The Dark

The dark web is a secretive, anonymous place where shadowy users access hidden services. It can be used for good or bad – but can it be cracked? And if so, how?

Highly-publicised cases such as the online illegal drugs store Silk Road have raised public awareness of the dark web, a layer on top of the web that offers anonymity to users.

But what technologies are used to power this mysterious alternate network? How are they used, who is using them, and do they have vulnerabilities of their own that could ultimately lead to their downfall? Danny Bradbury finds out.

Dark Adaptation

Dark adaptation reduces coupling (from space constant determinations, tracer coupling), smaller receptive-field sizes (surround-to-center ratio), and decreased sensitivity of horizontal cells.

From: Encyclopedia of the Eye, 2010

Related terms:

Electroretinography

Photopic Vision

Retinal Pigment Epithelium

Protein

Mutation

Photoreceptor

Retinal

Visual Acuity

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Vision II

A. Stockman, L.T. Sharpe, in The Senses: A Comprehensive Reference, 2008

2.06.3.1.1 Scotopic luminous efficiency

Scotopic luminous efficiency is comparatively straightforward, since it depends on the activity of a single univariant photoreceptor type, the rods. Thanks to univariance, scotopic luminous efficiency fulfils the basic requirement of any system of photometry that the luminous efficiency of any mixture of lights is the sum of the efficiencies of the components of the mixture; otherwise known as Abney's Law (Abney, W. d. W. and Festing, E. R., 1886; Abney, W. d. W., 1913). Figure 5 shows the scotopic CIE 1951 V′(λ) function (white line), which is based on original data from Crawford B. H. (1949) and Wald G. (1945).

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Electrogenesis of the Electroretinogram

Laura J. Frishman, in Retina (Fifth Edition), 2013

Origin of the scotopic threshold response

For very weak flashes from darkness, near psychophysical threshold in humans,33,90 small negative (n) and positive (p) STR dominate the ERG of most mammals that have been studied. This response, which is more sensitive than the b-wave (or a-wave) and saturates at a lower light level than either component, was thus named because of its sensitivity.63 As shown in Fig. 7.7, for the monkey and human, the nSTR at stimulus onset dominates the dark-adapted diffuse flash ERG in response to the weakest stimuli. The nSTR is distinct from the scotopic a-wave, although it can appear as a "pseudo a-wave" at light levels where it can be removed by suppressing inner retinal activity pharmacologically.121,122 The STRs occur in response to stimuli much weaker than those that elicit more distally generated waves of the ERG because convergence of the rod signal in the retinal circuitry increases the gain of responses generated by inner retinal neurons.

The nSTR was initially observed to be generated more proximally (IPL) than PII (INL) in intraretinal depth analysis studies in cats.63 As shown in Fig. 7.21, the field potential recorded in proximal retina in response to a weak diffuse stimulus was negative-going for the duration of the stimulus, and returned slowly to baseline after light offset. For stimuli too weak to elicit PII, the nSTR reversed polarity in midretina and became a positive-going signal in the mid- and distal retina. This reversal suggests a source proximal to, and a sink distal to, the reversal point (see description of the Müller cell mechanism, below). For stronger stimuli, the reversed nSTR in mid- and distal retina was replaced by PII, which then dominated the ERG. The similarity of the onset times and timecourse of the proximal retinal STR and the negative STR in the cat vitreal ERG can be seen in Fig. 7.21.

The nSTR also can be separated from PII using pharmacologic agents (GABA, glycine, or NMDA30,90,121) to suppress responses of the amacrine and ganglion cells proximal to bipolar cells. These agents remove the STR, but not PII (Fig. 7.15). In contrast, APB eliminates both the scotopic b-wave (PII) and STR, indicating that the STR will not be generated if the primary rod circuit is no longer mediated by rod bipolar cells (Fig. 7.2).

