Physarum polycephalum, a true slime mold, was tested to determine if it exhibits intelligent behavior as defined by an organism's ability to make decisions that increase fitness based on environmental variables. The experiments found that P. polycephalum 1) avoided areas marked by slime trail, even when incentivized, 2) moved directionally along spatial patterns like oat trails, and 3) efficiently utilized existing tubule networks when crossing areas marked by slime trails. These results suggest P. polycephalum is able to solve problems and make adaptive decisions based on its environment, meeting the operational definition of intelligence used in the study. The findings have implications for understanding the evolution of centralized nervous systems and applications in robotics.
Navigation and Pathfinding in a True Slime Mold Slide Show
1. Trail avoidance, spatial pattern
recognition, and tubule-crossing
efficiency in the true slime mold
Physarum polycephalum
HANNAH MCSHEA
2. WHAT IS INTELLIGENCE?
the ability to solve
problems and
increase organismal
fitness (philosophy
of biology)
goal-directed
adaptive behavior
(psychology)
the achievement of
behavioral sub-goals
that support the
system’s ultimate
goal in an uncertain
environment
(artificial
intelligence)
Operational definition: the
capacity to make behavioral
decisions based on
environmental variables
that result in the acquisition
of resources
Variables present
Decision made
Variance in
response
(occasional bad
decisions)
Criteria
3. WHAT IS A SLIME MOLD?
• My organism is Physarum polycephalum, a
protist and “true” (multinucleated unicellular)
slime mold.
• In the image at right, everything that is yellow is
the organism. It explores with search fronts,
bracketed at bottom left, then condenses its
biomass into the more efficient tubules visible
above and to the right of the search front.
• Oscillatory movement: P. polycephalum moves
by a process known as cytoplasmic streaming
whereby cytoplasm and constituent organelles
rush forward for about 90 seconds, backward for
about 90 seconds, like a battering ram.
• Precursor to internal memory: P. polycephalum
lays down a slime trail that serves as a chemical
map. This way of “remembering” environment is
thought to have served primitive organisms
before centralized organization (and eventually
cephalization) was metabolically feasible.
4. EXPERIMENTAL OBJECTIVES
• Trail serves as external memory map
• The likely evolutionary cause for this is that areas covered in trail can be
presumed to be depleted of food. Do we find this to be the case in the
individual? In other words, does the individual perform a logic operation
when it encounters slime trail, or is it simply chemically averted?
• Anticipates temporal events – after being presented with a
stimulus periodically, P. polycephalum was found to respond
in phase even after stimulus had ceased
• Does P. polycephalum also anticipate spatial events, e.g., recognize
patterns?
• Avoids trail…
• What about when there’s no choice?
OPTIMIZATION AND INTELLIGENCE
5. METHODS
• Single stock per trial
• Organisms were ordered from Carolina Biological and cultured on
non-nutrient agar with oat flakes in a dark drawer at room
temperature.
• Scoring table
• A scoring table was erected with a camera secured 20 cm above a
plate in the viewing stage, backed by a 3mm grid against which
movement was scored. Raw data was taken in the form of hourly
photographs of each plate.
• Grid system
• Photographs were analyzed to determine how many 3 mm grid
squares were occupied by the organism, and when pertinent, the
location of those grid squares.
6. 1. TRAIL AVOIDANCE: METHODS
Will Physarum polycephalum avoid areas covered in
slime trail, even if there is an incentive to traverse slime
trail?
Y-shaped traps were constructed with one arm covered
in slime trail and one arm with blank agar. In the
unincentivized trap (n=10), which had been tested
before by Reid et al. (2012), an oat was placed at the
end of each arm. In the new incentivized trap (n=25),
an oat was placed at the end of the slime arm only.
Data was taken every 6 hours and the area covered by
the plasmodia in each arm was scored, as well as
whether it had reached the oat. At 36 hours a final
choice was recorded. If 75% of the plasmodium’s area
was in a region (left arm, right arm, or stem) of the
trap, then that region was considered chosen.
