J Neurophysiol
88: 2322–2328, 2002; 10.1152/jn.00119.2002.
Encoding of Sound Localization Cues by an Identified Auditory
Interneuron: Effects of Stimulus Temporal Pattern
ANNIE-HÉLÈNE SAMSON AND GERALD S. POLLACK
Department of Biology, McGill University, Montreal, Quebec H3A 1B1, Canada
Received 19 February 2002; accepted in final form 1 August 2002
Sound localization in the horizontal plane is based on comparing sounds at the two ears. Sound arrives earlier at the ear
closer to the source, and in general stimulus intensity is higher
at that ear. Some animals can detect the small (tens of microseconds) binaural difference in arrival time (for review, see
Carr 1993); others, including insects, rely mainly on binaural
intensity difference as the cue for sound location (for review,
see Pollack 1998).
Two features of the neural response to acoustic stimuli vary
with stimulus intensity: spike count and/or rate increases with
increasing intensity and response latency decreases (Imaizumi
and Pollack 2001; Kiang et al. 1965). The binaural difference
in both of these measures varies systematically with sound
location, and either or both might code for sound direction
(Boyan 1979; Eggermont 1998; Mörchen 1980).
Crickets localize sound both to find mates and to evade
predators. Stridulating male crickets produce songs with relatively low carrier frequency (dominant frequency for Teleogryllus oceanicus: 4.5 kHz), consisting of repeated trains of
sound pulses at rates varying (for T. oceancius) between 8 and
32 pulses/s (Balakrishnan and Pollack 1996). Echolocating
bats produce ultrasound pulses (20 to ⬎100 kHz), repeated at
rates ranging from a few pulses per second to ⬎100 pulses/s
(Griffin et al. 1960). Behavioral experiments have demonstrated that crickets perform positive phonotaxis (movement
toward the sound source) in response to cricket-like stimuli and
negative phonotaxis (movement away from the sound source)
when presented with bat-like sounds (Moiseff et al. 1978;
Nolen and Hoy 1986; Pollack et al. 1984). The importance of
the frequency ranges of cricket song and bat sounds is reflected
by the organization of the cricket’s ear; nearly three-quarters of
the approximately 70 receptor neurons in each ear are tuned to
cricket-like frequencies, and more than one-half of the remainder are most sensitive to ultrasonic frequencies (Imaizumi and
Pollack 1999).
When sensory neurons are stimulated with a long series of
rapidly repeated stimuli, their response strength (spike count
and/or rate) decreases for successive stimuli, and response
latency increases (Coro et al. 1998; Givois and Pollack 2000;
Pasztor 1983). In cricket auditory receptors, as in mammals
(Eggermont and Spoor 1973), the decline in response strength
is greater the higher the sound level (Givois and Pollack 2000).
This implies that neurons ipsilateral to the sound source, where
stimulus intensity is higher, will experience a larger response
decrement. This may even lead to a reversal in the sign of the
binaural difference in response strength (contralateral response⬎ ipsilateral; Givois and Pollack 2000). The change in
latency of receptor-neuron responses induced by repeated stimulation does not depend on sound level; thus directional information encoded by binaural latency difference is not affected.
Phonotaxis responses are not driven directly by receptor
Address for reprint requests: G. S. Pollack, Dept. of Biology, McGill
University, 1205 Ave. Doctor Penfield, Montreal, Quebec H3A 1B1, Canada
(E-mail: [email protected]).
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.
INTRODUCTION
2322
0022-3077/02 $5.00 Copyright © 2002 The American Physiological Society
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Samson, Annie-Hélène and Gerald S. Pollack. Encoding of sound
localization cues by an identified auditory interneuron: effects of
stimulus temporal pattern. J Neurophysiol 88: 2322–2328, 2002;
10.1152/jn.00119.2002. An important cue for sound localization is
binaural comparison of stimulus intensity. Two features of neuronal
responses, response strength, i.e., spike count and/or rate, and response latency, vary with stimulus intensity, and binaural comparison
of either or both might underlie localization. Previous studies at the
receptor-neuron level showed that these response features are affected
by the stimulus temporal pattern. When sounds are repeated rapidly,
as occurs in many natural sounds, response strength decreases and
latency increases, resulting in altered coding of localization cues. In
this study we analyze binaural cues for sound localization at the level
of an identified pair of interneurons (the left and right AN2) in the
cricket auditory system, with emphasis on the effects of stimulus
temporal pattern on binaural response differences. AN2 spike count
decreases with rapidly repeated stimulation and latency increases.
Both effects depend on stimulus intensity. Because of the difference
in intensity at the two ears, binaural differences in spike count and
latency change as stimulation continues. The binaural difference in
spike count decreases, whereas the difference in latency increases.
