Contrast Threshold of a Brisk-Transient Ganglion Cell In Vitro

Abstract

. We measured the contrast threshold for mammalian brisk-transient ganglion cells in vitro. Spikes were recorded extracellularly in the intact retina (guinea pig) in response to a spot with sharp onset, flashed for 100 ms over the receptive field center. Probability density functions were constructed from spike responses to stimulus contrasts that bracketed threshold. Then an "ideal observer" (IO) compared additional trials to these probability distributions and decided, using a single-interval, two-alternative forced-choice procedure, which contrasts had most likely been presented. From these decisions we constructed neurometric functions that yielded the threshold contrast by linear interpolation. Based on the number of spikes in a response, the IO detected contrasts as low as 1% [4.2 Ϯ 0.4% (SE); n ϭ 35]; based on the temporal pattern of spikes, the IO detected contrasts as low as 0.8% (2.8 Ϯ 0.2%). Contrast increments above a very low "basal contrast" were discriminated with greater sensitivity than they were detected against the background. Performance was optimal near 37°C and declined with a Q 10 of about 2, similar to that of retinal metabolism. By the method used by previous in vivo studies of brisk-transient cells, our most sensitive cells had similar thresholds. The in vitro measurements thus provide an important benchmark for comparing sensitivity of neurons upstream (cone and bipolar cell) and downstream to assess efficiency of retinal and central circuits.

Spike Generator Limits Efficiency of Information Transfer in a Retinal Ganglion Cell

“…These methods have been described in detail previously (Dhingra et al, 2003). Briefly, a retina attached to sclera was removed from an anesthetized adult guinea pig, superfused with Ames medium (4 -7 ml/min at ϳ37°C), and visualized through infrareddifferential interference contrast optics.…”

“…Briefly, a retina attached to sclera was removed from an anesthetized adult guinea pig, superfused with Ames medium (4 -7 ml/min at ϳ37°C), and visualized through infrareddifferential interference contrast optics. Visual stimuli were generated in Matlab (MathWorks, Natick, MA) (Dhingra et al, 2003), displayed with a 12-bit precision on a color monitor (640 ϫ 480 pixels; 120 Hz), and projected onto photoreceptors. The mean background intensity was ϳ20,000 photons ⅐ m 2 ⅐ sec (equivalent to ϳ8000 R*/cone/sec) at 535 nm, well into the photopic range.…”

“…To understand how efficiently information is transferred and how neural mechanisms preserve signal quality, one could measure information loss at each retinal stage. One way is to compare the contrast sensitivity of different stages (Geisler and Davila, 1985;Geisler, 1989;Dhingra et al, 2003). The contrast detection threshold of a ganglion cell, measured in vivo and in vitro, is ϳ2-3% (Enroth-Cugell and Robson, 1966;Derrington and Lennie, 1982;Linsenmeier et al, 1982;Dhingra et al, 2003), but another stage is required for the comparison.…”

“…To investigate this, we measured contrast discrimination thresholds of guinea pig brisk-transient ganglion cells from graded potential and spike train. Thresholds were measured with an "ideal observer" that analyzed the temporal pattern and variability in the response of the cell Dhingra et al, 2003). We found that the spike train had twofold higher thresholds and carried 40% fewer gray levels compared with the graded potential response.…”

Retina is structured to process an excess of darkness in natural scenes

“…For all filter radii, negative contrasts (dark regions) were more numerous. To quantify this asymmetry, we measured the proportion of negative contrasts, setting response threshold to match measurements from ganglion cells (25)(26)(27). As filter width increased, the contrast distributions flattened, causing the number of subthreshold responses to decrease.…”