J Comp Physiol (1981) 142:347-357 Journal of Comparative Physiology. A �9 Springer-Verlag 1981

Discrimination of Sensory Signals from Noise in the Escape System of the Cockroach: The Role of Wind Acceleration

Mark R. Plummer* and Jeffrey M. Camhi Section of Neurobiology and Behavior, Division of Biological Sciences, Langmuir Laboratory, Cornell University, Ithaca, New York 14850, USA

Accepted December 30, 1980

Summary. 1. Cockroaches (Perip[aneta americana) that were restrained but were able to make normal walking movements were stimulated with wind puffs delivered to the cercal wind receptors. Some puffs were superimposed on a constant ' headwind' simulat- ing the relative wind that the walking cockroach would experience if it were not fixed in place. Puffs were either of variable peak velocity, or of fixed peak velocity but variable acceleration. In some experi- ments the movement responses of one metathoracic leg were recorded; in other experiments the responses of giant interneurons were recorded.

2. The threshold wind velocity of 3 mm/s for evok- ing a behavioral response in slowly walking cock- roaches (Camhi and Nolen 1981) was still effective even in the presence of an 80 mm/s headwind. Mea- surements of the background wind ( 'noise') near a cercus of a slowly walking cockroach in the headwind showed a mean velocity of about 80 mm/s plus peri- odic wind gusts of up to 30 mm/s produced by the stepping motions of the legs (Fig. 3). Although over 50% of these gusts have velocities greater than the threshold stimulus for evoking a behavior (Fig. 4A), the cockroach did not respond to these gusts. Howev- er, the maximal accelerations of these gusts were con- sistently less than that of the just-threshold wind puff (Fig. 4B).

3. Stimuli of high acceleration (greater than 600 mm/s 2) usually evoked running. Those of inter- mediate acceleration (approximately 300 mm/s 2) usually evoked a pause in walking. The lower the acceleration, the greater the incidence of no response (Fig. 7A). This discrimination of wind acceleration was generally the same even when the stimuli were superimposed on the headwind (Fig. 7B). The dis-

* Present address : Department of Psychology, Neurosciences Pro- gram, Stanford University, Stanford, California 94305, USA

Abbreviation: GI giant interneuron

crimination of acceleration was also independent of the direction of the wind stimuli (Fig. 7 C, D).

4. The wind made by the strike of a natural preda- tor, the toad Bufo marinus (known to be the cue by which cockroaches detect and respond to the toad; Camhi et al. 1978) produces a sufficiently high wind acceleration, sufficiently early, to account for the cockroach's running response to the wind made by the strike.

5. In electrical recordings from a single connective of the ventral nerve cord, it was possible to identify the action potentials from the group of three largest giant interneurons, No.'s 1, 2, and 3, which may me- diate the initiation of escape behavior (Camhi and Nolen 1981). The number and frequency of action potentials in these neurons were greater for wind stim- uli of higher acceleration, even though the peak wind velocity of all stimuli was the same (Fig. 10).

6. The results indicate that wind acceleration is a crucial cue by which the cockroach escape system discriminates wind signals, including the wind from the strike of a predator, from background noise, in- cluding that produced by the animal's own walking.

Introduction

Animals in natural environments often need to dis- criminate weak sensory signals from substantial back- ground noise. Such discrimination requires a differ- ence, however slight, between some physical parame- ters of the signal and of the noise. Parameters such as amplitude, frequency composition, and temporal patterning offer possibilities for this discrimination. In this paper we explore the mechanisms by which the cockroach Periplaneta americana discriminates small aperiodic wind signals which evoke its escape behavior from background wind (hereafter called

0340-7594/81/0142/0347/$02.20

348 M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System

" n o i s e " ) . S o m e o f this no i se is g e n e r a t e d as the insect

w a l k s ; the rest is a m b i e n t wind.

I t has been s h o w n tha t t e t h e r e d c o c k r o a c h e s are

m o r e sensi t ive to w i n d puffs p r e s e n t e d whi le they

are w a l k i n g t h a n whi le they are s t a n d i n g still. D u r i n g

wa lk ing , a Wind p u f f o f on ly 3 m m / s e v o k e s a pause

in the wa lk ing , a n d a p u f f o f 12 m m / s e v o k e s r u n n i n g

( C a m h i and N o l e n 1981). Th is h igh sens i t iv i ty u n d e r -

lies the c o c k r o a c h ' s abi l i ty to de tec t and escape f r o m

the s t r ike o f a n a t u r a l p r e d a t o r , the t o a d Bufo matin- us, by sens ing the w i n d m a d e by the t o a d ' s s t r ike

( C a m h i et al. 1979). T h e h e i g h t e n e d sens i t iv i ty d u r i n g

w a l k i n g m a k e s sense in t h a t whi le wa lk ing , a cock - r o a c h w o u l d be espec ia l ly v u l n e r a b l e to p r e d a t o r y

a t t ack , e n c o u n t e r i n g m o r e p r e d a t o r s a n d be ing m o r e

vis ib le to t h e m t h a n whi le s t a t i ona ry (e.g., E w e r t

1976). H o w e v e r , this h igh sens i t iv i ty p re sen t s a p r o b -

lem. I f the w a l k i n g c o c k r o a c h were n o t t e t he r ed in

place, b u t r a the r were free to m o v e t h r o u g h space,

this ve ry m o v e m e n t w o u l d p r o d u c e a ' r e l a t i v e w i n d '