The proportional changes in response strength and in latency are
greater at the interneuron level than at the receptor level, suggesting
that factors in addition to decrement of receptor responses are involved. Intracellular recordings reveal that a slowly building, longlasting hyperpolarization is established in AN2. At the same time, the
level of depolarization reached during the excitatory postsynaptic
potential (EPSP) resulting from each sound stimulus decreases. Neither these effects on membrane potential nor the changes in spiking
response are accounted for by contralateral inhibition. Based on
comparison of our results with earlier behavioral experiments, it is
unlikely that crickets use the binaural difference in latency of AN2
responses as the main cue for determining sound direction, leaving the
difference in response strength, i.e., spike count and/or rate, as the
most likely candidate.
TEMPORAL PATTERN AND SOUND LOCALIZATION CUES
2323
neurons, but rather by interneurons that receive input from
receptors. Negative phonotaxis in response to ultrasound stimuli is initiated by the identified, bilaterally paired interneuron
AN2 (Nolen and Hoy 1984). The effects of the changes in
receptor responses described above might be compensated or
exaggerated during signal transmission from receptors to interneurons, but as yet this issue has not been studied. In this
study, we investigate the effects of rapidly repeated stimulation
on encoding of sound localization cues by AN2.
METHODS
Animals
Electrophysiology
For extracellular recordings of AN2, the cervical connectives were
exposed and placed on a pair of silver-wire hook electrodes. AN2
produces the largest sound-evoked spikes in the cervical connectives,
and these are easily recognized by visual inspection (Moiseff and Hoy
1983, see inset of Fig. 1A). To record compound action potentials of
auditory receptor neurons, we inserted a Teflon-coated silver wire
(114 ␮m OD) into the anterio-dorsal surface of the femur, close to the
nerve branch carrying the axons of the receptor neurons from their
origin in the prothoracic tibia to the prothoracic ganglion. The indifferent electrode was placed proximally and ventrally in the femur (see
Pollack and Faulkes 1998 for further details). For intracellular recordings of AN2, the prothoracic ganglion was exposed and supported on
a metal platform. The ganglion was submerged in physiological saline
(Strausfeld et al. 1983) and recordings were made using microelectrodes (⬎30 M⍀) filled with 2 M K acetate. Electrophysiological
recordings were digitized (Digidata 1320A, 10 kHz sampling rate,
Axon Instruments) using the program AxoScope 8.0 (Axon Instruments). Responses were viewed and analyzed using custom-written
programs (GP) for Scilab (www-rocq.inria.fr/scilab/), and statistical
analysis were performed using Statistica for Windows (StatSoft,
Tulsa, OK).
FIG. 1. Effect of repeated stimulation on spike count (A) and first-spike
latency (B) of AN2 responses. Sound pulses were presented at 16 and 0.3 pps
(for clarity, responses to 0.3 pps are shown only for 90 dB). Each point is the
response to 1 sound pulse; data are from a single, representative experiment. In
A, for time points ⬎1 s, only every 5th data point is plotted for 16 pps. Solid
lines (for 16 pps,70 dB in A; 70 dB in B) are curves fit to an equation of the
form: r(t) ⫽ rss ⫹ ae⫺t/ss ⫹ be⫺t/ll, where r(t) is response at time t, rss is the
steady-state response, ss and ll are short and long time constants, and a and b
are constants. Inset: extracellular recording, from a different experiment,
showing AN2 spikes (top; first 8 responses; 90 dB, 16 pps) and stimulus
monitor (bottom).
the series. Stimuli were produced, using LabWindows programs (National Instruments), by a D/A circuit (ATMIO16F5, National Instruments, Austin; D/A update rate 200 kHz) and were attenuated (PA4,
Tucker-Davis, Gainesville, FL), amplified (D150A, Amcron, Elkhart,
IN), and broadcast through loudspeakers (RadioShack, 40 –1310B)
situated on the cricket’s left and right in the horizontal plane, perpendicular to the longitudinal axis, at a distance of 50 cm. The loudspeakers and cricket were housed in a chamber lined with echosuppressing mineral-wool wedges. Sound pressure level in dB re 2 ⫻
10⫺5 Nm⫺2 (SPL) was measured at the cricket’s position with a
Brüjel and Kjaer (Naerum, Denmark) 4135 microphone and 2610
measuring amplifier.
Stimuli
Data analysis
Stimuli consisted of trains of trapezoid-shaped sound pulses of
30-ms duration, including 5-ms rise and fall times. Carrier frequency
was 30 kHz, which is within the range of AN2’s greatest sensitivity
(Moiseff and Hoy 1983). Pulse trains were 30 – 45 s long and were
preceded by 60 s of silence. Pulses were presented at rates of 0.3, 8,
16, and 26 pulses/s (pps). The lowest pulse rate was presented at the
beginning, middle, and end of each experiment, and responses were
statistically indistinguishable in all cases. This ensured both that
responsiveness remained stable, and that the 60-s pause following
each stimulus was sufficient for return to baseline responsiveness. The
order of presentation of stimuli varied from experiment to experiment,
with the two highest pulse rates most often presented in the middle of
The response to each sound pulse was analyzed over a time window
that included the entire response as determined by visual inspection.
