Dark adaptation and increment threshold in a rod monochromat.

SUMMARY1. Recordings from single axons of retinal ganglion cells in the rat's optic tract were used to determine whether bleaching a small area of the receptive field reduced sensitivity globally or locally, near the bleached photoreceptors.2. When a suprathreshold test spot was alternated between two equally sensitive positions, the ganglion cell gave an approximately balanced response. The balance was upset if a small-spot bleach was selectively applied to one position. Recovery of the balanced condition was rapid.3. Varying the duration of a constant illuminance bleach varied the duration of the imbalance following the bleach.4. The recovery of sensitivity after small-spot bleaches was measured both at the location of the bleach and also at another location, initially equally sensitive. The recovery at the bleached location lagged recovery at the unbleached location; but even in the bleached location, the return of sensitivity was rapid.5. Recovery of sensitivity after half-field bleaches was measured in the bleached and unbleached halves of the receptive field. Recovery in the bleached half lagged that in the unbleached half.6. A comparison between the effects of a small-spot bleach and a half-field bleach of the same strength show that the duration of dark adaptation depends on the area of the bleach.

Section: Discussionmentioning

confidence: 99%

Dark adaptation within the receptive field centre of rat retinal ganglion cells.

Cicerone¹,

Green

1980

“…The equipment was the same as that recently described (Blakemore & Rushton 1965a). The subject was fixed in position with dental impression and brow rest and the lights entered his eye by Maxwellian view.…”

Section: Methodsmentioning

confidence: 99%

The separation of cone mechanisms in dark adaptation

Croz¹,

Rushton²

1966

SUMMARY1. In dark adaptation the threshold is raised as though the bleaching of visual pigment generated an equivalent background. Now Stiles has shown that real coloured backgrounds act selectively upon the various colour mechanisms, so we ask: 'Do equivalent backgrounds from coloured bleachings also act selectively?' 2. When dark adaptation was plotted using a blue test flash ( Fig. 1 (Fig. 3) the experimental results fitted the curves calculated on the assumption that the equivalent background of bleaching simply adds to the real background in raising the threshold-a condition already established for rods.4. In conditions where 7r4 and 7T were both present (blue test, yellowgreen background and white bleach) kinked dark adaptation curves were obtained (Fig. 2) where the upper branch (T4) coincided with those of Fig. 3 and the lower were due to irl.5. The blue mechanism recovers in dark adaptation at about the same rate as red and green, or slightly slower.6. Dark adaptation curves with red (Tr5) and green (OrT) limbs can be obtained after a deep red bleach (Fig. 4) using a red test flash and a green background. The red and the green limbs were also plotted alone in their entirety by slightly changing the conditions. 7. We are led to the idea of three colour mechanisms that adapt as independently one of another after bleaching as they do with backgrounds.8. Though this simple independence accounts for the wide and conspicu-

“…Blakemore & Rushton (1965) measured increment threshold in a rod-monochromat with 1 s duration, achromatic flashes of large and small target diameter. They found that the curve measured with a small (5 min of arc) diameter flash rose with a slope of 0-59, whereas that measured with a large (6 deg) diameter flash rose with a slope of 0-87.…”

Section: T Sharpe and Othersmentioning

confidence: 99%

Temporal and spatial summation in the human rod visual system.

Sharpe

Stockman

Fach

et al. 1993

SUMMARY1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the fovea with 520 nm targets of varying size and duration. Measurements were made under rod-isolation conditions in two normal observers and in a typical, complete achromat observer who has no cone-mediated vision. The purpose of these experiments was to determine how the temporal and spatial summation of rod-mediated vision changes with light adaptation.2. The absolute threshold and the rise in increment threshold with background intensity depend upon target size and duration, but the psychophysically estimated dark light of the eye (the hypothetical light assumed to be equivalent to photoreceptor noise) does not.3. The rise in increment threshold for tiny (10 min of arc), brief (10 ms) targets approaches the de Vries-Rose square-root law, varying according to the quantal fluctuations of the background light. The slope of the rod increment threshold versus background intensity (TVI) curves in logarithmic co-ordinates is about 0-56+0-04 (when cones are not influencing rod field adaptation). For large (6 deg) and long (200 ms) targets, a maximum slope of about 0 77 + 0 03 is attained.4. The steeper slopes of the rod-detected TVI curves for large, long targets implies some reduction in temporal or spatial summation. In fact, the change in summation area is much more critical: under conditions where only the rod system is active the TVI curve slope is independent of target duration, suggesting that temporal summation is practically independent of background intensity.5. The rise in threshold also depends on the wavelength of the background field in the normal observer but not in the achromat, confirming reports that the field adaptation of the rods is not independent of the quantal absorptions in the cones. The cone influence is most conspicuous on long-wavelength backgrounds and is found for all target sizes and durations, but is greater for large and long targets than for the other conditions. MS 9972