tha t w o u l d c o m p l i c a t e its t a sk o f de t ec t ing m i n u t e

w i n d puffs. D u r i n g typ ica l ly s low w a l k i n g at 80 m m / s

( D e l c o m y n 1971) the c o c k r o a c h w o u l d e n c o u n t e r a

re la t ive w i n d o f this s a m e speed, s o m e 25 t imes g rea t e r

t h a n the speed o f the t h r e s h o l d w i n d puff. N o t h i n g

is k n o w n o f the insec t ' s ab i l i ty to r e s p o n d to smal l

w i n d s ignals in the p resence o f such la rge b a c k g r o u n d

noise. In this p a p e r we r e p o r t t ha t the t h r e s h o l d w i n d

puffs o f 3 m m / s fo r a pause a n d 12 m m / s for a run

r e m a i n e f fec t ive even in the p resence o f re la t ive wind

o f 80 m m / s . M o r e o v e r , this re la t ive w i n d is p u n c t u a t -

ed by w i n d gusts o f up to 30 m m / s p r o d u c e d by the

m o v i n g legs, to w h i c h the c o c k r o a c h shows no b e h a v -

iora l r esponse . Th is c o m p l e x b a c k g r o u n d noise ,

t h o u g h h igh in wind ve loc i ty , c o n t a i n s w i n d acce le ra -

t ions tha t a re l ower t h a n t h a t o f the t h r e s h o l d wind

signals. W e d e m o n s t r a t e t ha t c o c k r o a c h e s d e c o d e and

use the acce l e r a t i on i n f o r m a t i o n c o n t a i n e d in w i n d

signals, i n c l u d i n g t hose f r o m p r e d a t o r y str ikes, to re-

s p o n d se lec t ive ly to these s ignals whi le i g n o r i n g the

b a c k g r o u n d noise .

Methods

Adult male Periplaneta americana were used in all experiments. The animals were procured and reared as described earlier (Camhi and Nolen 1981).

Cockroaches were prepared for recording of leg movements as described before (Camhi and Nolen 1981). After removing the wings, 4 pins placed vertically through the abdomen were affixed to a drop of wax on a lubricated lucite surface under which were photocells for recording the movements of one metathoracic leg (Fig. 1 B). Wind stimuli were produced by either of two sources. For wind puffs of low velocity but uncontrolled acceleration, we used the wind stimulator described previously (Camhi and Nolen 1981), which delivered puffs through a tube positioned just behind

the animal. Alternatively we used a wind stimulator capable of delivering ramp signals whose wind velocity increased from zero to a selected peak velocity at a controllable acceleration. This stimulator (Fig. 1 A) consisted of a box made of balsa wood (65 cm long, 25 cm wide, 19 cm high) that moved along two brass tracks on wheels having precision bearings. A single stimulus was pro- duced when the initially stationary box began to roll a short dis- tance down the tract, accelerating to some peak velocity and then decelerating to a stop. The cockroach was located inside the box, pinned in place on its lucite platform that remained stationary as the box moved around it. The lucite platform was supported by a rod that passed through a slot 1 cm wide in the floor of the box. The rod was attached to an underlying baseboard. The air inside the box, moving over the animal, constituted the wind stimulus. A removable lid permitted us to position the animal inside the box. A lucite window in the lid allowed us to observe the animal during an experiment.

The balsa box was driven by the gravitational force on a preselected mass that was permitted to drop through a predeter- mined distance (Fig. l A). The mass was attached to a steel-tipped rod connected to a wheel. The steel-tipped end of the rod was held in place initially by an electromagnet and was permitted to drop when the current through the magnet was switched from DC to AC. A fine guage belt connected the wheel to a second wheel that was attached to the box by a rack and pinion. Tension was maintained on the system by slightly inclining (0.5 ~ ) the base- board that held the tracks.

By increasing the mass on the steel-tipped rod, we could in- crease the wind acceleration as measured at the cockroach's cerci (Fig. 2A). By increasing the distance through which the rod fell, we could increase the peak wind velocity (Fig. 2B). For most exper- iments employing this balsa box stimulator, we used the same peak velocity, 40 mm/s, but five different accelerations between 120 and 1,700 mm/s 2 (Fig. 2A). The acceleration was constant on the rising phase of a given stimulus over at least 70% of the stimulus amplitude. Within any set of trials, the wind acceleration produced by a given mass on the rod varied by no more than 15%. Only responses occurring during the rising phase of the stimu- lus were of interest, so the irregularities of the falling phase (Fig. 2) were unimportant.

When an experiment employing the balsa box was begun, a cockroach was pinned in place on the platform and the lid of the box was closed. The insect was not touched again until experiment was over. We used only animals that spontaneously showed normal walking movements of 2 4 steps/s for long periods of time and that responded to an initial wind puff of high accelera- tion by running. This was approximately 80% of all the cock- roaches examined. A series of stimuli of constant peak velocity but variable acceleration was presented either in a highly irregular sequence or in a sequence that included alternation between the low and high acceleration, to help rule out fatigue as a contributor to the responses to different accelerations. Wind stimuli were pre- sented only while the cockroach was walking at 2-4 steps/s. The minimal interstimulus interval for all experiments was 2 min.

