Covers

For decades neurophysiologists have aspired to accomplish visualization of neuronal assemblies in action.  A neuronal assembly may be defined as a group of neurons that cooperate to perform a specific computation required for a specific task. The activity of cells in an assembly is timelocked (coherent). However, the cells that comprise an assembly may be spatially intermixed with cells in other neuronal assemblies that are performing different computational tasks. The visualization of the spatio-temporal pattern of ongoing or evoked coherent-activity in neuronal assemblies has been accomplished by making use of the fact that activity of the neurons in an assembly is timelocked. The firing of a single neuron served as a time reference to selectively visualize only activity that was synchronized with it.
To study the spatio-temporal organization of neuronal assemblies single unit recordings and optical imaging based on voltage sensitive dyes were combined.  Subsequently,  spike-triggered averaging of the optical images relative to the spike of the firing neurons were performed. With sufficient averaging the activity of other neuronal assemblies not timelocked to the reference neuron was averaged out, thus enabling the selective visualization of the reference neuron's coherent assembly.

Optical imaging based on intrinsic signals enables mapping of the geometric relationship between various modules underlying visual processing in primary visual cortices of monkeys (the cube) and cats (the ellipse). The suggestions that geometric relationships exists (as schematically depicted on the surface of the cube) were first put forward by Hubel & Wiesel and Hubel & Livingstone in their famous ice-cube model. They were later revealed by intrinsic optical imaging (as schematically depicted on the surface of the cube). Optical imaging based on intrinsic signals can be combined with anatomical methods such as biocytin labeling of single neurons elucidating the relationship between neural structure and function (bottom left square). It also permits chronic recordings of functional maps providing new insights into the postnatal development of the neocortex or recording from behaving monkeys explaining higher brain functions.

Measured in vivo by voltage-sensitive dyeimaging combined with whole-cell voltage recodings and neuron reconstructions.