Test Flash Research Articles

Application and Effectiveness of Virtual Reality in Teaching Basic Concepts to Children with Autism Spectrum Disorder

Background: This study compares traditional teaching methods with cutting-edge virtual reality (VR) technology to assess its effectiveness in teaching basic concepts to children with autism spectrum disorder (ASD). Methods: Thirty-two children with ASD were included in the study and randomly assigned to either the VR or the control group. For the VR group, a teaching material was developed as an easy-to-play VR game called Geometry Park to teach concepts of shapes and colors to children with ASD, by virtually traveling through a world filled with geometrical shapes in different colors and sizes. For the control group, similar teaching material was prepared using flashcards with shapes and colors similar to those in Geometry Park. Each subject underwent ten play-and-learn sessions. At the end of the sessions, the research assistant asked them to identify 23 shapes and colors using "test flash cards" and reported the scores for each item. Results: The result of the two-sample t-test did not show a statistically significant difference between the means of the two groups (P=0.45), which might be a false negative due to Type II error. However, a medium effect size was observed for the VR group compared to the control group (Cohen’s d=0.39). Conclusion: The results of this experiment did not demonstrate a statistically significant difference between the VR and the control groups. However, a medium effect size was found for the VR group, which is "clinically significant".

A motion-induced position shift that depends on motion both before and after the test probe.

Two versions of the flash grab illusion were used to examine the relative contributions of motion before and motion after the test flash to the illusory position shift. The stimulus in the first two experiments was a square pattern that expanded and contracted with an outline square flashed each time the motion reversed producing a dramatic difference in perceived size between the two reversals. Experiment 1 showed a strong illusion when motion was present before and after the flashed tests or just after the flashes, but no significant effect when only the pre-flash motion was present. In Experiment 2, motion always followed the flash, and the duration of the pre-flash motion was varied. The results showed a significant increase in illusion strength with the duration of pre-flash motion and the effect of the pre-flash motion was almost 50% that of the post-flash motion. Finally, Experiment 3 tested the position shifts when the linear motion of a disk before the flash was orthogonal to its motion after the flash. Here, the results again showed that the pre-flash motion made a significant contribution, about 32% that of the post-flash motion. Several models are considered and even though all fail to some degree, they do offer insights into the nature of the illusion. Finally, we show that the empirical measure of the relative contribution of motion before and after the flash can be used to distinguish the mechanisms underlying different illusions.

RF heat load compensation with electrical heater for XFEL accelerator - measurements at CMTB, AMTF and FLASH

A superconducting linac will provide electron bunches of 17.5 GeV beam energy for the operation of the European X-ray Free Electron Laser (XFEL) at DESY. 800 superconducting niobium 1.3 GHz nine cell cavities and 100 superconducting magnet packages will be cooled in a liquid helium II cooling bath at 2.0K temperature. Liquid helium II bath cooling at 2.0K is established by means of a 4 stages cold compressor system. For the stable operation of cold compressors sudden load changes should be avoided. Electrical heaters in the helium II bath will compensate dynamic heat load changes caused by the radio frequency (RF) operation of the cavities in the XFEL linac. The complementary load operation of electrical heaters is studied at the existing facilities at DESY. Measurements performed at Cryo Module Test Bench (CMTB), Accelerator Module Test Facility (AMTF) and FLASH accelerator are reported.

Increased visual sensitivity following periods of dim illumination.

We measured changes in the sensitivity of the human rod pathway by testing visual reaction times before and after light adaptation. We targeted a specific range of conditioning light intensities to see if a physiological adaptation recently discovered in mouse rods is observable at the perceptual level in humans. We also measured the noise spectrum of single mouse rods due to the importance of the signal-to-noise ratio in rod to rod bipolar cell signal transfer. Using the well-defined relationship between stimulus intensity and reaction time (Piéron's law), we measured the reaction times of eight human subjects (ages 24-66) to scotopic test flashes of a single intensity before and after the presentation of a 3-minute background. We also made recordings from single mouse rods and processed the cellular noise spectrum before and after similar conditioning exposures. Subject reaction times to a fixed-strength stimulus were fastest 5 seconds after conditioning background exposure (79% ± 1% of the preconditioning mean, in darkness) and were significantly faster for the first 12 seconds after background exposure (P < 0.01). During the period of increased rod sensitivity, the continuous noise spectrum of individual mouse rods was not significantly increased. A decrease in human reaction times to a dim flash after conditioning background exposure may originate in rod photoreceptors through a transient increase in the sensitivity of the phototransduction cascade. There is no accompanying increase in rod cellular noise, allowing for reliable transmission of larger rod signals after conditioning exposures and the observed increase in perceptual sensitivity.