In some experiments using either of the two wind stimulators, a steady 'headwind' was also introduced to simulate the wind that the insect would have experienced if it were unrestrained and thus free to move forward during walking. The headwind was delivered through a glass tube in front of and above the cockroach. This tube was attached by a clamp fixed to the lucite platform (Fig. I). The wind was delivered to the tube through a rubber hose which fit through the slot in the floor of the balsa box. The wind was generated by the building's air compressor and was filtered through cotton wool before arriving at the headwind tube. The flow rate of the headwind was set by a valve so that at least 80 mm/s of wind flow could be recorded just behind any part of either cercus, as measured in the protected environment

M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System 349

A Light

;./-- _

d

t

Wind probe

Wi nle

Photo cells

E[ectro 9 magnet

U; Fig. 1 A, B. Wind stimulator and experimental arrangement. A The wind stimulator was a balsa box that contained the cockroach. The box moved to the right along two tracks while the cockroach's position remained fixed. The stimulus was the movement past the cockroach of the air inside the box. The box moved in response to the falling of a weight, fixed to a lever arm, which was released from an electromagnet. The weight fell through a predetermined distance. Stimulus parameters were varied by varying the weight and the distance of the fall. The headwind, used in some experiments, was a steady stream of air directed over the cockroach from the front. Not drawn to scale. See text for details. B Arrangement for recording the wind (wind probe) flowing over the cerci and the movements of the right metathoracic leg (photocells). The headwind tube directed the headwind posteriorly and downward toward the cerci

A

20

0

B 4O 2O

0

I00 msec

Fig. 2A, 13. Wind stimuli. A Stimuli of 5 different accelerations, all with the same peak velocity, 40 mm/s. Accelerations shown (left to right) are 1700, 1000, 630, 300 and 140 mm/s 2. Only re- sponses to the rising phase of the wind are reported in this paper. B Five stimuli, all of the same acceleration (200 mm/s z) but differ- ent peak velocities

of the balsa box, or a simiIar enclosure for those experiments employing the other wind stimulator. The wind stimuli and head- wind were monitored by a hot-wire anemometer (Datametrics VTP). The wind measurements for the small wind puffs have been described elsewhere (Camhi and Nolen 1981). For measuring the wind of constant acceleration, created by the moving balsa box, the anemometer ' s active probe was placed 3.0 cm lateral to the posterior end of the cockroach (Fig. 1 B).

In some experiments we recorded the activity of axons in the left connective of the abdominal nerve cord in response to wind stimuli produced by the moving balsa box. The dorsal dissection of the abdomen that we used has been described elsewhere (Camhi and Nolen 1981). We kept the use of saline (CaIlec and Satelle i973) to a minimum, taking care not to wet the cerci or the lucite platform. A miniature micromanipulator (Narashigi MD4) was attached to the side of the lucite platform and was used to position two silver hook electrodes. The electrodes were insulated from the saline of the body cavity and from the opposite connective by a mixture of petroleum jelly and mineral oil. The leads from the electrodes were threaded through the slot in the bot tom of the box and were connected to an AC preamplifier (Grass P15). In order to assist in interpreting these recordings, some similar extracellular recordings were performed using our other wind stim- ulator, which had been used previously in making intracellular recordings from individually identified giant interneurons (Camhi and Nolen 1981). By comparing the intracellular and extracellular recordings to the same puffs from this stimulator, we were able to verify that most of the largest extracellularly recorded action potentials were from the giant interneurons of largest diameter, GI 's 1, 2 and 3, as reported below.

350 M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System

Leads from the anemometer, photocells and AC preamplifier were all connected to an instrumentation tape recorder (Hewlett Packard 3960). The FM channels used for ttae anemometer and photocell outputs had a frequency response of 0-2.5 kHz. Direct channels used for recordings from the nerve cord had a frequency response of 50 Hz-32 kHz. All recordings were displayed on an oscilloscope and filmed by conventional means.

For some observations the cockroach was prepared on the lucite platform as described above but was presented with wind stimuli not from either wind stimulator but rather from the strike of a toad. For this purpose the lucite platform was set into the floor of a chamber previously used for recording wind from the strikes of toads (Camhi et al. 1978). The methods for recording the wind from the toad and the responses of the cockroach's legs were similar to those used for recording within the balsa box. The only difference was that the wind probe was placed as close as possible to the cerci, since toad's wind was more focal than the wind inside the balsa box. Once the toad's tongue touched a cockroach, that insect was removed and replaced with another.

Results

Responses of Walking Cockroaches in a Headwind to Small Wind Puffs

As mentioned above, cockroaches fixed in place but making walking movements responded to wind puffs of 3 mm/s with a pause and 12 mm/s with a run (Cam- hi and Nolen 1981). We tested the responses to these same stimuli in the presence of a headwind of at least 80 mm/s. While exposed to this headwind, the cockroaches walked in a manner indistinguishable from that without this wind, except that they some- times raised their antennae near the stream of air. Surprisingly, in the presence of the headwind, wind puffs of 3 mm/s evoked pauses on 45% of the trials (n= 50, 4 animals) and 12 mm/s puffs evoked running on 85% (n---50, 4 animals). These values are not sig- nificantly different f rom those without the headwind. Since a 3 mm/s signal evoked a response in 80 mm/s of noise, the cockroach's wind detecting system can operate under a signal-to-noise velocity ratio of ap- proximately 1 : 25.

The responses described above were evoked by wind that was emitted outward from the wind tube positioned behind the animal, and thus had a direc- tion opposite of that of the headwind. Wind puffs of 5 mm/s drawn into the puff tube (and thus flowing posteriorly along with the headwind) evoked pauses on 50% of the trials. Inwardly drawn puffs of 15 mm/s evoked runs on 75% of the trials.

Background Noise

In order to analyze the background noise created by the movements of the cockroach's legs, we made recordings as close as possible (5 ram) behind one cercus of a slowly walking cockroach in the presence

of the headwind. For about half of the recording time, there was no clear pattern on the wind record- ing, as fluctuations of the ambient wind of up to 100 mm/s swamped the record. However, during the rest of the recording time, there was a mean wind speed of approximately 80 mm/s, superimposed upon which were periodic fluctuations that were temporally correlated with the stepping cycle (Fig. 3A). These gusts peaked at about the moment that the ipsilateral metathoracic leg was most posterior in its stepping cycle. These small gusts appeared to be produced by the ipsilateral metathoracic leg since, with the ab- domen pinned in place, these legs were the nearest potential source of wind. Moreover, simultaneous re- cordings f rom behind both cerci showed that the gusts at these two positions alternate (Fig. 3 C) as do the steps of the two metathoracic legs. We shall hence- forth refer to these gusts, in time with the stepping cycle, as ' leg gusts' . These leg gusts did not depend on the particular geometry of the headwind tube since they had about the same appearance regardless of the angle of this tube. Also, the leg gusts were present, though smaller, when the headwind was removed (Fig. 3 D).

