ON-OFF Directionally Selective Research Articles

Parallel Mechanisms Encode Direction in the Retina

In the retina, presynaptic inhibitory mechanisms that shape directionally selective (DS) responses in output ganglion cells are well established. However, the nature of inhibition-independent forms of directional selectivity remains poorly defined. Here, we describe a genetically specified set of ON-OFF DS ganglion cells (DSGCs) that code anterior motion. This entire population of DSGCs exhibits asymmetric dendritic arborizations that orientate toward the preferred direction. We demonstrate that morphological asymmetries along with nonlinear dendritic conductances generate a centrifugal (soma-to-dendrite) preference that does not critically depend upon, but works in parallel with the GABAergic circuitry. We also show that in symmetrical DSGCs, such dendritic DS mechanisms are aligned with, or are in opposition to, the inhibitory DS circuitry in distinct dendritic subfields where they differentially interact to promote or weaken directional preferences. Thus, pre- and postsynaptic DS mechanisms interact uniquely in distinct ganglion cell populations, enabling efficient DS coding under diverse conditions.

Two distinct types of ON directionally selective ganglion cells in the rabbit retina.

Mammalian retinas contain about 20 types of ganglion cells that respond to different aspects of the visual scene, including the direction of motion of objects in the visual field. The rabbit retina has long been thought to contain two distinct types of directionally selective (DS) ganglion cell: a bistratified ON-OFF DS ganglion cell that responds to onset and termination of light, and an ON DS ganglion cell, which stratifies only in the ON layer and responds only to light onset. This division is challenged by targeted recordings from rabbit retina, which indicate that ON DS ganglion cells occur in two discriminably different types. One of these is strongly tracer-coupled to amacrine cells; the other is never tracer-coupled. These two types also differ in branching pattern, stratification depth, relative latency, and transience of spiking. The sustained, uncoupled ON DS cell ramifies completely within the lower cholinergic band and responds to nicotine with continuous firing. In contrast, the transient, coupled ON DS ganglion cell stratifies above the cholinergic band and is not positioned to receive major input from cholinergic amacrine cells, consistent with its modest response to the cholinergic agonist nicotine. Much data have accrued that directional responses in the mammalian retina originate via gamma-aminobutyric acid (GABA) release from the dendrites of starburst amacrine cells (Euler et al., 2002). If there is an ON DS ganglion cell that does not stratify in the starburst band, this suggests that its GABA-dependent directional signals may be generated by a mechanism independent of starburst amacrine cells.

Robust directional computation in on-off directionally selective ganglion cells of rabbit retina

The spatial and temporal interactions in the receptive fields of On-Off directionally selective (DS) ganglion cells endow them with directional selectivity. Using a variety of stimuli, such as sinusoidal gratings, we show that these interactions make directional selectivity of the DS ganglion cell robust with respect to stimulus parameters such as contrast, speed, spatial frequency, and extent of motion. Moreover, unlike the directional selectivity of striate-cortex cells, On-Off DS ganglion cells display directional selectivity to motions not oriented perpendicularly to the contour of the objects. We argue that these cells may achieve such high robustness by combining multiple mechanisms of directional selectivity.

Synaptic organization of complex ganglion cells in rabbit retina: Type and arrangement of inputs to directionally selective and local‐edge‐detector cells