There is a similarly sensitive positive STR in the scotopic ERG of animals that have a negative STR.40,90 Because the pSTR is small and of opposite polarity to the nSTR, it can easily be cancelled out in the ERG. An instance of this can be seen in the dark-adapted macaque ERG in Fig. 7.7. The delayed onset of nSTR for the weakest stimulus is due to the presence of a pSTR that is slightly larger than the nSTR at early times. In response to a stimulus about 1 log unit higher (just above), the pSTR rides on the emerging PII as an early positive potential. Most pharmacologic agents that eliminate the nSTR also eliminate the pSTR.40,90 For example, NMDA eliminated both the nSTR and pSTR in the macaque ERG for responses such as those seen for the two weakest stimuli in Fig. 7.7 (Frishman, unpublished observations).

A linear model of the contributions of pSTR, nSTR, and PII to the dark-adapted cat ERG is shown in Fig. 7.22. The model assumes that each ERG component initially rises in proportion to stimulus strength, and then saturates in a characteristic manner, as has been demonstrated in single-cell recordings in mammalian retinas, as well as for ERG a- and b-waves in numerous studies. Only with the inclusion of a small pSTR does the model accurately predict the whole ERG at a given "fixed" time in response to a weak stimulus. The model was fit in Fig. 7.22 to responses measured at 140 ms after a brief full-field flash (<5 ms), which was the peak of the nSTR in the cat scotopic ERG. Similar models have been applied to mouse90 and human ERG.40

K+ Müller cell mechanism for generation of the STR

The STR, like the M-wave, is associated with Müller cell responses to [K+]o released by proximal retinal neurons. In intraretinal studies in cat, a proximal increase in [K+]o was observed that had clear similarities to the local STR that was simultaneously recorded: the dynamic range from "threshold" to saturation of the light-evoked proximal [K+]o increase was similar to that of the field potentials, and the retinal depth maxima for the two responses were the same.22,54 A causative role for the [K+]o increase in generating the nSTR (and a slow negative response in the vitreal ERG following the initial STR) was supported by the finding (Fig. 7.23) that Ba2+ removed the proximal retinal field potential and the nSTR in the ERG but did not, initially, eliminate the light-evoked increase in [K+]o. The cornea-negative polarity of the nSTR suggests a distally directed Müller cell K+ current (similar to M-wave and PIII currents in the vascularized cat retina). As noted above for the light-adapted M-wave, in the dark-adapted retina Ba2+ also appears to block K+ siphoning by the Müller cells. Whereas the proximal [K+]o increase remained intact when related field potentials were abolished, the distal [K+]o increase was eliminated by Ba2+.

Neuronal origins of the STR

Whether the neurons involved in the genesis of the nSTR and pSTR are amacrine or ganglion cells is species-dependent. In monkeys it is likely that the nSTR arises predominantly from ganglion cells. It was absent in eyes in which the ganglion cells were eliminated as a consequence of experimental glaucoma112 and by intravitreal injection of TTX to block Na+-dependent spiking activity of those neurons; the pSTR remained intact. In contrast, in cats and humans122 as well as in rodents,3 the nSTR is not eliminated by ganglion cell loss, and thus may be more amacrine cell-based. In rodents the pSTR relies upon the integrity of ganglion cells. A characteristic of Müller/glial cell-mediated ERG components is their slow timecourse. Glial cell mediation of the nSTR was demonstrated most directly in cat, but the timecourse of the nSTR is slow in all species. Glial mediation may explain the similarity in timecourse of nSTR across species regardless of the particular type of neuron producing the local changes in proximal [K+]o that generate the response.

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Primary Photoreceptor Degenerations: Terminology

M.E. Pennesi, ... R.G. Weleber, in Encyclopedia of the Eye, 2010

Dark Adaptation

Dark adaption is measured with a Goldmann–Weekers dark adaptometer. Each eye is tested separately by first bleaching the rod and cone photopigments with an intense light and then measuring the brightness of a second light needed to achieve a threshold response. A dark adaptation curve can be drawn by repeating this threshold measurement over time after the bleaching light has been turned off. The normal recovery curve can be separated into two segments. The first segment occurs as cones recover from the bleaching light. The rods are much slower to recover and contribute to the second segment of the curve.