Analysis of the leg gusts of 4 animals during slow walking showed that the peak-to-peak amplitudes of more than 50% of these gusts were greater than 3 mm/s, the minimally detectable puff size (Fig. 3 B, 4A). For one of these four animals, more than 75% of the leg gusts were larger than 12 ram/s, the size of the puffs which normally evoked escape running (Fig. 3 A, B, 4A). Yet in the presence of the headwind, as mentioned above, cockroaches walked normally and showed no unusual tendency to pause or run in the course of their stepping.

One possible mechanism by which a cockroach might discriminate signals from noise was revealed by plotting the maximal acceleration of each leg gust of 4 slowly walking animals (Fig. 4B). More than 99% of the leg gusts had accelerations lower than 300 mm/s 2, the acceleration of our 3 mm/s wind puff which was just supra-threshold for a pause response. All the leg gusts had accelerations lower than 750 mm/s 2, that of the 12 mm/s wind puff which con- sistently evoked running responses.

When a cockroach walked with stepping rates greater than 6/s (Fig. 3 C, right), it created wind gusts which, though not necessarily greater in amplitude than those made during slow walking (Fig. 5A), had much greater maximal accelerations (Fig. 5 B). If wind acceleration is the cue by which cockroaches discrimi- nate signals from noise, the rapid accelerations of the leg gusts during fast walking should lead to confu- sion of the noise with the signal. However, it has been shown that the threshold wind puff is higher

M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System 351

A R WIND .-"" 80 mmlsk= I05 mmls__ 3A mmls �9 mmls

R LEG " - " " - ~ ___ __ . . ~ .~ . ~ _ ~ __ . . , B X lO

A

C L W I N D ~ ....... ~ v ' .....

R WIND

R LEG ~ . . . .

R W I N D

L W I N D . . . . " . . . . .

R LEG

D R W I N D ~ - - - - - ~ " J L - - - - - - - - - I I I - - - - - - - ____,_.1 0 ,5 S e C

R LEG ~ - - ~ " . . . . . . . . . . A i

Fig. 3A-D. Recordings of wind near the cerci. A Small wind gusts (top trace) correlated with the stepping cycle (bottom trace; each dot represents the moment the leg was in its most retracted position). The instantaneous wind speeds during the trough and peak of one such gust are shown. The rising slope of one gust, used to compute wind acceleration, is also shown (dashed line). Headwind was on. B Wind puffs delivered from the puff tube and recorded near the cerci. Puffs of 3 mm/s and 12 mm/s (thresholds for pause and run, respectively) are recorded at the same sensitivity as the recording of background wind in A. Also shown is a recording of the 3 mm/s puff at 10 times the sensitivity. C Simultaneous recordings of wind from two anemometer probes, one behind each cercus, first during slow walking (left) and then during fast walking (right). Headwind was on. Sensitivity of both wind traces was half of that in A. D Similar recordings as in A and C, but with the headwind turned off. Sensitivity of wind trace was half that in C

A

160"

120- ~"~ LIJ

Z ".'.2

4 0

0 0 :5 12 15 3 0

t t P E A K - T O - P E A K A M P L I T U D E

( m m / s e c )

B

160

~ 0 I00 200 Z,00 4 0 0 ,500 600 700

t t M A X I M U M A C C E L E R A T I O N

( m m / s e c 2)

Fig. 4A, B. Analysis of wind gusts created by the legs during stepping at rates up to 4/s. Headwind was on. A Peak-to-peak amplitudes were often greater than the threshold puff size for a pause, 3 ram/s; or a run, 12 mm/s (arrows). B The maximal accelerations of almost all the leg gusts were lower than the maximal acceleration of the threshold puff for a pause (300 mm/s2). All were lower than the maximal acceleration of the threshold puff for a run (750 mm/s2). Data from 4 animals are plotted; different symbols represent different animals

d u r i n g fast w a l k i n g t h a n d u r i n g s lowing wa lk ing , of-

fe r ing an a p p a r e n t s o l u t i o n to this p r o b l e m ( C a m h i a n d N o l e n 1981).

Responses to Wind Stimuli of Different Accelerations

T h e r e sponses o f c o c k r o a c h e s to s t imul i f r o m the

ba l sa box, h a v i n g a c o n s t a n t p e a k wind ve loc i ty o f

40 m m / s bu t d i f fe ren t acce le ra t ions , d e p e n d e d u p o n

the w i n d acce le ra t ion . H i g h acce le ra t ions , a b o v e a b o u t 600 m m / s 2, u sua l ly e v o k e d a run (Figs. 6 A ,

7 A ) ; t hose o f a b o u t 3 0 0 m m / s z usua l ly e v o k e d a

pause in the o n g o i n g w a l k i n g (Fig. 6B, 7 A ) ; a n d the

l o w e r the acce l e r a t i on the g rea t e r the inc idence o f no r e sponse (Figs. 6C , 7 A ) . O n l y r e sponses tha t oc-

c u r r e d o n the r is ing phase o f the w i n d s t imul i were

c o u n t e d . 1 A r u n was de f ined as one o r m o r e cycles

o f r a p i d s t epp ing ( > 10/s) tha t o c c u r r e d as the first

r e sponse d u r i n g the r is ing phase . A pause was de f ined

as any de t ec t ab l e ces sa t ion ( inc idence = 9 0 % ) or s low- ing ( inc idence = 10%) o f w a l k i n g tha t o c c u r r e d as the

first r e sponse d u r i n g the r is ing phase . N o r e sponse

was de f ined as no de t ec t ab l e c h a n g e in the w a v e f o r m

o f s t epp ing d u r i n g the r is ing phase .