Time windows for AN2 responses ranged from 38 ms (26 pps) to 50
ms (0.3 pps) in duration and were constant for each pulse rate across
experiments; for receptor responses, windows ranged from 24 –26 ms,
and were constant for all measurements in an individual cricket.
Measuring AN2 spike count and latency was straightforward, because
each individual spike could clearly be detected (e.g., Fig. 1A, inset).
For receptor neurons, response strength was quantified by integrating
the recording over the response time window, following high-pass
filtering (100 Hz) and full-wave rectification. Receptor response latency was taken as the time, relative to stimulus onset, of the first
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Teleogryllus oceanicus were raised in the laboratory on a diet of
Purina Cat Chow and water. Unmated females, ages 12–20 days after
the final molt, were used in experiments. Crickets were anesthetized
by chilling on ice and were mounted on a wax support, ventral side up,
after removing their wings and mid- and hind legs. In most experiments, the femur of the fore legs was fixed with a bees’ waxcolophonium mixture, parallel to the body axis, and the tibia and
tarsus were held flexed against the femur, simulating their position
during flight. When recordings were made from receptor neurons, the
legs were rotated around the coxa-trochanter-femur joints so as to
project perpendicularly from the body axis, exposing the anterior
surface of the femur for electrode implantation (see Electrophysiology).
2324
A.-H. SAMSON AND G. S. POLLACK
negative peak of the compound action potential revealed by signal
averaging (see inset, Fig. 4B); this peak represents the nearly synchronous firing of many receptors neurons. See Givois and Pollack
(2000) and Pollack and Faulkes (1998) for further details.
RESULTS
Effects of repeated stimulation on spike count
and first-spike latency
Effects of intensity and pulse rate on changes in spike count
and latency
Figure 2A summarizes the decremented response as the
mean of responses to sound pulses delivered between 8 and
10 s after stimulus onset. As expected, the number of AN2
spikes/sound pulse increases with intensity. However, because
of the pulse-rate– dependent response decrement, the increase
is less pronounced the higher the pulse rate.
The increase in latency with repeated stimulation (Fig. 1B)
also depends on both pulse rate and intensity. The higher the
pulse rate, and the lower the intensity, the greater the increase
in latency (Fig. 2B).
Therefore the effects of repeated stimulation on spike count
and on first-spike latency vary with both intensity and pulse
rate. The effect of pulse rate is similar for both response
parameters: higher pulse rates produce larger response
changes. The effect of intensity, however, differs: at higher
intensities, the decrease in spike count is more pronounced, but
the change in latency is less pronounced.
Effects of repeated stimulation on localization cues
The stimulus intensity of a 30-kHz sound may be ⱕ15–20
dB higher at the ipsilateral than at the contralateral ear (WytJ Neurophysiol • VOL
FIG. 2. Effect of intensity and pulse rate on changes in AN2 responses.
Mean spike count and latency were calculated for responses to stimuli delivered between 8 and 10 s after stimulus onset, by which time the changes in
AN2’s response are well established. Points are mean ⫾ SE for 7 crickets.
Two-way ANOVA, intensity effect on spike-count: F(4,120) ⫽ 61.15, P ⬍
0.0001; pulse-rate effect on spike-count: F(3,120) ⫽ 276.78, P ⬍ 0.0001;
interaction: F(12,120) ⫽ 8.75, P ⬍ 0.0001; intensity effect on latency:
F(4,120) ⫽ 72.76, P ⬍ 0.0001; pulse-rate effect on latency: F(3,120) ⫽
116.98, P ⬍ 0.0001; interaction: F(12,120) ⫽ 5.73, P ⬍ 0.0001.
tenbach and Hoy 1999; personal observations). This, together
with the intensity dependence of the response changes described above, implies that the changes in response resulting
from repeated stimulation may alter binaural differences. Figure 3 shows binaural differences measured from simultaneous
recordings of the left and right AN2. Rapidly repeated stimulation results in a decrease in the binaural difference in spike
count, but an increase in the binaural latency difference. The
change in spike-count difference is monotonic with pulse rate,
whereas the latency difference reaches a maximum at 16 pps.
Response decrement is greater in interneuron AN2
than in receptor neurons
It seems probable that, as for low-frequency receptors
(Givois and Pollack 2000), responses of ultrasound receptors
decline with repeated stimulation (Coro et al. 1998). If so, then
the effects we describe for AN2 might be accounted for simply
by response decrement of receptor neurons. To determine
whether this is the case, we compared the decrement of compound action potentials recorded from the whole auditory
nerve with the decrement of AN2 responses. Ultrasound receptors comprise only a small proportion of the entire receptor
population (Imaizumi and Pollack 1999), and as a result, clear
whole-nerve responses, which reflect synchronous activity of
many receptors (Pollack and Faulkes 1998), can be recorded
reliably only for high stimulus levels late in the pulse train, by
which time responses have decremented. For this reason, we
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Responses of AN2 to stimuli presented at low repetition rate
are stable, both in spike count and in latency (Fig. 1, 0.3 pps).
