The philosopher Hume said that «nothing is more free than the imagination of man», and in earlier times another philosopher, Stilpon of Thebes, was calm while Alexander the Great sacked his city, because no-one would have been able to take away his wisdom. Thought is the product of the mind, of a structure radically distinguished from the brain from the time of Descartes, who was not able to reconcile the res cogitans with the res extensa, and compromised by joining them together in the pineal body.
Today the distance between mind and brain does not seem so great; progress in neurophysiology and neuropsychology has meant that we can roughly calculate how long an elementary mental operation takes, as well as giving an illustration of the brain zones involved.
Let us look at some examples, in which all the data come from daily practice at the St. John the Baptist Hospital in Rome (see picture).

Average reaction times in a normal population. Simple reaction times are in light blue, and reaction times to a choice of colours (test not invalidated by culture) in pink. The difference between them indicating the central time required to perform the discrimination (circa 0.1 of a second) is in white.

Reaction times
The subject has to press a button as soon as a red field appears at random for 18 msec on a screen with an 8' angle on the horizontal plane and a 6' one on the vertical plane. The times of 15 attempts are measured and the average and standard deviation are calculated. The mean value is called «simple reaction time»; it happens every day when the car in front of us suddenly brakes.
Subsequently, two random frames are shown, one of which is the red one described above and the other blue with the same exposure time. The subject has to press at the red and not at the blue. The times of 15 + 15 attempts are measured and the mean and standard deviation are calculated. The mean time is called «choice reaction time»; it happens every day when we reach a traffic light and have to decide whether to stop or go on.
If the simple reaction time is subtracted from the choice reaction time, we obtain a value very similar to the central discrimination time (see graphic in this paragraph).
Normally, a young subject in good health has a simple reaction time of 200 thousandths of a second (msec) and a choice reaction time of 300. The difference is therefore 100 msec. This is an example of measuring a mental operation, independently of perception and motor response. Experience tells us that training improves only the choice reaction time, precisely because it can lower those 100 central msec.

Right and left, typical P300 potentials (in this case with latency of 339 msec), registered by four zones of the two hemispheres. The vertical bars provide an immediate reading of the latency of the desired peak.

P300
The experiment we have described is also carried out, with a few variations, in an electrophysiological environment. In this case the «thought» will be graphically represented by an electric potential.
The operation goes as follows: a subject wearing an electrode on the top of his head and a reference electrode on his ear lobe is asked to look at a screen where 180 large and small (these latter less frequent) frames appear in a random sequence.



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