The type and topographic distribution of synaptic inputs to a directionally selective (DS) rabbit retinal ganglion cell (GC) were examined and were compared with those received by two other complex GC types. The percentage of cone bipolar cell (BC) input, presumably an index of sustained responses and simple receptive field properties, is much higher than expected for complex GCs in reference to previous reports in other species: approximately 20% for the type 1 bistratified ON-OFF DS GC and for a multistratified GC, and approximately 40% for the small-tufted local-edge-detector GC. Consistent with a previous study (Famiglietti [1991] J. Comp. Neurol. 309:40-70), no ultrastructural evidence is found for inhibitory synapses from starburst amacrine cells to the ON-OFF DS GC. The density of inputs to the ON-OFF DS GC is high and rather evenly distributed over the dendritic tree. Clustering of inputs brings excitatory and inhibitory inputs into proximity, but the strict on-path condition of more proximal inhibitory inputs, favoring shunting inhibition, is not satisfied. Prominent BC input and its regional variation suggest that BCs play key roles in DS neural circuitry, both pre- and postsynaptic to the ON-OFF DS GC, according to a bilayer model (Famiglietti [1993] Invest. Ophthalmol. Vis. Sci. 34:S985). Asymmetry of inhibitory amacrine cell input may signify a region on the preferred side of the receptive field, the inhibition-free zone (Barlow and Levick [1965] J. Physiol. (Lond.) 178:477-504), supporting a role for postsynaptic integration in the DS mechanism. Prominent BC input to the local-edge-detector, often without accompanying amacrine cell input, indicates presynaptic integration in forming its trigger feature.

A structural basis for omnidirectional connections between starburst amacrine cells and directionally selective ganglion cells in rabbit retina, with associated bipolar cells

Directionally selective (DS) ganglion cells of rabbit retina are of two principal types. ON DS ganglion cells prefer low velocity in one of three directions of movement and project axons to the accessory optic system (AOS), whereas ON-OFF DS ganglion cells prefer higher velocity in one of four directions and project to tectum and thalamus. Each has a distinct, recognizable dendritic morphology, based upon the correlation of form, physiology, and central projections. In previous Golgi studies, ON and ON-OFF DS cells were found to be partly co-stratified, and ON-OFF DS cells were found to co-stratify with starburst amacrine (SA) cells, the cholinergic amacrine cells of the retina, which also contain elevated levels of GABA. SA cells are radially symmetrical, have synaptic boutons in a distal annular zone of its dendritic tree, are presynaptic primarily to ganglion cell dendrites, co-stratify with ON-OFF DS ganglion cells, and contain the neurotransmitters shown pharmacologically to be involved in DS responses. For these reasons, SA cells are thought to play a role in the DS mechanism. Several models of this mechanism have utilized SA cell dendritic geometry in a centrifugal, radial format to impose directional inputs on DS ganglion cells. The opportunity to examine Golgi preparations containing ON DS ganglion cells that exhibit dendritic field overlap with both starburst amacrine cells and ON-OFF DS ganglion cells has resulted in several new findings. Co-stratification of ON DS ganglion cells and SA cells was demonstrated directly. Secondly, the boutons of single starburst amacrine cells make close contact in different lamellae of the starburst substratum in sublamina b of the inner plexiform layer (IPL) with three adjacent ON-OFF DS ganglion cells, which because of their considerable dendritic-field overlap must prefer different directions of motion. Thirdly, nearby presynaptic boutons of single SA cells make close contact with both ON and ON-OFF directionally selective ganglion cells. Single SA cells thus traverse all the lamellae of the starburst/cholinergic substratum. Fourthly, no directional bias is shown by vectors connecting the origins of dendritic sectors and distal synaptic boutons of starburst amacrine cells in those sectors that are in close contact with the dendrites of single ON or ON-OFF directionally selective ganglion cells. Fifthly, at least two distinct types of cone bipolar cell, nb1 and nb2, participate in the neural circuitry of directional selectivity for ON and ON-OFF DS ganglion cells, and nb1 cells co-stratify with ON DS cells. As a consequence of the second, third, and fourth points, starburst amacrine cells appear to be indiscriminate in their connections with DS ganglion cells, and therefore are unlikely to be the primary conduits for directionally selective information to retinal ganglion cells. This result is consistent with pharmacological studies showing that cholinergic antagonists do not block directional selectivity and a study showing that laser-ablation of SA cells does not reduce the directional selectivity of overlapping ON-OFF DS ganglion cells.

Responses of directionally selective retinal ganglion cells to activation of AMPA glutamate receptors.