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Basic Science, Inherited Retinal Disease, and Tumors

Laura J. Frishman, in Retina (Fourth Edition), 2006

A K+-Müller cell mechanism for generation of the scotopic threshold response

The STR, like the M-wave, is associated with Müller cell responses to [K+]o released by proximal retinal neurons.60,61 Frishman & Steinberg61 identified a proximal increase in [K+]o with obvious functional similarities to the STR that was simultaneously recorded in proximal retina: the dynamic range from "threshold" to saturation of the light-evoked proximal [K+]o increase was similar to that of the field potentials, and the retinal depth maxima for the two responses were the same.

A causative role for the [K+]o increase in generating the nSTR (and a slow negative response in the vitreal ERG following the initial STR) was supported by the finding that Ba2+ eliminated the proximal retinal field in the cat and the nSTR in the ERG but did not, initially, eliminate the light-evoked increase in [K+]o. The cornea-negative polarity of the nSTR suggests a distally directed Müller cell K+ current (similar to M-wave and PIII currents in the vascularized cat retina). As illustrated in Figure 6-31 for the light-adapted M-wave, Figure 6-36 shows that in the dark-adapted retina Ba2+ also appeared to block K+ siphoning by the Müller cells. Whereas the proximal [K+]o increase remained intact when related field potentials were abolished, the distal [K+]o increase was eliminated by Ba2+.

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Primary Photoreceptor Degenerations: Retinitis Pigmentosa

M.E. Pennesi, ... R.G. Weleber, in Encyclopedia of the Eye, 2010

Diagnostic Tests for RP

Dark Adaptation

Dark adaptation can be a useful test in patients with RP. Patients who manifest with a rod–cone dystrophy will usually have a detectable increase in final dark-adapted thresholds and show delayed dark-adaptation curves. Prolonged dark adaptation is especially common among patients with RHO mutations. Elevations of the early cone segment of the dark-adaptation curve may be particularly noticed by patients, more so than elevations of the rod segment (Figure 4).

Figure 4. Example of dark-adaptation curves in a normal subject (dashed lines represent the mean normal response, dotted lines represent the upper limit of normal) and patients with retinitis pigmentosa (solid lines). From Weleber, R. and Evan, K. G. (2006). Retinitis pigmentosa and allied disorders. In: Ryan, S. J. (ed.) The Retina, 4th edn., vol. 1, chap. 17, pp. 395–498. Philadelphia, PA: Elsevier.

Visual Fields

Visual fields are not only useful for making the diagnosis of RP, but are also one of the most useful objective methods for monitoring progression of the disease. Decreased visual-field sensitivity results from photoreceptor loss (Figure 5).

Figure 5. Example of a mildly abnormal kinetic visual field in a patient with early retinitis pigmentosa demonstrating the responses to different-sized targets. The gap between the size III4e and size I4e isopters is greater than normal, indicating loss of sensitivity in this region. The blind spot (region containing the optic nerve head and therefore no photoreceptor cells) is plotted in each eye just temporal to the fovea.

The earliest change seen as measured by kinetic perimetry is concentric constriction or decreased sensitivity with static perimetry in diffuse disease and relative midperipheral scotomas seen in the in regional disease. As these midperipheral scotomas or regions of decreased sensitivity enlarge and deepen, severe tunnel vision results. Eventually, macular function fails and visual field becomes difficult or impossible to measure by conventional perimetry. Although visual function may be reduced to light perception only, it is rare for patients to become completely blind. With the exception of female carriers in X-linked RP, visual-field loss is usually symmetrical. Marked asymmetry should raise concern for diseases that mimic RP (Figure 6).

