IS THE INPUT TO A GABAERGIC SYNAPSE THE SOLE ASYMMETRY IN TURTLES RETINAL DIRECTIONAL SELECTIVITY

We examined the effects of picrotoxin and pentylenetetrazol (PTZ) on t he responses to motions of ON-OFF directionally selective (DS) ganglio n cells of the turtle's retina. These drugs are antagonists of the inh ibitory neurotransmitter GABA. For continuous motions, picrotoxin mark edly reduced the overall directionality of the cells. In 21% of the ce lls, directional selectivity was lost regardless of speed and contrast . However, other cells maintained their preferred direction despite sa turating concentrations of picrotoxin. And in most cells, loss, mainte nance, or even reversal of preferred and null directions could occur a s speed and contrast were modulated. In 50% of the cells, reversal of preferred and null directions occurred at some condition of visual sti muli. However, picrotoxin did not tend to alter the preferred-null axi s for directional selectivity. For apparent motions, picrotoxin made m otion facilitation, which normally occurs exclusively in preferred-dir ection responses, to become erratic and often occur during null-direct ion motions. Finally, PTZ had effects similar to picrotoxin but with l ess potency. The results in this paper indicated that models of direct ional selectivity based solely on a GABAergic implementation of Barlow and Levick's asymmetric-inhibition model do not apply to the turtle r etina. Alternative models may comprise multiple directional mechanisms and/or a symmetric inhibitory one, but not asymmetric facilitation.