(Rivista Internazionale - December 1994: Mind and brain are closer now - 3/3)

Retroactive masking diagram. The figure shown to the subject, even for the minimum time of 18 msec, remains in the "buffer" memory system for 100 msec. New information arriving, in the form of a grid of dots, disturbs the processing in progress unless the subject is quick enough to extract it before the grid arrives.

On the other hand, if the exposition were to be shortened (making the task more difficult), the interval should lengthen. In effect, there is an interdependence between target stimulus and duration of the free interval. This is demonstrated in the constant data of the period between the start of the target stimulus and the masking. In the example given this value is 18 + 54 = 72 msec. The capacity of every subject's central processing can be assessed by determining the critical interval needed by the subject to avoid the masking. Certainly the old, and even more patients with brain damage, need greater intervals.
Our observations have shown that there is a difference of 71-75 msec between the young and old in tasks of identifying letters with retroactive masking. Other tests we have done demonstrate that the differences in age are even more pronounced when random numbers are shown instead of the letters of a word, even if the string has the same length. The reason is the absence of the semantic facilitation in this last case, or of the mechanism which enables us to complete an information (reading DRIN it can be found that only a K can make a word of five letters with a complete sense, a deduction which cannot be applied to the fifth unknown figure of random numbers). In young people an average exposition of 21 msec is required to read the word and 28 for the numbers. It is possible to influence perception times or, in other words, to enhance the ability to avoid the masking. In tests conducted in America it has been seen that the interval needed to avoid the masking is reduced by 33 msec (in practice from about 100 to 70 msec) in only five days of practice, even though this improvement did not cancel out the differences of age, because the young in turn dropped from about 65 to 40 msec.
We have thus seen how thought, moved by extremely complex events in the nervous system, can, sometimes be transcribed in the form of an electric potential or elementary chronometric calculation. None of this should displease the poet, who can be reassured by the fact that we are still some light years away from the creation of a model of mental functions, albeit we are no longer bound to the famous motto Ignoramus, ignorabimus according to which we don't know and will never know.

Prof. Salvatore Giaquinto
Head of Department for Neurorehabilitation SMOM
St. John the Baptist Hospital, Rome


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