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Abstract:
In typical adult visual processing, low-level visual features are integrated into a global construct that enables the recognition of an object. Behaviorally, young primates are impaired at integrating global form and motion cues. Also the neural machinery to support global processing is not fully developed. However, earlier studies using single-unit electrophysiology show that neuronal response properties are relatively mature compared to behavioral capability. Behavioral sensitivity to global stimuli continues to improve for months or years beyond the time that neuronal responses are adult-like. To understand this discrepancy, we used a larger scale method to investigate cortical development, functional MRI.
We tracked the development of BOLD activation in striate and extra-striate cortex of macaque monkeys (Macaca mulatta) longitudinally over 2-3 years. The animals were imaged at 4.7T while anesthetized and paralyzed. To segregate dorsal and ventral stream activity, we used stationary and dynamic Glass pattern stimuli. These have comparable local features (dipoles) but different global forms (concentric or radial) and responses were compared to random-dipole patterns having the same overall statistics. We analyzed responses to a variety of spatial and spatio-temporal stimuli using multi-voxel pattern analysis (MVPA). We determined how classification accuracy depended on the cumulative number of voxels from different cortical areas using a support vector machine (SVM). In young monkeys (age < 2 years), we observed high classification accuracies in primary visual cortex (V1) when Glass patterns were present or absent (stimulus vs. blank) but lower accuracy for static vs. dynamic patterns. Only 2/10 imaging sessions yielded accuracies significantly higher than chance for the same contrast in extrastriate area MT of young monkeys. These same comparisons consistently produced high classification accuracy in animals older than 2 years. These results indicate that BOLD signal differences can be measured at young ages with a Glass pattern stimulus. However, signals related to pattern type are not distinguished reliably until older ages. These results complement our earlier findings showing late onset of activation patterns in extrastriate cortex (Kourtzi et al., 2006, Mag Res Imaging).