Rodent Hyperglycemia-Induced Inner Retinal Deficits are Mirrored in Human Diabetes.

To evaluate the utility of low luminance stimuli to functionally probe inner retinal rod pathways in the context of diabetes mellitus in both rat and human subjects. Inner retinal dysfunction was assessed using oscillatory potential (OP) delays in diabetic rats. Scotopic electroretinograms (ERGs) in response to a series of increasing flash luminances were recorded from streptozotocin (STZ)-treated and control Sprague-Dawley rats after 7, 14, 20, and 29 weeks of hyperglycemia. We then evaluated OP delays in human diabetic subjects with (DR) and without (DM) diabetic retinopathy using the International Society for Clinical Electrophysiology in Vision (ISCEV) standard scotopic protocol and two additional dim test flashes. Beginning 7 weeks after STZ, OP implicit times in diabetic rats were progressively delayed in response to dim, but not bright stimuli. In many diabetic subjects the standard ISCEV dim flash failed to illicit measureable OPs. However, OPs became measurable using a brighter, nonstandard dim flash (Test Flash 1, -1.43 log cd s/m2), and exhibited prolonged implicit times in the DM group compared with control subjects (CTRL). Delays in scotopic OP implicit times are an early response to hyperglycemia in diabetic rats. A similar, inner retinal, rod-driven response was detected in diabetic human subjects without diabetic retinopathy, only when a nonstandard ISCEV flash intensity was employed during ERG testing. The addition of a dim stimulus to standard ISCEV flashes with assessment of OP latency during ERG testing may provide a detection method for early retinal dysfunction in diabetic patients.

Light adaptation increases response latency of alpha ganglion cells via a threshold-like nonlinearity

Adaptation is an important process of sensory systems to adjust sensitivity to ensure the appropriate information encoding. Sensitivity and kinetics of retinal ganglion cell (RGC) responses have been studied extensively using a brief flash superimposed on different but steady backgrounds. However, it is still unclear if light adaptation exerts any effect on more complex response properties, such as response nonlinearity. In this study, we found that the latency of spike responses to a repeated flashing spot stimulation increased by 30ms in the mouse ON α RGCs (An ON-type RGC is excited when a spot is turned on in the center of its receptive field). A single dimming event preceding the test flash on a steady adapting background could also produce similar effect in increasing latency of light responses. A simple computational model with a linear transformation of the light stimulus and a threshold-like nonlinearity could account for the experimental data. Moreover, the strength of the measured nonlinearity and the response latency were affected by the duration of light adaptation. The possible biological processes underlying this nonlinearity were explored. Voltage clamp recording revealed the presence of the increase in latency and threshold-like nonlinearity in the excitatory input of RGCs. However, no comparable nonlinearity was observed in the light responses of the ON cone bipolar cells. We further excluded GABAergic and glycinergic inhibition, N-methyl-d-aspartate receptor rectification and voltage-gated Na+ channels as potential sources of this nonlinearity by pharmacological experiments. Our results indicate the bipolar cell terminals as the potential site of nonlinearity. Computational modeling constrained by experimental data supports that conclusion and suggests the voltage-sensitive Ca++ channels and Ca++-dependent vesicle release in the bipolar cell terminals as mechanistic basis.