With more rapid stimulation, responses change systematically
through time. An initial very rapid decrement in spike count is
followed by a slower asymptotic decline that last for several
seconds (Fig. 1A). Short and long time constants, derived from
double-exponential curve fits (see Fig. 1 legend), were 303 ⫾
266 ms and 11.71 ⫾ 8.18 s (mean ⫾ SD; pooled data for 70
and 90 dB, and 8, 16, and 26 pps, averaged from 7 crickets).
Neither time constant varied with either sound level or pulse
rate (ANOVA, P values range from 0.27 to 0.36).
First-spike latencies lengthen with repeated stimulation (Fig.
1B) and also show rapid and slower phases of increase (time
constants: 1.39 ⫾ 0.64 ms; 47.87 ⫾ 15.72 s). Note that the
variability of latency decreases significantly with intensity
(mean variance for 7 crickets, of responses between 8 and 10 s
after stimulus onset is 4.37 ms, at 90 dB; 17.32 ms at 70 dB;
F6 ⫽ 6.9, P ⫽ 0.01; we focus on the interval 8 –10 s after
stimulus onset to facilitate comparison of our results with
earlier behavioral experiments; Pollack and El-Feghaly 1993;
see DISCUSSION). In addition, variability of latency increases as
the train of stimuli continues with larger increases at lower
intensities and higher pulse rates (ANOVA on mean variance
of responses between 8 –10 s after stimulus onset: pulse-rate
effect, F6 ⫽ 23.9, P ⬍ 0.001, intensity effect, F6 ⫽ 13.2, P ⬍
0.001). Variability of spike count is similar for the range of
pulse rates and intensities examined (ANOVA; pulse-rate effect, P ⫽ 0.4; intensity effect, P ⫽ 0.5).
TEMPORAL PATTERN AND SOUND LOCALIZATION CUES
2325
In parallel with the build-up of hyperpolarization in AN2,
the level of depolarization reached during successive excitatory
postsynaptic potentials (EPSPs) decreases [Fig. 5, B (thick
arrow) and C). Like the decrement in spike count, the membrane potential at the peak of the EPSP, during the period 8 –10
s after stimulus onset (thick arrow in Fig. 5B), varies with pulse
rate and intensity (Fig. 5E). The decrease in EPSP amplitude
likely reflects a combination of factors, including decrement of
receptor responses (Fig. 4), shunting of excitatory current
through channels responsible for the slow hyperpolarization,
and perhaps depression of receptor-to-AN2 synaptic transmission as well.
DISCUSSION
Our results show that ultrasound localization cues encoded
by AN2 are affected by rapidly repeated stimulation. The
spiking response decreases with time (Fig. 1), and this change
compare responses of AN2 and receptors only for high stimulus levels (90 –100 dB). To compare AN2 response strength
(spike-count) to the auditory response (integrated whole-nerve
recording, see METHODS), we normalized the decremented responses with respect to the mean response to a low-pulse-rate
stimulus (0.3 pps) at the same intensity; as shown in Fig. 1A,
stimulation at this repetition rate does not induce response
decrement. As shown in Fig. 4, response decrement is more
severe for AN2 than for the auditory receptors. In addition, the
effects of pulse rate on response decrement and change in
latency are stronger for AN2 than for receptors [2-way
ANOVA: interaction effect between pulse rate and level of
measurement (receptor or AN2) on relative response magnitude: F(3,62) ⫽ 86.27, P ⬍ 0.0001; on latency: F(3,62) ⫽
14.59, P ⬍ 0.0001].
Changes in membrane potential
The observation that response decrement of AN2 is more
pronounced than that of receptors suggests that factors in
addition to decremented receptor responses may also be involved. We examined this possibility by recording from AN2
intracellularly.
During the course of stimulation, a slowly building, longlasting hyperpolarization builds up in AN2. This is most
clearly evident as a strong hyperpolarization following the
termination of stimulation, shown in Fig. 5A. An additional
indication is the decrease in membrane potential reached between sound pulses, shown in Fig. 5, B (thin arrow) and C.
A known source of inhibition of AN2 is the interneuron ON1
which is excited by input from the contralateral ear (Faulkes
and Pollack 2000; Selverston et al. 1985). Following removal
of the contralateral ear, both the decrease in membrane potential between successive responses (Fig. 5C) and the inhibition
following the termination of stimulation (data not shown) were
still present. Cutting the contralateral leg nerve, which includes
the axons of auditory receptors, affected neither the decline of
AN2’s spike count, nor the increase in latency (Fig. 5D). Thus
neither the hyperpolarization of AN2, nor the decrement in
AN2’s spiking response with repeated stimulation, are due to
contralateral inhibition.