Previous studies in the rabbit retina have shown that drugs which block AMPA glutamate receptors abolish directional selectivity in ON-OFF directionally selective (DS) ganglion cells. The effects of activation of AMPA receptors on the directionally selective responses of these ganglion cells had not been studied. In the present study, extracellular recordings of the responses of ON-OFF DS ganglion cells to a moving bar of light were made in an in vitro rabbit retinal preparation. In control solution, bath application of AMPA (7-10 microM) abolished the light responses of most ON-OFF DS ganglion cells. On washout of AMPA, the light responses rapidly returned; however, the cells temporarily lost the ability to discriminate the direction of the moving bar of light. That is, the cells responded equally to movement in the preferred and null directions. Pretreatment of retinas with the glycine receptor antagonist strychnine (1-2 microM) did not alter the effects of AMPA. On the other hand, in retinas pretreated with the GABA(A) receptor antagonist SR95531 (0.2-0.25 microM), AMPA did not abolish the light responses of ON-OFF DS ganglion cells but instead abolished directional selectivity in these cells by bringing out a response to movement in the null direction. This finding suggests that an AMPA-induced GABA efflux from cells in the retina was responsible for the suppression of the light responses by AMPA. In control solution, application of the selective AMPA receptor agonist (S)-5-fluorowillardiine (2-3 microM) only temporarily abolished the light responses of ON-OFF DS ganglion cells. As the light responses returned, it was clear that directional selectivity had been abolished by (S)-5-fluorowillardiine. In control solution, blocking AMPA receptor desensitization with cyclothiazide (80-100 microM) greatly reduced the light responses of ON-OFF DS ganglion cells. As the light responses slowly returned on washout of cyclothiazide, directional selectivity was clearly reduced although not abolished. In retinas pretreated with SR95531, application of cyclothiazide abolished directional selectivity. Diazoxide (700-1000 microM), another blocker of AMPA receptor desensitization, abolished directional selectivity in ON-OFF DS ganglion cells without the need of adding SR95531 to the bathing solution. It is concluded that, in the rabbit retina, AMPA receptors play an important role in generating directional selectivity in ON-OFF DS ganglion cells. Moreover, excessive activation of AMPA receptors greatly compromises the mechanism for directional selectivity in ON-OFF DS ganglion cells.

Potassium-evoked directionally selective responses from rabbit retinal ganglion cells

Previous studies have shown that directionally selective (DS) retinal ganglion cells cannot only discriminate the direction of a moving object but they can also discriminate the sequence of two flashes of light at neighboring locations in the visual field: that is, the cells elicit a DS response to both real and apparent motion. This study examines whether a DS response can be elicited in DS ganglion cells by simply stimulating two neighboring areas of the retina with high external K+. Extracellular recordings were made from ON-OFF DS ganglion cells in superfused rabbit retinas, and the responses of these cells to focal applications of 100 mM KCl to the vitreal surface of the retina were measured. All cells produced a burst of spikes (typically lasting 50-200 ms) when a short pulse (10-50 ms duration) of KCl was ejected from the tip of a micropipette that was placed within the cell's receptive field. When KCl was ejected successively from the tips of two micropipettes that were aligned along the preferred-null axis of a cell, sequence-dependent responses were observed. The response to the second micropipette was suppressed when mimicking motion in the cell's null direction, whereas an enhancement during apparent motion in the opposite direction frequently occurred. Sequence discrimination in these cells was eliminated by the GABA antagonist picrotoxin and by the Ca(2+)-channel blocker omega-conotoxin MVIIC, two drugs that are known to abolish directional selectivity in these ganglion cells. The spatiotemporal properties of the K(+)-evoked sequence-dependent responses are described and compared with previous findings on apparent motion responses of ON-OFF DS ganglion cells.

Effect of ON pathway blockade on directional selectivity in the rabbit retina.

