Imaging plasticity and structure of cortical maps in cat and mouse visual cortex

The study reported in the first part of this thesis utilized optical imaging of intrinsic signals to visualize changes in orientation maps in cat visual cortex induced by pairing a visual stimulus with an intracortical electrical stimulation. We found that the direction of plasticity within orientation maps depends critically on the relative timing between visual and electrical stimulation on a millisecond time scale: a shift in orientation preference towards the paired orientation was observed if the cortex was first visually and then electrically stimulated. In contrast, the cortical response to the paired orientation was diminished if the electrical preceded the visual cortical stimulation. Spike-time-dependent plasticity has been observed in single cell studies; however, our results demonstrate an analogous effect at the systems level in the live animal. Thus, timing-dependent plasticity needs to be incorporated into our conception of cortical map development. While the pairing paradigm induced pronounced shifts in orientation preference, the general setup of the orientation preference map remained unaltered. In order to unravel potential factors contributing to this overall stability, we determined the distribution of plasticity across the cortical surface. We found that pinwheel centers, points were domains of all orientation meet, exhibited less plasticity than other regions of the orientation map. The resistance of pinwheel centers to changes in orientation preference may support maintenance of the general structure of the orientation map. The study that forms the second part employs optical imaging to visualize the retinotopy in mouse visual cortex. We were able to resolve the pattern of retinotopic activity with high precision and reliability in the primary visual cortex (area 17). Functional imaging of the position, size and shape of area 17 corresponded exactly to the location of this area in stained histological sections. The imaged maps were also confirmed with electrophysiological recordings. The retinotopic structure of area 17 showed very low inter-animal variability, thus allowing averaging maps across animals and therefore statistical analysis. These averaged maps greatly facilitated the identification of at least four extrastriate visual areas. In addition, we detected decreases in the intrinsic signal below baseline with a shape and location reminiscent of lateral inhibition. This decrease of the intrinsic signal was shown to be correlated with a decrease in neuronal firing rate below baseline. Both studies were facilitated by the development of a signal analysis technique (part III), which improves the quality of optical imaging data. Intrinsic signal fluctuations originating from blood vessels were minimized based on their correlation with the actual superficial blood vessel pattern. These fluctuation components were then extracted from images obtained during sensory stimulation. This method increases the reproducibility of functional maps from cat, rat, and mouse visual cortex significantly and might also be applied to high resolution imaging using voltage sensitve dyes or functional magnetic resonance

Versatile 3D-Printed Micro-Reference Electrodes for Aqueous and Non-Aqueous Solutions

While numerous reference electrodes suitable for aqueous electrolytes exist, there is no well-defined standard for non-aqueous electrolytes. Furthermore, reference electrodes are often large and do not meet the size requirements for small cells. In this work, we present a simple method for fabricating stable 3D-printed micro-reference electrodes. The prints are made from polyvinylidene fluoride, which is chemically inert in strong acids, bases, and commonly used non-aqueous solvents. We chose six different reference systems based on Ag, Cu, Zn, and Na, including three aqueous and three non-aqueous systems to demonstrate the versatility of the approach. Subsequently, we conducted cyclic voltammetry experiments and measured the potential difference between the aqueous homemade reference electrodes and a commercial Ag/AgCl-electrode. For the non-aqueous reference electrodes, we chose the ferrocene redox couple as an internal standard. From these measurements, we deduced that this new class of micro-reference electrodes is leak-tight and shows a stable electrode potential

Pairing-Induced Changes of Orientation Maps in Cat Visual Cortex

AbstractWe have studied the precise temporal requirements for plasticity of orientation preference maps in kitten visual cortex. Pairing a brief visual stimulus with electrical stimulation in the cortex, we found that the relative timing determines the direction of plasticity: a shift in orientation preference toward the paired orientation occurs if the cortex is activated first visually and then electrically; the cortical response to the paired orientation is diminished if the sequence of visual and electrical activation is reversed. We furthermore show that pinwheel centers are less affected by the pairing than the pinwheel surround. Thus, plasticity is not uniformly distributed across the cortex, and, most importantly, the same spike time-dependent learning rules that have been found in single-cell in vitro studies are also valid on the level of cortical maps