FUNCTIONAL-STUDY OF THE RAT CORTICAL MICROCIRCUITRY WITH VOLTAGE-SENSITIVE RYE IMAGING OF NEOCORTICAL SLICES

The computations performed within cortex are likely to be determined b y its internal dynamics in addition to its pattern of afferent input. As a step toward characterizing these dynamics, we have imaged electri cal activity in slices from rat primary visual cortex stained with the voltage-sensitive dye di-II-ANEPPS. In response to electrical stimula tion two fluorescence signals of similar maximum amplitude are elicite d. (i) A fast signal that peaks in a few milliseconds, is dependent on membrane voltage, and has a significant presynaptic component. This s ignal can be used to image electrical activity ratiometrically. (ii) A slow signal that peaks a few seconds after stimulation, does not refl ect voltage changes, and may originate from changes in scattering prop erties of the slice and from interactions of the dye with the cells. T he spatial pattern of fast signals obtained in response to focal stimu lation of coronal slices is consistent with known interlaminar project ion patterns. In tangential slices, imaging of fast signals reveals cl ustered horizontal responses. Finally, imaging of fast signals during epileptiform activation of the disinhibited circuit reveals propagatin g responses, without evidence for modular activation.