J Neurophysiol • VOL
FIG. 4. Response decrement of auditory receptors and AN2. Relative response is computed as the mean response [spikes-count for AN2 and integrated
responses for leg nerve (see METHODS)] induced by sound pulses delivered
8 –10 s after stimulus onset, normalized with respect to the mean of 15
responses to 0.3 pps at the same intensity. Points are mean ⫾ SE of 5 crickets.
Inset in A: responses to 3 successive pulses (90 dB, 16 pps) at the onset of the
pulse train and 10 s later for leg nerve (a and b, respectively) and for AN2 (c
and d). Inset in B: averaged compound action potentials (8 –10 s) recorded in
leg nerve. Arrows indicate onset of sound pulse and first negative peak of the
response; time difference between these is latency.
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FIG. 3. Binaural differences in spike count and latency vary with pulse rate.
Points are means ⫾ SE (n ⫽ 7 crickets) of differences between ipsilateral and
contralateral spike counts and latencies, for sound pulses delivered between 8
and 10 s after stimulus onset. Two-way ANOVA, pulse-rate effect on spike
count difference: F(3,43) ⫽ 35.62, P ⬍ 0.0001; intensity effect on spike-count
difference: F(1,43) ⫽ 0.4, P ⫽ 0.53; pulse-rate effect on latency difference:
F(3,43) ⫽ 5.81, P ⬍ 0.005; intensity effect on latency difference: F(1,43) ⫽
23.12, P ⬍ 0.001.
2326
A.-H. SAMSON AND G. S. POLLACK
is more pronounced at higher intensity and pulse rate (Fig. 2).
First-spike latency increases with time and, as for the change in
spike count, the increase in latency is greatest for high pulse
rates. However, the effect of intensity is opposite to that on
spike count; the change in latency decreases with increasing
intensity (Fig. 2).
The intensity dependence of these response features is of
particular importance when considering binaural cues used in
sound localization. The decrement in AN2’s spiking response
is greater for the neuron ipsilateral to the sound source, and as
a result, the binaural difference in spike count decreases. In
contrast, because the change in latency with repeated stimulation is smaller at higher intensities, the binaural difference in
response latency increases as stimulation continues.
The larger latency shift at lower intensities may be due to the
shape of the relationship between latency and stimulus intensity. For AN2, as for most interneurons, latency decreases
asymptotically as stimulus intensity increases (Moiseff and
Hoy 1983), presumably because the rate of rise of the EPSP
increases to a maximum (perhaps set by membrane biophysics)
as excitatory input increases. Consequently, intensity changes
have a larger effect on latency at the low end of the intensity
range. Excitatory input to AN2 decreases during repeated
stimulation, in part because of decrementing responses of
receptor neurons (Fig. 4), and possibly also because of synaptic
depression. The decrease in excitation should be functionally
equivalent to a decrease in stimulus intensity. This decrease,
occurring at an already low stimulus intensity, can be expected
to produce a large shift in latency. Consistent with this interpretation, our intracellular recordings show that the rise time of
J Neurophysiol • VOL
the EPSP increases for successive stimuli, particularly for
lower stimulus intensities (data not shown).
Our intracellular recordings show that in addition to decreasing excitatory input, a hyperpolarizing current may also play a
role in AN2’s response decrement. Two sources of inhibitory
synaptic input to AN2 have been described previously: 1)
contralateral inhibition, originating in the contralateral identified neuron ON1 (Faulkes and Pollack 2000; Selverston et al.
1985), and 2) ipsilaterally derived inhibition of unknown origin
(Moiseff and Hoy 1983). The hyperpolarization persists after
removing the contralateral ear (Fig. 5C); thus it is either
ipsilateral in origin and/or intrinsic to AN2. The ipsilateral
inhibition described by Moiseff and Hoy (1983) is recruited
most effectively by low-carrier-frequency stimuli. However,
some low-frequency-tuned receptor neurons are stimulated by
intense ultrasound (Imaizumi and Pollack 1999); thus we cannot rule out this source. Slowly building, long-lasting hyperpolarization similar in time course to that which we observed
in AN2 has been described in other auditory neurons of insects
(Pollack 1988; Römer and Krusch 2000). In the ON1 neuron of
Acheta domesticus, this hyperpolarization is triggered by activity-associated Ca2⫹ accumulation (Sobel and Tank 1994).
