Optical imaging methods based on voltage-sensitive dyes (VSD) allow one to monitor the activity of entire neuronal populations concurrently. Thanks to newly developed dyes and advanced cameras, this technique affords high spatiotemporal resolution and high signal/noise ratios. While current methods of VSD imaging have resulted in substantial advances in our understanding of primary visual cortex (V1), these methods can be further enhanced (Aim 1), and applied to new areas of inquiry (Aim 2).
The first aim i s to develop methods of stimulation and data analysis that will increase signal/noise ratio and allow a broader range of measurements. In preliminary measurements, we found that the power distribution of the physiological noise in VSD measurements is concentrated at low frequencies. Noise power decreases with frequency. Traditional methods of stimulation in VSD experiments evoke responses at low frequencies - not an optimal choice. Instead, I propose to employ stimuli that evoke responses oscillating at high frequencies, where they are well clear of the noise. We will test whether indeed these methods are more efficient than the traditional ones, and if they afford increased spatiotemporal resolution. Along these lines, I also propose to apply to VSD imaging powerful methods of white-noise analysis originally developed for electrophysiological techniques. In particular, I propose to obtain maps of orientation preference using white noise in the orientation domain and maps of retinotopy using multi-focal m-sequences. These techniques will allow us to measure the evolution of orientation selectivity in time, and the complex interactions between different retinotopic locations.
The second aim i s to apply VSD imaging to the study of functional connectivity between brain regions. We can achieve this by stimulating or recording electrically in one region, while obtaining event-triggered maps with VSD imaging in the other region. I propose to develop this method and to apply it to the functional connectivity between lateral geniculate nucleus (LGN) and V1. To develop event-triggered VSD imaging, at first we will concentrate on extremely large electrical events: induced interictal events similar to those seen in epilepsy. We will induce these events with focal iontophoresis in V1 of gabazine, a selective GABAA antagonist. Then, we will measure event-triggered maps corresponding to activity in LGN. Activity in LGN can be induced electrically, or through visual stimulation, or can be spontaneous. The resulting spike-triggered maps of VSD signals in V1 will reflect and illuminate the rules of connectivity between LGN and V1. These methods ultimately blur the distinction between anatomy and physiology. If successful, they could in principle be applied to any pair of brain regions, as long as one of them is on the surface of the brain. ? ?
| Benucci, Andrea; Frazor, Robert A; Carandini, Matteo (2007) Standing waves and traveling waves distinguish two circuits in visual cortex. Neuron 55:103-17 |
