Neural Processing, Neuromodulation and sensory plasticity
Dir. Daniel Shulz

Using the barrel cortex of the rat as a system model, our research is centered on the study of neuronal processes responsible for the coding of sensory information and perception, as well as their regulation through the interaction of the animal with the environment.

 

  • We are interested in the propagation and integration of neuronal information in the primary somatosensory cortex and the emergence of collective properties in response to spatially distributed stimuli on the receptor surface.
  • In addition, we study the functional and synaptic adaptation described by Hebbian and non-Hebbian plasticity algorithms.
  • Finally, we include in this research, the study of permissive factors linked to the attentional and behavioral state of the animal which are mediated by ascending neuromodulatory systems, with a special focus on cholinergic and noradrenergic systems.


Among our recent observations, we have provided evidences for a plasticity of the cortical representation of certain temporal parameters of tactile stimuli that depends for its expression on the presence of Acetylcholine, a neuromodulator implicated in the regulation of the attentional state of the animal. In addition, we have shown in vivo the occurrence of a synaptic plasticity controlled by the temporal contiguity between the synaptic inputs and the postsynaptic action potentials. Finally, the recent development of a 25-whisker stimulator allowed us to reveal a new kind of cortical integration of tactile information spatially distributed on the whole whisker pad. This last results led us to question the classical view of the organization of the somatosensory system of the rat as a sum of independent cortical columns and to propose a more integrated view in which horizontal information transfer in the thalamus and the cortex ensures the detection of natural complex stimuli.

 

As they search and locomote through their environment, rats move their vibrissae in co-ordinated
waves at a dominant frequency of 5 to 10 Hz, generating multi-whisker multiple contacts with
obstacles and objects. Through these contacts, the animal extracts information about their position
as well as their form and texture. (Gao et al., 2001). Sensory whiskers in the mystacial pad of
rodents are mapped onto layer IV of the postero-median region of the primary somato-sensory
cortex as discrete units named "barrels". The detailed description of the organization of the barrel
cortex into discrete architectonic modules (Woolsey and van der Loos, 1970) has triggered a large
number of functional studies in the last thirty years, taking advantage of its unique punctuate
characteristic. In particular, the vibrissal system of the rodent has become one of the dominant
models for investigating the mechanisms of sensory information processing within the putative
basic computational unit of the cortex, the cortical column (Petersen and Sakmann, 2001).
Numerous anatomical and extra-cellular electrophysiological studies have demonstrated a one-toone
correspondence between a mystacial vibrissa and its corresponding cortical barrel (Killackey
and Belford, 1979, Simons, 1985). Thus, the barrel cortex has been classically considered as a
collection of independent anatomo-functional columns. Recently, whole cell recordings of synaptic
responses evoked by individual whisker deflections were obtained from neurons in layers 2 to 5 of
the primary somato-sensory cortex of adult rats in a few laboratories across the world, including
our own (see Figure below). Despite its original conception as a uniquely segregated cortex, the
convergence of information onto single neurons was found to be extensive, spanning several
vibrissae from the centre of the receptive field (Moore and Nelson, 1998, Zhu and Connors, 1999,
Jacob et al., 2004). This spread of spatio-temporal subthreshold RFs in the vibrissal cortical
representation is similar to that described in visual cortex by the team of Yves Frégnac (Bringuier
et al, 1999), and suggests that rat barrel cortex has a wide array of dynamic cortico-cortical
interactions that provides a potential substrate for complex temporal interactions over millisecond
to second time-scale. Such context-dependent higher-order interactions have been characterized
so far in the visual and the auditory cortices. They were not investigated extensively in the barrel
cortex largely because stimulation techniques did not allow one to stimulate more than a few
whiskers simultaneously. To solve this problem, a vibrissal stimulation matrix (described in next
sections) that allows independent stimulation of up to 25 vibrissae has been developed in the last
few years by my team.

Our research program addresses the question of the neuronal coding of complex natural patterns
of sensory inputs in the primary somato-sensory cortex. Our working hypothesis is double: first, the
cortical response is contextual in the sense that it will be different if the stimuli are presented in a
spatial and temporal complex sequence or isolated. Second, we propose that the statistics of the
whisker contacts when the rat explores natural tactile scenes should have a significant impact on
the cortical processing of information and on plasticity of the barrel cortex. Our prediction is that if
we probe the system with controlled “natural” sensory inputs in an in vivo preparation, progresses
would be obtained in our understanding of the functional architecture of the system and the
plasticity rules that underlie perceptual learning.

Selected Publications

Luc Estebanez, Sami El Boustani, Alain Destexhe and Daniel Shulz, Correlated input reveals coexisting coding schemes in a sensory cortex, Nature Neuroscience 15 No12: 1691-1699, (2012) [pdf]

Vincent Jacob, Julie Le Cam, Valerie Ego-Stengel and Daniel Shulz, Emergent Properties of Tactile Scenes Selectively Activate Barrel Cortex Neurons, Neuron 60: 1112-1125, (2008) [pdf] [abstract]

Valerie Ego-Stengel, Julie Le Cam and Daniel Shulz, Coding of apparent motion in the thalamic nucleus of the rat vibrissal somatosensory system, The Journal of Neuroscience 32: 3339-3351, (2012) [pdf]

Vincent Jacob, Daniel J. Brasier, Irina Erchova, Daniel E. Feldman and Daniel Shulz, Spike timing-dependent synaptic depression in the in vivo barrel cortex of the rat, J Neurosci 27: 1271-84, (2007) [pdf] [abstract]

Vincent Jacob, Luc Estebanez, Julie Le Cam, Jean Yves Tiercelin, Patrick Parra, Gérard Parésys and Daniel Shulz, The Matrix: a new tool for probing the whisker-to-barrel system with natural stimuli, J Neurosci Methods 189 : 65-74, (2010) [pdf] [abstract]