The presumed function of negative phonotaxis in crickets is
bat evasion (Hoy 1992). Bat echolocation sounds span a broad
range of temporal patterns, varying both with the species of bat
and with the progression of the bat-insect interaction (Griffin et
al. 1965). Before a bat has encountered a potential prey, during
the “search” phase of echolocation, echolocation cries are
emitted over roughly the same range of pulse rates as in our
experiments (6 –35 pps) (Jones 1999). During the “terminal”
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FIG. 5. Intracellular recordings of AN2. A: responses to a 90
dB, 16 pps sound stimulus, (indicated below the trace), as well
as the poststimulus period. B: first 3 responses (thick line),
superimposed with 3 responses beginning 10 s after stimulus
onset (thin line). Thick arrow indicates peak depolarization
during the excitatory postsynaptic potential (EPSP); thin arrow
indicates minimum membrane potential between EPSPs. C: as
in B, after removal of the contralateral ear. Bottom: stimulus
monitor (applies to B as well). D: effect of removing contralateral inhibition on spike count and latency (means of stimuli
delivered 8 –10 s after stimulus onset). Points are mean ⫾ SE
(n ⫽ 6 crickets), of responses to 90-dB stimuli, presented at 8
and 26 pps. t-test for paired samples on intact and cut habituated
responses; 8 pps: t ⫽ 1.31, P ⫽ 0.26; 26 pps: t ⫽ 1.51, P ⫽
0.23; on intact and cut habituated latency; 8 pps: t ⫽ 1.62, P ⫽
0.18; 26 pps: t ⫽ 1.63, P ⫽ 0.19. E: maximum depolarization
(thick arrow in B) during EPSPs between 8 and 10 s after onset
of stimulus. Points represent means ⫾ SE for 3 crickets, each
stimulated at both 70 and 90 dB SPL. Two-way ANOVA,
intensity effect on depolarization: F(1,16) ⫽ 13.5, P ⫽ 0.002;
pulse rate effect on depolarization: F(3,16) ⫽ 25.9, P ⬍ 0.001;
interaction: F(3,16) ⫽ 3.25, P ⫽ 0.04.
TEMPORAL PATTERN AND SOUND LOCALIZATION CUES
J Neurophysiol • VOL
ably, binaural difference in spike count and latency of AN2,
and in particular their changes throughout the course of the
pulse trains, were similar in their experiments to that which we
describe. Pollack and El-Feghaly found that the initial phonotaxis response to stimulation was reliably directed away from
the sound source for all pulse rates tested. However, at high
pulse rates, i.e., under conditions that produce the largest
changes in response of AN2, these initial, negative, responses
were transient (approximately 1 s in duration), and by 10 s after
the onset of stimulation, phonotaxis was on average toward,
rather than away from, the sound source. Our physiological
results show that the binaural difference in response latency
after 10 s of stimulation is larger than that at stimulus onset,
suggesting that if the crickets relied mainly on latency difference as the cue for sound direction, their behavioral responses
should not have reversed in direction. By contrast, the binaural
difference in spike count, which initially strongly favors the
ipsilateral AN2, is near zero after 10 s of stimulation. It seems
likely, then, that the reversal in behavioral response direction is
due in part to the loss of directional information encoded as
binaural spike-count difference. It is unclear, however, why the
response reverses in direction, rather than simply disappearing.
Nevertheless, comparison of our physiological results with
these earlier behavioral studies suggests strongly that binaural
latency difference is not the chief cue for determining sound
direction. This agrees with recent experiments that indicate that
the same may be true for localization of cricket song (Pollack
2001), but contrasts with a recent model of phonotaxis in
crickets, in which latency difference is the pertinent cue (Webb
and Scutt 2000).
This work was supported by the Natural Science and Engineering Research
Council of Canada.
REFERENCES
BALAKRISHNAN R AND POLLACK GS. Recognition of courtship song in the field
cricket, Teleogryllus oceanicus. Anim Behav 52: 353–366, 1996.
BOYAN GS. Directional response to sound in the central nervous system of the
cricket Teleogryllus commodus (Orthoptera: Gryllidae). I. Ascending interneurons. J Comp Physiol 130: 137–150, 1979.
CARR CE. Processing of temporal information in the brain. Annu Rev Neurosci
16: 223–243, 1993.
CORO F, PÉREZ M, MORA E, BOADA D, CONNER WE, SANDERFORD MV, AND
AVILA H. Receptor cell habituation in the A1 auditory receptor of four
noctuoid moths. J Exp Biol 201: 2879 –2890, 1998.
EGGERMONT JJ. Azimuth coding in primary auditory cortex of the cat. II.
Relative latency and interspike interval representation. J Neurophysiol 80:
2151–2161, 1998.
EGGERMONT JJ AND SPOOR A. Cochlear adaptation in guinea pigs: a quantitative description. Audiology 12: 193–220, 1973.
ENGEL JE AND HOY RR. Experience-dependent modification of ultrasound
auditory processing in a cricket escape response. J Exp Biol 202: 2797–
2806, 1999.
FAULKES Z AND POLLACK GS. Effects of inhibitory timing on contrast enhancement in auditory circuits in crickets (Teleogryllus Oceanicus). J Neurophysiol 84: 1247–1255, 2000.
