ON-OFF UNITS IN THE 1ST OPTIC CHIASM OF THE BLOWFLY .2. SPATIAL PROPERTIES

We recorded from the spiking on-off unit in the first optic chiasm (be tween lamina and medulla) in the blowfly Calliphora vicina, and invest igated its spatial properties. The receptive field extends over (11.4/-0.9)-degrees horizontally and (8.7+/-0.6)-degrees vertically, i.e. a bout 7 by 5 interommatidial angles. The line spread function of the on -off unit - calculated from its response to moving sinusoidal gratings - has a half-width of (2.3+/-0.2)-degrees. This half-width is slightl y broader than that of the photoreceptor. Lateral inhibition occurs wh en two different areas of the receptive field are stimulated simultane ously. Fast temporal adaptation (i.e. adaptation to trains of short li ght pulses) takes place independently in different areas of the recept ive field.