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