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Abstract:
To study the time course of activation in different visual areas and the effects of increasing stimulus coherence in the activity of temporal cortices, a stimulus was generated that continuously transforms from one that has no globally recognizable form, into one that does. The stimuli appeared as fragmented images that show periodic increases and decreases in coherence through continuous rotation. In particular, we decomposed faces and objects into a "bull‘s-eye" pattern. Adjacent rings rotated in opposite directions, such that either a coherent face or object would come into phase for a brief duration. Imaging was performed in a 4.7T/40cm Biospec vertical scanner (Bruker, Inc), with 50mT/m gradients, using quadrature transmit/receive RF coils. Voxel volume was 0.5x0.5x2mm, TE=20ms, TR=740ms, and FA=20deg. Multi-shot, multi-slices T2* weighted images (15 horizontal slices) of the entire brain of anesthetized monkeys were collected using gradient-recalled EPI fMRI sequences. Areas that responded well to face or object stimuli were first localized using a standard block design. An optimal slice containing activated portions of both the occipital and temporal areas was then selected for a one-shot, (FOV=12.5, matrix=64x64 or 128x128) time-resolved imaging (TE=40ms, TR=250ms). Observation periods typically lasted 64 to 128 seconds permitting one or more full transitions from fragmented to fully coherent stimulus images. The time course of activation in different areas of the temporal pathway, its latency, as well as hysteresis and predictive activation effects will be discussed.