7. 1. TRAIL AVOIDANCE: SETUP
Incentivized trap
Blank arm Slime arm
Stem
Unincentivized trap
8. 1. TRAIL AVOIDANCE: RESULTS
Why such complete avoidance? It
could be that…
• Food is detectable and slime trail is
associated with depleted food. This
is not likely because if it were the
case, the presence of food would
have logically overridden the trail-
avoidance mechanism.
• Food is detectable and slime trail is
BAD but not associated with food.
This could be! Food does not
logically override “BAD” as it did
“depleted food” above. It might be
that the individual organism does not
retain evolutionary reason for
avoiding slime trail.
• Slime trail renders food
undetectable. It could be that the
chemical “scent” of the slime trail is
so strong that no logical operation
need take place.
0
20
40
60
80
100
blank slime stem
percentplasmodialmass
area of trap
Biomass distribution at 36-hr decision point
unincentivized
incentivized
9. 2. SPATIAL PATTERN RECOGNITION: METHODS
Will P. polycephalum move directionally forward after
following a trail of 4 oats?
Plates were prepared with 4 oats placed 1 cm
apart in a line (n=21), and control plates were
prepared with 5 oats (n=22). P. polycephalum
was placed 1 cm behind the 1st oat. Plates were
monitored until plasmodia reached the 4th
oat, at which point photographs were taken
every hour until 3 hours after the 5th oat (or
equivalent point) was reached. Lateral and
longitudinal distance moved beyond the 4th oat
were recorded. Directionality was defined as a
ratio of movement forward over movement
laterally; ratios greater than 1 indicate
directional forward movement.
10. 2. SPATIAL PATTERN RECOGNITION: SETUP
5
4 4
3 3
2 2
1 1
5-oat group 4-oat group
11. 2. SPATIAL PATTERN RECOGNITION: RESULTS
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 5 10 15
directionalityratio
time (hrs)
4-oat group
*dashed line indicates a directionality ratio of 1. Points above
this line can be said to represent directional motion forward in
the direction of the oat trail. Increase in directionality over time
is significant for both groups, and directionality was not
significantly different between the two groups.
12. 2. SPATIAL PATTERN RECOGNITION: RESULTS
Some mechanism besides chemotaxis is present, because the 4-oat group moved
with the same directionality as the 5-oat (chemotactic) group.
• Optimizes foraging because food sources often arise in patterns (fungi along a log).
• Directionality increases in both groups after the organism reaches the 5th oat or the
equivalent point in space.
0
0.5
1
1.5
2
2.5
3
before 5th oat after 5th oat
averagedirectionalityratio
4 oats
0
0.5
1
1.5
2
2.5
3
before 5th oat after 5th oat
averagedirectionalityratio
5 oats
13. 3. TUBULE-CROSSING EFFICIENCY: METHODS
Within a network of slime, will
P. polycephalum seek out
pockets of unexplored space, or
will it utilize old tubules to
cross the agar?
The arrow at right indicates a P.
polycephalum tubule exploring
empty space. In the red circle, P.
polycephalum is interacting with
slime trail. Instances of
interaction, where the organism
is on top of or within a slime trail
tubule, were considered to be
occupancy.
14. 3. TUBULE-CROSSING EFFICIENCY: SETUP
Organisms were placed on plates that had been
completely covered in slime trail network by a
plasmodium that had been removed (n=20). This
procedure produced a plate with a patchy distribution
of tubules and empty spaces among the tubules such
that it was impossible for the migrating plasmodia to
avoid old slime trail. After 24 hours of growth, each
9mm2 grid square containing plasmodium was
examined to see whether the organism was
interacting with existing tubules or was on blank agar
space. Data was taken every 6 hours from hour 24 to
hour 60. Variance in mass between plasmodia meant
that occupancy ratios were most meaningfully
expressed as a percentage.
15. 3. TUBULE-CROSSING EFFICIENCY: RESULTS
Newly-documented
characteristic of navigation:
• Organisms initially seek
out unexplored space, then
consolidate biomass
into/onto existing tubules.
