Prefrontal cortex and symbol learning:

Terry Deacon on the neurological roots of hyper-association

What makes a symbol?

• As we have said previously, a sign can refer:– Iconically: By resemblance– Indexically: By contiguity in time or space– Symbolically: But HOW?

• Prefrontal cortex = All cortex anterior to BA 6 (premotor cortex)– = BA 8, 9, 10, 44, 45, 46, 47, + orbital region

Our big brains

-Humans have relatively huge Humans have relatively huge frontal lobes.frontal lobes.-A common A common moiety (portion or share) underlies massive increases in the cerebellum and frontal lobes

So what?

• This suggests two things:i.) Selective pressure on either the cerebellum or frontal

lobes may bring the other along - this opens the possibility of bridging the non-language/language gap

ii.) Small genetic changes may underlie the large difference between human and other primate brains- Only a few K of information (a fraction of a percentage difference) differentiates chimp DNA from human DNA

How’d we get so much free brain?

- Our sensory demands did not increase as our brain size did, because our brain grew disproportionately to our body- For example, our visual tracts match our body size, not

our brain size• This has freed up neurons for non-sensory stuff

– Nonsensory regions can get more 'votes' on which cells they can recruit

Association tissue is good!

• One result of our free tissue and early birth is that most information in our brain is not hard-wired but rather is shaped by experience– The new-born brain is over-wired – Useful pathways are selected out from the over-

abundance by deleting what we aren’t using

Frontal lobes: A coalition of willing

• Since our frontal lobes are a.) really big and b.) largely association tissue, they get lots more ‘votes’ than normal- making us 'front-heavy'

• What does pre-frontal cortex do?– Many things: eye gaze control, short term memory– But it also modulates processing of many other regions, giving

it a role in bringing motor output under the control of internally-shifted valence modulation

– As we might say in ordinary English, ‘bringing behavior under the control of attention’

Example: Delayed alternation• In a delayed alternation task, an animal must get reward in the

place it last wasn't– Animal swith frontal lobes damage have trouble with this task– It is not a memory problem, since it is not random: they tend

to perseverate– It is a a problem in shifting behaviour under control of

internal states• Many related tasks have been studied, all involving shifting

behaviour in a way that is not marked independently of the actor– We might say: they all have to do with 'invented context' or

internally-marked context

Babies are brain-damaged apes• Adele Diamond has shown that very young human infants are almost

incapable of successfully completing tasks which tap this functionality.– Infants under 9 months behave very much like adult rhesus

monkeys with surgical lesions in the prefrontal cortex. – Rhesus monkeys attain normal adult levels of performance on this

task between 1.5- 4 months. – Human infants do not attain that level of performance until 7.5 -

12 months. • Performance on this and many other tests requiring context-sensitive

behavioral switches suggests that the prefrontal cortices are much slower to develop in human infants than in other primates.

Symbols are portable context• Why doe this matter to language?

– Words (and other symbols) are contextual-markers– They mark how something is to be taken, rather than just mapping

on to how a thing looks, thereby bring interpretation under internal control

– Remember the rat who could eat when the buzzer rang unless the blue light was on• We suggested this was closer to a symbol than just eating when

the buzzer rang, because now context was modulating meaning– When context is internal- when the blue light exists only inside

ourselves- we have a symbol

Analogy is context

• Think of trying to explain something complicated to me by analogy– What you are doing is saying that the meaning you want

me to attach to a new symbol X is like the meaning that I already attach to a symbol Y that I already know

– Example 1: Think of explaining division as a serial subtraction: we bootstrap from an understanding of one relation (subtraction) to an understanding of another relation (division)

– Example 2: My son's one trial learning of the generality of dipping

Deacon, meet Wittgenstein.• Symbols are like tools (sound familiar?)• Both ‘intelligent’ real-world (behavioral) semantics, and

lexical semantics are made up of these kinds of internal symbol-symbol analogies– A brick is either a door stop, a weapon, or a hammer,

depending on ‘how we look at it’– A simple-minded creature who had learned to look only for real

weapons as weapons would be at a loss if he had ‘only’ a hammer or a brick or a stone

– He would be unable to over-ride the sensory array- indexical relations- and see a hammer/brick/stone as something other than what he had learned it was

How not to be goaded.

• The term ‘stimulus’ derives from the Latin term meaning ‘goad’• The ability to make behavior contingent on internal changing

context (to decide if a rock is a hammer, a weapon, or just a rock) release us from being ‘goaded’ and turns us into ‘goaders’

• As we have seen with delayed alternation, there is strong consensus about the neurological substrate that underlies this ability to make behavior contingent on internal changing context: the dorsolateral prefrontal cortex (DLPFC).

• Dorsal = ‘back’• Lateral = ‘side’ • DLPFC = BA 46/9

Limbic circuit

- DLPFC gets massive input through mediodorsal thalamic nucleus (MD) from limbic regions, most notably amygdala (A) and septum (S)

-both are implicated in calculating sensations of pleasure, pain, and fear

The DLPFC goads the brain• The DLPFC is reciprocally connected to almost the entire

cortex, especially the ‘new’ association cortex• The DLPFCs (many) functional descriptions all involve

modulating the nature of imminent behavior in response to changing task demands, including those specified by internally-generated (change of) plans.

• Chris Frith (2000) has proposed an elegant term for this general task performed by DLPFC: he calls it “sculpting the response space”.– We can call it ‘goading the brain’, because in a sense it is a

(virtual) stimulus-creator (but ‘sculpting’ is a better metaphor, since the frontal lobe is not a dictator, but a synthesizer and triage area)

Another description: Fitness evaluation

• Another good way of thinking of the DLPFC function is as a ‘fitness evaluator’ for cortical activity

• The region mediates between calculations carried out in the cortex and the valence (fitness) of those calculations as calculated by limbic systems

• In this sense, DLPFC is neither ‘controller’ (‘executive function’ or ‘goader’) nor ‘silent slave’ (‘goaded’): but rather one component of a system for generating and selecting possible interpretations of (messages about) phenomena

“Tasks sensitive to prefrontal damage...involve short-term memory, attention, suppression of responses, and context sensitivity. But they all have one important feature in common: each involves a kind of negation relationship between stimuli or stimulus-behavior relationships. They all have to do with using information about something you’ve just done or seen against itself, so to speak, to inhibit the tendency to follow up that correlation and instead shift attention and direct action to alternative associations. Precisely because one association works in one context or trial, it is specifically excluded in the next trial or under different stimulus conditions. An implicit ‘not’ must be generated to learn these tasks, not just an inhibition”

Terry Deacon - The Symbolic Species

Symbols as rules for negation[‘This is not the territory.’]

This is the territory

This is a message

about the territory

This dividing lineis how we discern the difference between messages and things.

A synthesis?

• You should try to understand how we can relate Fauconnier’s mappings, Wittgenstein’s behavioral symbol-grounding, Bateson’s ‘frame as negation’ definition of symbols, and Deacon’s emphasis on DLPFC function as underlying symbolism

• These ideas are so closely related that a coherent viewpoint starts to take shape