NMDA-mediated facilitation in the echo-delay tuned areas of the auditory cortex of the mustached bat

ELSEVIER

Hearing Research 110 (1997) 219-228

NMDA-mediated facilitation in the echo-delay tuned areas of the auditory cortex of the mustached bat Atsushi Tanahashi *, Junsei Horikawa \ Nobuo Suga Department of Biology, Washington University, One-Brookings Drive, St. Louis, MO 63130, USA

Received 6 May 1996; revised 20 February 1997; accepted 28 Apri11997

Abstract We recorded the responses of single delay-tuned neurons in the dorsal fringe (DF) area and the FM-FM area of the auditory cortex of the mustached bat using multi-barreled carbon-fiber electrodes. An iontophoretic application of N-methyl-o-aspartate (NMDA) or kainate (KA) to a DF neuron evoked a burst of discharges from the neuron. The burst of discharges evoked by NMDA was always smaller than that evoked by KA. Simultaneous application of o-2-Amino-5-phosphonovalerate (APV) with NMDA and KA abolished the NMDA-evoked but not the KA-evoked discharges. APV did not evoke any significant changes in the auditory responses of 43 out of the 47 delay-tuned neurons studied in the DF area, and in all 20 neurons studied in the FM-FM area. In the remaining four DF neurons, however, APV either increased the initial discharges of their auditory response or decreased the late discharges of their response. These results indicate that in the majority of neurons in the DF and FM-FM areas NMDA receptors do not playa significant role in the processing of target-distance information, and that their facilitative auditory responses are basically created by synaptic interactions occurring in the subcortical auditory nuclei. Keywords: Auditory cortex; o-2-Amino-5-phosphonovalerate; Facilitation; Mustached bat; N-methyl-o-aspartate; Ranging

1. Introduction

The biosonar pulse of the mustached bat consists of four harmonics (H l - 4 ), and each harmonic consists of a long constant frequency (CF) component followed by a short, frequency modulated (FM) component. Since each biosonar pulse is composed of eight components in total (CF l - 4 and FM l - 4 ), its echo potentially consists of eight components (Fig. 1). The auditory cortex (AC) of the mustached bat contains several functional areas for the processing of different types of biosonar information (Suga, 1990). Among these areas, neurons tuned to paired stimuli (pulse-echo pairs) with particular time intervals (echo delays) are found in the FMFM, dorsal fringe (DF) and ventral fringe (VF) areas

* Corresponding author. Present address: Medical School, Department of Otorhynolaryngology, Nagoya University, 65 Tsurumai-cho, Shouwa-ku, Nagoya 466, Japan. Tel.: +81 (52) 744 2323; fax: +81 (52) 744 2325. 1 Present address: Medical Research Institute, Tokyo Medical and Dental University, 2-3-10, Kanda-Surugadai, Tokyo 101, Japan

(Edamatsu et aI., 1989; O'Neill and Suga, 1979, 1982; Suga and Horikawa, 1986; Suga and O'Neill, 1979; Suga et aI., 1978, 1983). These neurons, called 'delaytuned (FM-FM)' neurons, show the maximum facilitative responses to a pulse-echo pair with a particular echo delay. The precursors of delay-tuned neurons have been found in the inferior colliculus (IC) (Mittmann and Wenstrup, 1994; Yan and Suga, 1996). Collicular delay-tuned neurons project to a certain portion of the medial geniculate body (MGB) (Wenstrup and Grose, 1995). Thalamic delay-tuned neurons in tum project to the cortical FM-FM, DF and VF areas (Olsen, 1986). The amount of facilitation and sharpness in delay tuning are larger for thalamic delay-tuned neurons than for collicular delay-tuned neurons (Yan and Suga, 1996). The facilitative responses of thalamic delay-tuned neurons are mostly mediated by N-methyl-oaspartate (NMDA) receptors in the MGB (Butman, 1992). The amount of facilitation evoked by a pulseecho pair is somewhat stronger for the AC delay-tuned neurons than for the MGB delay-tuned neurons (Olsen,

0378-5955/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved PII SO 3 7 8 - 5 9 5 5 (97) 00073-7

A. Tanahashi et al.! Hearing Research 110 (1997)

220

Pulse 120

Echo I

\ --------,

CF4

N

$2 90

H3

>()

0-

\

CF3

c c ~ 60

2. Materials and methods

I --------,

H4

H2

~ LL

FM4

\

\

The materials and methods of the present study were basically the same as those described in Suga et al. (1983), except for the iontophoretic drug injections.

