Cone-specific mediation of rod sensitivity in trichromatic observers

PURPOSE. The slope of the rod threshold versus the illuminance (TVI) function changes with the wavelength of the background light. This study was conducted to determine whether the changes in slope are due to the stimulation of specific cone classes. METHODS. An eight-channel optical system was used to generate lights that differed in cone and rod photoreceptor illuminance. Rod flicker TVI functions were measured in normal trichromatic observers at mesopic light levels. The independent variables were (1) the relative contribution of the short (S)-and long (L)-wavelength cones to the background light (i.e., the background lights varied along S-only and L-only lines), and (2) the temporal frequency of the flickering lights (4, 7.5, and 15 Hz). RESULTS. The 4-Hz rod flicker TVI function had a slope of 0.87 when measured near W (MacLeod-Boynton chromaticity of 0.66, 1.0). At 4 and 7.5 Hz, an increase in the relative L-cone illuminance steepened the slope of the rod-only TVI curve, but an increase in the relative S-cone illuminance had no effect. The slope of the 7.5-Hz TVI function decreased at higher illuminance levels. At 15 Hz, the thresholds could be measured over only a limited range. CONCLUSIONS. The L-cone system contributes to the desensitization of the rod system at mesopic light levels, whereas, in the range of lights used in these experiments, the S-cone system apparently does not. The possibility that S-cone stimulation desensitizes the response to rod signals at higher levels of S-cone illumination cannot be eliminated. (Invest Ophthalmol Vis Sci. 2002;43:898 -905) T he primate visual system operates over a range of 10 log units. This ability is due in part to the duplex retina in which scotopic (i.e., rod-dominated) vision operates at low light levels and photopic (i.e., cone-dominated) vision operates at high light levels. In several early studies, researchers proposed that these two systems behave independently of each other under many conditions, 1-4 but there is now clear evidence of the rods' influence on the cone systems and the cones' influence on the rod system. Visual signals originating in the rod photoreceptors do not have their own pathway to the brain but instead combine with neural signals originating in the cone photoreceptors. Signals originating with the rod photoreceptors are transmitted to the retinal ganglion cells through at least two anatomic pathways. One pathway combines through second-order cells. Rod photoreceptors connect to rod bipolar cells, which in turn connect to rod (AII) amacrine cells. The rod amacrine cells have gap junction connections with on-center ganglion cells in sublamina b of the inner plexiform layer, and have inhibitory synapses with off-center ganglion cells in sublamina a. Rod signals may also enter the cone circuit through gap junctions between rod spherules and cone pedicles (see Refs. 5-7). There is also recent evidence in rodents of a third pathway connecting the rod photoreceptors directly to OFF cone bipolar cells. 8,9 The general perceptual consequences of interaction between rods and cones have been documented extensively. We know, for instance, that the rod photoreceptor system influences cone-mediated sensitivity 10 -13 and vice versa 14 -18 ; that interaction between the rod and cone systems is more evident with flashed lights than with steady lights 19 ; and that location, spatial extent, and temporal frequency play an important role in determining the magnitude of rod and cone interaction. 17,20 -24 Rod-cone interaction (how rods influence cones) and cone-rod interaction (how cones influence rods) have become umbrella terms that characterize many classes of visual processing. One historical difficulty with experiments that investigate rod-cone (and cone-rod) interaction is that the narrowbandwidth lights (i.e., lights of a few spectral wavelengths) used as experimental stimuli often stimulate more than one class of photoreceptor. These experiments therefore do not lend themselves as easily to physiological interpretation. Many previous researchers have addressed such topics by measuring rod sensitivity to lights to which the rod system is much more sensitive than the cone systems (e.g., Ref. 25) or by investigating the responses of monochromatic and dichromatic observers. 26 -28 To investigate questions concerned with cone-rod interaction, I used an approach based on the cone-rod photoreceptor space defined by Shapiro et al. For this article, I examined rod TVI functions for 4-Hz flickering lights. Aguilar and Stiles 25 measured a rod TVI function by optimizing experimental parameters to isolate the rod system. One of these optimizations was to desensitize the cone systems with a long-wavelength adaptation light. They found that the slope of a major portion of the curve (i.e., when the adaptation light is between Ϫ2 and 2.2 log scotopic trolands [td]) is approximately 1.0. However, Sharpe et al