i Actually, we counted not only the responses that began during the rising phase, but also those that began up to 14 ms after the peak of the stimulus. 14 ms is the mean behavioral latency seen in response to wind puffs of greater velocity and acceleration than those used in this study (Camhi and Nolen 1981). Thus we judged that responses occurring within these 14 ms had to be evoked by the rising phase of the wind stimulus

352

A

Z

t~

Z

B 15.~

I0 ' Z W a i

Z 5"

M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System

. . . . . . 0 0 3 12 15 30 0 I00 200 :300 400 500 600 700

t t t t

P E A K - T O - P E A K AMPLITUDE MAXIMUM A C C E L E R A T I O N

(mrn/sec) (mrn/sec z)

Fig. 5A, B. Analysis of leg gusts during stepping rates greater than 6/s. Headwind w a s o n .

A Peak-to-peak amplitudes as during slow walking (Fig. 4A) were usually greater than the threshold puff size for a pause (3 mm/s). B The maximal accelerations also were greater than the maximal acceleration of the threshold puff for a pause (300 mm/s2). Data from three animals are plotted; different symbols represent different animals

A LEG - - ~ WJNO ~ \

I

B

C p r , ,, , , . . , , , , ,

D

E

,f= i i ~

j- 5 0 0 rnsec

Fig. 6A-E. Response of the right metathoracic leg of slowly walking cockroaches to wind stimuli of different accelerations. For each of the five trials A-E the top trace is a recording of the position of the right metathoracic leg (p protraction phase; r retraction). Middle trace: instantaneous wind velocity. Bottom

trace. a recording of the same wind as the middle trace, but inverted and at 5 • the sensitivity, permitting one to see more clearly the time of onset of the wind (arrows). However,the actual measurements were made from films having a time scale 2.5 x greater than is shown here, affording more accurate time measurements. The blank space in the two wind traces just prior to the stimulus is an artifact of the signal to the electromagnet. A Peak velocity 40 mm/s, acceleration 1500 mm/s 2. Running response was evoked early in the rising phase of the wind. B Peak velocity 40 ram/s, acceleration 320 mm/s z. Pause was evoked during the rising phase of the wind, followed by running after the end of the falling phase. C Peak velocity 40 mm/s, acceleration 120 mm/s 2. No response during the rising phase of the wind, followed by a running response late in the falling phase. The run began 40 ms after the stimulus peak and thus, by criteria explained in the text, appears to have been evoked by the falling phase of the wind. D Peak velocity 24 mm/s, acceleration 150 mm/s 2. A pause was evoked during the rising phase. E Peak velocity 60 ram/s, acceleration 180 mm/s 2. Response same as D, except that the ruff was delayed until the beginning of the falling phase

There was a special concern regarding instances of no response, since these were usually followed by a pause and/or run that occurred during the falling phase of the wind. It was possible that this 'second- ary ' response occurring on the falling phase was in fact a long latency response to the rising phase. How-

ever, we demonstrated that this generally was not the case in the following experiment. For seven ani- mals (36 trials) we either increased or decreased the duration of wind stimuli of low acceleration and we carefully examined the temporal relationships of the wind and the insect's responses. If the falling phase

M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System 353

of the wind signal could evoke responses (and thereby change a no response into a pause or a run) one would expect that the secondary response would oc- cur at a relatively fixed latency after the peak of the wind. On the other hand, if the secondary re- sponse was independent of the falling phase, one would expect it to remain at a relatively fixed latency with respect to the onset of the rising phase. In most cases (69%) the secondary response occurred within 20-50 ms after the peak (Fig. 6D, E). z In two cases ( = 6 % ) with long duration ramps the secondary re- sponse occurred approximately 80 ms after the peak. In 9 cases (=25%) a secondary running response did occur within the rising phase, though it was al- ways preceded by a pause. In no trial, either in this series or any other, was running the initial response seen within the rising phase of our lowest acceleration wind puff. On the basis of these data we concluded that (1) the falling phase of the wind can evoke a response independent of the rising phase; and (2) an animal which fails to respond to a wind puff until after the peak of the wind signal 1 is most likely re- sponding to the falling phase, not the rising phase.

Although our stimulator was unable to deliver accelerations below 120 mm/s 2 less controlled obser- vations indicated that the incidence of no response increased for accelerations lower than this. These ob- servations involved simply removing the lid of the balsa box and relying upon air currents in the room as a source of wind. In general, acceleration of the wind was lower than 50 mm/s 2. Only very rarely did gusts of these lower accelerations evoke any response from the animal, even for velocities up to 100 mm/s.

These behavioral responses were evoked by wind and not by other cues since; (1) covering the cerci with petroleum jelly abolished all responses to the movement of the box; (2) covering various other parts of the body with petroleum jelly left the response intact; (3) leaving the cerci uncovered but covering the entire animal with a transparent lucite chamber (sealed to the lucite platform with wax) abolished all responses to movement of the box; (4) cockroaches are known to use their cercal wind receptors to re- spond to stimuli having peak wind velocities at least 10 times lower than the present stimuli (Camhi and Nolen 1981).