\

FM3

\

\

2.1. Preparation

t --------" \ \ CF2 t

FM2

'"---"

FM1

Four Jamaican mustached bats, Pteronotus parnellii parnellii, weighing 11 to 13 g each, were used. The animals were anesthetized with a neuroleptanalgesic (Innovar-vet: Fentanyl 0.06 mg/kg body weight (b.wt) and Droperidol 3 mg/kg b.wt), and the dorsal part of the skull was surgically exposed. A 1.5 cm long stainlesssteel post was glued to the top of the skull with cyanoacrylate glue. The surgical wound was treated with an antibiotic ointment (Furacin) and the skin was sutured. After the surgery, the bat was injected with a cortico-steroid hormone (Prednisolone) to reduce the possibility of post-surgery shock, and was kept separated from other bats in a humidity- and temperaturecontrolled chamber during recovery from the anesthesia and the surgery. Four days after surgery, each bat was placed into a styrofoam restraint suspended by a rubber band at the center of an echo-attenuated soundproof room maintained at 31 o C. The head was immobilized using the stainless-steel post. A hole (,.., 50 flm in diameter) for an electrode penetration was made in the skull overlying the DF or FM-FM area of the AC using a fine needle and with the aid of a surgical microscope. The unanesthetized animal showed no signs of distress during this procedure, although it responded to accidental touching of the surgical wounds and/or the face. An indifferent electrode was placed anterior to the AC.

Doppler-shift

30

H1 CF 1 ...

o

~

o

Delay

219~228

"-

10 20 Time in ms

30

Fig. I. Schematized spectrogram of the biosonar pulse (Pulse) and its Doppler-shifted echo (Echo) of the mustached bat. The pulse consists of four harmonic components: H l - 4 . Each harmonic consists of a constant frequency (CF) component (CF l - 4 ) and a frequency-modulated (FM) component (FM l - 4 ). Delay: echo delay.

1986). The response properties of cortical delay-tuned neurons are very similar to those of thalamic delaytuned neurons (Butman, 1992; Olsen, 1986; Olsen and Suga, 1991 ;). The response properties of delaytuned neurons in the FM-FM, DF and VF areas are also very similar to each other (Edamatsu et aI., 1989; Suga and Horikawa, 1986). The aim of the present study was to examine whether NMDA receptors are involved in the facilitative responses of cortical delaytuned neurons to paired acoustical stimuli.

P-E pairs with different echo delays P P

Od 1d i 2d

3d! 4d

I il I il I il

i 5d

; 6d i 7d

11

I I. I il

ad i 9d

i

E

N

I!

E

o

2

4

6

8

10

12 X150 ms

Fig. 2. Acoustic stimulus train. The train consists of 13 time blocks: Pulse alone (P), 10 pulse-echo pairs with different echo delays (P~E), echo alone (E), and no stimulus (N). Echo delay was varied from 0 ms to 9xd ms, where d was either 0.5, 1.0, 1.5 or 2.0. Each block was 150 ms long. The train was delivered every 2 s.

A. Tanahashi et al. 1Hearing Research 110 (1997) 219-228

160 50

140

100

en Q)

80

..:.:::

•-K

K

K

K

0 Ql

Ul

-;;; 30 Ql

Ul

'5

N

N

N

.§ 20 '0 ~ 10

a. en 60

40

500

20 0 0

100

200

300

400

500

Current in nA Fig. 3. Relationship between impulse counts and electric currents applied through a barrel containing isotonic saline. Electric currents of less than 100 nA showed no effect on the background discharges of a delay-tuned neuron in the DF area, but currents of more than 200 nA increased its background discharges.

