338
J. DIAMOND
Our conclusion that fatigue is a genuine property of the Mauthner cell
is strengthened by our observation that the VIIIth nerve electrical and
chemical inhibition and excitation of the Mauthner cells was quite unaffected at frequencies of stimulation higher than stimulation frequencies
which very rapidly caused all the fatiguable systems mentioned above
to cease operating completely in the same experimental preparations.
B. Directionality Sensing: An Error
In Section IV, C we saw how the minimum discrimination time
measurcs the ability of the spinal circuitry to resolve (as separate
events) two impulses, one in each Mauthner axon; this time can be as
small as 150 pst'c. Consider a sound, originating to one side of the head,
on a line with both vestibular systems, If we ignore the influence of the
swim bladder and accept the situation discussed in Section VIII of an
asymmetrically effective stimulus, we can estimate approximately
whether the sound could cause the nearer Mauthner cell to fire an
impulse 150 psec or more iii advance of the opposite cell. Only if this
happens can there be a response from the animal giving information as
to the side from which the sound originated. The sound would travel
through water, or goldfish tissue, at about 1500 meters/sec; if the
vestibular systems were 15 mm apart it would affect the nearer one some
10 psec before the other. Even if we allow for some loss in the amplitude of the stimulus in its transmission through (and around) the head,
and thus for a very slight effect on the relative magnitude of the resultant
VIIIth nerve volleys (Section VIII, D ) , it is difficult to believe that the
10 psec differential could be stretched to 150 psec. The system is clearly
not designed for directioiiality sensing of sound. ( I n the following section we mention the possibility of optical stimuli reinforcing sound as
an orientating mechanism.) The suggestion made by Moulton and
Dixon (1967) that the Mauthner cells provide a basis for directionality
sensing must therefore be rejected; moreover since Mauthner cell activity
was measured 'by the occurrence of a tail flip, which our observations
show can result from the activation of other cclls in the brain and spinal
cord, their evidence must be regarded as inadequate.
C. The Avoidance Reaction: A Genuine Function
When is the Mauthner reflex likely to be of greatest value? One
likely proposition relates to birds of prey, and this was first suggested
J. DIAMOND
Our conclusion that fatigue is a genuine property of the Mauthner cell
is strengthened by our observation that the VIIIth nerve electrical and
chemical inhibition and excitation of the Mauthner cells was quite unaffected at frequencies of stimulation higher than stimulation frequencies
which very rapidly caused all the fatiguable systems mentioned above
to cease operating completely in the same experimental preparations.
B. Directionality Sensing: An Error
In Section IV, C we saw how the minimum discrimination time
measurcs the ability of the spinal circuitry to resolve (as separate
events) two impulses, one in each Mauthner axon; this time can be as
small as 150 pst'c. Consider a sound, originating to one side of the head,
on a line with both vestibular systems, If we ignore the influence of the
swim bladder and accept the situation discussed in Section VIII of an
asymmetrically effective stimulus, we can estimate approximately
whether the sound could cause the nearer Mauthner cell to fire an
impulse 150 psec or more iii advance of the opposite cell. Only if this
happens can there be a response from the animal giving information as
to the side from which the sound originated. The sound would travel
through water, or goldfish tissue, at about 1500 meters/sec; if the
vestibular systems were 15 mm apart it would affect the nearer one some
10 psec before the other. Even if we allow for some loss in the amplitude of the stimulus in its transmission through (and around) the head,
and thus for a very slight effect on the relative magnitude of the resultant
VIIIth nerve volleys (Section VIII, D ) , it is difficult to believe that the
10 psec differential could be stretched to 150 psec. The system is clearly
not designed for directioiiality sensing of sound. ( I n the following section we mention the possibility of optical stimuli reinforcing sound as
an orientating mechanism.) The suggestion made by Moulton and
Dixon (1967) that the Mauthner cells provide a basis for directionality
sensing must therefore be rejected; moreover since Mauthner cell activity
was measured 'by the occurrence of a tail flip, which our observations
show can result from the activation of other cclls in the brain and spinal
cord, their evidence must be regarded as inadequate.
C. The Avoidance Reaction: A Genuine Function
When is the Mauthner reflex likely to be of greatest value? One
likely proposition relates to birds of prey, and this was first suggested