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Abstract:
Introduction: Neurons in the deep layers of superior colliculus (SC) respond during somatosensory input, so we expected that deep-layer responses would be evoked by a somatovisual integration task. Based on animal experiments, we also expected the deep-layer activation to correspond to the retinotopic position of the hands for the subject. Methods: Visual stimuli were 4 square grids of red and green dots, with patterns changing every 0.5 s. Usually, the dot patterns were random. Each grid was 3° on a side, and offset 2.5° in both x and y from fixation. Subjects fixated on a central dot, and were cued to attend to the left or right pair of grids by tactile stimulation with a train of air puffs delivered alternately to the index and ring fingers of the current hand. Every few seconds, subjects (N = 2) were cued to attend either the upper or lower grid in the current hemifield by a double air puff to one of the two fingers. After a variable delay, the dot pattern took on either a x or + shape. Only occurrences of the rarer + shape were counted by the subject, and reported at the end of each run. The left-right alternation occurred every 30 s and repeated 8 times in each 4-min run. During each run, fMRI data was collected on a 3T scanner using a dual-echo spiral sequence (1.4-mm voxels) on 13 quasi-axial slices that covered SC. Results: We observed two regions of significant responses (Figure attached). Early responses (~3-s lag with respect to right stimulation onset) on the left SC, and later responses (~18-s lag) on the right SC. They are located near the caudo-lateral edges of both SCs. Activations extend ~5 mm within SC, but also include more superficial responses in the same region. Discussion: The somatovisual integration task successfully evoked strong lateralized responses that included the deep layers of SC. The results confirm the expected retinotopic organization of deep layer neuronal responses for hand stimulation: strongly caudal because hand position is at high eccentricity, and at the lateral margins of SC because of their position in the lower visual field. Future experiments are planned at 9.4T to extend these results.
