Rd. Smith et al., IS THE INPUT TO A GABAERGIC SYNAPSE THE SOLE ASYMMETRY IN TURTLES RETINAL DIRECTIONAL SELECTIVITY, Visual neuroscience, 13(3), 1996, pp. 423-439
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.