Corollaries between ish and human behaviour are easily
imagined. Given what we know about the problems of merely
observing behaviours, we need to be very cautious in inter-
preting preliminary observations of a small number of trout.
In a lesser-known and subsequent study undertaken in 2008,
Sneddon had to increase the concentration of acetic acid 50
times before she observed the occasional anomalous behaviour
in carp (and this time only in two of the ive ish injected with
acid). In contrast, zebraish never showed any anomalous behav-
iour after acid injection.
In her latest repeats of this experiment in 2011 and 2014,
Sneddon failed to report this anomalous behaviour in either
trout or cod after acid injection. These disparate results high-
light the inherent diiculties in drawing conclusions too early
from casual observations of a small number of ish.
We also need to be circumspect when extrapolating from
the effects of analgesics since they can act in a variety of ways.
They are known to disrupt the early stages of transmission of
signals from the skin to the brain (which is analogous to how
cutting the cord that connects a keyboard to a computer prevents
signals from activating central programs). Altered behaviour
following drug administration merely informs us that there
has been a break in the transmission of information somewherewithin the nervous system. It
does not indicate that ish feel
pain. While drugs may prevent
a paraplegic with a complete
spinal cord injury from
responding to a pinprick, it is
clearly wrong to subsequently
conclude that they can feel pain.
If we can’t trust behavioural
observations and we can’t trust
the use of analgesics, then what
are we expected to trust? To
answer this question we need
to clarify where in the brain the
feeling of pain emerges. Modern
human brain imaging studies
have conclusively revealed that
pain is dependent on highly
dynamic and complex patterns
of electrical activity within the
forebrain. These electrical
signals travel along an intricate
network of ibres that precisely
interconnects various specialised
forebrain subregions.
In some respects, the human
forebrain is like a major metro-
politan road network. During the day (akin to being conscious
and aware) there is traic lowing back and forth along a grid
consisting of motorways between large districts, major arterial
roads between suburbs, and side roads within the suburbs. In
contrast, at night (comparable to being non-conscious and
unaware of pain) traic is reduced, lows mostly in one direc-
tion, and is predominantly restricted to major roads.
The answer to the question about what we can trust as a
measure of the ability of ish to feel pain now turns out to be
quite simple – it has its origins in the study of anatomy. We can
ask the question: are ish equipped with the specialised fore-
brain subregions as well as the bidirectional network of ibres
and interconnections that are necessary for awareness and the
feeling of pain?
Before we continue any further, a preparatory lesson is needed
on the anatomy of a generic vertebrate brain. For our purposes,
the brain can be divided into a front part called the forebrain
and a rear part called the brainstem (Fig. 1). The brainstem is
the evolutionary old part of the brain and is present in both
ish and mammals. Given that ish and mammals shared a
common ancestor about 450 million years ago, its preservation
over this timescale highlights the functional importance of this
brain region. The reason that the brainstem is so vital is that it32 | APRIL 2016
Figure 1. The forebrains of fish and
humans are so disparate that it’s difficult
to find any structural similarities.