Flash illusions induced by visual, auditory, and audiovisual stimuli

When two objects are flashed at one location in close temporal proximity in the visual periphery, an intriguing illusion occurs whereby a single flash presented concurrently at another location appears to flash twice (the visual double-flash illusion: Chatterjee et al., 2011, Wilson & Singer, 1981). Here, for the first time, we investigate the time course of the effect, and directly compare it to the time course of the auditory (sound-induced flash illusion) effect, for both fission (single test flash, double inducer) and fusion (double test flash, single inducer) conditions, across stimulus onset asynchronies (SOAs) of 30 to 250 ms. In addition, using a novel audiovisual stimulus, we directly compare the cue strength of the two modalities, and whether they are additive in effect. The results show that the time course of fission and fusion is different for visual inducers, but not for auditory inducers. In audiovisual conditions, in situations of uncertainty, observers tended to follow the more reliable (auditory) cue. There was little evidence for a superadditive effect of auditory and visual cues; rather, observers tended to follow one or the other modality. The results suggest that the visually induced flash illusion and the auditory-induced effect may both stem from perceptual uncertainty, with the difference in time courses attributable to the lower temporal resolution of vision compared to audition.

Neural correlates of motor–sensory temporal recalibration

The relative timing of a motor–sensory event can be recalibrated after exposure to delayed visual feedback. Here we examined the neural consequences of lag adaptation using event-related potentials (ERPs). Participants tapped their finger on a pad, which triggered a flash either after a short delay (0ms/50ms) or a long delay (100ms/150ms). Following the exposure phase, they judged the temporal order of a synchronous tap–flash test stimulus. The synchronous flash was more often perceived to occur before the tap after exposure to long than short delays, indicating that the temporal relation between the tap and the flash was realigned. ERPs evoked by the synchronous tap–flash test stimulus showed that adaptation to delayed flashes caused an early attenuation of the visual P1 (85ms–150ms), and a later negativity at central electrodes (N450). The P1-attenuation may reflect the unexpected earliness of the test flash, or a violation of “cause-before-consequence”. The N450 may be due to realignment of the adapted and the actual timing of the tap–flash interval. We conclude that motor–visual temporal recalibration has consequences at early perceptual levels of visual processing and involves a high-level recalibration mechanism.

Rod photoreceptor temporal properties in retinitis pigmentosa

One of the characteristic signs of retinitis pigmentosa (RP) is the progressive loss of night vision. We have previously shown that the gain of rod photoreceptor activation is moderately reduced in some patients with RP, but this decrease in activation kinetics is not sufficient to account for the night blindness. Recently, single rod recording from animal models of RP showed rods under degeneration remain saturated for shorter periods than normal rods; i.e. are less able to sustain the rod photoresponse. Using paired-flash ERG, here we determine whether rod phototransduction inactivation parameters might also be abnormal in patients with RP. Inactivation parameters were derived from 13 subjects with normal vision, 16 patients with adRP, and 16 patients with autosomal recessive/isolate (rec/iso) RP. The adRP cases included 9 patients with rhodopsin mutations and 7 patients with peripherin/RDS mutations. The inactivation phase was derived using a double-flash paradigm, with a test flash of 2.7 log scot td-s followed at varying intervals by a 4.2 log scot td-s probe flash. Derived rod photoresponses to this just-saturating test flash in normal subjects exhibit a critical time to the initiation of recovery (T sat) of 525 ± 90 (SD) ms. The values of T sat were 336 ± 104 (SD) ms in patients with adRP ( P < 0.001) and 271 ± 45 (SD) ms ( P < 0.001) in patients with rec/iso RP. When T sat values were categorized by mutations, the values were 294 ± 91 (SD) ms ( P < 0.001) for rhodopsin mutations, and 389 ± 100 (SD) ms ( p = 0.01) for peripherin/RDS mutations. Overall, T sat in patients with RP was significantly correlated with the amplitude of ISCEV standard rod response ( r = 0.56; P < 0.001) and the gain of the activation phase of phototransduction ( r = 0.6, P < 0.001). T sat may be a useful marker for therapeutic efficacy in future clinical trials in RP.

