The neural circuitry underlying movement detection was inferred from studies of amacrine cells under whole-cell patch clamp in retinal slices. Cells were identified by Lucifer yellow staining. Synaptic inputs were driven by “puffing“ transmitter substances at the dendrites of presynaptic cells. Spatial sensitivity profiles for amacrine cells were measured by puffing transmitter substances along the lateral spread of their processes. Synaptic pathways were separated and identified with appropriate pre- and postsynaptic pharmacological blocking agents.

Two distinct amacrine cell types were found: one with narrow spread of processes that sustained excitatory synaptic current, the other with very wide spread of processes that transient excitatory synaptic currents. The transient currents found only in the wide-field amacrine cell were formed presynaptically at GABAB receptors. They could be blocked with baclofen, a GABAB agonist, and their time course was extended by AVA, a GABAB antagonist. Baclofen and AVA had no direct affect upon the wide-field amacrine cell, but picrotoxin blocked a separate, direct GABA input to this cell.

The narrow-field amacrine cell was shown to be GABAergic by counterstaining with anti-GABA antiserum after it was filled with Lucifer yellow. Its narrow, spatial profile and sustained synaptic input are properties that closely match those of the GABAergic antagonistic signal that forms transient activity (described above), suggesting that the narrow-field amacrine cell itself is the source of the GABAergic interaction mediating transient activity in the inner plexiform layer (IPL). Other work has shown a GABAB sensitivity at some bipolar terminals, suggesting a population of bipolars as the probable site of interaction mediating transient action.

The results suggest that two local populations of amacrine cell types (sustained and transient) interact with the two populations of bipolar cell types (transient forming and nontransient forming). These interactions underlie the formation of the change-detecting subunits. We suggest that local populations of these subunits converge to form the receptive fields of movement-detecting ganglion cells.

Small-tufted (ST) ganglion cells of rabbit retina are divided into eight types based upon morphology, branching pattern, level of dendritic stratification, and quantitative dimensional analysis. Only one of these types has been previously characterized in Golgi preparations, and four may be discerned in the work of others. Given their small dendritic-field size, and assuming uniform mosaics of each across the retina, ST cells comprise about 45% of all rabbit ganglion cells, and are therefore of major functional significance. Four ST cells occur as two paramorphic (a/b) pairs, and thus belong to class III, as previously defined. Four branch in sublaminae a and b of the inner plexiform layer (IPL) and therefore belong to class IV. ST cells have small cell bodies 10–15 μm in diameter, small axons 0.7–1.3 μm in diameter, and small dendritic-field diameters, 40–110 μm in mid-visual streak. The dendrites of ST cells are highly branched, and bear spines and appendages of varying length, but vary from type to type. Class III.2 cells and class III.3 cells are partly bistratified. Class IV small-tufted cells differ characteristically in multiple features of dendritic branching and stratification. Class III small-tufted cells apparently have concentric (ON-center and OFF-center) receptive fields and may have “sluggish-transient” (class III.2) and “sluggish-sustained” (class III.3) physiology. Class IV cells include the “local-edge-detector” (LED) (class IVst1), and are all expected to give ON–OFF responses to small, centered, slowly moving visual stimuli. Based upon systematic variation in dendritic-field size across the retina, ST cells may be divided into two groups. In this “universal prey” species, they may belong to two systems of motion detection, typified by ON–OFF directionally selective and LED ganglion cells, respectively, specialized for detection of rapid motion at the horizon for land-based predators, and slow motion for airborne predators.