GIVOIS V AND POLLACK GS. Sensory habituation of auditory receptor neurons:
implications for sound localization. J Exp Biol 203: 2529 –2537, 2000.
GRIFFIN DR, FRIEND JH, AND WEBSTER FA. Target discrimination by the
echolocation of bats. J Exp Zool 158: 155–168, 1965.
GRIFFIN DR, WEBSTER FA, AND MICHAEL CR. The echolocation of flying
insects by bats. Anim Behav 8: 141–154, 1960.
HOFFMANN RS, GARDNER AL, BROWNELL RL, KOOPMAN KF, MUSSER GG, AND
SCHLITTER DA. Mammal Species of the World. Lawrence, KS: Allen Press,
1982, p. 136 –148.
88 • NOVEMBER 2002 •
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Downloaded from http://jn.physiology.org/ by 10.220.33.1 on June 17, 2017
phase, just before capture, pulse rate may exceed 100/s (Griffin
et al. 1960). Pulse durations of echolocation sounds are generally shorter than the 30-ms sounds we used (which we chose
to facilitate comparisons with earlier behavioral work), and it
seems reasonable to suppose that pulse duration might affect
the extent of response decrement. However pulse durations of
several to tens of milliseconds are typical of high-duty-cycle
bats (Jones 1999). Northern Australia, where T. oceanicus live
(Hill et al. 1972), is home to at least five species of Hipposidaridae (Hoffmann et al. 1982), a subfamily characterized by
high-duty-cycle calls (Jones 1999). Thus it is plausible that T.
oceanicus might be exposed, in nature, to bat cries that would
induce considerable decrement of AN2 responses.
Habituation of ultrasound-induced behavioral responses of
T. oceanicus has been studied in the laboratory (Engel and Hoy
1999; May and Hoy 1991). These studies showed that negative
phonotactic responses declined when crickets were stimulated
with pulses (either 10 or 50 ms in duration) presented at rates
of 1.3 pps and 2/s. We saw no decrement in the response of
AN2 when 30-ms pulses were presented at a rate of 0.3/s, and
considerable decrement when the pulse rate was 8/s. Unfortunately, we have no data on AN2’s response to the pulse rates
used in the habituation studies, so we cannot say to what extent
the decline in AN2 response might contribute to behavioral
habituation. May and Hoy (1991) suggest that behavioral habituation is not the result of declining AN2 responses, based on
an earlier physiological study of AN2 (Nolen and Hoy 1987)
that, they claim, failed to find significant response decrement
even at a pulse rate of 15/s. Our results are clearly at odds with
this. However, the data on which the claim for lack of decrement in AN2 is apparently based, Fig. 12 of Nolen and Hoy
(1987), show a substantial decline in AN2’s response, from
approximately 12 spikes to the first sound pulse to 7 spikes to
the second. Nevertheless, the phenomenology of AN2’s response decrement and behavioral habituation do differ, suggesting that habituation is not accounted for entirely by AN2’s
response decrement. In particular, behavioral habituation is
greatest at low stimulus intensities (May and Hoy 1991),
whereas the decline in AN2’s response is most pronounced at
high intensities (Fig. 2).
It is unclear to what extent the decrement in AN2’s response
might affect natural behavior. The initial response to ultrasound, at lower intensities, is a short-latency turn away from
the sound source, and it is possible that this would remove the
cricket from the sound field of the bat’s echolocation cry before
substantial response decrement could occur (cf. Miller 1983).
When stimulated with higher-intensity ultrasound, crickets react in a nondirectional manner (Nolen and Hoy 1986), so the
effects of response decrement on sound localization cues might
be irrelevant. Estimating the degree to which stimulus-induced
changes in localization cues might play a role in bat-cricket
interactions must await field observations, which are currently
lacking.
No matter what its relevance to natural behavior, the decrement we describe, when compared with earlier laboratory
experiments, helps to illuminate how ultrasound location is
represented at the level of the AN2 neurons. Pollack and
El-Feghaly (1993) described the effects of pulse rate on phonotactic responses of T. oceanicus to ultrasound stimuli. Their
stimuli, like ours, consisted of trains of 30-ms duration sound
pulses, with pulse rates ranging from 8 to 32 pulses/s. Presum-
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J Neurophysiol • VOL
PASZTOR VM. Graded potentials and spiking in single units of the oval organ,
a mechanoreceptor in the lobster ventilatory system. III. Sensory habituation
to repetitive stimulation. J Exp Biol 107: 465– 472, 1983.
POLLACK GS. Selective attention in an insect auditory neuron. J Neurosci 8:
2635–2639, 1988.
POLLACK GS. Neural processing of acoustic signals. In: Comparative Hearing:
Insects, edited by Hoy RR, Popper AN, and Fay RR. New York: Springer
Verlag, 1998.