Deactivation of the rod response in retinopathy of prematurity

It is known that retinopathy of prematurity (ROP) alters the activation of rod photoreceptors, but the effect of ROP on deactivation has not been investigated. We studied deactivation using an electroretinographic (ERG) paired flash procedure in 22 subjects (12 infants and 10 older subjects) with a history of preterm birth and ROP. The amplitude of the rod-isolated a-wave response to a flash presented 2-120 s after a test flash was measured, and the time at which it reached 50% of the single flash amplitude (t(50)) was determined by linear interpolation. Deactivation results were compared to those in former preterms who never had ROP (n = 6) and term-born controls. In infants, t(50) values of ROP subjects did not differ from those in subjects who never had ROP or term-born controls. Among mature ROP subjects, eight of 12 had t(50) values longer than any control subject. Prolonged deactivation in these mature ROP subjects may indicate lack of maturation of the deactivation process (t(50)) or progressive compromise of retinal function with increasing age.

The achromatic mechanisms do not combine cone signals additively: a new experimental approach

Many standard models of color vision include a “luminance” mechanism that combines signals from the L and M cones additively (e.g., Boynton, 1979; Guth, 1991; Lennie, Pokorny, & Smith, 1993). Experiments that employ “equiluminant” stimuli generally assume linearity, explicitly or implicitly. The luminance mechanism is often studied using moderate to high frequency flicker or motion stimuli. However, brief, weak equichromatic flashes are also generally believed to be detected by this mechanism (Eskew, McLellan, & Giulianini, 1999; Krauskopf, Williams, & Heeley, 1982). The present experiments were undertaken to determine if the mechanism that detects “white” Gaussian blobs, on a gray background, combines the L and M cones linearly, using a new approach that was developed to study the linearity of mechanisms detecting S-cone isolating flashes (Giulianini & Eskew, 2007). A test flash is embedded in masking noise (flickering lines covering the test region). In different runs, the noise consists of L-cone modulations only, M cone modulations only, or the superposition of the two cone component noises (L&M noise). If the detection mechanism combines its L and M cone inputs linearly, the threshold elevation produced by the compound L&M noise is predictable from a specific combination of the elevations produced by the two component noises. Previous results with this method found that S cone mechanisms are nonlinear, but that the ‘green’ mechanism is remarkably linear. In the present study, we measured the masking effect of 8 different compound noises and their associated L and M components. The results show that the mechanism that detects achromatic flashes does not combine its L and M cone inputs linearly. This result, along with others using flicker detection (Stockman & Plummer, 2005; Stromeyer, Chaparro, Tolias, & Kronauer, 1997), calls into question the generality of the idea of equiluminance.

Attentive tracking can modulate the illusory misalignment of a flash

Purpose: The perceived position of a static flash can be shifted by nearby motion (flash-shift: Whitney & Cavanagh, 2000) but the role of attention in the flash-shift effect has not been explored. To investigate whether attention drawn to a moving, tracked stimulus can modulate the position information of static objects, the perceived misalignment of a flash was measured while the offset between the attentively tracked stimulus and the flash was varied. Methods: Subjects were instructed to attentively track a pair of stimulus bars (dark bars) of the rotating radial grating (a twelve-cycle sinusoidal luminance grating), indicated by a pointer. The pointer rotated with the target pair for 667ms and subjects continued to track the indicated bars with attention after the pointer disappeared. A further 833ms later, two horizontal lines were briefly flashed adjacent to the grating at the 3 and 9 o'clock positions. The perceived misalignment of two flashes was measured as a function of the angular position of the tracked bars at the moment of the flashes. At the moment of the flash, the radial grating was always at the same orientation (light bars aligned with vertical and horizontal) and the location of the tracked bar was an integer number of cycles of the grating away from the flash (−2 to +2). Results: When the test flashes were presented before the tracked bars had reached their location, the two flashes showed a misalignment in the direction of the motion of the grating. However, when the flashes were presented after the tracked bars had passed their location, the illusory displacement was considerably reduced or reversed. Conclusion: Even though the moving stimulus was physically identical for each of the offsets between the tracked bar and the test, the perceived position of the tests depended on the locus of attention. This result shows that attention to a moving stimulus can influence the perceived position of a nearby, briefly presented stationary object.