Figure 6. Kinetic visual fields obtained from patient with retinitis pigmentosa. Note the relative preservation of inferior fields, which correlated with preserved superior retina. From Weleber, R. and Evan, K. G. (2006). Retinitis pigmentosa and allied disorders. In: Ryan, S. J. (ed.) The Retina, 4th edn., vol. 1, chap. 17, pp. 395–498. Philadelphia, PA: Elsevier.

The rate of visual-field loss has been shown to be exponential. This rate is thought to be similar for the different forms of inheritance once correction has been made for the critical age of onset. Massof and Finkelstein found that patients lost about 50% of their visual field every 4.5 years. The superior visual field, which corresponds to the inferior retina, is often more affected than inferior visual fields. Based on this finding, it has been suggested that increased levels of light may play a role in accelerating retinal degeneration and this in turn may play a role in the forms of RP with greater damage in the inferior retina.

Electroretinograms

ERGs play a crucial role in the diagnosis of RP because these electrophysiological recordings are sensitive enough to detect decreased photoreceptor function early in the disease when fundus findings and visual fields may be minimally altered. In addition, ERGs are particularly useful to assess visual function in preverbal infants and children. Almost all patients with symptomatic RP will have detectable changes on the ERG at the time of diagnosis. While the ERG is useful for the diagnosis of RP, visual fields are better for monitoring of the course of the disease. In severe cases of RP, such as LCA, the ERG may be not recordable.

Patients with RP can show decreased amplitude and timing of the major components of the ERG. Caution must be taken when interpreting decreases in the amplitude of an ERG because poor contact of the electrodes, deviations of the eye, and high myopia can affect the amplitude of the signal. When present, delayed timing tends to be a more robust indicator of dysfunction.

By analyzing the different components of the ERG, different forms of RP can be classified. Degeneration of the rod and cone photoreceptors leads to a decrease in the amplitude of different waveforms of the ERG and can also increase the timing or latency of the peaks of these waveforms. The most common forms of RP manifest as a rod–cone dystrophy and the first detectable changes will be apparent on the scotopic ERG. Decreases in the b-wave amplitude and timing of the peak of the b-wave are indicative of early rod photoreceptor death. Further loss of rod cells leads to further decreases in the b-wave amplitude and decreased amplitude of the a-wave responses at higher intensities. Patients with a cone–rod dystrophy have normal, or lesser defect of b-wave responses to dim scotopic stimuli, but typically have more markedly abnormal ERGs to 30-Hz flicker or single-flash stimuli measured under photopic conditions (Figures 7–9).

Figure 7. ERGs recorded from a patient with autosomal recessive RP (left column) compared to a control patient (right column). This patient is demonstrative of a rod–cone dystrophy. There is a flat response to the dim blue flash under scotopic conditions, which specifically stimulates rods. The bright flash under scotopic conditions normally elicits mixed responses from both rods and cones. In this case, the response is severely attenuated and the small amount of signal is likely coming from the cone system. Under light-adapted conditions (photopic single flash and 30-Hz flicker), which selectively stimulate the cones, the response is only slightly decreased consistent with the categorization of a rod–cone dystrophy.

Figure 8. ERGs recorded from a patient with peripherin/RDS null mutation (left column) compared to a control patient (right column). This patient is demonstrative of a rod–cone dystrophy where the rods and cones are equally affected. It is of importance that peripherin is expressed in both rods and cones. There is a severely diminished response to the dim white flash under scotopic conditions, which specifically stimulates rods. The bright flash under scotopic conditions normally elicits a mixed response from both rods and cones. In this case, the response is severely attenuated. Under light-adapted conditions (photopic single flash and 30-Hz flicker), which selectively stimulate the cones, the response is also severely decreased consistent with the categorization of an equal rod–cone dystrophy.

Figure 9. ERGs recorded from a patient with autosomal recessive RP (left column) compared to a control patient (right column). This patient is demonstrative of a cone–rod form of RP. There is a mildly diminished response to the dim blue flash under scotopic conditions, which specifically stimulates rods. The bright flash under scotopic conditions normally elicits a mixed response from both rods and cones. In this case, the response is only moderately attenuated. Under light-adapted conditions (photopic single flash and 30-Hz flicker), which selectively stimulate the cones, the response is severely decreased consistent with the categorization of a cone–rod dystrophy.