POLLACK GS. Sound localization in crickets: the dominant neural cue is
binaural difference in response strength, not latency difference. Intl Soc
Neuroethol Abstr 98, Bonn, July 29 –Aug. 3, 2001.
POLLACK GS AND EL-FEGHALY E. Calling song recognition in the cricket
Teleogryllus oceanicus: comparison of the effects of stimulus intensity and
sound spectrum on selectivity for temporal pattern. J Comp Physiol 173:
759 –765, 1993.
POLLACK GS AND FAULKES Z. Representation of behaviorally relevant sound
frequencies by auditory receptors in the cricket. J Exp Biol 201: 155–163,
1998.
POLLACK GS, HUBER F, AND WEBER T. Frequency and temporal patterndependent phonotaxis of crickets (Teleogryllus oceanicus) during tethered
flight and compensated walking. J Comp Physiol 154: 13–26, 1984.
RÖMER H AND KRUSCH M. A gain-control mechanism for processing of chorus
sounds in the afferent auditory pathway of the bushcricket Tettigonia viridissima (Orthoptera: Tettigoniidae). J Comp Physiol 186: 181–191, 2000.
SELVERSTON A, KLEINDIENST H-U, AND HUBER F. Synaptic connectivity between cricket auditory interneurons as studied by selective photoinactivation. J Neurosci 5: 1283–1292, 1985.
SOBEL EC AND TANK DW. In vivo Ca2⫹ dynamics in a cricket auditory neuron:
an example of chemical computation. Science 263: 823– 826, 1994.
STRAUSFELD NJ, SEYAN HS, WOHLERS D, AND BACON JP. Lucifer Yellow
histology. In: Functional Neuroanatomy, edited by Strausfeld NJ. Berlin:
Springer Verlag, 1983, p. 132–155.
WEBB B AND SCUTT T. A simple latency-dependent spiking-neuron model of
cricket phonotaxis. Biol Cybern 82: 247–269, 2000.
WYTTENBACH RA AND HOY RR. Laser-vibrometric measurements of the response of cricket ears to ultrasound. Soc Neurosci Abstr 25(1–2), Miami
Beach, Oct. 23–28, 1999.
88 • NOVEMBER 2002 •
www.jn.org
Downloaded from http://jn.physiology.org/ by 10.220.33.1 on June 17, 2017
HILL KG, LOFTUS-HILLS JJ, AND GARTSIDE DF. Pre-mating isolation between
the Australian field crickets Teleogryllus commodus and. T. oceanicus
(Orthoptera: Gryllidae). Aust J Zool 20: 153–163, 1972.
HOY RR. The evolution of hearing in insects as an adaptation to predation from
bats. In: The Evolutionary Biology of Hearing, edited by Webster DB, Fay
RR, and Popper AN. Berlin: Springer-Verlag, 1992.
IMAIZUMI K AND POLLACK GS. Neural coding of sound frequency by cricket
auditory receptors. J Neurosci 19: 1508 –1516, 1999.
IMAIZUMI K AND POLLACK GS. Neural representation of sound amplitude by
functionally different auditory receptors in crickets. J Acoust Soc Am 109:
1247–1260, 2001.
JONES G. Scaling of echolocation calls in bats. J Exp Biol 202: 3359 –3367,
1999.
KIANG NY-S, WATANBE T, THOMAS EC, AND CLARK LF. Discharge Patterns of
Single Fibers in the Cat’s Auditory Nerve. Cambridge, MA: MIT Press,
1965.
MAY ML AND HOY RR. Habituation of the ultrasound-induced acoustic startle
response in flying crickets. J Exp Biol 159: 489 – 499, 1991.
MILLER LA. How insects detect and avoid bats. In: Neuroethology and Behavioral Physiology, edited by Huber F and Markl H. Berlin: SpringerVerlag, 1983.
MOISEFF A AND HOY RR. Sensitivity to ultrasound in an identified auditory
interneuron in the cricket: a possible link to phonotactic behavior. J Comp
Physiol 152: 155–167, 1983.
MOISEFF A, POLLACK GS, AND HOY RR. Steering response of flying crickets to
sound and ultrasound: mate attraction and predator avoidance. Proc Natl
Acad Sci USA 75: 4052– 4056, 1978.
MÖRCHEN A. Spike count and response latency. Two basic parameters encoding sound direction in the CNS of insects. Naturwissenschaften 67: 469,
1980.
NOLEN TG AND HOY RR. Initiation of behavior by single neurons: The role of
behavioral context. Science 226: 992–994, 1984.
NOLEN TG AND HOY RR. Phonotaxis in flying crickets. I. Attraction to the
calling song and avoidance of bat-like ultrasound are discrete behaviour.
J Comp Physiol 159: 423– 439, 1986.
NOLEN TG AND HOY RR. Postsynaptic inhibition mediates high-frequency
selectivity in the cricket Teleogryllus oceanicus: implications for flight
phonotaxis behavior. J Neurosci 7: 2081–2096, 1987.