Startle Reaction Times in the Starling (Sturnus vulgaris)

Startle response times of Starlings (Sturnus vulQaris) to auditory and visual stimuli have been determined. Birds were placed into an anechoic chamber and exposed to either a one msec flash of unfiltered white light, or a three msec pure tone burst. An electronic detection system for monitoring and recording the activity of the birds was designed and used for the experiments. Display modes of the recording apparatus allowed for an accuracy to I0-4 seconds in measuring reaction times. The mean reaction time of birds to light stimuli was 76.38 msec ± 15.32 msec. The mean reaction time to sound stimuli was 80.64 msec + 14.40 msec.

. The assumption of the use of auditory or visual signals to coordinate the apparently synchronous turning and wheeling movements of birds in flocks is contingent upon knowledge of the response times of t he species involved.
Current data, and related hypotheses concerning reaction times (RTs) in birds are based upon speculations on the physiological and anatomical characteristics of the avian sensory systems (Pumphrey, 1961), or from observations of the temporal properties of vocalization of various species of birds (Thorpe, 1963;Grimes, 1965;Greenewalt, 1968). Thorpe (1963)  Thorpe suggested that the high degree of precision of time-keeping in the duets could render recordings of duetting pairs of birds useful in the establishment of auditory RT. Using spectographic analysis of recordings of duetting pairs of Black-headed Gonoleks as evidence for auditory RT, Thorpe (1963) reported a mean RT of 144 msec, with a standard deviation (SD) of 12.6 msec for the fastest single pair performance recorded. Grimes (1965)

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The latent period of startle response, this being the time from occurrance of stimulus to elicitation of response, was used as an index of RT. The criteria for a response did not require the birds to move from any prestartle position on the perch, as initial experiments demonstrated that birds' startle responses uniformly consisted of a rapid and virtuall y unnoticable sequence of flexor contractions such that the bird appeared to go into a semi-crouch position, resembling the startle pattern described for mammals by Landis and Hunt (1939).
A comparison of RT measurements in humans (Costa, Vaughan and Gilden, 1965) using both electromyographic and microswitch sensors noted a 7.5 msec difference in results, due to the lag time of the mechanical system. My efforts were thus focused on the design of a completely non-mechanical electronic system for the continuous monitoring of the activity of caged birds.
Experiments were conducted in an anechoic chamber (Fig. I).
Calibrations of the ambient noise level in this chamber, and the intensities of the auditory and visual startle stimuli were done with a Bruel & Kjaer precision sound level meter, and the digital readout photometer of the Electrical Engineering laboratories at the University of Rhode Island. Auditory calibration was performed in a manner similar to that described by Hoffman and Searle (1968).
The experimental cage was placed in the anechoic chamber. The sound level meter was then placed inside this cage and a large number of measurements taken. The meter was moved after each measurement, until the entire area of the cage had been sampled. The intensity of the ambient noise in the cage was defined as the mean of the distribution of the measurements for ambient noise intensity.
Sound pressure readings of the room in which the chamber was located for sounds in the 100 Hz to 20,000 Hz range were consistently 60 to 65 dB. This range was of crucial interest because the range of sounds audible to birds is approximately 200 Hz to 20,000 Hz (Sturkie, 1965). The noise level inside the chamber (28 to 32 dB) was 30 dB below that in the room. A fixed position measurement at the point in the chamber occupied by the birds was used to determine the SPL of the auditory stimulus. A reading of 120 dB+ I dB was obtained.
The mean of a series of trials was used as an index of the intensity of the light stimulus. The value obtained for 10 trials was 27 ft-  A controlled level of diffuse backround illumination was obtained from a variable intensity lamp which was mounted behind a gauze screen, and built into the ceiling of the inner area of the anechoic chamber.
All experiments were conducted with a backround illumination of five ft-Lamberts + 15%.

Auditory Stimulus
The acoustic stimuli originated from a modified Avid Pulser/Mixer-Variable Pitch Tone Burst Generator. This unit provided for control of tone frequency (0 Hz to 16,000 Hz) and duration (3 msec to 30 msec) of the burst of sound. A 2,000 Hz pure tone signal, duration three msec, traveled through a Heath 25 watt amplifier before reaching a midrange speaker of 12.5 cm diameter. The intensity of the sound burst was 123 dB. The attenuation of the burst traveling from the source to the bird inside the cage was two dB to three dB.
Thus the actual burst arriving at the birds' heads was close to 120 dB + dB.
Following Hoffman and Fleshler's (1963) suggestion that a backround of stead y noise facilitates acoustic startle by masking out random pulses of noise, white noise was added to the low level of ambient noise in the inner chamber. The white noise was generated by an Audiolab random noise generator, and fed through the Heath amp-1 ifier to a speaker mounted inside the chamber. This speaker also served as the acoustic stimulus transducer. The combined level of ambient noise and generated white noise inside the cage was 45 dB .± ldB.

