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. In collaboration with Carlo Porro at the University of Udine, an fMRI study found that somatosensory cortex in humans is activated not only during the actual delivery of tactile stimuli, but also during mental imagery of the same stimuli, in the absence of touch (Porro et al., 1996). Fascinated by the finding that somatosensory cortex is not limited to the on-line processing of sensory data, the research projects turned progressively more to the issue of the representation, storage, and recall of information in cortex. Justin Harris (laboratory member, 1999-2002) discovered the principle of "topographic learning": in humans, the neuronal modifications that underlie perceptual learning are distributed according to the spatial framework of the cortical body map (Diamond et al., 2001; Harris et al. 2001a).
Subsequent work extended the principle to short-term memory: the somatosensory cortical map is involved with the storage of information when the stimulus features need to be transiently conserved for subsequent analysis (Harris et al. 2001b, Harris et al., 2002). The somatosensory cortical map thus takes its place as one part of the working memory circuit controlled by prefrontal cortex. Current psychophysical work (Harris et al. 2006) concerns the question of how the perceptual properties of vibration (i.e. frequency, regularity vs noisiness) emerge from cortical activity, and the mechanisms underlying fast perceptual learning - rapid improvements in vibration judgments during training (unpublished).
While human psychophysics is invaluable, problems in neuronal coding and representation cannot be approached directly without experiments in laboratory. The laboratory has been investigating the vibrissal system of rats, selected because the tactile capacities of this sensory system are comparable to those of human finger tip. The lab documented the spatial and temporal distribution of neuronal activity in the tactile region of rat cerebral cortex ("barrel" cortex") using a 100-channel electrophysiological recording system (Rousche et al., 1999; Petersen and Diamond, 2000). Comparing barrel cortex functional organization to the distribution of tactile learning, Diamond's laboratory showed that the "topographic learning" principle generalizes from humans to rats - tactile information is stored within the spatial structure of maps (Harris et al., 1999; Diamond et al., 1999). In a related set of experiments with Rasmus Petersen and Stefano Panzeri, the laboratory used rigorous Information Theory methods to investigate exactly which features of ensemble neuronal activity report information about whisker stimuli (e.g. Panzeri et al., 2001; Petersen et al., 2001; Petersen et al., 2002).