Why the middle finger has such a slow connection

Each part of the body has its own nerve cell area in the brain -- we therefore have a map of our bodies in our heads. The functional significance of these maps is largely unclear. What effects they can have is now shown by Ruhr-University Bochum (Germany) neuroscientists through reaction time measurements combined with learning experiments and "computational modeling." They have been able to demonstrate that inhibitory influences of neighboring "finger nerve cells" affect the reaction time of a finger.

The fingers on the outside – i.e. the thumb and little finger - therefore react faster than the middle finger, which is exposed to the "cross fire" of two neighbours on each side. Through targeted learning, this speed handicap can be compensated. The working group led by PD Dr. Hubert Dinse (Neural Plasticity Lab at the Institute for Neuroral Computation) report in the current issue of PNAS.

The researchers set subjects a simple task to measure the speed of decision: they showed them an image on a monitor that represented all ten fingers. If one of the fingers was marked, the subjects were to press a corresponding key as quickly as possible with that finger. The thumb and little finger were the fastest. The middle finger brought up the rear. "You might think that this has anatomical reasons or depends on the exercise" said Dr Dinse, "but we were able to rule that out through further tests. In principle, each finger is able to react equally quickly. Only in the selection task, the middle finger is at a distinct disadvantage."

To explain their observations, the researchers used computer simulations based on a so-called mean-field model. It is especially suited for modelling large neuronal networks in the . For these simulations, each individual finger is represented by a group of nerve cells, which are arranged in the form of a topographic of the fingers based on the actual conditions in the somatosensory cortex of the brain. "Adjacent fingers are adjacent in the brain too, and thus also in the simulation", explained Dr. Dinse. The communication of the nerve cells amongst themselves is organised so that the nerve cells interact through mutual excitation and inhibition.

The computer simulations showed that the longer reaction time of the middle finger in a multiple choice task is a consequence of the fact that the middle finger is within the inhibition range of the two adjacent fingers. The thumb and little finger on the other hand only receive an inhibitory effect of comparable strength from one adjacent finger each. "In other words, the high level of inhibition received by the of the middle fingers mean that it takes longer for the excitement to build up – they therefore react more slowly" said Dr. Dinse.

From the results of the computer simulation it can be concluded that weaker inhibition from the neighbouring would shorten the reaction time of the middle finger. This would require a so-termed plastic change in the brain – a specialty of the Neural Plasticity Lab, which has been studying the development of learning protocols that induce such changes for years. One such protocol is the repeated stimulation of certain nerve cell groups, which the laboratory has already used in many experiments. "If, for example, you stimulate one finger electrically or by means of vibration for two to three hours, then its representation in the brain changes" explained Dr. Dinse. The result is an improvement in the sense of touch and a measurable reduction of the inhibitory processes in this brain area. This also results in the enlargement of the representation of the finger stimulated.

The Bochum researchers then conducted a second experiment in which the middle finger of the right hand was subjected to such stimulation. The result was a significant shortening of the of this finger in the selection task. "This finding confirms our prediction" Dr. Dinse summed up. Thus, for the first time, Bochum's researchers have established a direct link between the so-called lateral inhibitory processes and decision making processes. They have shown that learning processes that change the cortical maps can have far-reaching implications not only for simple discrimination tasks, but also for decision processes that were previously attributed to the so-called "higher" cortical areas.

More information: Claudia Wilimzig, Patrick Ragert, and Hubert R. Dinse. Cortical topography of intracortical inhibition influences the speed of decision making, PNAS (2012), doi/10.1073/pnas.1114250109

add to favorites email to friend print save as pdf

Related Stories

Researchers solve mystery of disappearing bird digit

Sep 04, 2011

Evolution adds and subtracts, and nowhere is this math more evident than in vertebrates, which are programmed to have five digits on each limb. But many species do not. Snakes, of course, have no digits, and ...

Finger length clue to motor neuron disease

May 11, 2011

(Medical Xpress) -- People with the commonest form of motor neuron disease (MND) called amyotrophic lateral sclerosis (ALS) are more likely to have relatively long ring fingers, reveals research from the Institute of Psychiatry ...

Acute pain is eased with the touch of a hand

Sep 23, 2010

There may be a very good reason that people naturally clutch their hand after receiving an injury. A new report published online on September 23 in Current Biology shows that self-touch offers significant relief for acute ...

Profound reorganization in brains of adults who stutter

Aug 15, 2011

Hearing Beethoven while reciting Shakespeare can suppress even a King's stutter, as recently illustrated in the movie "The King's Speech". This dramatic but short-lived effect of hiding the sound of one's own speech indicates ...

Learning through mere exposure

May 11, 2011

In cooperation with colleagues from the Leibniz Institute for Employment Research of the TU Dortmund, neuroscientists in Bochum have demonstrated that human visual perception and attention can be improved without training. ...

Recommended for you

Surprising new role for calcium in sensing pain

17 hours ago

When you accidentally touch a hot oven, you rapidly pull your hand away. Although scientists know the basic neural circuits involved in sensing and responding to such painful stimuli, they are still sorting ...

Neurons in human skin perform advanced calculations

Sep 01, 2014

Neurons in human skin perform advanced calculations, previously believed that only the brain could perform. This is according to a study from Umeå University in Sweden published in the journal Nature Ne ...

User comments

Adjust slider to filter visible comments by rank

Display comments: newest first

geokstr
1 / 5 (1) Feb 07, 2012
Dese joiks dint test nobody from Brooklyn or dey woulda seen da fastest middle fingas in the woild.