ON SPIKING UNITS IN THE 1ST OPTIC CHIASM OF THE BLOWFLY .3. THE SUSTAINING UNIT

We recorded from the spiking sustaining unit in the optic chiasm betwe en lamina and medulla in the brain of the blowfly Calliphora vicina, a nd investigated both temporal and spatial properties of the light-adap ted cell. The sustaining unit fails to follow the highest temporal fre quencies followed by the photoreceptor, but its temporal resolution is substantially better than that of the on-off unit. The sustaining uni t does not display the fast temporal adaptation as previously describe d in the on-off unit. As compared with the on-off unit, the sustaining unit has a high sensitivity to small contrasts. Although the sustaini ng unit continues spiking as long as the light is on, its response is also transient as it adapts rapidly after a change of intensity. The r eceptive field and the line spread function of the sustaining unit hav e a similar size and profile: a central lobe with a half-width of appr oximately 2-degrees surrounded by a circular inhibitory zone located a t about 3-degrees off-axis.