2.2. Recording of action potentials and iontophoretic injections of drugs Multi-barreled carbon-fiber electrodes (ArmstrongJames and Millar, 1979; Armstrong-James et aI., 1980) were used for both recording action potentials and iontophoretically applying drugs. The electrode consisted of a glass capillary tube with a carbon fiber (7 )lm in diameter, Thornel, Amoco) for the recording and five glass capillary tubes surrounding it for the injections. The action potentials of a single or a few neurons recorded from the DF or FM-FM area were separated using a window discriminator, and their shapes were monitored on the screen of a digital storage oscilloscope during the recording session. For sound stimulation, the sweeps and amplitudes of paired FM sounds were set at the best frequency sweeps and best amplitudes of the neuron, and the time interval between the paired FM sounds (corresponding to echo delay)

N

N

l

~

1,000 Time in second

N

N

1,500

Fig. 4. Effects of N-methyl-D-aspartate (NMDA), kainate (KA) and D-2-Amino-5-phosphonovalerate (APy) on a DF neuron. NMDA (N, -50 nA, 30 s) evoked a burst of discharges that was less than that evoked by KA (K, -20 nA, 30 s). APV (double headed horizontal arrow, -100 nA, 630 s) abolished selectively the discharges evoked by NMDA (downward arrows), but not by KA. Short bars indicate 30 s applications of NMDA and KA.

was changed in a specific way, named a delay (D)-scan by a computer (Fig. 2). The response to the D-scan was recorded before, during, and after an application of either NMDA, kainate (KA) and/or n-2-Amino-5-phosphonovalerate (APV) was applied to the neuron using an iontophoretic pump (Neuro Phore BH-2 system). The concentration, pH and injection current for these drugs are shown in Table 1. Each of the barrels of the electrode was filled with either one of these or isotonic saline solution. The saline solution was used for automatic current balancing and also for a sham injection to confirm that any changes in the auditory response were not evoked by the electric current for a drug application, but by the drug applied. We found that a 100 nA current did not excite any DF neurons, but 200 nA current excited 7 out of 10 DF neurons (Fig. 3). Therefore, current used for iontophoresis was set at 100 nA for all 47 neurons studied. It was also confirmed, by applying the current to them through the barrel containing saline solution, that the 100 nA current itself did not excite the neurons. To study the effect of APV on the responses to acoustic stimuli, APV was continuously injected for

Table I Drugs injected Concentration (mM)

Agonist NMDA Kainate Antagonist D-APV Control Saline

K

K

§ 40

en

to

APV

•

K

'0

120 0

N: NMDA K: Kainate

221

pH

Injection Current (nA)

Duration (s)

20 50

7.5-8.0 7.5-8.0

-50 -20

30 30

50/100

7.5-8.0

-100

600

7.5-8.0

-100

600

A. Tanahashi et at. / Hearing Research 110 (1997) 219-228

222

B

A

20

80 1: control

10

40

:; E ~

0

0

p

'"

(])

'"

1

2

3

4

5

6

7

8

9

E

N

2: under APV 100 mM -100 nA 600 - 800 s

40

0

75

150

0

75

150

o

75

150

30

80

0 0

--'"

0

15

:; a.

.§

0

0

z

0

p

80

0 0

1

2

3

4

5

6

7

8

9

E

N 20

3: recovery 1200 - 1400 s 10

40

P

I0

1

2

3

4

5

6

Echo delay in ms

7

8

9

I

E

N

Time in ms

Fig. 5. Effect of APV on a delay-tuned neuron in the DF area. A: The arrays of peristimulus time (PST) histograms displaying the responses of a DF neuron to the pulse-echo (P-E) stimulus trains shown in Fig. 2. The neuron shows a strong facilitative response to a P-E pair at a 5.0 ms E-delay. The responses recorded before (control condition), during (under APV) and after an APV injection (recovery condition) are represented by I, 2 and 3, respectively. The response pattern was affected slightly by APV, but delay tuning was not affected at all (A). B: PST histograms of responses to P-E pairs at a 5.0 ms E-delay. Dashed lines indicate the position between fast and slow responses.

