in bin 0) Mean = 28.44 ms
40 -
20 -
I 0
dA
5
I
15
10
100 firs! spikes in bin 0) Mean = t 9.57 ms
(+
20_~ 0
10
5
15
0 60 -
60
(+
100 first spikes in bin 0) Mean = 15.60 ms
0
40 -
5OdB N = 184 spikes (+ 100 first spikes in bin 0) Mean = 13.73 ms
5
10
15
r
E 60 ”
40 dB N = 248 spikes (+ 100 first spikes in bin 0) Mean = 14.27 ms
60 -
40 *
F 60 de N = 123 spikes (+ 100 first spikes in bin 0’ Mean
= t3.35
ms
20 -
0
10
5
15
H -
60
80 dB N = 70 spikes
100 first spikes in bin 0)
(+
40 -
(+ 100
first spikes in bin 0) Mean = 12.87 ms
Mean = 13.09 ms
0
5
10
15
0
5
10
15
TIME AFTER FIRST SPIKE (ms) Fig. 3. Detailed data on neuron RR408. Each panel shows the NRH for responses to 100 trials of a tone pulse whose amplitude is specified in each panel. Other text within the panels indicates the number of spikes in the histogram, the number of first spikes, and the mean latent period of the first spike.
25
Mechanisms shaping spike burst responses The neurons in this sample discharged bursts of spikes locked in time to tone onset. The intervals between these spikes were often highly regular (Fig. l), and commonly had lengths between 1.5 and 3.5 ms (Table I). The neurons in the sample all had CFs in excess of 10 kHz, indicating that the periodicities in the response were not tied to the fine structure of the tones (cf. Kitzes et al., 1978), i.e., they were not phase-locked responses to the first few cycles of the sinusoidal signals. It is also unlikely that the spike regularity is a cortical expression of basilar membrane ‘ringing’ (i.e., decaying oscillations driven by tone onset). This is for two reasons. One is that although the tones were brief, they had rise-times of 2.5 ms, which are unlikely to generate significant such oscillations, let alone at the stimulus levels used here (e.g., Fig. 1B). The second is that at the level of cochlear output, periodicity in neural responses evoked by mechanical ringing is most marked in low-CF cells. where the frequency of the oscillating neural response matches the cell’s CF (Kiang et al., 1965).
TABLE
I
DESCRIPTIVE STATISTICS ON THE LENGTH OF MEAN ISIS SEEN IN RESPONSES OF 20 NEURONS TO STIMULI EVOKING MAXIMAL SPIKE COUNTS Population
measure
Mean of interval means (ms) Range of interval means (ms) Mean of interval s.d.s (ms) Range of interval s.d.s (ms) Mean of interval cvs Range of interval cvs Number of cells
First IS1
Second
IS1
Third IS1
2.34
2.26
2.02
1.45 to 3.68
1.58 to 3.20
1.10 to 2.65
1.14
1.09
1.12
0.29 to 2.33
0.47 to 1.82
0.67 to 1.80
0.46
0.47
0.57
0.16 to 0.79 20
0.25 to 0.73 14
0.44 to 0.86 9
Note that these stimulus conditions were not always optimal for spike time regularity. since 40% of the data come from nonmonotonic cells whose maximal discharge rates were achieved using low amplitude tones.