Discriminating colors through a red filter by protanopes and colour normals

Individuals with color vision deficiency have difficulties in differentiating colour in their daily activities. Through certain coloured filters, dichromats may report an improvement of their capacity to differentiate colors, but it is not known if this is achieved by means of a chromatic mechanism. The present study attempts to explain the mechanism by which a coloured filter can produce a beneficial effect in dichromatic visual perception and what is the nature of this improvement. Four male protanopes and four normal trichromats (two males and two females) participated in the present study. We evaluated the effect of the red filter (with a spectral transmittance similar to that of the X-Chrom filter) on the detection thresholds for monochromatic light stimuli from 420 to 660 nm in 20 nm steps. The increment spectral sensitivity functions were measured for 1.2 degrees diameter test flashes presented for 300 ms on a 60-cd m(-2) illuminant C background using an optical bench with a monochromator, for both filter and no filter conditions. The capacity to correctly name green, yellow and red for the monochromatic lights of 550, 575 and 625 nm presented for 300 ms on a 60 cd m(-2) illuminant C background screen was also evaluated with and without the red filter. The spectral sensitivity data suggest that, the use of a red filter improves the protanope's capacity to detect long wavelength light stimuli. The results on the colors naming procedure demonstrate that the red filter modifies colour perception in normal and protanope subjects. In normals, only the red color perception is preserved, and typical colour perception for the green and the yellow is lost. Without the filter, all the protanopes demonstrated a residual colour perception for red and green colours. Through the red filter only red colour perception remains. A red filter does not improve the protanopic red-green perception, but it does improve the ability of the protanope to detect long-wavelength light. This improvement seems to arise by means of the luminance mechanism.

Dark adaptation recovery of human rod bipolar cell response kinetics estimated from scotopic b‐wave measurements

We recorded ganzfeld scotopic ERGs to examine the responses of human rod bipolar cells in vivo, during dark adaptation recovery following bleaching exposures, as well as during adaptation to steady background lights. In order to be able to record responses at relatively early times in recovery, we utilized a 'criterion response amplitude' protocol in which the test flash strength was adjusted to elicit responses of nearly constant amplitude. In order to provide accurate and unbiased measures of response kinetics, we utilized a curve-fitting procedure to fit a smooth function to the measured responses in the vicinity of the peak, thereby extracting both the time-to-peak and the amplitude of the responses. Following bleaching exposures, the responses exhibited both desensitization and accelerated kinetics. During early post-bleach recovery, the flash sensitivity and time-to-peak varied according to a power-law expression (with an exponent of 6), as found in the presence of steady background light. This light-like phenomenon, however, appeared to be set against the backdrop of a second, more slowly recovering 'pure' desensitization, most clearly evident at late post-bleach times. The post-bleach 'equivalent background intensity' derived from measurements of flash sensitivity faded initially with an S2 slope of approximately 0.24 decades min(-1), and later as a gentle S3 tail. When calculated from kinetics, the results displayed only the S2 slope. While the recovery of rod bipolar cell response kinetics can be described accurately by a declining level of opsin in the rods, the sensitivity of these cells is reduced further than expected by this mechanism alone.

A novel GCAP1(N104K) mutation in EF-hand 3 (EF3) linked to autosomal dominant cone dystrophy

The GUCA1A gene encodes a guanylate cyclase activating protein (GCAP1) that is involved in regulation of phototransduction in the vertebrate retina. We discovered a novel C312A transversion in exon 2 of the human GUCA1A gene, replacing Asn-104 (N104) in GCAP1 with Lys (K), in two affected members of a family with dominant cone dystrophy. The mutation N104K is located in the third EF-hand motif (EF3) shown previously to be instrumental in converting Ca 2+-free GCAP1 to a GC inhibitor in the Ca 2+-bound form. In one patient, rod ERGs were fairly stable over a 12-year-period whereas 30 Hz flicker ERG and single-flash cone ERGs declined. In both patients, double-flash ERGs showed that rod recovery from an intense test flash was significantly delayed. The EC 50 for GC stimulation shifted from ∼250 nM in wild-type GCAP1 to ∼800 nM in the GCAP1(N104K) mutant suggesting inability of the mutant to assume an inactive form under physiological conditions. The replacement of N104 by K in GCAP1 is the first naturally occurring mutation identified in the EF3 loop. The rod recovery delays observed in double-flash ERG of affected patients suggest a novel dominant-negative effect that slows GC stimulation.