Data Recording
A single-throw, double-pole visual stimulus switch permitted simultaneous closing of the flash circuit and a 12 volt DC trigger circuit wired to a do uble-throw, double-pole switch. The Avid Pulser/Mixer was equiped with a built-in trigger circuit which was also wired to the double-throw, doub le-pole switch. The corrrnon poles of this switch were wired to the starter pickup of a digita l timer, and one channel each of a Grass polygraph and Tektronic dual sweep oscilloscope. This design facilitated easy change from auditory to visual stimulus utilization within the system (Fig. 3). Startle reaction times to light flash stimuli for 18 Starlings are shown in Figure 6. The mean of the means and mean stand-     RTs from the birds. As there was no significant difference between the mean RTs of different birds to light stimuli, the mean of the means (76.6 msec) is assumed as a representative fi gure (± 5%) for the mean RT of Starlings to light flash stimuli. Thorpe (1963) recorded duetting bird songs in the field. He assumed that the birds were equidistant from the microphone, and estimated this distance to be between 10 and 20 meters. Incorporating the speed of sound with possible distance errors encountered, Thorpe predicted the true mean RT of the birds recorded to be between 90 msec and 135 msec (minimum RTs between 70 msec and 116 msec), with a SD of 12.6 msec. The results reported here for auditory RTs are in close agreement with the estimates of avian auditory RT proposed by Thorpe. Fleshler (1965) concluded that startle reaction in the rat is invariant over a wide range of stimulus durations. The time at which the stimulus reaches and remains at peak intensity is critical only in that it occurs in an initial critical period, 12 msec in the rat. This initial period is equal to, or less than, the RT minus the time for neural transmission involved in the perception of stimulus and evocation of response. In the rat, the initial period is about 75% of the total RT (Fleshier, 1965). The RT values obtained in the present experiments wou ld yield an initial period of over 75% of the total RT. Theref6re, the acoustic bursts of three msec duration reached peak intensity well within the probable limits necessary to elicit minimum or near minimum startle reaction times.
Startle response latency for rats as determined by Fleshler (1965), Hoffman and Searle (1963), and Landis and Hunt (1939), is approximately four to five times faster than that obtained for birds in this investigation. Fleshier (1965) makes a conservative estimate that 25% of the total RT to acoustic startle stimuli in the rat is involved in the time required for neural transmission. Investigations of neural transmission rates of nerve fibers in mammals (Prosser and Brown, 1966) has shown that mylinated fibers transmit impulses at 100 to 120 m/sec (large diameter fibers), and 25 to 50 m/sec (smal l diameter fibers). Birren and Wall (1956) reported a conduction velocity of 60 m/sec in the rat.
One would seemingly have to assume a slower rate of transmission, or a proportionally longer distance of travel, or a combination of both in birds, to account for the longer latency of response. Graf (1956), investigating representative sections of the peripheral nervous system in the Rock Dove (Columba livia), reported an absence of larger diameter fibers. Sturkie (1965) concludes from this observation that the conduction velocities of nerve fibers shoul~ be less in birds than in mammals. Investigations of conduction velocity of nerve impulses in chickens would support this hypothesis (Carpenter and Bergland, 1957). No data on conduction velocity of impulses in Starlings is presently available. However, using Carpenter's measure of 40 m/sec, and assuming an approximately equal distance of impulse travel as that estimated by Fleshier (1965) for in the rat, a time of six msec ut ilized for nerve transmission is calculated.
This represents only 8% of the total startle response time of Starlings to auditory stimuli. This suggests that differences 29 in nerve conduction veloci t y and distance of impulse travel may account for as little as 12% of the observed increase in RT seen in birds. Hoffman and Searle (1963) suggested that the organization of startle occurs in some specific brain center, and it is the level of activity of this center t hat determines the elicitation of startle by the individual, and the brevity of response time.
Differences in the functioning of a center such as this could account for the increase in RT noted. Circumstantial evidence {Pomeroy, unpublished) has indicated that birds may be able to maintain two discrete functioning levels of this center, and thus respond to the same stimuli at very different rates.