In contrast to data obtained from responses to long tones in the cochlear nuclear complex (Goldberg and Greenwood, 1966; Pfeiffer and Kiang, 1965), the inter-spike intervals in the transient response of cortical cells showed little dependence on stimulus variables, except in threshold responses, where the intervals lengthened slightly (Fig. 2). The very fact that the cortical responses are transient necessarily restricts the range of ISIS to be expected, but it need not impose either the regularity or the stimulus independence seen here. The significance of this relative insensitivity is that it was expressed over SPL ranges which could have profound effects on the number of spikes, especially in nonmonotonic cells. Accordingly, quite separate mechanisms must have controlled the number of spikes in the burst, and the relative spacing of the spikes in the responses. In monotonic cells, ISIS typically reached minimal lengths quite close to threshold. The further implication of this is that for both cell types, information about the SPL of the tones resided largely in the number of spikes, and not in their relative timing. The slopes of the SD/mean regression lines for ISI length (or first-spike latency) describe the rate at which the precision of spike timing becomes poorer with increases in interval length (or latent period). The slopes of interval SD/mean regression lines in the present data were close to, or greater than 1.0, which is in contrast to those for the first-spike latency (which are typically less than 0.8: Phillips and Hall, 1990). This suggests that different processes shape the timing of the first spike and the relative timing of the later ones. In this respect, the timing of the first spike is a sensitive function of many stimulus variables (Fig. 3; Brugge et al., 1969; Phillips, 1990; Phillips et al., 1989; Phillips and Hall, 1990) while the present study suggests that the relative timing of the later spikes is not. As such, stimulus conditions set both the time when the neuron’s response is initiated, and the number of spikes in the discharge, but the internal time structure of the spike train is controlled by other factors. The processes that determine the inter-spike intervals of cortical auditory neurons are not known. The afferent pathway to cortical neurons is extraordinarily convergent (Phillips, 1988a), which raises the possibility that the ISIS might be
26
shaped by factors similar in principle to those which produce comparably brief and regular ISIS in some chopper units of the cochlear nucleus (Rhode and Kettner, 1987; Rhode and Smith. 1986a,b: Rhode et al., 1983: Young et al., 1988). Another possible mechanism is an adapting, excitatory feedback circuit. Because of the brevity and regularity of the intervals seen in this study, such a circuit would probably have to contain only one or two synapses. A further possibility is some form of damped oscillatory mechanism. whose frequency is an intrinsic property of the neuron. but whose expression is determined by the duration of the excitatory drive on the cell. This hypothesis might eventually be testable in a brain slice preparation, where the intrinsic electrical properties of a cortical neuron can be partially isolated from the properties of the afferent pathway to it. Functional significance of spike burst responses A further question concerns the functional significance of this interval behavior, particularly since the intervals themselves appear to carry little information about the stimulus. One clue lies in the brief length of the intervals. It is becoming increasingly clear that cortical unit responses are often best driven by transient stimulus events (Phillips. 1988b. 1990). We recently showed that the precision with which the onset latency of cortical neurons times these events is comparable to that described for cochlear nerve fibers (Phillips and Hall, 1990). The present study shows that the burst responses are composed of spikes whose temporal separations are almost as short as biophysical limitations permit. This means that optimal stimuli evoke not only a larger number of spikes from a given neuron, but also that those spikes are sufficiently closely spaced as to provide a salient neural signal against a background of ongoing or spontaneous discharges whose ISIS are likely much longer. It might, therefore, be a mechanism which enhances the neural representation of transient events in the cortex. These observations might help us further understand the contribution of the auditory cortex to speech perception. It is clear that for the vast majority of cortical cells. steady-state temporal responses (time-locking to stimulus periodicities)
