Trail avoidance, spatial pattern

recognition, and tubule-crossing

efficiency in the true slime mold

Physarum polycephalum

HANNAH MCSHEA

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

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.

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

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.

1. TRAIL AVOIDANCE: METHODSWill 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.

1. TRAIL AVOIDANCE: SETUP

Incentivized trap

Blank arm Slime arm

Stem

Unincentivized trap

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

perc

ent

pla

sm

odia

l m

ass

area of trap

Biomass distribution at 36-hr decision point

unincentivized

incentivized

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.

2. SPATIAL PATTERN RECOGNITION: SETUP

5

4 4

3 3

2 2

1 1

5-oat group 4-oat group

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

directionalit

y r

atio

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.

2. SPATIAL PATTERN RECOGNITION: RESULTSSome 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

avera

ge d

irectionalit

y r

atio

4 oats

0

0.5

1

1.5

2

2.5

3

before 5th oat after 5th oat

avera

ge d

irectionalit

y r

atio

5 oats

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.

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.

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

perc

en

t tu

bule

utiliz

ation

time after 24 hours (hrs)

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

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.

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.

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.

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

REFERENCES

Albus, J.S. 1991. Outline for a theory of intelligence. IEEE Trans. Systems, Man and Cybernetics, 21: 473–509.

Chung, J. and Choe, Y. 2009. Emergence of memory-like behavior in reactive agents using external markers. IEEE 21st International Conference 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. European Molecular Biology Association Reports 12: 1221-1225.

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