The analysis of visual stimuli begins in the primary visual cortex of the mammalian forebrain. Here neurons for the first time in the visual pathway show a strong selectivity for the orientation and the direction of movement of a visual stimulus. The intracortical mechanisms involved in the generation of these features are still highly controversial. Previous studies suggest that the intracortical mechanisms involved in orientation and direction selectivity can be may be independent and show a different developmental time scale. The role of intracortical inhibitory circuits in setting up these receptive field properties appear to be different. Whereas a nonspecific mechanism is sufficient to create orientation tuning direction selectivity in the upper cortical layers requires specific inhibitory inputs from cortical regions tuned to the non-preferred direction. Direction and orientation selectivity are first created in cortical layer 4, i.e. the initial synaptic mechanisms involved should be manifested here, e.g. spatially asymmetric inhibition has been proposed as a potential mechanism for creating cortical direction selectivity in layer 4. To test this hypothesis we propose a combination of in vivo and in vitro experiments examining the relationship between intrinsic circuits and functional maps during postnatal development of the ferret visual cortex. orientation and direction preference maps will be recorded in vivo and the spatial distribution and orientation/direction tuning of intracortical excitatory and inhibitory inputs to individual neurons will be mapped in vitro.
In aim 1 we determine if intracortical inhibitory inputs to layer 4 neurons show asymmetries in their strength and spatial distribution. To do this we will analyze the spatial pattern of inhibitory synaptic inputs to layer 4 stellate cells in coronal and tangential slice preparations using scanning photostimulation.
In aim 2 we test the hypothesis that direction and orientation tuning in layer 4 depend on different intracortical mechanisms. We will examine the orientation and direction specificity of inhibitory synaptic inputs to individual neurons. The properties of inhibitory connections will be analyzed in view of current models of orientation and direction selectivity.
In aim 3 we investigate whether intracortical connections cluster in a direction specific manner, and whether orientation and direction specific clustering occur at the same time. To do this we will perform intracortical injections of extracellular tracers in orientation and direction domains in vivo following optical imaging of orientation and direction preference maps. The orientation and direction tuning of local, medium and long-range axonal projections will be analyzed by calculating the tuning difference between the injection sites and the location of neurons projecting to that location.
|Pallas, Sarah L; Wenner, Peter; Gonzalez-Islas, Carlos et al. (2006) Developmental plasticity of inhibitory circuitry. J Neurosci 26:10358-61|
|Roerig, Birgit; Chen, Bingzhong; Kao, Joseph P-Y (2003) Different inhibitory synaptic input patterns in excitatory and inhibitory layer 4 neurons of ferret visual cortex. Cereb Cortex 13:350-63|