Biology subjects

Sabbah, S.

Publications and source records attributed to Sabbah, S..

5 recordsLinked to original sources

bioRxiv2025

Spherical code of retinal orientation-selectivity enables decoding in ensembled and retinotopic operation

Selectivity to orientations of edges is seen at the earliest stages of visual processing, in retinal orientation-selective ganglion cells (OSGCs), thought to prefer vertical or horizontal orientation. However, as stationary edges are projected on the hemispherical retina as lines of longitude or latitude, how edge orientation is encoded, and decoded by the brain, is unknown. Here, by mapping the OS of thousands of OSGCs at known retinal locations in mice, we identified three OSGC types whose preferences match two longitudinal fields, and a fourth type matching two latitudinal fields, the members of each field-pair being non-orthogonal. A geometric decoder revealed that two OS sensors yield optimal orientation decoding when approaching the deviation from orthogonality we observed for OSGC field-pairs. Retinotopically-organized decoding generated type-specific variation in decoding efficiency across the visual field. OS tuning was greater in the dorsal retina, possibly reflecting an evolutionary adaptation to an environmental gradient of edges.

neuroscience↗

bioRxiv2022

Intrinsically photosensitive retinal ganglion cells evade temporal filtering to encode environmental light intensity

The retina encodes environmental light intensity to drive innate physiological responses. The synaptic basis of such coding remains obscure. Intrinsically photosensitive retinal ganglion cells (ipRGCs) are the only retinal output neurons stably encoding intensity. They do so even without their melanopsin photopigment, so specializations in their synaptic drive from bipolar cells (BCs) must also contribute. Here, we shed new light on mechanisms responsible for this unique intensity-coding drive. By ultrastructural reconstruction, we show that specific BC types and unusual ribbon synapses carry photoreceptor signals to ipRGCs. By glutamate imaging and electrophysiology, we show that their light responses are unusually persistent. Still, we find that virtually all BCs encode intensity. Intensity coding becomes restricted to ipRGCs primarily because other RGCs filter out steady-state intensity signals postsynaptically. Thus, neural "pinholes" in global, persistent neural "masking" allow intensity signals to be encoded by ipRGCs and sent to specific centers of the visual brain.

neuroscience↗

bioRxiv2020

Synapse-specific direction selectivity in retinal bipolar cell axon terminals

The ability to encode the direction of image motion is fundamental to our sense of vision. Direction selectivity along the four cardinal directions is thought to originate in direction-selective ganglion cells (DSGCs), due to directionally-tuned GABAergic suppression by starburst cells. Here, by utilizing two-photon glutamate imaging to measure synaptic release, we reveal that direction selectivity along all four directions arises earlier than expected, at bipolar cell outputs. Thus, DSGCs receive directionally-aligned glutamatergic inputs from bipolar cell boutons. We further show that this bouton-specific tuning relies on cholinergic excitation and GABAergic inhibition from starburst cells. In this way, starburst cells are able to refine directional tuning in the excitatory visual pathway by modulating the activity of DSGC dendrites and their axonal inputs using two different neurotransmitters.

neuroscience↗

bioRxiv2020

Luxotonic signals in human prefrontal cortex: A possible substrate for effects of light on mood and cognition

Animal studies revealed a mood-regulating neural pathway linking intrinsically photosensitive retinal ganglion cells (ipRGCs) and the prefrontal cortex (PFC), involved in the pathophysiology of mood disorders. As humans too have luminance-encoding ipRGCs, we asked whether a similar pathway exist in humans. Here, fMRI was used to identify PFC regions and other areas exhibiting luminance-dependent signals. We report 29 human brain regions where activation either monotonically decreased or increased with luminance. Luxotonic activity was identified across the cerebral cortex, in diverse subcortical structures, and in the cerebellum, regions that have functions related to visual image formation, motor control, cognition, emotion, and reward processing. Light suppressed PFC activation level, the activation monotonically decreasing with increasing luminance. The sustained time course of light-evoked PFC responses, and their susceptibility to prior light exposure, most closely resembled those of ipRGCs. These findings offer a functional link between light exposure and PFC-mediated cognitive and affective phenomena.

neuroscience↗

bioRxiv2018

Synaptic circuits for irradiance coding by intrinsically photosensitive retinal ganglion cells

We have explored the synaptic networks responsible for the unique capacity of intrinsically photosensitive retinal ganglion cells (ipRGCs) to encode overall light intensity. This luminance signal is crucial for circadian, pupillary and related reflexive responses light. By combined glutamate-sensor imaging and patch recording of postsynaptic RGCs, we show that the capacity for intensity-encoding is widespread among cone bipolar types, including OFF types.\n\nNonetheless, the bipolar cells that drive ipRGCs appear to carry the strongest luminance signal. By serial electron microscopic reconstruction, we show that Type 6 ON cone bipolar cells are the dominant source of such input, with more modest input from Types 7, 8 and 9 and virtually none from Types 5i, 5o, 5t or rod bipolar cells. In conventional RGCs, the excitatory drive from bipolar cells is high-pass temporally filtered more than it is in ipRGCs. Amacrine-to-bipolar cell feedback seems to contribute surprisingly little to this filtering, implicating mostly postsynaptic mechanisms. Most ipRGCs sample from all bipolar terminals costratifying with their dendrites, but M1 cells avoid all OFF bipolar input and accept only ectopic ribbon synapses from ON cone bipolar axonal shafts. These are remarkable monad synapses, equipped with as many as a dozen ribbons and only one postsynaptic process.

neuroscience↗