The Efference Copy Paper

Here is the paper I wrote while doing a little bit of research on efference copies. Each sentence was restricted to fewer than 17 words. While trying to stay under the limit makes for slow going, it’s a rewarding exercise. The sentences end up sounding a bit simple, but the primary purpose of any paper is to be understood. After that, you can worry about trying to dazzle a reader with your insights (or confuse them into believing you have any).

Efference and Objectification

Perception begins with bugs. Spiders, praying mantises, and bees have all demonstrated the ability to perceive the world around them.[1] This is impressive. It prods us to reconsider which species have perceptual capabilities similar to our own. Yet, there remains a high standard of proof for attributing perception. The existence of efference copies in simple organisms appears to be evidence of perceptual ability. If this is correct, types of worms and slugs will be granted perception. Close examination suggests this conclusion is misguided. The existence of efference copies alone is not a sufficient mark of perception.

We check if organisms have perception by seeing if they exhibit perceptual constancies. A perceptual constancy is an ability. Imagine I have a red water bottle. The top half is in sunlight, while the other half is in shade. Therefore, the top appears a light red, while the bottom looks darker. The human visual system attributes the same shade of red to the entire bottle. This happens despite it looking like it is two different shades of red. The ability to do this is a perceptual constancy. The visual system can attribute the same redness despite different kinds of light hitting the eye. This light is called the “proximal stimulus” and is registered the moment it strikes retinal sensors. For example, the two halves of the bottle cause different registrations of proximal stimulus. In realizing a perceptual constancy, the visual system also engages in “objectification.”[2] It picks out elements in the proximal stimulus relevant to the object. It also ignores elements specific to its perspective of the object. In terms of the example, the visual system did two things in exercising objectification. First, it identified which parts of the registered stimulus were due to the bottle’s redness. Second, it discarded parts caused by circumstantial factors like shade and perspective.

Using the new vocabulary, an explanation of perceptual constancies takes shape. A perceptual constancy is the ability to represent accurately despite variation in registrations of proximal stimulus. The thing represented can be a particular or an attribute.[3] Objectification also happens if and only if there are perceptual constancies. Therefore, it is a reliable indicator of perceptual constancies, and thus perception.[4]

Some maintain organisms like the nematode worm exhibit objectification, as evidenced through efference copies. Efference copies arise to handle conflicting behavior associated with sensory input. When tactile sensors in the head of the worm are stimulated, it moves backward. If similar sensors are activated in its tail, it moves forward. The result is a cruel dilemma. Moving forward stimulates its head sensors, inducing backward movement. Backward movement stimulates its tail sensors, triggering forward movement. Under these circumstances, the poor worm would be unable to move meaningfully in either direction. Fortunately, its sensory system, and those like it, has overcome the problem. Imagine the worm receives stimulus from its tail and activates the move forward reflex. To avoid the feedback loop described, the activation sends an efference copy to the sensory system. This copy functions as a report to the rest of the organism. It indicates that the worm is moving forward.[5] Therefore, any stimulation of head sensors is due to movement, not obstacles or predators. The activation of the second, move-backward reflex is then inhibited, allowing the worm to travel peacefully.

Supporters claim the worm’s sensory system discriminates between different types of proximal stimulus. Due to the efference copy, the worm “knows” further stimulus is caused by its own movement. This amounts to a type of separation between stimulus caused by distal objects versus circumstantial factors.[6] The result is to inform appropriate worm behavior. Efference copies, it seems, are evidence of objectification.

We can call what happens in the worm “objectification.” Yet, it bears little relation to the stronger kind displayed in perceptual constancies. There is a high standard for marking processes as perceptual objectification. It is set in the science of visual psychology.

