The human brain, which works by processing discrete nerve impulses, is often regarded as a digital computer. Daedalus wants to measure its processing power in operations per second. Modern magnetic-resonance imaging can detect a region of high brain activity by its increased metabolic rate, but cannot deduce the corresponding digit rate. Daedalus now has an improvement.

He points out that the basic processing unit of the whole nervous system is the nerve pulse. At each point it involves a radial movement of sodium ions lasting about a millisecond. Sodium nuclei have spin and magnetic moment. In a suitable magnetic field, a resonating radio-frequency can elevate them to an excited state, from which they will later relax back to the ground state. Suppose that radio-frequency is 1 kilohertz, corresponding to a time constant of a millisecond. Then relaxation should be decidedly stimulated if the nuclei are being vibrated with the same time constant — as by being in a nerve along which a pulse is passing.

The magnetic field for which sodium ions resonate at 1 kHz is a modest 0.9 microtesla. Daedalus's nuclear magnetic psychometer places the subject's head in such a field and irradiates it at 1 kHz. It measures the relaxation times of the sodium ions in the various regions of his brain. The greater his mental activity, the faster the ions will relax; or more exactly, the stronger the peak of their relaxation-time spectrum within the 1 ms region. Once properly calibrated, the instrument will give the total rate of working for the subject's brain, in pulses per second.

Psychology will at last have a sound numerical basis. Daedalus expects that subjects with a high IQ will show a greater processing rate than those of low IQ. But asked to solve a problem, their rate of working will rise less — their algorithms will be more efficient. Yet intuitive types with an unexceptional IQ may still show high firing rates, from their active imaginations. As the subject learns a new skill, the firing rate in the relevant brain region will rise, and then decline as he automates his new ability. Skilled meditators may be able to drive their brain activity right down; but even a sleeping or drugged subject will still show the processing needed for the brain's steady ‘housekeeping’ — maintaining heart rate, temperature, peristalsis and so on. All the values should exceed a billion pulses per second, showing up modern computers for the primitive devices they are.