How a person remembers a touch

"A new touch does not erase the memory of a previous touch from working memory.  Rather, new and old tactile memories can persist independently of each another, once a person's attention has registered the touches," said the study leader, whose work is now published in the current issue of the journal PNAS.

The scientists of the Department for Neurology and the Bernstein Center for Computational Neuroscience at the Charité pursued the question, in which form multiple touch sensations are represented in human working memory. Working memory is responsible, for example, for the temporary storage of information, which is important to understand the environment currently surrounding us. In the present experiment, the test persons were stimulated by tactile stimulation devices (i.e. animating to the sense of touch), such as those used for reading Braille, delivering vibrations of two different frequencies to the index fingers. After stimulation, the participants were told which of the two frequencies they should compare with a subsequent test frequency.

In early brain regions in the "feeling center," where the information of the sense of touch is first directed and processed, systematic changes in cerebral activity occurred when subjects remembered a touch. These changes in activity, which were seen in the so-called alpha rhythm in the early brain regions, were however still unspecific with respect to the task-relevant information.

The memory of different touches, with the distinction between the two frequencies with which the subjects have been stimulated, takes place in higher regions of the brain, in the so-called frontal lobes. Here the researchers could identify brain waves (oscillations) of a specific wavelength, the so-called beta rhythm, which were systematically modulated by the memory of the two different vibration frequencies. Of particular interest was the fact that the frontal beta activity is not limited to the most recently presented frequency. The test persons were also able to reproduce a previous frequency if they were asked to remember. These results indicate the existence of a quantitative tactile memory representation in the human frontal lobes. This memory representation can be controlled consciously. It is subject to the active control of individuals on the current contents of their memory.