Contribution of voltage‐gated sodium channels to the b‐wave of the mammalian flash electroretinogram

Voltage-gated sodium channels (Na(v) channels) in retinal neurons are known to contribute to the mammalian flash electroretinogram (ERG) via activity of third-order retinal neurons, i.e. amacrine and ganglion cells. This study investigated the effects of tetrodotoxin (TTX) blockade of Na(v) channels on the b-wave, an ERG wave that originates mainly from activity of second-order retinal neurons. ERGs were recorded from anaesthetized Brown Norway rats in response to brief full-field flashes presented over a range of stimulus energies, under dark-adapted conditions and in the presence of steady mesopic and photopic backgrounds. Recordings were made before and after intravitreal injection of TTX (approximately 3 microm) alone, 3-6 weeks after optic nerve transection (ONTx) to induce ganglion cell degeneration, or in combination with an ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 200 microm) to block light-evoked activity of inner retinal, horizontal and OFF bipolar cells, or with the glutamate agonist N-methyl-D-aspartate (NMDA, 100-200 microm) to reduce light-evoked inner retinal activity. TTX reduced ERG amplitudes measured at fixed times corresponding to b-wave time to peak. Effects of TTX were seen under all background conditions, but were greatest for mesopic backgrounds. In dark-adapted retina, b-wave amplitudes were reduced only when very low stimulus energies affecting the inner retina, or very high stimulus energies were used. Loss of ganglion cells following ONTx did not affect b-wave amplitudes, and injection of TTX in eyes with ONTx reduced b-wave amplitudes by the same amount for each background condition as occurred when ganglion cells were intact, thereby eliminating a ganglion cell role in the TTX effects. Isolation of cone-driven responses by presenting test flashes after cessation of a rod-saturating conditioning flash indicated that the TTX effects were primarily on cone circuits contributing to the mixed rod-cone ERG. NMDA significantly reduced only the additional effects of TTX on the mixed rod-cone ERG observed under mesopic conditions, implicating inner retinal involvement in those effects. After pharmacological blockade with CNQX, TTX still reduced b-wave amplitudes in cone-isolated ERGs indicating Na(v) channels in ON cone bipolar cells themselves augment b-wave amplitude and sensitivity. This augmentation was largest under dark-adapted conditions, and decreased with increasing background illumination, indicating effects of background illumination on Na(v) channel function. These findings indicate that activation of Na(v) channels in ON cone bipolar cells affects the b-wave of the rat ERG and must be considered when analysing results of ERG studies of retinal function.

Test of the paired-flash electroretinographic method in mice lacking b-waves

Previous studies of rod photoreceptors in vivo have employed a paired-flash electroretinographic (ERG) technique to determine rod response properties. To test whether absence versus presence of the ERG b-wave affects the photoreceptor response derived by the paired-flash method, we examined paired-flash-derived responses obtained from nob mice, a mutant strain with a defect in signal transduction between photoreceptors and ON bipolar cells that causes a lack of the b-wave. Normal littermates of the nob mice served as controls. The normalized amplitude-intensity relation of the derived response determined in nob mice at the near-peak time of 86 ms was similar to that determined for the controls. The full time course of the derived rod response was obtained for test flash strengths ranging from 0.11 to 17.38 scotopic cd s m(-2) (sc cd s m(-2)). Time-course data obtained from nob and control mice exhibited significant but generally modest differences. With saturating test flash strengths, half-recovery times for the derived response of nob versus control mice differed by approximately 60 ms or less about the combined (nob and control) average respective values. Time course data also were obtained before versus after intravitreal injection of L-2-amino-4-phosphonobutyrate (APB) (which blocks transmission from photoreceptors to depolarizing bipolar cells) and of cis 2,3-piperidine dicarboxylic acid (PDA) (which blocks transmission to OFF bipolar cells, and to horizontal, amacrine and ganglion cells). Neither APB nor PDA substantially affected derived responses obtained from nob or control mice. The results provide quantitative information on the effect of b-wave removal on the paired-flash-derived response in mouse. They argue against a substantial skewing effect of the b-wave on the paired-flash-derived response obtained in normal mice and are consistent with the notion that, to good approximation, this derived response represents the isolated flash response of the photoreceptors in both nob and normal mice.