600 s while the D-scan (a train of acoustic stimuli) was delivered every 2 s. To demonstrate the specific effect of APV on NMDA receptors, APV was also applied to neurons while NMDA and KA were alternately injected for 30 s each with a 90 s interval, without acoustic stimulation. 2.3. Acoustic stimuli

The details of the acoustic stimulation system are described elsewhere (Suga et aI., 1983). The acoustic stimuli used were paired FM sounds (FM 1-FM n, n =2, 3,4) mimicking the FM components in the species-specific orientation pulse (P) and its echo (E) (Fig. 1). The amount of the frequency sweep was 20% of the starting frequency of FM1 and FMn. The duration and rise-fall time of the FM sounds were 3.0 ms and 0.5 ms, respec-

tively. The amplitudes of these acoustic stimuli were set using a manual attenuator (Hewlett Packard 350D) or a computer-controlled custom-made digital attenuator. A train of FM1 and FMn sounds (D-scan) was delivered using a two-channel sound delivery system (Fig. 2). The D-scan consisted of 13 time blocks: pulse FM1 alone, lO pulse FM 1-echo FMn pairs with gradually increasing echo delay, echo FM n, and no stimulus. Each block was 150 ms long, so that the D-scan lasted 1.8 s. The echo delay of a pulse-echo pair was incremented by steps of either 0.5, 1.0, 1.5 or 2.0 ms, depending upon the length of the best delay of a given neuron. 2.4. Data acquisition and processing

The response of neurons to individual D-scans deliv-

A. Tanahashi et al. / Hearing Research 110 (1997) 219-228

A

B

300

20

•

•

1: control

•

•

•

•

150

~

223

10

•

•

•

•

O~--~·~-T~~L---~--~--~--~~~~~~--~--~·~~L-~

E

ti 300

Po

o o

1

2

EN

3456789

2: under APV 50 mM -100 nA 600 - 800 s

-( /)

(l)

~ 150

o 4--....1.11--..........-'-----, 150 o 75

60

30

a.

.5

o

c::i

z

o

I ....

P

1

.J..

o

a,. .

1

7

6

o

9

8

N

E

20

•

3: recovery 1200 - 1400 s

•

5

4

3

2

300

150

i

.w.

150

10

• 3

75

•

•

•

o

4

5

6

Echo delay in ms

• 7

• 8

• 9

IE

•

o 4-..1.-"1111--.---'--....1..&\ N

o

75

150

Time in ms

Fig. 6. Effect of APV on a delay-tuned neuron in the DF area. A: The arrays of PST histograms displaying the responses of a DF neuron to P-E stimulus trains. The neuron shows a strong facilitative response to a P-E pair at a 5.0 ms E-delay. The responses recorded before (control condition), during (under APV) and after an APV injection (recovery condition) are represented by 1, 2 and 3, respectively. The response pattern was affected strongly by APV, but delay tuning was not affected at all. Dots in A (I and 3) indicate normalized impulse counts. B: PST histograms of responses to P-E pairs at a 5.0 ms E-delay. Dashed lines indicate the position between fast and slow responses.

ered 100 times were recorded by a computer (IBM 286) with a data acquisition interface (Modular Instruments) with a 0.1 ms resolution and gate markers of drug injections. The responses were displayed on a computer monitor as peri stimulus-time (PST) histograms. The number of impulses, response latency, best delay and width of the delay-tuning curve were measured from these histograms.

3. Results Data were obtained from 47 neurons in the DF area of 4 animals (6, 7, 16 and 18 neurons, respectively) and 20 neurons in the FM-FM area of 3 animals (5, 7, and 8 neurons, respectively). These neurons were recorded from cortical depths of 165-685 !-lm

(380 ± 90 !-lm, mean ± S.D.) in the DF area and 210-800 !-lm (430 ± 140 !-lm, mean ± S.D.) in the FM-FM area.

3.1. Responses of cortical delay-tuned neurons to NMDA, KA and APV

NMDA and KA were applied alternately to 10 out of 47 delay-tuned neurons recorded in the DF area and 10 out of 20 delay-tuned neurons recorded in the FM-FM area. Each application of NMDA and KA evoked a burst of discharges from these neurons. The number of discharges evoked by NMDA (mean ± S.D.: 19 ± 7 and 9 ± 3 impulses/s for DF and FM-FM neurons, respectively; n =10) was only a half of that evoked by KA (39 ± 8 and 19 ± 4 impulses/s for DF and FM-FM neurons, respectively) despite the NMDA injection (-50 nA, 30 s) larger than the KA

