What is PCT?

The closed loop…

This system that corrects error to help us reach our goals can be shown in a diagram. It’s called a negative feedback loop or a closed loop.

Closed loops come up in lots of places – from the system in your body that helps to control its temperature – to the thermostat in your house. Even guided missiles use the same idea.


In PCT, closed loops are the only component of the theory - they form the fundamental structure of the way that the brain and nerves of a person work according to the theory.

Let’s have a look at a simple closed loop working for temperature control.

closed loop

The reference value for the temperature is 37˚

The input is the temperature – 35˚ in this case due to a cool breeze – the disturbance from the environment

The error is 2˚

This drives the output – in this case shivering more. The shivering warms the skin, getting the body temperature nearer 37˚

All the parts of this loop are working at once in the body, as are the loops controlling many other important variables we need to keep just right…

The closed loop in detail…

We have explained a simple loop for controlling temperature…

But how does PCT control experiences – perceptual variables?

The diagram below, taken from Bill Powers’ most recent book, shows the full details of a closed loop in PCT.

It’s essentially the same with a few more details… First, it shows what are signals and what are functions. The functions have mathematical formulas that change one signal, or a set of signals into a new signal.

Second, it shows that the environment can provide feedback to the person, not just a disturbance. One example of feedback is when you talk into an amplifier – it feeds back your output (voice)!

rat Here is another colourful example. Albino rats have poor vision with their pink eyes owing to low levels of melanin in the retina. So, to see properly they sway their head from side to side; this helps them to see in 3D as the image to each eye changes with the headsway - this behaviour is the control of their perception. Click here to see! In an academic article, Robert Patterson explains why a control theory approach is vital to understand 3D vision (steropsis) in all animals.

Third, this diagram illustrates that goals can operate without any requirement for 'awareness' or 'consciousness' in the system. This is consistent with emerging evidence from cognitive science, summarised here in a paper by Custers and Aarts (2010).

Fourth, and really important, is that this loop is at the bottom of many more loops – it sends information up to them and receives its information from them…

So what is going on here?