Chromatic parameters derived from increment spectral sensitivity functions

We propose a mathematical model to derive the chromatic parameters from increment spectral sensitivity functions. This model was applied to determine the effective red, green, blue, and yellow mechanism contribution to the detection of the spectral stimuli of five normal trichromatic subjects. Detection thresholds were measured for a 300 ms, 1.2 degrees circular test flash presented on a 100 cd/m2 white background for spectral wavelengths between 410 and 660 nm. The model analysis confirmed that in the red-green wavelength area, the detection of our chosen stimuli was mediated by two distinct (L-M) antagonistic mechanisms: a red-green and a yellow, from the blue-yellow system. We inferred that the red-green mechanism receptive fields consisted of a single L- or M-cone center with a homogeneous or heterogeneous surround devoid of S-cone projections. For the receptive fields of the yellow half of the blue-yellow mechanism, we propose a similar configuration but with S-cone projections present in the surround. This proposal is not concordant with what is currently understood regarding retinal physiology. However, two L-M antagonistic mechanisms in the red-green wavelengths as proposed by our results predict what would appear as an intuitive yellow mechanism with a maximal sensitivity at the 578 nm wavelength, where the red-green mechanism sensitivity is null.

Dark adaptation of human rod bipolar cells measured from the b‐wave of the scotopic electroretinogram

To examine the dark adaptation of human rod bipolar cells in vivo, we recorded ganzfeld ERGs to (a) a family of flashes of increasing intensity, (b) dim test flashes presented on a range of background intensities, and (c) dim test flashes presented before, and up to 40 min after, exposure to intense illumination eliciting bleaches from a few per cent to near total. The dim flash ERG was characterized by a prominent b-wave response generated principally by rod bipolar cells. In the presence of background illumination the response reached peak earlier and desensitized according to Weber's Law. Following bleaching exposures, the response was initially greatly desensitized, but thereafter recovered slowly with time. For small bleaches, the desensitization was accompanied by acceleration, in much the same way as for real light. Following a near-total bleach, the response was unrecordable for >10 min, but after approximately 23 min half-maximal sensitivity was reached, and full sensitivity was restored between approximately 35 and 40 min. With smaller bleaches, recovery commenced earlier. We converted the post-bleach measurements of desensitization into 'equivalent background intensities' using a Crawford transformation. Across the range of bleaching levels, the results were described by a prominent 'S2' component (0.24 decades min(-1)) together with a smaller and slower 'S3' component (0.06 decades min(-1)), as is found for dark adaptation of the scotopic visual system. We attribute the S2 component to the presence of unregenerated opsin, and we speculate that the S3 component results from ion channel closure by all-trans retinal.

Calcium-Sensitive Downregulation of the Transduction Chain in Rod Photoreceptors of the Rat Retina

In vertebrate rod outer segments phototransduction is suggested to be modulated by intracellular Ca. We aimed at verifying this hypothesis by recording saturated photosignals in the rat retina after single and double flashes of light and determining the time t c to the beginning of the signal recovery. The time course of Ca i after a flash was calculated from a change of the spatial Ca 2+ concentration profile recorded in the space between the rods. After single flashes t c increased linearly with the logarithm of flash intensity, confirming the assumption that t c is determined by deactivation of a single species X* in the phototransduction cascade. The photoresponse was shortened up to 45% if the test flash was preceded by a conditioning preflash. The shortening depended on the reduction of Ca i induced by the preflash. The data suggest that the phototransduction gain determining the amount of activated X* is regulated by a Ca i-dependent mechanism in a short time period (<800 ms) after the test flash. Lowering of Ca i by a preflash reduced the gain up to 20% compared to its value in a dark-adapted rod. The relation between phototransduction gain and Ca i revealed a K 1/2 value close to the dark level of Ca i.