The cockroach's discrimination of different wind accelerations was generally similar in the presence o f a headwind (Fig. 7B) as without it (Fig. 7A). Thus cockroaches were able to decode acceleration even in the presence of a background wind flow of about 80 mm/s. The discrimination of acceleration was also

2 These data include instances of an initiai no response followed by a secondary pause and/or run, as well as an initial pause followed by a run

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WIND ACCELERATION (mm/sec a ) Fig. 7A-D. Incidence of running, pausing and no response as a function of wind acceleration. All stimuli had the same peak velocity, 40 mm/s. A Wind from behind. No headwind. 11 animals, 181 trials. B Wind from behind. Headwind on. 4 animals, 81 trials. C Wind from 90 ~ right. No headwind. 9 animals,61 trials. D Wind from the front. No headwind. 9 animals, 61 trials. Under all condi- tions the highest acceleration gave the greatest incidence of running, the lowest acceleration gave the greatest incidence of no response, and intermediate accelerations generally gave the greatest incidence of pauses

similar when wind stimuli were presented from 90 ~ right (Fig. 7C) or from in front of the animal (Fig. 7D) rather than behind. There was, however, generally a higher incidence of no response for wind from 90 ~ than from the other directions (Fig. 7C). The direction of stimulation was varied by rotating through 90 or 180 ~ the lucite platform upon which the cockroach walked. No headwind was used and the anemometer probe was moved as needed to keep it near the cerci and oriented properly, orthogonally to the wind and not obstructing the wind flow over the cerci.

Responses to the Acceleration of a Predator's Wind

Having demonstrated the role of different constant wind accelerations in determining the cockroach's re- sponse, we wished to examine the response to a natu- ral source of wind, whose acceleration was not con- stant. The cockroach is known to give a running re-

354 M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System

sponse to the wind created by the strike of a na tura l predator , the toad Bufo marinus (Camhi et al. 1978).

We measured the acceleration of the toad 's wind at different momen t s dur ing the strike. We then looked for a correspondence between the latency of the cock- roach's response to this wind of varying acceleration and the latencies of the responses to winds of cons tan t acceleration produced by the balsa box.

The latency of the response to cons tant wind accel- erat ions was generally shorter and less variable the higher the acceleration (Fig. 8). How do the specific

values of latency found compare to the latency in response to the wind from the toad 's strike? Figure 9 shows an example of one wind puff f rom a toad 's strike, measured at the posi t ion of a cockroach at which the toad was striking (from Camhi et al. 1978). The cockroach had been anesthetized with CO2. The arrow labeled " T " points to the m o m e n t when the toad 's tongue first emerged from its mouth , as deter- mined from cine films at 64 frames/s (Camhi et al. 1978). The arrow labeled " R " points to the m o m e n t

that is the mean time prior to T when freely moving cockroaches begin their evasive responses to strikes by toads (Camhi et al. 1978). We scanned this record-

ing from left to right to determine the first m o m e n t when a wind acceleration of 600 mm/s 2 occurred (dashed line, Fig. 9). This is the acceleration at which

50% of the stimuli f rom the balsa box evoked runn ing responses (Fig. 7A). 3 This m o m e n t is indicated by the arrow labeled ' S ' on Fig. 9. The time from S to R in Fig. 9 is 56 ms. On all 18 toad strikes recorded like that of Fig. 9 (Camhi et al. 1978), the mean time f rom S to R was 41 ms. This is close to the behavioral

latency expected for wind puffs having an acceleration of 600 mm/s 2, namely 44 ms (Fig. 8). 4 This corre-

spondence suggests that wind acceleration is an im- por tan t feature by which the cockroach detects the wind made by a toad 's strike. In fact, the match

3 The acceleration at which 50% of the stimuli evoked running varied with the stimulus conditions. It was 600 mm/s 2 for wind from behind, with no headwind (Fig. 7A). It was slightly less for wind from behind, with a headwind (Fig. 7B). However, it was greater about 800 mm/s 2 for wind from the front (Fig. 7D). Cockroaches usually encounter toads by walking to- ward them (Camhi et al. 1978a). Thus the wind stimulus would normally come from near the front. However, we regard the detailed shape of the curve for wind from behind (Fig. 7A) as much more accurate than that for wind from the front (Fig. 7D) since it is based upon three times as many data points. (Most data points of Fig. 7 D are the means of only four trials.) Thus, although it is possible that wind from the front requires a slightly higher acceleration, our data do not demonstrate this

4 Actually these puffs didn't reach these accelerations of 600 mm/s 2 until nearly the end of this 44 ms period (Fig. 2). Therefore, the mean acceleration was actually less than 600 mm/s 2. Howev- er, even if it had been possible to deliver a pure 600 mm/s 2 stimulus, the latency probably would have been about the same, since the latencies measured for greater accelerations were not significantly less than for our 600 mm/s 2 stimulus (Fig. 8)

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Fig. 9. Wind produced by the strike of a toad at a cockroach anestheized with CO2 (modified from Camhi et al. 1978). The cock- roach, attached to a thread, had been moved slightly, moments before, to attract the toad's attention and evoke the strike. The wind was recorded by an anemometer whose active wire was locat- ed within 1 cm of the cockroach. The three traces show the same wind recorded at high, medium and low sensitivity. From cine films of the strike it is known that the toad's tongue first emerged from its mouth at time T (arrow). Arrow R is drawn at a time prior to T equal to the mean time before the emergence of the tongue that freely ranging cockroaches began their escape responses to toads. Arrow S represents the first moment that the toad's wind achieved an acceleration of 600 mm/s 2. This acceleration is indicated by the dashed line. The interval from S to R on this trial was 57 ms

is much less good between 44 ms and the mean time from the onset of the min imal wind velocity that evokes r unn i ng (12 mm/s ; Camhi and Nolen 1981) to the onset of movement by the cockroach. This mean time was 63 ms (Camhi, unpubl ished) . This mis- match again supports wind acceleration and not ve- locity, as the key st imulus parameter.