Cone-specific mediation of rod sensitivity in trichromatic observers

PURPOSE. The slope of the rod threshold versus the illuminance (TVI) function changes with the wavelength of the background light. This study was conducted to determine whether the changes in slope are due to the stimulation of specific cone classes. METHODS. An eight-channel optical system was used to generate lights that differed in cone and rod photoreceptor illuminance. Rod flicker TVI functions were measured in normal trichromatic observers at mesopic light levels. The independent variables were (1) the relative contribution of the short (S)-and long (L)-wavelength cones to the background light (i.e., the background lights varied along S-only and L-only lines), and (2) the temporal frequency of the flickering lights (4, 7.5, and 15 Hz). RESULTS. The 4-Hz rod flicker TVI function had a slope of 0.87 when measured near W (MacLeod-Boynton chromaticity of 0.66, 1.0). At 4 and 7.5 Hz, an increase in the relative L-cone illuminance steepened the slope of the rod-only TVI curve, but an increase in the relative S-cone illuminance had no effect. The slope of the 7.5-Hz TVI function decreased at higher illuminance levels. At 15 Hz, the thresholds could be measured over only a limited range. CONCLUSIONS. The L-cone system contributes to the desensitization of the rod system at mesopic light levels, whereas, in the range of lights used in these experiments, the S-cone system apparently does not. The possibility that S-cone stimulation desensitizes the response to rod signals at higher levels of S-cone illumination cannot be eliminated. (Invest Ophthalmol Vis Sci. 2002;43:898 -905) T he primate visual system operates over a range of 10 log units. This ability is due in part to the duplex retina in which scotopic (i.e., rod-dominated) vision operates at low light levels and photopic (i.e., cone-dominated) vision operates at high light levels. In several early studies, researchers proposed that these two systems behave independently of each other under many conditions, 1-4 but there is now clear evidence of the rods' influence on the cone systems and the cones' influence on the rod system. Visual signals originating in the rod photoreceptors do not have their own pathway to the brain but instead combine with neural signals originating in the cone photoreceptors. Signals originating with the rod photoreceptors are transmitted to the retinal ganglion cells through at least two anatomic pathways. One pathway combines through second-order cells. Rod photoreceptors connect to rod bipolar cells, which in turn connect to rod (AII) amacrine cells. The rod amacrine cells have gap junction connections with on-center ganglion cells in sublamina b of the inner plexiform layer, and have inhibitory synapses with off-center ganglion cells in sublamina a. Rod signals may also enter the cone circuit through gap junctions between rod spherules and cone pedicles (see Refs. 5-7). There is also recent evidence in rodents of a third pathway connecting the rod photoreceptors directly to OFF cone bipolar cells. 8,9 The general perceptual consequences of interaction between rods and cones have been documented extensively. We know, for instance, that the rod photoreceptor system influences cone-mediated sensitivity 10 -13 and vice versa 14 -18 ; that interaction between the rod and cone systems is more evident with flashed lights than with steady lights 19 ; and that location, spatial extent, and temporal frequency play an important role in determining the magnitude of rod and cone interaction. 17,20 -24 Rod-cone interaction (how rods influence cones) and cone-rod interaction (how cones influence rods) have become umbrella terms that characterize many classes of visual processing. One historical difficulty with experiments that investigate rod-cone (and cone-rod) interaction is that the narrowbandwidth lights (i.e., lights of a few spectral wavelengths) used as experimental stimuli often stimulate more than one class of photoreceptor. These experiments therefore do not lend themselves as easily to physiological interpretation. Many previous researchers have addressed such topics by measuring rod sensitivity to lights to which the rod system is much more sensitive than the cone systems (e.g., Ref. 25) or by investigating the responses of monochromatic and dichromatic observers. 26 -28 To investigate questions concerned with cone-rod interaction, I used an approach based on the cone-rod photoreceptor space defined by Shapiro et al. For this article, I examined rod TVI functions for 4-Hz flickering lights. Aguilar and Stiles 25 measured a rod TVI function by optimizing experimental parameters to isolate the rod system. One of these optimizations was to desensitize the cone systems with a long-wavelength adaptation light. They found that the slope of a major portion of the curve (i.e., when the adaptation light is between Ϫ2 and 2.2 log scotopic trolands [td]) is approximately 1.0. However, Sharpe et al

