CORRESPONDENCE BETWEEN VISUALLY EVOKED VOLTAGE-SENSITIVE DYE SIGNALS AND SYNAPTIC ACTIVITY RECORDED IN CORTICAL PYRAMIDAL CELLS WITH INTRACELLULAR MICROELECTRODES

Fast, multiple-site optical recording of voltage-sensitive dye (VSD) s ignals and intracellular microelectrode recordings were combined to ch aracterize visually evoked neuronal responses in the visual cortex of the pond turtle, Pseudemys scripta. By using an in vitro, eye-brain pr eparation stained with the merocyanine oxazolone voltage-sensitive dye , NK-2495 or a close analog, NK-2761, large VSD signals relatively fre e of vibrational noise could be recorded in single trials following a stroboscopic light flash to the contralateral eye. VSD signals recorde d from the same cortical location in repeated trials exhibited conside rable variability in the onset, duration, and amplitude of secondary d epolarizations. Because of this variability, secondary depolarizations were largely absent in signal-averaged responses. Superposition of VS D signals with intracellular recordings obtained from cortical pyramid al cells revealed a close correspondence between their signal waveform s. The two signals were virtually identical in their onset, initial ra te of rise, and time-to-peak. At later periods (>500 ms), the correspo ndence was less close, especially for large cortical depolarizations. Some of this disparity could be attributed to contamination of the VSD signal by a large intrinsic optical response. A second contribution w as a failure of the VSD signal to register asynchronous regenerative e ffects occurring in single pyramidal cells. It is suggested that the c lose correspondence between the microelectrode and optical recordings in the early phase of the response may reflect the organization of pyr amidal cells into clusters that receive virtually identical synaptic i nputs.