Wessel, Ralf
Coauthors(s): Harvey Karten, Dep Neuroscience, UCSD David Kleinfeld, Dep Physics, UCSD Harald Luksch, University of Aachen, Germany
UCSD
Physics
Dep Physics, 0319 UCSD 9500 Gilman Dr La Jolla, CA92093

Biophysics of visual motion analysis in avian tectum

Neurons in the deep layers of the avian optic tectum (SGC) respond preferentially to small moving objects and their response is suppressed by whole-field motion in vivo. We studied the circuit comprised of anatomically defined SGC type I cells and their presynaptic small retinal ganglion cells with respect to their ability for motion detection using a slice preparation. Type I SGC cells responded with one or multiple bursts (“chattering”) to depolarizing somatic current injection. Electrical stimulation of the retinal ganglion cell axons caused an EPSP/spike response in the SGC I soma with a sharp onset and a latency of 12 ± 2 ms. Direct electrical stimulation of the SGC I dendrites caused a response in the soma similar in shape to synaptic stimulation, however with a shorter latency of 5 ± 1ms. The sharp onset response to dendritic stimulation persisted when the soma was hyperpolarized below –100 mV, but was abolished in saline containing 1 uM TTX. The duration of the sharp onset response increased when Ca2+ in the saline was replaced with Ba2+. For two pulse electrical stimulation of the retinal ganglion cell axons at the same location, the second pulse did not cause a response in the SGC I soma for an interval of less than 700 ± 600 ms (phasic response). However, for two pulse direct electrical stimulation of the SGC I dendrites at the same location, the second pulse did cause a response for an interval down to 21 ± 8 ms (tonic response). When separate groups of retinal ganglion cell axons were stimulated at different times, the second pulse did cause a response for an interval down to 23 ± 12 ms. Together the data suggest that (a) cells from deep tectal layers which have been shown to respond in-vivo with bursts (“chattering”) to small fast moving spots are identified as SGC I cells, (b) dendritic signal transfer is mediated by Na, Ca, and K(Ca) channels, and (c) the signal transmission from the retinal ganglion cell axons to the distal dendrites of these cells is phasic, whereas the transmission from the distal dendrites to the soma is tonic. These features yield a differential response to small moving objects and background motion: A small moving object excites different dendrites consecutively, leading to sustained somatic bursting, whereas whole-field motion excites all dendrites simultaneously and leads to a short initial response followed by a suppressed response. These features also set the limit of speed sensitivity to approximately 50 deg/s, a value close to the in-vivo observation.