• Old tubules have already
mapped environment with
maximal efficiency, so it
makes sense to use them
as a guide.
• External memory map is
more complex than
previously supposed – it
involves both avoidance
and utilization functions. 0
10
20
30
40
50
60
70
80
90
100
20 25 30 35 40 45 50 55 60
percenttubuleutilization
time after 24 hours (hrs)
16. ARE THEY INTELLIGENT?
the ability to solve
problems and
increase organismal
fitness (philosophy
of biology)
goal-directed
adaptive behavior
(psychology)
the achievement of
behavioral sub-goals
that support the
system’s ultimate
goal in an uncertain
environment
(artificial
intelligence)
The capacity to make
behavioral decisions based
on environmental
variables that result in the
acquisition of resources
Variables present
Decision made
Variance in
response
(occasional bad
decisions)
Criteria
17. CONCLUSIONS: INTELLIGENT BY
OPERATIONAL DEFINITION
1. Trail avoidance in P. polycephalum is strong enough to
overpower positive chemotaxis.
2. P. polycephalum recognizes spatial patterns.
3. When P. polycephalum must cross slime trail, it uses
tubules to do so efficiently.
18. IMPLICATIONS AND APPLICATIONS
Emergent intelligence – complex properties arising from
simple repeated components:
• How did centralized neural networks arise? IF Physarum
polycephalum is a precursor to these, then it could help us
explain their evolution.
• Human intelligence is an emergent property of our dendritic
network. By what mechanism does higher-order organization
emerge from simple units like slime mold protoplasmic
oscillation or human synapses? The slime mold is an incredibly
simple system worth understanding for its applicability to other
emergent systems.
• Robots have been modelled on P. polycephalum and controlled
by it. This study’s findings can be used to create robots that are
more robust and adaptable, more able to navigate complex
environments.
19. FURTHER STUDIES
• A computer model of Physarum polycephalum could be used to test
thresholds of emergence. I have created a simple Netlogo model of
slime mold behavior that I intend to refine further.
• Y-traps could be created with components of slime trail – different
proteins, lipids, cellulose – to determine what sort of reaction takes
place to trigger trail avoidance.
• Threshold tubule size for viable guidance could be determined – how
large does a tubule have to be for utilization? How does efficiency
correlate with tubule size?
• When slime molds were exposed to temporally-administered
stimuli, response faded as stimuli failed to occur at the expected time.
Does spatial memory degrade in the same way? In other words, how
is this sort of information stored in the organism? I suspect it has to
do with protoplasmic streaming oscillations.
20. ACKNOWLEDGMENTS
• Dr. Amy Sheck for her help in project development and for
acting as mentor and teacher of the Research in Biology
sequence at NCSSM
• Dr. John Tyler Bonner for guidance and wisdom when first
developing the project and for inspiration throughout
• Ms. Korah Wiley for guidance during Glaxo Summer
Research Fellowship
• Glaxo endowment to NCSSM for providing lab space and
time to conduct project over the summer
• Glaxo Fellows and Research in Biology classmates
Allen, Baslious, Brewster, Feng, Kirollos, and Wu, for their
critique and support
• Research in Biology mentors Ge, Harrison, Lin, Maynor, and
Tsui, for their mentorship, critique, and support
21. REFERENCES
Albus, J.S. 1991. Outline for a theory of intelligence. IEEE Trans.
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Chung, J. and Choe, Y. 2009. Emergence of memory-like behavior in
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on Tools with Artificial Intelligence: 404-408.
Reid, C., T. Latty, A. Dussutour, and M. Beekman. 2012. Slime mold uses an
externalized spatial "memory" to navigate in complex environments.
Proceeding of the National Academy of Sciences 109(43): 17490-17494.
Saigusa, T., A. Tero, T. Nagagaki, and Y. Kuramoto. 2008. Amoebae
anticipate periodic events. Physical Review Letters 100: 018101.
Sternberg, R., and W. Salter. 1982. Handbook of human intelligence.
Cambridge, UK: Cambridge University Press.
Trewavas, A., and F. Baluska. 2011. The ubiquity of consciousness.
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