Rodents explore their environment with an array of whiskers, inducing complex patterns of whisker deflections. Cortical neuronal networks can extract global properties of tactile scenes. In the primary somatosensory cortex, the information relative to the global direction of a spatiotemporal sequence of whisker deflections can be extracted at the single neuron level. To further understand how the cortical network integrates multi-whisker inputs, we imaged and recorded the mouse barrel cortex activity evoked by sequences of multi-whisker deflections generating global motions in different directions. A majority of barrel-related cortical columns show a direction preference for global motions with an overall preference for caudo-ventral directions. Responses to global motions being highly sublinear, the identity of the first deflected whiskers is highly salient but does not seem to determine the global direction preference. Our results further demonstrate that the global direction preference is spatially organized throughout the barrel cortex at a supra-columnar scale.

We demonstrate here the usefulness of the method for voltage sensitive dye imaging of the mouse barrel cortex. The method could also benefit other experimental approaches and model species.

Signal processing in neuritic trees is ruled by the concerted action of passive and active membrane properties that, together, determine the degree of electrical compartmentalization of these trees. We analyzed how active properties modulate spatial propagation of graded signals in a pair of nonspiking (NS) neurons of the leech. NS neurons present a very extensive neuritic tree that mediates the interaction with all the excitatory motoneurons in leech ganglia. NS cells express voltage-activated Ca(2+) conductances (VACCs) that, under certain experimental conditions, evoke low-threshold spikes. We studied the distribution of calcium transients in NS neurons loaded with fluorescent calcium probes in response to low-threshold spikes, electrical depolarizing pulses, and synaptic inputs. The three types of stimuli evoked calcium transients of similar characteristics in the four main branches of the neuron. The magnitude of the calcium transients evoked by electrical pulses was a graded function of the change in NS membrane potential and depended on the baseline potential level. The underlying VACCs were partially inactivated at rest and strongly inactivated at -20 mV. Stimulation of mechanosensory pressure cells evoked calcium transients in NS neurons whose amplitude was a linear function of the amplitude of the postsynaptic response. The results evidenced that VACCs aid an efficient propagation of graded signals, turning the vast neuritic tree of NS cells into an electrically compact structure.