Abstract
A small number of humans are born without an optic chiasm. In these individuals, the entire visual field is represented in both cerebral hemispheres, unlike the contralateral organization in typically developed controls. We investigated the representation of visual and somatosensory space in the superior colliculus (SC) of a subject (age 23) with congenital achiasma. SC is a brainstem structure critical for spatial orientation. Methods: Visual stimulation was measured using a blocked protocol: the subject fixated while an expanding wedge of moving dots (150° polar angle, speed 2°/s) was presented for a single hemifield for 12 s, alternating with 12-s blank periods. In alternating runs, stimuli were presented in either the left or right hemifield. Stimuli were presented only to the subject's dominant (right) eye; the left eye was patched. To manipulate attention, the subject either performed a challenging task at fixation or at the peripherally located stimulus. Somatosensory responses were evoked using piezoelectric stimulation of one of the subject's palms, alternated with rest. Attention was focused on the stimuli using a challenging rate-discrimination task. High-resolution (1.2-mm) fMRI data were collected using a spiral sequence. Results: Visual stimuli of the right eye evoked strong responses only in the right colliculus; responses for both hemifields showed similar spatial patterns upon rostral SC. Directing attention onto the stimulus increased the stimulus-evoked response in right SC, and evoked a non-localized, somewhat delayed response over both colliculi. Somatosensory stimulation of the subject's dominant right palm produced significant activation in left SC; left palm stimulation produced both contra- and ipsilateral SC activation. Conclusions: In an achiasmatic subject, the SC ipsilateral to the stimulated eye represents both visual hemifields, much as in cerebral cortex. However, somatosensory stimuli are represented by the contralateral SC. The findings challenge the need for consistent spatiotopic maps in SC across sensory modalities.
Meeting abstract presented at VSS 2014