Amacrine cells in the external plexiform layer of the fly's
lamina have been intracellulary recorded and dye-filled for the first
time. The recordings demonstrate that like the lamina's short
photoreceptors R1–R6, type 1 lamina amacrine neurons exhibit
nonspiking, “sign-conserving” sustained depolarizations in
response to illumination. This contrasts with the sign-inverting responses
that typify first-order retinotopic relay neurons: monopolar cells
L1–L5 and the T1 efferent neuron. The contrast frequency tuning of
amacrine neurons is similar to that of photoreceptors and large lamina
monopolar cells. Initial observations indicate that lamina amacrine
receptive fields are also photoreceptor-like, suggesting either that their
inputs originate from a small number of neighboring visual sampling units
(VSUs), or that locally generated potentials decay rapidly with
displacement. Lamina amacrines also respond to motion, and in one
recording these responses were selective for the orientation of moving
edges. This functional organization corresponds to the anatomy of amacrine
cells, in which postsynaptic inputs from several neighboring photoreceptor
endings are linked by a network of very thin distal processes. In this
way, each VSU can receive convergent inputs from a surround of amacrine
processes. This arrangement is well suited for relaying responses to local
intensity fluctuations from neighboring VSUs to a central VSU where
amacrines are known to be presynaptic to the dendrites of the T1 efferent.
The T1 terminal converges at a deeper level with that of the L2 monopolar
cell relaying from the same optic cartridge. Thus, the localized spatial
responses and receptor-like temporal response properties of amacrines are
consistent with possible roles in lateral inhibition, motion processing,
or orientation processing.