The attenuation of rod signals by backgrounds

1. The paper which precedes this investigated the nerve interaction between two flashes, λ at centre (Fig. 1a) and ϕ on the surround region (but not on the centre). The size of the inhibitory nerve signal V generated by ϕ is given by V = ϕ(ϕ + σ), where σ is the semi-saturation constant.

2. A former paper (Alpern & Rushton, 1967) had shown that when the flash ϕ falls upon a steady background θ, V suffers attenuation in the G-box (Fig. 1b) down to the fraction θ_D/(θ_D + θ) where θ_D is the eigengrau or receptor noise. Thus, in general, the nerve signal N is given by [Formula: see text].

3. This formula had only been established for a moderate range of values. In this paper we use extreme values to explore the limits of its validity. We find the equation to be true over the entire intensity range where N is measurable.

4. Six different types of experiment have been performed to test various features of the equation. For instance, if log N is plotted against log ϕ for various fixed values of θ, the curve is always the same with simply a vertical shift. And the shift is equal to log(1 + θ/θ_D) for all values both of θ and of ϕ.

5. The most interesting curve is the plot of log ϕ against log θ for fixed N. This is similar to the Weber—Fechner increment threshold but the criterion is not that ϕ be strong enough to be detected, but strong enough to generate an N signal just sufficient to inhibit some fixed λ flash. These curves (below the onset of saturation) are all the same except for vertical separation, and prove that the condition for flash detection is that a fixed signal, N₀, is generated of size 10^-5 of the maximum signal obtainable (i.e. with ϕ large and θ zero).

6. With strong backgrounds the curves of (5) above exhibit a marked saturation of the Aguilar & Stiles' type (1954). The family of curves each with a fixed N value shows a remarkable symmetry (Fig. 8) which in fact follows from the equation in (2) above. It has nothing to do with bleached pigment, but follows from the equation in (1) above. V there cannot exceed unity, thus when scaled by the G-box below the criterion level, further increase in ϕ will not bring improvement.

     

Safety of Hibitane II. Human experience

Chlorhexidine has been widely used in medical practice since its introduction on to the marked in the early 1950s. Primarily it has been used for topical antisepsis, e.g. pre-surgery skin preparation, burns prophylaxis, and prior to obstetrical/gynaecological procedures. This extensive experience has demonstrated the virtual absence of sensitization and a low irritancy potential for the compound. Only one significant adverse effect has been identified during medical use, namely, the production of sensorineural deafness after direct instillation into the middle ear, a property shared by several commonly used antiseptics. The encouraging reports of the use of chlorhexidine for plaque control prompted further safety investigations. It has been shown that absorption after oral ingestion is very low, and long-term oral use has not produced changes in haematological and biochemical parameters. Occasional oral intolerance of mouthrinse formulations has been reported, although no histological abnormalities were present in gingival biopsies taken after 18 months' daily use. Very occasionally, a reversible swelling of the parotid glands has been reported after use of chlorhexidine in mouthrinse formulations, but not after other methods of administration. Tooth discoloration, which shows wide inter-individual variation is seen after all dental formulations. This undesirable cosmetic effect would appear to represent the only significant argument against everyday prophylactic oral use of chlorhexidine.     

The separation of cone mechanisms in dark adaptation

1. 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. 1b) following bleaching by orange light a kinked curve was obtained. The upper branch was shown to have the same dark adapted threshold as Stiles blue (pi(1)) mechanism and the lower branch as his green (pi(4)) mechanism. The pi(4) dark adaptation curve alone (unkinked) was obtained using a white instead of an orange bleach.3. Dark adaptation curves were obtained in which the test flash was presented upon various steady backgrounds. In conditions where only pi(4) was involved (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 pi(4) and pi(1) were both present (blue test, yellow-green background and white bleach) kinked dark adaptation curves were obtained (Fig. 2) where the upper branch (pi(4)) coincided with those of Fig. 3 and the lower were due to pi(1).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 (pi(5)) and green (pi(4)) 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 conspicuous range of adaptive phenomena, we have encountered some special conditions (not here described) that seem to imply a measure of interaction between the different colour mechanisms.