Scientists explain the natural world. Good explanations implicate only what is necessary to explain the phenomena in question, and no more. Imagine frogs croaking in a pond. We can explain their croaks as plangent pleas to a wizard to retransform them into humans. This is unlikely. Nothing about the croaks suggests the presence of magic or that the frogs were once human. An explanation that doesn’t posit the existence of wizards and animal-human transmutation would suffice. Viewing the croaks as mating calls is simpler, and does the same explanatory work. It fits with our existing biological knowledge. Scientists would have to be presented with compelling circumstances to resort to the anthro-amphibian explanation. Perhaps the croaks sound like “help” and wizards were spotted in the area. Otherwise, the mating call description remains the most likely to be accurate.

Visual psychologists must settle on explanations of animal behavior. Behaviors can often be explained in terms of the proximal stimulus and an animals’ neurology. This is in contrast to explanations that implicate objectification or perception. Consider olfactory navigation by salmon. We can sufficiently explain how they traverse oceans back to their home stream. Olfactory proximal stimulus causes the neurons to fire in a certain pattern, driving accurate navigation. No reference is made to external objects or perception. This neuro-causal explanation is the simplest, and most descriptive science has to offer.[7] If science only postulates a neuro-causal explanation, objectification or perception are probably not taking place. By contrast, the science does implicate external objects in some causal explanations of behavior. In these circumstances, we can be confident objectification and perception are present.

No reference to objectification is present in the scientific explanation of nematode worm behavior. It is a neuro-causal explanation that does not implicate objects in the distal environment. The worm does not separate aspects of the proximal stimulus. Its sensory system does not distinguish which elements are perspectival. It merely reacts to the stimulus. This is true despite the presence of efference copies. They only inhibit reflexes, and have no bearing on how stimulus is processed.[8] We can, however, still maintain the worm’s actions are relevant to environmental objects. Its sensory system functions to keep it from bumping into things. This is a functional explanation of its behavior, and implicates external objects. Indeed, any behavior, perceptual or non-perceptual, can be explained functionally. Yet, we’re interested in what causes worm behavior. The science only appeals to the stimulus received in its causal explanation. There is no compelling evidence to reference external objects. This suggests efference copies alone aren’t indicators of objectification, and thus perception.

Efference copies without perception are observed in other species. Consider crayfish. While more complex than worms, they utilize efference copies in a similar way. The lower abdomens of crayfish are covered in sensitive hairs that trigger a tail-flipping escape response.[9] This leaves them susceptible to a similar type of feedback loop described above. Efference copies prevent this scenario. As self-initiated movement commences, a crayfish’s sensory system blocks signals from the hairs. The registration of stimulus does not progress far enough in the system to trigger a response.[10] Clearly, objectification is not present in this situation either. There is no evidence crayfish distinguish proximal stimulus caused by objects versus circumstantial factors. The causal explanation of crayfish behavior does not implicate objects in the world. Functionally, the crayfish is escaping predators. Yet, the behavior can be sufficiently explained with respect only to the initial proximal stimulus. Hypothetical predators need not enter the conversation. Objectification and perception are equally absent from the causal explanations of crayfish and worm behavior.

Efference copies are a fascinating biological feature. They allow species to better interact with the environment. Yet, their presence is not an indicator of perception. That capacity still begins with bugs, and seemingly not earlier.

Works Cited

Burge, Tyler. Origins of Objectivity. Oxford University Press, 2010.

—. “Perception: Where the Mind Begins.” The Royal Institute of Philosophy. 2014.


Crapse, Trinity B and Marc A Sommer. “Corollary discharge across the animal kingdom.” National Neuroscience Review (2008): 587-600.

[1]Burge, Perception: Where the Mind Begins

[2]Burge 397, Origins of Objectivity

[3]Burge 1, Perceptual Constancy

[4]Burge 2, Perceptual Constancy

[5]Crapse and Sommer

[6] Burge 11, Perceptual Constancy

[7] Burge 425, Origins of Objectivity

[8] Burge 12, Perceptual Constancy

[9] Crapse and Sommer

[10] Crapse and Sommer

Efference Copies Everywhere

I’m writing a paper right now about efference copies in nematode worms and what they can tell us about perception. The more I learn, the more impressed I am with the little guys, and all of the strange minor procedures organisms undergo so they can function.