1. In this report we describe extracellular recordings made from directionally selective (DS) ganglion cells in the rabbit retina during perfusion with 2-amino-4-phosphonobutyric acid (APB) to block ON channels through the retina. 2. Application of 100 microM APB selectively and reversibly abolished the responses of ON ganglion cells in the rabbit retina. In addition, 100 microM APB completely and reversibly blocked ON component responses of ON-OFF DS ganglion cells to both stationary and moving stimuli. These results are consistent with the idea that APB blocks ON pathways through the retina. 3. Under ON pathway blockade with APB, OFF component responses of ON-OFF DS ganglion cells remained DS. DS OFF responses retained the same preferred direction as the pre-APB ON-OFF responses and could be driven using either normal or reversed contrast stimuli. 4. Extracellular responses of ON DS ganglion cells were completely blocked by APB. Under APB, these cells showed no response to stationary or moving stimuli. 5. Application of the gamma-aminobutyric acid-A (GABAA) antagonist 2-(3-Carboxypropyl)-3-amino-6-(4-methoxyphenyl)pyridazinium bromide (SR95531) reversibly abolished directional selectivity of ON DS and ON-OFF DS ganglion cells in the rabbit retina. This finding is consistent with previous data for picrotoxin. 6. Application of SR95531 during ON channel blockade by APB caused OFF component responses of ON-OFF DS ganglion cells to lose their directional selectivity. Under these conditions, OFF responses to movement in the preferred and null directions became virtually identical. 7. These results indicate that simultaneous ON and OFF layer input is not required to generate directional responses in ON-OFF DS ganglion cells. In addition, it appears that a GABAA-dependent mechanism for directional selectivity may operate independently in the two separate dendritic layers of the ON-OFF DS ganglion cell.

Dendritic co-stratification of ON and ON-OFF directionally selective ganglion cells with starburst amacrine cells in rabbit retina.

The morphology, dendritic branching patterns, and dendritic stratification of retinal ganglion cells have been studied in Golgi-impregnated, whole-mount preparations of rabbit retina. Among a large number of morphological types identified, two have been found that correspond to the morphology of ON and ON-OFF directionally selective (DS) ganglion cells identified in other studies. These cells have been characterized in the preceding paper in terms of their cell body size, dendritic field size, and branching pattern. In this paper, the two kinds of DS ganglion cell are compared in terms of their levels of dendritic stratification. They are compared with each other and also with examples of class III.1 cells, defined in the preceding paper with reference to our previous studies. Studies employing computer-aided, 3D reconstruction of dendritic trees, as well as analysis of a pair of ON DS and ON-OFF DS ganglion cells with overlapping dendritic trees show that the two types of DS ganglion cell partly co-stratify in the middle of sublamina b (stratum 4). The report that some ON DS ganglion cells extend a few dendrites into sublamina a is confirmed. The study of pairs of ON-OFF DS ganglion cells and starburst amacrine cells with overlapping dendritic trees reveals a precise co-stratification of these two cell types, and many points of close apposition of starburst boutons with ON-OFF DS ganglion cell dendrites in both sublaminae of the inner plexiform layer (IPL). This is confirmed by high-resolution light microscopy and by electron microscopy. It is possible to conclude, therefore, that ON DS are also partly co-stratified with type b starburst (cholinergic) amacrine cells, and are apparently also partly co-stratified with type a starburst amacrine cells, when occasional dendrites rise to that level. The co-stratification of the two kinds of DS ganglion cell is consistent with the sharing of some inputs in common, including some cone bipolar cell inputs. The co-stratification of both with starburst amacrine cells agrees with the physiological demonstration of the powerful pharmacological effects upon ON and ON-OFF DS ganglion cells reported for cholinergic agonists. The major difference in the dendritic stratification of bistratified ON-OFF DS ganglion cells and generally unistratified ON DS ganglion cells is consistent with the bisublaminar organization of ON and OFF pathways in the IPL. The problem of occasional branches of ON DS cells in sublamina a is discussed in terms of a threshold for OFF responses.(ABSTRACT TRUNCATED AT 400 WORDS)