A blinking "spotlight" of re-entrant activity in the avian tectum
Description
Re-entrant connections are inherent to nervous system organization; however, a comprehensive understanding of their operation is still lacking. In birds, topographically organized re-entrant signals, carried by axons from the nucleus-isthmi-parvocellularis (Ipc), are distinctly recorded as bursting discharges across the optic tectum (TeO). Here, we used up to 48 microelectrodes regularly spaced on the superficial tectal layers of anesthetized pigeons to characterize the spatial-temporal pattern of this axonal re-entrant activity in response to different visual stimulation. We found that a brief luminous spot triggered repetitive waves of bursting discharges that, appearing from initial sources, propagated horizontally to areas representing up to 28 deg of visual space, widely exceeding the area activated by the retinal fibers. In response to visual motion, successive burst-waves started along and around the stimulated tectal path, tracking the stimulus in discontinuous steps. When two stimuli were presented, the burst-wave sources alternated between the activated tectal loci, as if only one source could be active at any given time. Because these re-entrant signals boost the retinal input to higher visual areas, their peculiar dynamics mimics a blinking "spotlight", just as the classic metaphor alluded to explain spatial attention. Tectal re-entry from Ipc is thus highly structured and intrinsically discontinuous, and higher tectofugal areas, which lack retinotopic organization, will thus receive incoming visual activity in a sequential and piecemeal fashion. We anticipate that analogous re-entrant patterns, perhaps hidden in less bi-dimensionally organized topographies, may organize the flow of neural activity in other parts of the brain as well. Extracellular recordings were performed using two arrays of regularly spaced tungsten microelectrodes (4x4 and 4x8, 1-2 MOhms, 125 µm thick, 250-300 µm electrode tip separation (Microprobes, Gaithersburg, MD)) and a RHD2000-EVAL multi-channel amplifier (Intan Technologies, Los Angeles, CA). Each array was inserted at approximate 500 µm depth in the tectum. A silver wire was implanted in the bone in contact with the saline as a reference electrode. Neural signals were amplified, band passed between 10 Hz and 10 KHz and digitized at 20 KHz. The recordings are stored as Numpy arrays along with a description of the stimulus in JSON format.