A. Tanahashi et al.I Hearing Research 110 (1997) 219-228

224

B

A

30

40

1: control 15

20

0

0 "'5 E 40 ~

P

0

1

2

3

4

5

6

8

7

9

E

N

0

75

150

0

'75

150

20

2: under APV 100 mM -100 nA 600 - 800 s

(I)

8.....

gJ 20

10

.!!1 ::J

c-

.S

'0 0

0

z 40

p

0 0

1

2

3

4

5

6

7

8

9

E

30

3: recovery 1200 - 1400 s 15

20

1

2

3

4

5

6

Echo delay in ms

7

8

N

Time in ms

Fig. 7. Effect of APV on a delay-tuned neuron in the FM-FM area. The parameters for the PST histograms were the same as those given in Fig. 5. The response pattern and delay tuning were not affected significantly by APV.

injection (-20 nA, 30 s). When APV was applied in addition to NMDA and KA, the NMDA-evoked response was completely suppressed, but the KAevoked response was not affected (Fig. 4). Application of APV alone evoked neither excitation nor inhibition of these neurons. 3.2. Effect of APV on the facilitative responses of cortical delay-tuned neurons to pulse-echo pairs The effects of APV on facilitative responses to pulseecho pairs were studied in 47 DF neurons and 20 FMFM neurons. All of these neurons showed strong facilitative responses to pulse--echo pairs with particular echo delays, but no, or weak responses to either the pulse or echo alone. In most of the neurons (32/47 DF neurons and 16/20 FM-FM neurons), the facilitative responses consisted of a fast component (discharges within 20 ms after the initial discharges) and

a slow component (discharges occurring later than 20 ms after the initial discharges) that were separated by a short period of no, or a decreased response (e.g., control of Fig. 5). While recording the facilitative responses, APV was injected to each neuron three times at 30 min intervals. In 43 out of the 47 neurons in the DF area, injection of APV caused no noticeable changes in either best delay, response latency, discharge rate or delay-response curve width. The remaining four neurons, however, showed a statistically significant change (Wilcoxon test, P < 0.05) in response magnitude but not in best delay, response latency and delay-response curve width. The responses of two neurons out of the four are shown in Figs. 5 and 6. In Fig. 5B, the PST histograms show a decrease in the slow component evoked by an APV injection. The shape of the delayresponse curve was not changed significantly by APV (Fig. 5A). Fig. 6B shows a remarkable APV-induced increase in

A. Tanahashi et al.! Hearing Research 110 (1997) 219-228

A : No. of impulses / stimulus at BO

•

10

0

0

•

L.

(J.)

"0

c

:::>

00

,hdA

2

0

8

~

~

00

~o

> 6 a. c:(

t

L.

(J.)

"0

c

ll.

4

C : Best delay in ms 10

a > a. 6 c:(

:::>

4

~ll.

ll.ll.

2

0

all.

c?

0 0

0 4

2

8

6

10

0

o :Width of delay-response curve 20

0

in ms at background + s. d.

ll.

10

A~

> a.

c:( L.

(J.)

"0

•

> a.

ll.ll.

c:( L.

(J.)

i

6

10

"0

AQ

c :::>

0<%

15

4.~ll.ll.

a

c :::>

~~

cy.6 5

4

oe

10 a 6 Before APV injection

0 0

12

~tc 6

0

ll.~

4

10

Before APV injection

8 : Latency of facilitative response inms

a

6

4

2

Before APV injection

12

225

5

10

15

20

Before APV injection

APV 50 mM, -100 nA, 600 sec N=20

• A significant change in A

ll.A APV 100 mM, -100 nA, 600 sec N=27 Fig. 8. Effects of APV on response magnitude (A), response latency (B), best delay (C) and width of delay-response curve (D) of delay-tuned neurons in the DF area. The filled and open symbols represent neurons that showed significant changes in response magnitude and those that showed insignificant changes, respectively.

the fast component and slight decrease in the slow component. Other response parameters, such as latency, best delay and delay-response curve width, were unchanged (Fig. 6A). The dotted curve in Fig. 6A (1 and 3) shows normalized impulse counts for easier comparison in delay tuning between the control, APV and recovery data.

