FIGURE 5

FIGURE 5

Working models of orientation-selective retinal circuits in vertebrate retinae. (A) Proposed circuit diagrams underlying the firing selectivity of different OSGC types in rabbit (left), mouse (middle), and zebrafish (right). Photoreceptors are represented in yellow; bipolar cells (BCs) in green; amacrine cells (ACs) in blue; and ganglion cells (GCs) in red. Cell numbering is used to relate each cell type with the corresponding tuning profile shown in (B) below. Excitatory synapses are indicated by ‘+’ symbols (full circles), whereas inhibitory synapses are indicated by ‘–’ (empty circles). Electrical synapses between mouse OFF orientation-selective amacrine cells (AC1) and OFF OSGCs are indicated by the jagged line. Potential but speculative connectivity between AC1 and BC terminals in larval zebrafish is represented by the dashed blue line. (B) Response profiles of the cell types represented in A to oriented visual stimuli. ‘+’ and ‘–’ symbols indicate along which axes excitatory (+, green) and inhibitory (–, blue) inputs contribute to the tuning of each cell type. The neurotransmitter identities of the various amacrine cell types are also reported (question marks indicate predicted neurotransmitter identities). GABA, gamma-aminobutyric acid. Tonic inputs are specified in small gray writings. Unless specifically indicated in small gray writings, the response polarity of the various cell types is the same as that of the respective downstream OSGC. In rabbit and zebrafish circuits, orientation selectivity is proposed to originate from orientation-selective GABAergic amacrine cells (AC1), which subsequently generate tuned responses in downstream amacrine, bipolar and/or ganglion cells through inhibitory synapses. In the mouse OFF circuit, AC1 amacrine cells convey orientation selectivity to OFF OSGCs by electrical coupling through gap junctions. Rabbit ON horizontal OSGCs possess two horizontally oriented flanking subfields generated by tonically active amacrine cells (AC3), which are predicted to invert the ON pathway signal by disinhibiting center bipolar cells (BC1) and, therefore, render negative contrast stimuli in the flanking subfields excitatory (dashed box).