Consciousness as simulation of behaviour and perception: Lecture notes
Dpt of Physiological Sciences
Summary of lecture given in
Karolinska institutet 1999-03-10
A The problems of consciousness
The problem of consciousness actually consists of several problems. For many neuroscientists as well as for philosophers and laymen, the following are usually considered to be among the most important problems:
How does the inner world arise?
The central problem for most people. How can we see things which have no material existence?
What are mental objects?
What is the object we see with our "inner eye" or think about if it is not a material object?
What is the function of the inner world?
Could evolution have produced an organism like a human being and responding just like a human to any given certain input, but with no inner world? If so, why didn’t it?
Do animals have an inner world?
Can machines have an inner world?
B The central message of the lecture
These questions can be given plausible answers with the help of ideas which have been around for a long time, mainly in associationistic psychology and the behaviourist tradition. The central idea is that the brain can generate its own input. I will describe a physiological mechanism which can briefly be described as simulation of behaviour and perception. It has three components:
1) Simulation of behaviour.
Thinking that one is doing something is similar to actually doing it. It is covert behaviour.
2) Simulation of perception.
Imagining that one is seeing (hearing, feeling) something is very similar to actually seeing (hearing, feeling) it and activates the same brain structures.
We assume an associative mechanism which enables simulation of both
behaviour and perception to elicit other perceptual activity. An important
consequence of this is that simulation of behaviour can elicit perceptual
activity which resembles the activity which probably would have occurred
if the the simulated actions had actually been performed.
C Simulation of behaviour
Physiological experiments such as blood flow measurements provide direct
evidence for a similarity between imagined and overt behaviour.
D Simulation of perception
Similarly, we may have visual perceptions which are not caused by external events. If we activate the visual pathways, such as may occur if we apply a mild pressure to the eyeball, we may ’see’ stars or flashes, because we simulate the activity in the optic nerve which normally occurs when we actually see stars or flashes.
No assumption is being made here about the nature of perceptual activity. We have not assumed that the brain constructs an ‘image’ or a ‘representation’ which is being watched by a homunculus in the brain.
E How can perceptual simulation be elicited?
Perceptual activity can be elicited by
a) exteroceptive and interoceptive stimuli (normal perception)
b) other perceptual activity
c) covert behaviour
Consider the simple organism in fig. 1, which reacts to a stimulus
S with a certain behavioural response R. S will cause
a series of neuronal events which are designated by s. These will
cause other neuronal activity r which in turn will cause the overt
Figure 1: simple organism
Think of s as activity in the sensory cortex and of r as ‘preparatory’ or ‘incipient’ behaviour, i.e. neural activity in the pre-motor and supplementary motor areas of the frontal lobe.
Evidence that perceptual activity can be elicited by other perceptual
activity is sensory pre-conditioning. A stimulus S1 is
first repeatedly presented before a second stimulus S2.
may be said to ‘predict’ S2. We then establish a conditioned
response R to S2. If we now present S1,
it will elicit R. In the first demonstration of this phenomenon,
Brogden presented dogs with repeated paired tone and light stimuli. He
then paired the tone with a foot shock which elicited leg flexion until
the tone elicited a conditioned flexion response. When he finally presented
the light alone, it too elicited a conditioned response. Since light had
not been paired with the foot shock, the simplest explanation for this
phenomenon is that the light elicited some activity which was similar to
that normally elicited by a tone, say a "tone-perception", and that this
became a link in the chain from light to leg flexion.
Figure 2: Sensory preconditioning
Thus, one way in which a simulated perception of S might be triggered is simply by some other sensory activity, which has been associated with S. For instance, hearing a car, a bird or a musical instrument might activate visual cortical areas and elicit simulated perceptions of the corresponding objects. A special, but important, case would be the sound of a word.
The sensory consequences of behaviour are predictable. Walking into
walls usually give rise to similar stimuli. Suppose the organism O
which performs the response R in the situation S and that this results
in the new situation S. S' is potentially harmful and O
has learned to perform an important defensive response R'. If this
sequence of events occurs regularly, it would be useful for O if
the predictable stimulus S' could be simulated in response to the
preparatory or incipient behaviour r, preceding R, because
it would enable O to make the defensive response R' faster.
We will assume an associative mechanism called ‘anticipation’ which consists
in the ability of early stages in a behavioural response being able to
elicit sensory activity which resembles that activity which would normally
be elicited by the corresponding overt behaviour.
Figure 3: Anticipation
When a response has been performed, the situation for the animal will change from S into, say, S' which causes a new behaviour, R' etc. In this way, a single initial stimulus can trigger a long sequence of responses. The successive situations and responses are illustrated in fig. 3. If there is a consistent relation between stimuli and responses, there will also, as a consequence, be a correlation between types of internal events. Thus when a response R performed in situation S causes the situation to change into S', the ‘preparatory’ response r', performed in the presence of s, will be followed by the internal stimulus s'. We will assume that this regularity can be learned, so that performing the neural activity r in the presence of s, by some associative mechanism elicits the ‘expected’ activity s' (fig. 3B).This may have effects on the animal’s behaviour which are similar to the effects of actually perceiving S', i.e. the animal may perform r'.
