In recent years, researchers have developed a number of methods that enable them to remotely control specified groups of neurons and to probe the workings of neuronal circuits.
This involves creating genetically engineered mice expressing algal proteins called Channelrhodopsins in specified groups of neurons.
The technique, known as optogenetics, allowed the scientists to artificially cause groups of neurons to fire in unison by pulsing light into the brains of the mice.
More recently, at Duke, the neuroscientist Warren Meck has suggested that timing is governed by groups of neurons that oscillate at different frequencies.
The plan was to use the device, which had two L.E.D.s, to study two groups of neurons in a region called the nucleus accumbens, which humans also possess.
Pulses of laser light can then be used to activate or silence specified groups of neurons on a millisecond-by-millisecond timescale.
These first stimulations may trigger other stimulations that associate the earlier ones with one another; in turn, these groups of neurons become elements in ever more intricate mappings.
With impressive technical advances such as optogenetics, in which light beams excite or silence targeted groups of neurons, scientists are beginning to control memories at an unprecedented level.
The brain develops in a similar way, he suggests: connections among groups of neurons that are most effective in their reactions to certain stimulations become strengthened and succeed in affecting perception and behavior.
TMS, positron emission tomography (PET) and fMRI all are ways of visualising what's going on in the human brain, down to the firing of tiny groups of neurons, before, during and after various interventions.
To analyze the role of small groups of neurons, scientists have relied on a method not unlike the one that Hess used with his cats: stimulating targeted brain areas, in experimental animals, with thin electrodes.