Fundus Photography/Fluorescein Angiography

Documentation by fundus photography can assist in monitoring changes in patients with RP. Fluorescein angiography in patients with RP will demonstrate hyperfluorescence in areas of RPE atrophy and can highlight areas of cystoid macular edema. However, fluorescein angiography has largely been supplanted by optical coherence tomography (OCT) for detecting cystoid maculopathy. In addition, concerns about light exposure accelerating certain forms of RP in animal models have prompted many ophthalmologists to exercise caution in obtaining excessive photographs.

Optical Coherence Tomography

OCT provides a noninvasive cross-sectional image of the retina. It is very useful in patients with RP when there is a question of cystoid macular edema. The ability to detect cystoid macular edema by OCT often obviates the need to get a fluorescein angiogram.

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Functional vision changes in the normal and aging eye

Bruce P. Rosenthal, Michael Fischer, in A Comprehensive Guide to Geriatric Rehabilitation (Third Edition), 2014

Dark adaptation

Dark adaptation, or the ability, to adjust to new levels of illumination, such as going from the outdoors to the indoors, may be very apparent and debilitating from retinal diseases (e.g. macular degeneration, macular edema, diabetic and hypertensive retinopathy). Adapting to changes in lighting levels is exemplified by the effect of a camera flash into the eye or entering a dark movie theatre. The response time in adapting, especially for elderly, may result in a fall from an object not obvious in the environment. Absorptive lenses and filters may be beneficial in minimizing the adaptation time as well as enhancing the contrast. Stereoacuity vision loss may often result in the vision being much poorer in one eye. This disparity between the two eyes may manifest itself in such tasks as threading a needle or tying shoelaces.

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Environmental design

Mary Ann Wharton PT, MS, in Geriatric Physical Therapy (Third Edition), 2012

Dark adaptation.

Dark adaptation, or the ability of the eye to become more visually sensitive after remaining in darkness for a period of time, is delayed in older persons. One reason for this visual change is the smaller, miotic pupil, which limits the amount of light reaching the periphery of the retina. It is this area of the retina that contains the rods, which are sensitive to low light intensities. Another reason for delayed dark adaptation in older individuals is the metabolic changes in the retina. The oxygen supply to the rod-dense area of the retina diminishes as a result of vascular changes, which, in turn, affect the efficiency of the rods to respond to low levels of illumination. As a result of these changes, older persons have difficulty adapting to darkness and to abrupt and extreme changes in light.2-5

Use of a night-light is recommended to assist in overcoming the decreased ability for the eyes to adapt to the dark. Red light stimulates the cones but not the rods, allowing an older person to see well enough by red light to function in the dark. Therefore, a red bulb is suggested because it reduces the time required for adaptation to the dark and while permitting the older individual to see well enough to function. It is also recommended that older individuals carry a pocket flashlight to aid in transition to dimly lit environments. Improving lighting at the point of entry to an area, through pull cords or light switches near the entrance to a room, is also recommended. Automatic timers or keeping a light on at all times in dimly lit areas can prevent older individuals from having to enter a darkened room.3,5,15

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Basic Science, Inherited Retinal Disease, and Tumors

Joel Pokorny, Vivianne C. Smith, in Retina (Fourth Edition), 2006

Temporal and spatial characteristics

The dark adaptation curve also depends on the size and presentation time of the test light. A greater range of sensitivity change is measured for large, long test lights than for small, brief test lights. Interpretation of the dark adaptation function Following a full bleach of the visual photopigments, the time course of dark adaptation is primarily determined by the rate of regeneration of the photopigment. The recovery is often described by the Dowling–Rushton equation, which states that the logarithm of the relative threshold is proportional to the photopigment still in the bleached state:

(4)log(/th//abs)=k(1-p)

where Ith is the threshold, Iabs is the final dark-adapted threshold, 1–p is the proportion of bleached photopigment, and k is a constant. Hood and Greenstein37 discuss the limitations of the Dowling–Rushton equation; for example, the Dowling–Rushton equation does not describe regeneration for pigment bleaches less than about 10%. Further, the time course of dark adaptation depends on the bleaching history. Adaptation is not totally explained by the state of the visual photopigment. This is clear because the range of adaptation depends on the test field characteristics, and substantial sensitivity change occurs even after negligible bleaching. These sensitivity changes are called neural adaptation and are thought to reflect sensitivity control or gain changes as opposed to pigment kinetics.

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Unique Specializations – Functional: Dynamic Range of Vision Systems

A.C. Arman, A.P. Sampath, in Encyclopedia of the Eye, 2010

Signal transfer from rods to rod bipolar cells

At scotopic threshold, vision relies on a sparse number of photons at the retina that produce few photon absorptions per thousands of rods within the 0.2-s integration time of the rod photoresponse. Under these conditions, the transmission of a small, graded hyperpolarization upon photon absorption requires that rod synapse is appropriately optimized. The transmission of small, graded single-photon responses at the rod synaptic terminal is aided by two specializations. First, the resting dark membrane potential, or voltage, sits at approximately –40 mV, near the steepest point in the relationship between voltage and L-type Ca2+ channel opening (Figure 2). Thus, small changes in membrane potential produce substantial changes in the number of open channels, thereby altering glutamate release. Second, if the rod bipolar cell is sensing reductions in glutamate release due to photon absorption, then statistical lapses of glutamate release in darkness would mimic light absorption. Thus, the high rate of glutamate release generated in darkness by the specialized synaptic ribbon in the rod spherule reduces the probability of these lapses. Together, these synaptic properties allow the small, light-evoked signals from rods to be reproducibly transferred to downstream neurons.

Despite the rod synaptic specializations for the transmission of single-photon absorptions, the depolarization in darkness due to open cyclic guanosine monophosphate (cGMP)-gated channels is also a complicating factor in the detection of these sparse signals. Open cGMP-gated channels in turn will report internal fluctuations in cGMP, produced by the phototransduction mechanism, which are commonly referred to as dark noise. Since rods generate a small, graded hyperpolarization upon photon absorption, the downstream convergence of thousands of rod signals would cause the light-evoked response from a single rod to be overwhelmed by the dark noise of the majority. Given the magnitude of dark noise in individual rods, it has been proposed that some type of nonlinear combination of rod signals would be required to increase the detection of the single-photon responses in downstream cells. Since rod photoreceptors are relatively depolarized in darkness, the steady release of glutamate from the synapse provides some insights into potential mechanisms. Postsynaptic saturation at the rod-to-rod bipolar synapse would allow noise generated by open cGMP-gated channels in the rod outer segment to be eliminated. It was proposed that the saturation of postsynaptic glutamate receptors would provide a nonlinear way to eliminate the rod noise, since the synapse would not be able to relay small changes in membrane potential that reflect rod noise. Later work suggested that such thresholding is critical for maximizing the detection of the single-photon response in retinal neurons downstream of the rods (Figure 1). In particular, the extent of nonlinear signaling appears to be set to separate optimally the rod single-photon response from rod noise, allowing scotopic vision to reach the highest possible sensitivity.

Figure 1. Convergence at the rod-to-rod bipolar synapse. (a) A rod bipolar cell pools inputs from many rods, but near absolute visual threshold only one rod may absorb a photon (red), while the remaining rods are generating electrical noise (blue). A nonlinear threshold (dashed) may improve photon detection at this synapse by retaining responses in rods absorbing a photon and discarding responses of the remaining rods. (b) Nonlinear signal processing can improve the fidelity of rod signals. If rod outputs from (a) are simply summed, the resulting trace is noisy, but when summed after applying a threshold for each rod in (a), the response is more detectable. From Okawa, H. and Sampath, A. P. (2007). Optimization of single-photon response transmission at the rod-to-rod bipolar synapse. Physiology 22: 279–286. With kind permission from The American Physiological Society.