The effects of APV on facilitative responses to pulseecho pairs were also studied in 20 delay-tuned neurons in the FM-FM area. In all 20 neurons, APV injection caused no statistically significant changes (Wilcoxon test, P> 0.1) in either response magnitude, best delay, response latency, discharge rate, or delay-response curve width. Fig. 7 shows a typical example of such a

A. Tanahashi et al.I Hearing Research 110 (1997) 219-228

226

C : Beat delay in

A: No of impulses / stimulus at BD

ms

6

12

0

0 0

5

10 0

8

> a..

«.....

0

«..... Q)

era

"C C

:::J

> a..

0

0 00<0

6

Q)

0

0

0

3

c

:::J

0

OCD 0

"C

000

4

0

4

0

0 0

0

2

COO

fb

8

2

0

0 0

4

2

10

8

6

12

0

6

D : Width of delay-response curve

response

in ms

8

5

Before APV injection

Before APV injection

B : Latency of facilitative

4

3

2

in ms at bacground + s. d.

10 00 0

7

Q)

CD

0

> a..

«.....

0

8

0

6

0

"C

c

> a..

0

«.....

00

6

8

Q)

"C

c

:::J

:::J

CO

coCO


2

0

0

<0 O(Q

4

0

5

O~

00

0

4 4

5

6

7

8

Before APV injection

o

0

2

4

6

8

10

Before APV injection

APV 100 mM, -100 nA, 600 sec N=20

Fig. 9. Effects of APV on response magnitude (A), response latency (B), best delay (C) and width of delay-response curve (D) of delay-tuned neurons in the FM-FM area. None of the neurons showed any significant change.

neuron in the FM-FM area, that showed no significant changes after APV injection. Figs. 8 and 9 show the comparisons of response magnitude (A), latency (B), best delay (C) and a delay-tuning curve width (D) between the data obtained before and during an APV injection in the DF and FM-FM

areas, respectively. Each symbol represents a single neuron. The symbols on the diagonal line running through the origin of the coordinates indicate no change. These graphs show that the effect of APV was insignificant, except for the response magnitude of the four DF neurons, shown by the filled symbols in Fig. 8A.

A. Tanahashi et al. / Hearing Research 110 (1997) 219-228

227

4. Discussion

Acknowledgments

APV abolished NMDA-evoked, but not KA-evoked responses. This suggests strongly that the facilitative responses of cortical delay-tuned neurons to paired acoustic stimuli would be greatly reduced if these responses were mediated by NMDA in the DF and FM-FM areas. However, APV showed no effect on 91% (43/47) of the delay-tuned neurons studied in the DF area and all (20/20) of those in the FM-FM area. These results indicate that the facilitative responses of the majority of the DF and FM-FM neurons to paired stimuli were formed by synaptic interactions occurring in the subcortical nuclei, that the excitation of these neurons is predominantly mediated by non-NMDA receptors, and that the excitation of 9% DF neurons is enhanced by NMDA-receptors. The reduction in the slow component induced by an APV application can be explained by the blockage of the NMDA-receptormediated response (Armstrong-James et aI., 1993; Butman, 1992). The increase in the fast component, however, may be explained by the assumption that APV suppressed the activity of the cortical inhibitory neurons converging upon these DF neurons. Butman (1992) observed that an application of 6-cyano-7 -nitroquinozaline, a non-NMDA receptor antagonist, to delay-tuned neurons in the FM-FM area of the cortex abolished completely their responses to pulse-echo paIrs. The DF area receives a projection from the MGB (Fitzpatric et aI., unpublished data) and the cortical FM-FM area, which also receives a projection from the MGB (Olsen, 1986). The response properties of DF neurons are similar to those in the FM-FM area except for the parameters of response latency and the length of best delays (Suga and Horikawa, 1986). Facilitative responses of delay-tuned neurons in the MGB are mostly suppressed by APV. In particular, the slow component of these responses is completely eliminated by APV, without exception, indicating that these responses are mostly mediated by NMDA receptors in the MGB (Butman, 1992). Therefore, most of the facilitative responses of the delay-tuned neurons in the DF and FM-FM areas are created by synaptic interactions occurring in the subcortical auditory nuclei. In the visual cortex, NMDA receptors are found in all layers in kittens, but only in layers II and III in adult cats (Fox et aI., 1989). Sensitivity to APV significantly decreases for neurons in the adult visual cortex (Tsumoto et aI., 1987; Armstrong-James et aI., 1993). These results paralleled the present result that the NMDAreceptor-mediated responses were found rarely in DF and FM-FM neurons in adult bats. We, however, did not examine the layer-specific effect of APV in the present study.

