Spatial distribution of functional superficial-deep connections in the adult ferret superior colliculus

Numerous studies have identified connections between the superficial visual and deeper multisensory layers of the superior colliculus (SC), but the functional distribution of the superficial-deep projection has not been mapped. This question was assessed in the present study using extracellular electrophysiological stimulation and recording techniques. In vitro slices from adult ferrets were used to functionally map the rostro-caudal, medio-lateral, and dorso-ventral distribution of these superficial-deep connections. For each coronal (n=6) or parasagittal (n=10) slice, single and multi-unit responses to electrical stimulation of a point in the superficial layers were systematically recorded at different locations along a grid (approximately 300 microm intervals) across the slice. Recording sites with similar activation thresholds were grouped on the histological reconstruction of each slice to plot the functional access of superficial stimulation site to the deeper layers. Low intensity stimulation (defined as a current threshold < or =75 microA) activated areas of the subjacent intermediate layers in most cases (75%; 12/16). Higher intensity stimuli (> 75-600 microA) accessed larger areas which, in 50% of the slices, extended into the deepest layers of the SC. However, regardless of the rostro-caudal or medio-lateral position of the superficial layer stimulation site, the proportion of the deeper layers activated remained remarkably constant, although the volume of activated deep layer tissue was shifted in each case toward the central regions of the SC. This last observation argues against the precise alignment of the superficial and deep layer visual maps, suggesting instead that the arrangement of the superficial layer projection may more closely relate to the organization of deep layer auditory and/or somatosensory representations.