I’ll start with the issue that faces the worms. First, they are super simple creatures. They only have 302 neurons (compare to 250,000 in a fruit fly) and thus have a limited repertoire of behavior. For example, if tactile sensors in the head are stimulated, the worm moves backwards. If sensors in the tail are stimulated, it moves forwards. These reflexes are useful for obvious reasons. The worms need a way to avoid predators and obstacles, and this simple behavioral schema seems to do the trick.

But, a dilemma lurks below the surface. Suppose a worm’s head sensors are activated and it starts moving backwards, only to have its tail sensors activate by virtue of moving through the soil. Now, it reverses direction and moves forwards, to have its head sensors activate and trigger the move backward reflex. In moving backwards… its tail sensors activate… and now it tries to move forwards…

Like someone trying to squeeze into a parking space, the worm would stop and start, making little movements forward and backwards in vain as it struggles in a vicious cycle. These seemingly reasonable stimuli responses would render the worm static for eternity, unable to feed or find a mate.

Yet, the nematodes have not gone extinct. They continue to thrive in rotting fruit and be bred for all sorts of scientific experiments. What, then, keeps them mobile?

The answer lies in efference copies. When a worm’s tail sensors are activated, for example, it triggers the move forward reflex, and signals the nervous system to inhibit the move backward response. This signal is an efference copy. (Note: from my understanding, the worm’s head sensors still register stimulus, but it is only the corresponding behavior that is blocked by the copy.)

We can crudely think of efference copies as the neurological equivalent of the nervous system CCing the rest of the body so everybody is on the same page.

More interesting applications of efference copies are present in complex organisms. For example, crickets make noise by rubbing their wings together in a process called stridulation (which is a cool word). The ruckus they create is loud for us, but much more so for them. To ensure they don’t lose their sense of hearing, the signal to make a song that is sent to the cricket’s motor neurons is simultaneously routed to the auditory system. The auditory system then prevents signals from the tympanum (eardrum) from being processed, effectively cutting off hearing. This example differs from the case of the worm, as the cricket only inhibits the processing of stimulus. The worm processes stimulus, but prevents corresponding reflex-based actions.

Efference copies are present in humans, too. They play a role in vision and movement, but my favorite example is tickling.

Feeling tickled is the result of tactile stimulation on sensitive parts of your body. It’s apparently a simple process. If your feet/stomach/armpits, receive the correct stimulus, you feel giggly. Why, then, can you not tickle yourself? You can provide the same type of stimulus as anyone else, so we can’t we bring ourselves to fits of laughter?

Efference copies. Whenever we act, our sensory systems create a “prediction” of how that action will create additional sensory input. We’re just like the worm in this regard. We need a way of distinguishing stimulus created by external objects, versus us. If you run a little brush across your own palm, it’s not very tickly. Your sensory system has already predicted the stimulus associated with the action and is primed to ignore it.

It is possible to tickle yourself, though. You just need to be indirect about it.

Scientists studying this phenomena created a tickling robot. In the robot’s arm is a little brush. Underneath its arm is your open, right palm. In your left hand, you have a small stylus you can use to draw a pattern. The robot will then take the same pattern drawn with your left hand and trace it on your right palm with the brush, hopefully tickling you.

What the researchers found was that if the robot traces the pattern as you’re drawing it, it’s not very tickly. However,as the delay between you drawing the pattern and the robot tracing it increased, the more tickly the result was. A delay of 300ms between you telling the robot how to tickle and it tickling increased subjective feelings of ticklishness by ~50% (full disclosure: I’m eyeballing the graph from the study for this number. The authors don’t provide it. From what I can see, it jumps from 2.1ish to around 3.4 on the tickle rating rank).

I hope you can see why I find efference copies so interesting now. Beyond fulfilling functions just described, they also play a big role in generally distinguishing self from the environment. There is a study describing efference copies’ role in internal speech, as well as a section in the tickling-robot section detailing how auditory illusions with schizophrenia can be attributed to issues with efference copies.

If you want a survey of efference copies across the animal kingdom, you can check out this article.