This work was supported by research grants from the National Institute on Deafness and Other Communicative Disorders (DCOOI75) and the Office of Naval Research (NOOOI4-90-J-1068) to N. Suga. The authors thank I. Saitoh for his technical assistance in the initial stage of the present research. The protocol of our research was approved by the Animal Studies Committee of Washington University (Approval No. 92279).

References Armstrong-James, M. and Millar, J. (1979) Carbon fiber microe1ectrodes. J. Neurosci. Methods 1, 279-287. Armstrong-James, M., Fox, K. and Millar, J. (1980) A method for etching the tips of carbon fiber microe1ectrodes. J. Neurosci. Methods 2,431-432. Armstrong-James, M., Welker, E. and Callahan, C.A. (1993) The contribution of NMDA and non-NMDA receptors to fast and slow transmission of sensory information in the rat SI barrel cortex. J. Neurosci. 13, 2149-2160. Butman, A.J., 1992. Synaptic mechanisms for target ranging in the mustached bat, Pteronotus Parnellii. Doctoral dissertation, Washington University, St. Louis. Edamatsu, H., Kawasaki, M. and Suga, N. (1989) Distribution of combination-sensitive neurons in the ventral fringe area of the auditory cortex of the mustached bat. J. Neurophysiol. 61, 202207. Fox, K., Sato, H. and Daw, N. (1989) The location and function of NMDA receptors in cat and kitten visual cortex. J. Neurosci. 9, 2443-2454. Mittmann, D.H. and Wenstrup, J.J. (1994) Combination-sensitive neurons in the inferior collicu1us of the mustached bat. Assoc. Res. Otolaryngol. Abstract 17, 93. O'Neill, W.E. and Suga, N. (1979) Target range-sensitive neurons in the auditory cortex of the mustached bat. Science 203, 69-73. O'Neill, W.E. and Suga, N. (1982) Encoding of target-range information and its representation in the auditory cortex of the mustached bat. J. Neurosci. 2, 17-31. Olsen, J.F., 1986. Processing of biosonar information by the medial geniculate body of the mustached bat. Doctoral dissertation, Washington University, St. Louis. Olsen, J.F. and Suga, N. (1991) Combination-sensitive neurons in the medial geniculate body of the mustached bat: encoding of target range information. J. Neurophysiol. 65, 1275-1296. Suga, N. (1990) Cortical computational maps for auditory imaging. Neural Netw. 3, 3-21. Suga, N. and Horikawa, J. (1986) Multiple time axes for representation of echo delays in the auditory cortex of the mustached bat. J. Neurophysiol. 55, 776-805. Suga, N. and O'Neill, W.E. (1979) Neural axis representing target range in the auditory cortex of the mustached bat. Science 206, 351-353. Suga, N., O'Neill, W.E., Kujirai, K. and Manabe, T. (1983) Specialization of 'combination-sensitive' neurons for processing of complex biosonar signals in auditory cortex of the mustached bat. J. Neurophysiol. 49, 1573-1626. Suga, N., O'Neill, W.E. and Manabe, T. (1978) Cortical neurons sensitive to combinations of information bearing elements of biosonar signals of the mustached bat. Science 200, 778-781. Tsumoto, T., Hagihara, K., Sato, H. and Hata, Y. (1987) NMDA

228

A. Tanahashi et at.! Hearing Research 110 (1997) 219-228

receptors in the visual cortex of young kittens are more effective than those of adult cats. Nature 327, 513-514. Wenstrup, J.J. and Grose, C.D. (1995) Inputs to combination-sensitive neurons in the medial geniculate body of the mustached bat: the missing fundamental. J. Neurosci. 15, 4693-4711.

Yan, J. and Suga, N. (1996) Corticofugal modulation of time-domain processing of biosonar information in bats. Science 273, 11001103.