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J.B. Demb, K.A. Zaghloul, M. Manookin, K. Boahen; Peripheral Contrast Adaptation in Subthreshold Responses of Mammalian Y–Type Retinal Ganglion Cells . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2313.
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© ARVO (1962-2015); The Authors (2016-present)
In retinal ganglion cells, contrast presented over the receptive field periphery suppresses the spiking response to a stimulus over the receptive field center. Here, we measured the effect of peripheral contrast on the ganglion cell’s subthreshold membrane potential and tested whether the effect of peripheral contrast could be explained by an increased conductance in the ganglion cell, reflecting postsynaptic inhibition.
We made intracellular or whole–cell recordings of Y–type ganglion cells in the in vitro guinea pig retina (n = 22 cells). A cell was stimulated with a white–noise modulation of a central spot (0.5 mm diameter). In alternate 10–second periods, the receptive field periphery (1.0 mm inner diameter; 3.0–3.7 mm outer diameter) contained a 0% or 100% contrast drifting grating (2–Hz drift; 4.3 or 5.0 cyc/mm, square–wave). We quantified the impact of peripheral contrast on the subthreshold response to the central spot using a linear–nonlinear analysis. We measured the conductance change during the peripheral grating using current pulses or voltage steps.
Peripheral contrast suppressed the response to the central stimulus by two mechanisms: a reduced gain of the subthreshold response (OFF cells: 30.3 +/– 0.03%, n = 16; ON cells: 13.8 +/– 0.04%, n = 6) (mean +/– sem) and a tonic membrane hyperpolarization (OFF cells: –1.5 +/– 0.2 mV; ON cells: –0.06 +/– 0.40mV). Initially (50–200 msec), the peripheral grating increased membrane conductance over baseline by 15.6 +/– 5.7%. Afterwards, the grating increased conductance over baseline by 6.9 +/– 2.1% (n = 2 ON cells, 9 OFF cells). In the same eleven cells, the grating reduced the gain of the subthreshold response by 34.8 +/– 3.0%.
During a sustained presentation of contrast to the peripheral receptive field, a ganglion cell increased its membrane conductance by ∼7%. This increase could explain only a minor portion of the ∼35% reduction in response gain to a central stimulus. Thus, peripheral contrast does not act largely by causing a postsynaptic inhibition of the ganglion cell. It is likely that peripheral contrast acts by inhibiting the ganglion cell’s presynaptic bipolar terminals.
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