344
PE. Nachtigall et aI.
model would predict. These high repetition rate data suggest a time constant of about 250/ls.
Another method for examining temporal integration presents clicks in
pairs. If the two clicks are within the integration window, that is, if they are
separated by less than the time constant, then the threshold for detecting
the two clicks should be lower than the threshold for detecting a single click.
In one experiment of this type (Au et al. 1988) the minimum intensity for
single clicks was first determined using a staircase procedure. The staircase
procedure was then repeated for each of eight double-click, echo-like signal
types in which the separation between the two clicks (~t) varied between
50 and 600/ls.At separations less than about 250/ls, the threshold for detecting double-click signals was approximately 3 dB below the threshold for
detecting single-click signals. At separations above 250/ls the double-click
threshold began to shift toward the threshold for single clicks and matched
this threshold by separations of 300/ls. The best fit to these data yielded a
time constant, 1, of 264/ls, which corresponds reasonably with the 250/ls
integration found for click trains (Vel'min et al. 1975), but is about 1% of
the value found for tonal signals (Johnson 1967).
Au et al. (1988) used a "phantom echo" technique to assess the effects of
single versus double clicks. An electronic apparatus was constructed to
detect the dolphin's outgoing echolocation signal and to activate the singleor double-click stimulus after an appropriate delay. In this way, the returning signal mimicked the echo the animal would receive from an object
with either one (single click) or two (double click) reflecting surfaces.
The phantom echo was used in response to the speculation that dolphins
might have two hearing systems, one for communicative signals and one for
echolocation signals (Vel'min and Dubrovskiy 1975). The communicative
system is presumably used for passive hearing and the echolocation system
is used for returns from active echolocation. To the extent that this experiment was successful and the animal did actually engage those hearing
mechanisms used in active echolocation, these data argue against this speculation because the same time constant was found for combining clicks that
were perceived passively (Vel'min et al.1975) as well as for those perceived
as responses to outgoing echolocation clicks.
Essentially the same time constant was also found using a backward
masking technique (Moore et al. 1984). If the dolphin integrates a noise
event with the returning echo, then the presence of the noise will mask or
interfere with the detection of the click signal by reducing the signal-tonoise ratio. A dolphin was trained to echolocate on a water-filled aluminum
cylinder located about 46 m in front of it and to report its presence or
absence. A broadband noise burst was timed arrive at the dolphin 100, 300,
500, or 700/ls after the arrival of the echo from the cylinder. Delays of 500
or 700/ls had little effect on the dolphin's ability to detect the cylinder, but
the shorter delays interfered with the animal's detection of the cylinder.
Detection performance was at chance for the 100/ls delay and the calcu-
PE. Nachtigall et aI.
model would predict. These high repetition rate data suggest a time constant of about 250/ls.
Another method for examining temporal integration presents clicks in
pairs. If the two clicks are within the integration window, that is, if they are
separated by less than the time constant, then the threshold for detecting
the two clicks should be lower than the threshold for detecting a single click.
In one experiment of this type (Au et al. 1988) the minimum intensity for
single clicks was first determined using a staircase procedure. The staircase
procedure was then repeated for each of eight double-click, echo-like signal
types in which the separation between the two clicks (~t) varied between
50 and 600/ls.At separations less than about 250/ls, the threshold for detecting double-click signals was approximately 3 dB below the threshold for
detecting single-click signals. At separations above 250/ls the double-click
threshold began to shift toward the threshold for single clicks and matched
this threshold by separations of 300/ls. The best fit to these data yielded a
time constant, 1, of 264/ls, which corresponds reasonably with the 250/ls
integration found for click trains (Vel'min et al. 1975), but is about 1% of
the value found for tonal signals (Johnson 1967).
Au et al. (1988) used a "phantom echo" technique to assess the effects of
single versus double clicks. An electronic apparatus was constructed to
detect the dolphin's outgoing echolocation signal and to activate the singleor double-click stimulus after an appropriate delay. In this way, the returning signal mimicked the echo the animal would receive from an object
with either one (single click) or two (double click) reflecting surfaces.
The phantom echo was used in response to the speculation that dolphins
might have two hearing systems, one for communicative signals and one for
echolocation signals (Vel'min and Dubrovskiy 1975). The communicative
system is presumably used for passive hearing and the echolocation system
is used for returns from active echolocation. To the extent that this experiment was successful and the animal did actually engage those hearing
mechanisms used in active echolocation, these data argue against this speculation because the same time constant was found for combining clicks that
were perceived passively (Vel'min et al.1975) as well as for those perceived
as responses to outgoing echolocation clicks.
Essentially the same time constant was also found using a backward
masking technique (Moore et al. 1984). If the dolphin integrates a noise
event with the returning echo, then the presence of the noise will mask or
interfere with the detection of the click signal by reducing the signal-tonoise ratio. A dolphin was trained to echolocate on a water-filled aluminum
cylinder located about 46 m in front of it and to report its presence or
absence. A broadband noise burst was timed arrive at the dolphin 100, 300,
500, or 700/ls after the arrival of the echo from the cylinder. Delays of 500
or 700/ls had little effect on the dolphin's ability to detect the cylinder, but
the shorter delays interfered with the animal's detection of the cylinder.
Detection performance was at chance for the 100/ls delay and the calcu-