This in terpre ta t ion of earlier results was derived f rom a correlat ion of two sets of data collected sepa- rately: first, the m o m e n t - t o - m o m e n t accelerat ion of the wind produced by the strikes of toads at anesthe- tized cockroaches (e.g., Fig. 9); second, the latencies of responses of unanes thet ized cockroaches to the strikes of toads, where the wind f rom the toad could

M.R. Plmnmer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System 355

not simultaneously be measured (mean 41 ms). In the present work we also made simultaneous recordings of the toad's wind and the cockroach's response, as described in the Methods section. The toad's strikes were from behind the cockroach, which was re- strained but able to make normal leg movements that were monitored by photocells. For each of the five strikes at slowly walking cockroaches we determined the moment when the wind acceleration reached 600 mm/s 2, and then measured the latency from that moment to the onset of the cockroach's response. The mean latency was again 41 ms, a close match to 44ms, the latency in response to stimuli of 600 mm/s 2 from the balsa box. This further suggests that cockroaches respond specifically to the accelera- tion of the wind made by the toad.

Prior to reaching an acceleration of 600 mm/s 2, the toad's wind had to pass through lower accelera- tions which, when presented alone, usually evoked a pause in walking (e.g., 300 mm/s2; Fig. 7). For the 18 toad strikes previously recorded, the wind reached an acceleration of 300 mm/s 2, on average, 81 ms be- fore the cockroach began its response. Since the mean latency of the pause response to a 300 mm/s 2 ramp was 78 ms (Fig. 8), there should be little tendency by a cockroach to pause prior to running in response to the strike of a toad. In fact, of the five runs just described, made by restrained cockroaches in re- sponse to toad strikes, all occurred without detectable prior pauses.

Responses of Giant Interneurons to Winds of Different Constant Accelerations

What is the physiological basis of the cockroach's discrimination of different wind accelerations? One approach to this question is to ask whether the giant interneurons (GI's), which appear to contribute to evoking the escape behavior (Westin et al. 1977 ; Ritz- mann and Camhi 1978; Ritzmann 1979; Camhi and Nolen 1981) are more strongly activated by stimuli of high than of low acceleration. Since all the stimuli had the same maximal wind velocity, and those of lower acceleration had a greater total air displace- ment, any preferential responsiveness of the GI's to stimuli of higher acceleration should be related to the cockroach's behavioral response to these wind.

Of particular interest were ventral GI's 1, 2, and 3, since these neurons have been implicated as possi- bly initiating the escape behavior (Camhi and Nolen 1981). As it was not possible to record intracellularly from the GI's while the cockroach was in the balsa box, we had to rely upon extracellular recordings. Since the axons of each of the GI's 1, 2, and 3 have approximately twice the diameter of any other axons in the abdominal connectives (e.g., Fig. 5 of Westin

et al. 1977), we expected their extracellularly recorded action potentials to be large relative to all others. To verify this we made the following tests. Recordings were made with hook electrodes from the left abdom- inal A~_ 5 connective of cockroaches that walked in place. It is known that the dorsal GI's (5, 6, 7, and others) but not the ventral GI's (1, 2, 3, and 4) give action potentials at high frequency during walking (Delcomyn and Daley 1979; Daley and Delcomyn 1980a; Camhi unpublished observations). Therfore we tentatively regarded spikes occurring at high fre- quency during slow walking in the absence of wind stimuli as those of the dorsal GI's and smaller axons. Only GI's 1, 2, and 3 should produce action potentials larger than these. Thus, it was these larger action potentials that we counted. We were able to verify that this procedure was legitimate by comparing these extracellular responses with previously recorded intra- cellular responses from identified GI's. In those prior intracellular studies (Camhi and Nolen 1981) a puff of 12 mm/s delivered from behind to a standing cock- roach evoked a mean of about 4 action potentials, total, in GI's 1, 2, and 3 of one side. s The same puff delivered to a slowly walking animal evoked just two action potentials in the same three GI's. Puffs evoked fewer spikes during rapid than during slow walking. In our extracellular recordings, using these same wind puffs, we counted means of 6 spikes during standing, 4 during slow walking and 2 during fast walking- a reasonable match to the earlier intracellu- lar recordings. Since the responses to wind of the dorsal GI's do not decrease, but actually increase during walking (Daley and Delcomyn 1980b), there was apparently little contamination of our counting from spikes of dorsal GI's. Therefore we are confident that our counting procedure gives fairly accurate esti- mates of the responses of the left GI's 1, 2, and 3.

In general, both the number of action potentials and the mean frequency of action potentials recorded from the ventral GI's were greatest for stimuli of high acceleration (Fig. 10A, B). The GI response to wind acceleration that most often evoked a pause in walking (300 mm/s2; Fig. 7) gave a mean of only 2 action potentials, and these were separated by a mean of about 40 ms. Running responses, evoked most often by higher wind accelerations, apparently required more action potentials at higher frequencies as had been suggested earlier (Camhi and Nolen 1981). As a further suggestion of the role of the GI's in these behaviors, the latency to the first GI action potential that we counted had a similar relationship to wind acceleration (Fig. 10C) as did the behavioral latency (Fig. 8), approaching an asymptote at acceler- ations greater than about 600 mm/s 2.

