Long-term voltage-sensitive dye imaging reveals cortical dynamics in behaving monkeys.

A novel method of chronic optical imaging based on new voltage-sensitive dyes (VSDs) was developed to facilitate the explorations of the spatial and temporal patterns underlying higher cognitive functions in the neocortex of behaving monkeys. Using this system, we were able to explore cortical dynamics, with high spatial and temporal resolution, over period of <or=1 yr from the same patch of cortex. The visual cortices of trained macaques were stained one to three times a week, and immediately after each staining session, the monkey started to perform the behavioral task, while the primary and secondary visual areas (V1 and V2) were imaged with a fast optical imaging system. Long-term repeated VSD imaging (VSDI) from the same cortical area did not disrupt the normal cortical architecture as confirmed repeatedly by optical imaging based on intrinsic signals. The spatial patterns of functional maps obtained by VSDI were essentially identical to those obtained from the same patch of cortex by imaging based on intrinsic signals. On comparing the relative amplitudes of the evoked signals and differential map obtained using these two different imaging methodologies, we found that VSDI emphasizes subthreshold activity more than imaging based on intrinsic signals, that emphasized more spiking activity. The latency of the VSD-evoked response in V1 ranged from 46 to 68 ms in the different monkeys. The amplitude of the V2 response was only 20-60% of that in V1. As expected from the anatomy, the retinotopic responses to local visual stimuli spread laterally across the cortical surface at a spreading velocity of 0.15-0.19 m/s over a larger area than that expected by the classical magnification factor, reaching its maximal anisotropic spatial extent within approximately 40 ms. We correlated the observed dynamics of cortical activation patterns with the monkey's saccadic eye movements and found that due to the slow offset of the cortical response relative to its onset, there was a short period of simultaneous activation of two distinct patches of cortex following a saccade to the visual stimulus. We also found that a saccade to a small stimulus was followed by direct transient activation of a cortical region in areas of V1 and V2, located retinotopically within the saccadic trajectory.