Research (for beginners)

What language does the brain use to generate sensations? Does the language change as we accumulate new sensory experiences? Concentrating on the sense of touch, the laboratory is tackling these problems using methods ranging across electrophysiology (measuring cellular activity), computational modeling (understanding the mathematical rules of brain function), and human psychophysics (studying how the subjective impression of a stimulus relates to its physical properties).

Ancient history

The somatosensory cortex is a long strip-shaped brain region responsible for the tactile sense, the sense of touch. Different zones of somatosensory are activated by stimulation of different sites on the skin. The organization of the connections from skin to cortex is not random; rather, nearby points on the skin are represented by nearby regions in the somatosensory cortex. This led people to speak of a somatosensory "map".

In the 1990's, Diamond, Favorov and Whitsel found that this map is not continuous: distinct blocks in the brain (called columns) represent distinct patches of the skin. Within one of these columns, all nerve cells respond to the same skin site. Cortical columns are connected to each other, relaying messages back and forth. A major discovery in the 1980's from the laboratories of Mike Merzenich and Jon Kaas was that cortical maps are not fixed - they can be modified by an experience as simple as repetitive stimulation of the skin. This flexibility of the brain is called plasticity. Diamond found that cortical map plasticity is brought about, at least in part, by modifications in the connections between the columns.

More recently...

In 1996 Diamond began to set up the SISSA Tactile Perception and Learning Laboratory to continue investigating cortical organization. The main guiding strategy has been to carry out physiological and behavioral experiments in laboratory animals (rats) and psychophysical experiments in humans to look for organizational rules that unify these distant species. Together with Carlo Porro (University of Udine), Diamond found that, when a person imagines his skin being touched, the resulting activity in somatosensory cortex is similar to when the touch really occurs. This finding made them think of somatosensory cortex as not just a mirror of the ongoing skin stimulation, but rather a place where felt stimulations can be represented, stored and later remembered.

Justin Harris (lab member from 1999-2002) discovered that somatosensory cortical maps are even involved in learning. This was shown in an experiment during which people were trained to distinguish rough and smooth surfaces with one fingertip. When asked to apply the learned skill to another fingertip, the closer the second finger was to the first ("trained") finger, the better the subjects performed. This is a hint that somatosensory cortex, where neighboring fingers are represented in neighboring brain regions, is involved in this learning process.

Our current work is concerned with the question of which properties of stimulation at the fingertips (frequency, regularity) are picked up by the brain and how this affects the improvement in a person's ability to distinguish between different stimuli.


Experiments with humans have the advantage that we can simply ask the subject what he feels (you can also ask a laboratory mouse, but he won’t answer). But human experiments have the great drawback that we can see little of how the brain works. We can control the input through the senses and measure the output through a person's reactions, but in between, the brain's mechanisms remain hidden in a "black box". Methods like fMRI or EEG can tell us roughly which brain regions are active, but cannot tell us how the cells in the brain behave during a sensory experience. Only in animals can we introduce electrodes into the brain to measure directly the activity of single nerve cells or groups thereof.

Rats are among the preferred laboratory animals in neuroscience. They use their whiskers, the long, stiff hairs sticking out from their snouts, to feel what is around them, with at least the same sensitivity that we have in our fingertips. Moreover, rats have a map of the whiskers in the cortex just like we have a map of the hand.

Harris and Diamond performed experiments with the rats that were analogous to the human fingertip experiment described above. Rats where trained to do a sensory task with one whisker hair and their ability to transfer the learned task to another whisker was measured. In a perfect parallel to the human experiment, the rats did better when the second whisker was close to the original "trained" one. Again, this was a hint that somatosensory cortex, where neighboring whisker hairs correspond to neighboring cortical columns, plays an important role in the process of learning and recalling.

Now

The laboratory has been developing a quantitative understanding of how neurons at multiple levels of the sensory pathway encode dynamic, complex whisker stimuli in their spike trains. This involves analyses of neuronal activity during computer-controlled "noisy" stimuli as well as with natural textural stimuli. We have monitored the vibrations produced in the shaft as the whisker sweeps across different surfaces and have identified distinctive patterns of neuronal activity induced by such vibrations. These patterns seem to be the neuronal "signature" for textures (Arabzadeh et al., 2005).

Experiments like these, carried out in anesthetized animals, serve as the starting point for studies of neuronal activity in awake, behaving animals. We have been trying to figure out the "code" used by brain cells to represent the properties of what the whiskers are touching. There are many challenges – in the behaving animal, the whisker sensory system is ‘‘active’’: the animal generates sensory signals by palpating objects through self-controlled whisker motion (just as we move our fingertips along surfaces to measure their tactile features). In our experiments, rats touched rough or smooth textures with their whiskers and turned left or right for a reward according to the texture identity. Monitoring behavior with high-speed videography, we have found that on trials when the rat correctly identified the stimulus, the firing rate of cortical neurons varies during a window of a few hundred milliseconds before making a decision according to the contacted texture: high for rough and lower for smooth. This firing-rate code is reversed on error trials (lower for rough than smooth). So when cortical neurons report the wrong stimulus, the rat, ‘‘feeling’’ the signals of its cortical neurons, fails to identify the stimulus. We conclude that barrel cortex firing rate on each trial predicts the animal’s judgment of texture. This work provides us with many new insights about how to "decode" what the rat has touched only by seeing its neuronal activity. This, after all, is exactly what the rat's brain must accomplish! We are now particularly interested in higher-level representations of tactile stimuli – how the physical parameters of whisker motion become transformed to a representation of object identity.

last modified: 20 October 2011