The mechanism that underlies the nonlinear threshold at the rod synapse has been studied to some extent, but is hindered by a lack of identification of the components of the signaling pathway. Light-evoked signaling between rod photoreceptors and rod (ON) bipolar cells results in a membrane depolarization, effectively inverting the sign of the rod's hyperpolarizing light response. The postsynaptic mechanism underlying this sign inversion is a G-protein signaling pathway initiated by the metabotropic glutamate receptor, mGluR6. mGluR6, in turn, activates a guanine nucleotide-binding protein, Goα, which leads to a series of unidentified events that close a cationic transduction channel of unknown identity. Thus, upon light-absorption, glutamate release from rods is reduced, thereby reducing the activity of the mGluR6 signaling pathway and allowing transduction channels to open and depolarize the cell (Figure 2).

Figure 2. Structure and signal transfer at the rod-to-rod bipolar synapse. (a) The rod spherule is a specialized invaginating structure where the dendrites of horizontal (H) and rod bipolar cells (RB) are apposed to a glutamate release site controlled by a ribbon. Ca2+ channels (CaV1.4) are located near the active zone (AZ) and allow the continuous release of glutamatergic vesicles in darkness. In ON rod bipolar cells, glutamate is sensed by mGluR6 receptors located near the mouth of the invagination. (Inset) Release of glutamate is dependent on Ca2+ influx through Ca2+ channels, which is graded by voltage over the physiological range. (b) The signaling cascade in rod bipolar cell dendrites is poorly understood. mGluR6 activation leads to the activation of Goα, which through unknown mechanisms leads to the closure of a nonselective cation channel whose identity is also unknown. The light-evoked reduction in glutamate release relieves activity in this cascade and opens cation channels leading to depolarization. From Okawa, H. and Sampath, A. P. (2007). Optimization of single-photon response transmission at the rod-to-rod bipolar synapse. Physiology 22: 279–286. With kind permission from The American Physiological Society.

In the context of the mGluR6 signaling cascade, it now appears that the nonlinear threshold that eliminates rod noise is due to saturation within the signaling cascade, and not at the level of the glutamate receptors. Furthermore, evidence from axotomized rod bipolar cells indicates that nonlinear signal transfer does not arise due to feedback in the inner plexiform layer. Saturation of the mGluR6 signaling cascade allows the elimination of noise by making the rod bipolar cell insensitive to small fluctuations in glutamate, driven by noise in the rod photoreceptor. Only when the rod's membrane potential is hyperpolarized sufficiently does the glutamate concentration in the synaptic cleft reduce enough to relieve the synapse from saturation. Such an operation thus allows larger hyperpolarizations due to light absorption to cross the rod synapse, while masking smaller fluctuations that are more likely due to noise in the rod photocurrent or synaptic transmission. Near absolute visual threshold, such synaptic processing is necessary to maximize the detectability of rod signals.

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Vision: Light and Dark Adaptation

A. Reeves, in Encyclopedia of Neuroscience, 2009

Electrophysiological Evidence

The phenomena of dark and light adaptation have been investigated electrophysiologically, using single-unit recording, single-cell recording, and electroretinography in various animal species, or just electroretinography in humans, with the goal of explaining the changes that occur with extended exposure to light or darkness. Retinal potentials often show a progressive increase in responsiveness that advances with dark adaptation. They are thus useful because they provide objective information regarding the visual process away from the threshold conditions to which psychophysics is limited. Although the changes seen with light and dark adaptation resemble those seen psychophysically, exact comparisons between data provided by the two methods are difficult because conditions are rarely comparable: retinal electrophysiology is typically conducted with large, bright stimuli to obtain measurable responses, whereas psychophysics is usually conducted with localized near-threshold test flashes. This is not necessarily the case, however. Experiments with suitable procedures have effected such comparisons and thus have helped to clarify understanding of the adaptation process. Even the reduction of blue sensitivity found psychophysically at the start of dark adaptation (transient tritanopia) has been seen in the electroretinogram by D van Norren. Thus although a complete description of light and dark adaptation remains some way off, various aspects of these processes have yielded to intense scrutiny at both the electrophysiological and the behavioral levels