are too poor to represent the time waveform of speech sounds on a temporal basis (Steinschneider et al., 1980). The onset latencies of cortical cells are sufficiently regular, however, to be demonstrably capable of indicating the timing of the phonetically important elements in speech sounds, while the spectra1 identity of those sound elements presumably resides in which neurons of the tonotopic array are activated (Steinschneider et al.. 1982: Phillips and Hall, 1990). A neural specialization which produces spike-bursts might generate a salient neural signal of the timing of these events. In this sense, the present data are compatible with the notion that the temporal representation of closely spaced sounds might be an important function of the cortex. Interestingly, there is evidence that patients with auditory cortical lesions have profound deficits in speech discrimination, but that what underlies the deficit is a failure in the discrimination of any sound whose identity lies in acoustic events whose durations or spacings are in the ms to tens-of-ms range (see Phillips and Farmer, 1990, for review). This range happens jointly to be the temporal grain of the phonetically-important speech elements, and encodable by the onset response timing of cortical neurons. That is, the lesions deprive the listener of the cortical representation of the timing of events with this grain, and this deprivation underlies the perceptual failure. It remains to query the extent to which our use of general anesthesia has influenced the spike timing properties of the cells in this sample. Certainly, spontaneous rates are higher in the alert animal, and this can serve to contaminate evoked ISIS with neural noise (Rhode and Kettner, 1987; Phillips and Hall, 1990). Whether genera1 anesthesia additionally alters the pattern of excitability of cortical neurons by affecting membrane properties is an issue that might best be addressed by empirical examination of the ISIS of cortical cells in the awake animal, or in a brain slice study. Acknowlegements
Thanks are due to S.E. Hall and to Dr. S.R. Shaw for helpful discussions of some of the issues raised here. C. Zinck wrote the data acquisition
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and analysis software. This research was supported by NSERC equipment and operating grants, and by a Dalhousie Research Development award, to DPP. References Brugge, J.F.. Dubrovsky, N.A.. Aitkin, L.M. and Anderson, D.J. (1969) Sensitivity of single neurons in auditory cortex of cat to binaural tonal stimulation; effects of varying interaural time and intensity. J. Neurophysiol. 32. 10051024. Brugge, J.F. and Merzenich, M.M. (1973) Responses of neurons in auditory cortex of the macaque monkey to monaural and binaural stimulation. J. Neurophysiol. 36. 1138% 1158. Goldberg, J.M. and Greenwood. D.D. (1966) Response of neurons of the dorsal and posteroventral cochlear nuclei of the cat to acoustic stimuli of long duration. J. Neurophysiol. 29. 72-93. Kiang, N.Y-S.. Watanabe. T.. Thomas. E.C. and Clark, L.F. (1965) Discharge Patterns of Single Fibers in the Cat’s Auditory Nerve. MIT Research Monograph No. 35. Cambridge, MA, MIT Press. Kitzes, L.M., Gibson. M.M., Rose, J.E. and Hind, J.E. (1978) Initial discharge latency and threshold considerations for some neurons in cochlear nuclear complex of the cat. J. Neurophysiol. 41, 116551182. Kitzes. L.M.. Wrege. KS. and Cassady, J.M. (1980) Patterns of responses of cortical cells to binaural stimulation. J. Comp. Neural. 192. 455-472. Pfeiffer. R.R. and Kiang, N.Y-S. (1965) Spike discharge patterns of spontaneous and continuously stimulated activity in the cochlear nucleus of anesthetized cats. Biophys. J. 5, 301-316. Phillips, D.P. (1988a) Introduction to anatomy and physiology of the central auditory nervous system. In: A.F. Jahn and J.R. Santos-Sacchi (Eds.), Physiology of the Ear. New York, Raven Press. pp. 407-429.
Phillips, D.P. (1988b) Effect of tone-pulse rise time on rate-level functions of cat auditory cortex neurons: Excitatory and inhibitory processes shaping responses to tone onset. J. Neurophysiol. 59. 1524-1539. Phillips, D.P. (1990) Neural representation of sound amplitude in the auditory cortex: Effects of noise masking. Behav. Brain Res. 37, 197-214. Phillips, D.P. and Farmer, M.E. (1990) Acquired word deafness. and the temporal grain of sound representation in the primary auditory cortex. Behav. Brain Res. 40. 85-94. Phillips, D.P. and Hall, S.E. (1990) Response timing constraints on the cortical representation of sound time structure. J. Acoust. Sot. Am. 88, 1403-1411. Phillips, D.P., Hall, SE. and Hollett, J.L. (1989) Repetition rate and signal level effects on neuronal responses to brief tone pulses in cat auditory cortex. J. Acoust. Sot. Am. 85, 1524-1539. Rhode. W.S. and Kettner. R.E. (1987) Physiological study of neurons in the dorsal and posteroventral cochlear nucleus of the unanesthetized cat. J. Neurophysiol. 57. 414-442. Rhode, W.S., Oertel. D. and Smith, P.H. (1983) Physiological response properties of cells labeled intracellularly with horseradish peroxidase in cat ventral cochlear nucleus. J. Comp. Neural. 213, 448-463. Rhode. W.S. and Smith, P.H. (1986) Encoding timing and intensity in the ventral cochlear nucleus of the cat. J. Neurophysiol. 56. 261-286. Rhode. W.S. and Smith, P.H. (1986) Physiological studies on neurons in the dorsal cochlear nucleus of cat. J. Neurophysiol. 56. 287-307. Steinschneider. M., Arezzo, J. and Vaughan, H.G. Jr. (1980) Phase-locked responses to a human speech sound and lowfrequency tones in the monkey. Brain Res. 198, 75-84. Steinschneider. M., Arezzo, J. and Vaughan, H.G.Jr. (1982) Speech evoked activity in the auditory radiations and cortex of the awake monkey. Brain Res. 252. 353-365. Young, E.D.. Robert, J-M. and Shofner. W.P. (1988) Regularity and latency of units in ventral cochlear nucleus: Implications for unit classification and generation of response properties. J. Neurophysiol. 60, l-29.