5 Since GI 3 responds only to wind from the front (Westin et al. 1977), these recordings represent responses only to GI's 1 and 2

356 M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System

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Fig. 10A-C. Responses of putative left GI's 1, 2, and 3 to wind stimuli of different accelerations. A The higher the wind accelera- tion, the greater the number of action potentials. Mean + 1 S.D. B The higher the wind acceleration the greater the spike frequency. Median values are shown, since any response consisting of only 1 action potential representing a spike frequency of zero, would grossly distort the mean value. C The higher the wind acceleration the shorter the latency from the onset of wind to the first action potential. Mean +1 S.D. shown. Data of A, B, and C based on the same 7 animals and 119 trials

Discussion

We have reported here that cockroaches can detect wind signals approximately 25 times smaller in ampli- tude than background noise. The wind signals, how- ever, have greater accelerations than are generally contained in the noise, suggesting a possible means of discrimination. We have verified that wind acceler- ation is a crucial parameter in this discrimination, by delivering stimuli of controlled acceleration. The

discrimination of different wind accelerations permits the cockroach to maintain high sensitivity during in- herently windy periods of locomotion. This is most clearly demonstrated by the observation (Fig. 7B) that different wind accelerations are discriminated even when superimposed on a headwind of at least 80 mm/s, simulating the relative wind that a cock- roach would experience during slow walking.

Our results show that a slowly walking cockroach stimulated by wind has three behavioral options. First, it can run, and does so if the wind acceleration is sufficiently high. Freely ranging cockroaches begin such runs by turning away from the wind source and in so doing escape from predators (Camhi and Tom 1978; Camhi et al. 1978). Secondly, the cockroach can pause, and it does so in response to intermediate wind accelerations. It has been suggested that pauses may occur in response to winds that evoke insufficient numbers of action potentials in the directionMly selec- tive giant interneurons to permit directional localiza- tion of the wind source (Camhi and Nolen 1981). A pause may make the cryptically colored cockroach invisible to a nocturnal predator (Ewert 1976) and thus may be the next best option to an oriented run- ning response. Third, the cockroach may not respond, as is the case with the lowest wind accelerations, such as those contained in background noise.

There are two possible ways that the cockroach could attain selectivity to high wind acceleration: (1) by a mechanical sensitivity of the receptor hair to wind of high accelerations, or (2) by a neuronal com- putation of wind acceleration at the stage of mechano- electrical transduction or later in the sensory process- ing. Concerning the first possibility, the cereal filiform hairs have been shown to have a resonance frequency of 100-200 Hz (J. Tautz, personal communication). Thus if a high acceleration stimulus contained signifi- cantly more energy in this frequency range than did a low acceleration stimulus, the former would deflect the hairs by a greater amount for a given stimulus velocity. However, even for the fastest ramps we used, the rising phase approximated a sine wave of only 5 Hz (Fig. 2). Moreover, 5 Hz appears to be fairly close to the highest frequency for which evolution has designed this system, since the graphs of both percentage running response (Fig. 7) and behavioral latency (Fig. 8) are nearly saturated by our most rap- idly accelerating stimuli. 6

Concerning the second possibility, a theoretical analysis (Fletcher 1978) suggests that the displace- ment of a filiform hair should be proportional to air

6 Wind puffs of much higher acceleration and much higher velocity do evoke escape responses with a mean latency of only 14 ms (Camhi and Nolen 1981). However, we do not know whether the acceleration or the velocity is the critical feature evoking these short latency responses

M.R. Plummer and J.M. Camhi: Wind Acceleration and the Cockroach Escape System 357

velocity, not acceleration. Since peak velocity was the same for each of our standard ramp stimuli (Fig. 2), it is likely that the hairs were deflected ap- proximately the same amount by each stimulus. Thus selectivity to wind acceleration would then have to be computed by the nervous system, perhaps at the level of mechano-electrical transduction. Although the amount of hair displacement was probably the same for all our stimuli, the velocity of hair displace- ment should be greater the higher the wind accelera- tion. Thus one possible solution would be for the sensory neurons to respond in proportion to the ve- locity of hair deflection. Many mechano-receptors are known to be selectively sensitive to the velocity of movement of the receptor apparatus (e.g., vertebrate muscle spindles - Hunt 1974; tibial thread hairs of c r icke t s - Gaffal and Theiss 1978; trochanteral hairs of cockroaches - French and Wong 1977). Indeed, recordings made from axons of cercal wind receptors in response to wind puffs indicate that at the moment the wind is maximally accelerating, the sensory re- sponse is maximal. When the wind reaches its highest velocity, but zero acceleration, the sensory response has decreased to almost zero (Dagan and Camhi 1979). Thus the selective responsiveness to high wind acceleration may occur through filtering by the senso- ry cells, and this could come about as part of the transductive process.

Aside from the noise presented to the cockroach's wind receptors by the insect's own walking, ambient wind currents present a potential problem. In the cockroach's native tropical habitat, however, wind acceleration appears to be low, particularly at night when these insects are active. Estimates put the high- est acceleration under normal conditions at 2 mm/s 2 (Camhi et al. 1979). Even if these estimates are low by an order of magnitude, environmental wind would not present a problem although its velocity can be higher than the cockroach's behavioral threshold (Camhi et al. 1979). It is possible that part of the cosmopolitan success this species has had in establish- ing secondary habitats in buildings, ships, sewers, mines, and the like (Roth and Willis 1960) is attribut- able in some small part to the protected wind environ- ments that these structures offer. Numerous species of predator are found in many such habitats, includ- ing the toad Bufo marinus which co-habits tropical caves and mines with cockroaches (J. Simmons, per- sonal communica~:ion). In such an environment the

Note Added in Proof

cockroach should be able to detect the strike of such a predator and often escape from it.

We thank R. Ritzmann, C. Comer, D. Daley, N. Vardi and S. Volman for reading the manuscript. E. Sherman contributed to early experiments in this paper. T. Nolen contributed the data on intracellular recordings of the GI's. S. Volman provided techni- cal assistance. This work was supported by NIH No. NS09083.

References

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The response of cercal wind-receptor axons to ramp displacements of their hairs has recently been shown to be proportional to the velocity of hair movement (Brufio et al. 1981). This supports the indirect evidence, presented in the Discussion, that pointed to the same conclusion.

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