The light/dark paradigm is based on the innate aversion of rodents to brightly illuminated areas and on the spontaneous exploratory behaviour of the animals, applying mild stressors i.e. novel environment and light. The test apparatus consists of a small dark secure compartment (one third) and a large illuminated aversive compartment (two thirds).

The test was developed with male mice. The strain, weight and age may be crucial factors.

The extent to which an anxiolytic compound can facilitate the exploratory activity depends on the baseline level in the control group. Differences between the type and severity of external stressors might account for variable results reported by different laboratories.

In conclusion, the black and white test may be useful to predict anxiolytic-like or anxiogenic-like activity in mice. Transitions have been reported to be an index of activity-exploration because of habituation over time and the time spent in each compartment to be a reflection of aversion. Classic anxiolytics (benzodiazepines) as well as the newer anxiolytic-like compounds (e.g. serotonergic drugs) can be detected using this paradigm. It has the advantages of being quick and easy to use, without requiring the prior training of animals.Social media affords brands and users a variety of benefits. However, a recent stream of research identifies a multidimensional dark side to social media. We contribute to this growing body of research in four key ways. First, we empirically investigate user perceptions of the dark side of social media in terms of the risks proposed by Baccarella et al. (2018), confirming the existence of six of the seven risks. Second, we identify and empirically investigate the strategies with which users seek to reduce the social media risks. Third, we develop scales to assess both the social media risks and user reduction strategies. Finally, we conduct segmentation analysis to empirically investigate how users differ in terms of their perceived social media risks and risk reduction strategies. Taken together, our findings provide a validated framework of, and scales to measure, user perceptions of, and responses to, the dark side of social media media risks, being: Sharing, Conversations, Relationships, Groups, Reputation, and Privacy.Of the offensive yet non-pathological personalities in the literature, three are especially prominent: Machiavellianism, subclinical narcissism, and subclinical psychopathy. We evaluated the recent contention that, in normal samples, this 'Dark Triad' of constructs are one and the same. In a sample of 245 students, we measured the three constructs with standard measures and examined a variety of laboratory and self-report correlates. The measures were moderately inter-correlated, but certainly were not equivalent. Their only common Big Five correlate was disagreeableness. Subclinical psychopaths were distinguished by low neuroticism; Machiavellians, and psychopaths were low in conscientiousness; narcissism showed small positive associations with cognitive ability. Narcissists and, to a lesser extent, psychopaths exhibited self-enhancement on two objectively scored indexes. We conclude that the Dark Triad of personalities, as currently measured, are overlapping but distinct constructs.Summary

From intense sunlight in bright snow down to a moonless night in a dark forest, we can use light to recognize objects and guide our actions. This remarkable range mainly rests on having two different types of photoreceptors, the rods and the cones. The cones are active under daylight conditions, allowing high acuity and color vision. Rods are mainly active under very dim illumination conditions and have an exquisite sensitivity to light [1]. There are obvious detriments to visual perception in near darkness, such as a central scotoma, reduced motion perception [2], and most of all a lack of color [3]. There is only one type of rod, and thus intensity and wavelength differences cannot be disentangled when only the rods are active. This is captured well by the old saying "at night all cats are gray", meaning that different colors inevitably get mapped onto different shades of gray. Here we show that the perception of lightness is also different for night vision: our results indicate that surfaces that appear to be white under daylight conditions, at best, appear medium gray under night vision, suggesting that activation of the cones is necessary for the perception of white.

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