Such a mechanism could have a great survival value, because it would enable an organism to ‘anticipate’ the consequences of a response and to emit behaviour adequate to the new situation with greater speed or to suppress behaviour which has negative consequences.
Example: In a classic experiment Tolman let a rat explore a T
maze terminating with one black and one white chamber, both of which contain
food. The rat is then placed in a black chamber and given painful stimulation.
When the rat is later placed in the T maze it does not go the left at the
branching point, even though it has never been punished for this behaviour.
It is as if it ‘foresees’ the consequence of going left.
Figure 4: Tolman's experiment
A plausible interpretation of this experiment is that the rat at the
branching point prepares walking to the left, that this (via the anticipation
mechanism) elicits a perception of the black chamber and that this (via
a classical conditioning mechanism) elicits anxiety. In a sense, the rat
really does ‘foresee’ the consequence of walking left.
F Simulating chains of behaviour
A stimulus S elicits a response R which causes a new stimulus
etc. producing a chain of stimuli and responses.
Figure 5: Behavioural chain
Once the mechanism of anticipation is in place, it will enable the organism
to simulate a behavioural chain by performing a series of covert responses
and corresponding simulated perceptions. S is followed by s and
the incipient response r. This will elicit the normal consequence
of R, s', even if R is suppressed. s' can
then elicit a new incipient response r' etc.
Figure 6: Simulation of behavioural chain
Suppose that a simple organism, starting at the bottom of the maze below can perform two responses, moving forward to the left (L) and forward to the right (R). It has a visual system which is capable of generating a unique activity pattern for each letter in the maze and for each of these activity patterns form a connection with one of the two possible responses. Suppose also that it has previously moved through the maze and learned a number of associations such as the following:
Figure 7: Maze
Even if the organism had never walked from B to G via D, it could simulate this response chain. Performing the covert left move when seeing B, would trigger ‘seeing’ D, which would trigger a new movement.
The animal may have been subjected to operant conditioning which had
made the response l in d slightly stronger than r. If F
was not a reinforcing position to be in, the relative strength of r
in D would increase and the next time the organism started out from
it would emit r, thus finding itself ‘seeing’ G.
Example: Simulating a conversation
Consider two persons carrying on a conversation. A person utters something that is a stimulus for B to respond, thereby providing a stimulus for a new utterance from A and so on. The brain mechanism would not be different if A were to talk to himself, listening and responding to his own utterances. If activity in Brocas area on its way to the motor cortex could be fed directly into the auditory cortex, A could simulate the conversation without producing any audible utterance at all, yet utilising the same mechanisms as when taking part in an overt conversation.
An interesting consequence of the anticipation mechanism is that we
will "hear" a verbal response twice, first when the preparatory response
activates the auditory cortex and then when we actually hear the sound
produced by the overt utterance. This may be the explanation for the idea
that there is a mental event, a ‘thought’, which precedes and causes the
G The problems of consciousness again
How does the inner world arise?
The inner world is an obvious and unavoidable consequence of the simulation process. If covert behaviour can generate perceptual activity which resembles the activity generated by perception of the external world, then one will perceive something similar to external world, even if it does not exist.
Notice that this conclusion is independent of the mechanisms of perception. In particular, we do not have to make any assumptions at all about the existence "images" or "representations". If we can simulate the perception of x, then it will seem to us that we are observing x, regardless of how the brain normally goes about it to make us see x, that is regardless of whether it uses "images" or "representations".
What are mental objects?
The inner world does not presuppose the existence of mental objects.
The ‘zombie’ problem: why is the inner world necessary?
The answer to this question is simply that an inner world is an unavoidable consequence of simulation. Once we can perform covert movements and these are accompanied with activity in sensory cortex which mimicks perception, the inner world is there.
Do animals have an inner world?
Yes, if they can generate their own sensory input via either a sensory conditioning mechanism or via an anticipation mechanism
Can machines have an inner world?
Same answer as for animals.
I An additional advantage of the simulation hypothesis
There is growing evidence that the cerebellum and basal ganglia, usually
regarded as strictly motor structures, participate in cognitive functions.
This has seemed puzzling to many but is a logical consequence of the view
presented in this lecture. If movement simulation is movement preparation,
it must involve the same brain structures. This also explains the cognitive
symptoms of cerebellar or basal ganglia disease.
Previous versions of the simulation hypothesis were presented in
Hesslow G. (1994) Will neuroscience explain consciousness? Journal of Theoretical Biology. 171:29-39.
Hesslow, G. (1996) Hjärnan och själen: kan fysiologin förklara den inre världen. I Widén, L. (red) En bok om hjärnan. Sockholm, Tiden.
Many of the critical ideas can be found in the behaviourist literature,
Skinner BF (1953) Science and Human Behavior. Macmillan, New York
Skinner BF (1974) About Behaviorism. Knopf, New York
Much of the empirical evidence for covert behaviour cited in the
lecture can be found in
Jeannerod M (1994) The representing brain: Neural correlates of motor intention and imagery. Behavioral.and.Brain Sciences. 17: 187-245
Evidence for simulation of perception is reviewed in
Kosslyn SM (1994) Image and Brain: The Resolution of the Imagery Debate. MIT Press, Cambridge