The separation of monocular and binocular contrast

AbstractThe contrast asynchrony is a stimulus configuration that illustrates the visual system’s separable responses to luminance and luminance contrast information (Shapiro, 2008; Shapiro et al., 2004). When two disks, whose luminances modulate in phase with each other, are each surrounded by a disk, one light and one dark, observers can see both the in-phase brightness signals and the antiphase contrast signals and can separate the two. Here we present the results of experiments in which observers viewed a similar stimulus dichoptically. We report that no asynchrony is perceived when one eye is presented with modulating disks and the other eye is presented with the black and white surround rings, nor is an asynchrony perceived in gradient versions of the contrast asynchrony. We also explore the “window shade illusion” (Shapiro, Charles, & Shear-Heyman, 2005) dichoptically and find that when a modulating disk is presented to one eye and a horizontally split black/white annulus is presented to the other, observers perceive a “shading” motion up and down the disk. This shading can be seen in either direction in the binocular condition, but it is almost always seen as moving towards low contrast in the monocular condition. These findings indicate the presence of separable retinal and cortical networks for contrast processing at different temporal and spatial scales

Exploration and exploitation in the presence of network externalities

This paper examines the conditions under which exploration of a new, incompatible technologyis conducive to firm growth in the presence of network externalities. In particular, this studyis motivated bythe divergent evolutions of the PC and the workstation markets in response to a new technology: reduced instruction set computing (RISC). In the PC market, Intel has developed new microprocessors bymaintaining compatibilitywith the established architecture, whereas it was radicallyr eplaced byRISC in the workstation market. History indicates that unlike the PC market, the workstation market consisted of a large number of power users, who are less sensitive to compatibilitythan ordinaryusers. Our numerical analysis indicates that the exploration of a new, incompatible technologyis more likelyto increase the chance of firm growth when there are a substantial number of power users or when a new technologyis introduced before an established technologytakes off. (; ; ;

Entanglement between Demand and Supply in Markets with Bandwagon Goods

Whenever customers' choices (e.g. to buy or not a given good) depend on others choices (cases coined 'positive externalities' or 'bandwagon effect' in the economic literature), the demand may be multiply valued: for a same posted price, there is either a small number of buyers, or a large one -- in which case one says that the customers coordinate. This leads to a dilemma for the seller: should he sell at a high price, targeting a small number of buyers, or at low price targeting a large number of buyers? In this paper we show that the interaction between demand and supply is even more complex than expected, leading to what we call the curse of coordination: the pricing strategy for the seller which aimed at maximizing his profit corresponds to posting a price which, not only assumes that the customers will coordinate, but also lies very near the critical price value at which such high demand no more exists. This is obtained by the detailed mathematical analysis of a particular model formally related to the Random Field Ising Model and to a model introduced in social sciences by T C Schelling in the 70's.Comment: Updated version, accepted for publication, Journal of Statistical Physics, online Dec 201

Global Jacquet-Langlands correspondence, multiplicity one and classification of automorphic representations

In this paper we show a local Jacquet-Langlands correspondence for all unitary irreducible representations. We prove the global Jacquet-Langlands correspondence in characteristic zero. As consequences we obtain the multiplicity one and strong multiplicity one theorems for inner forms of GL(n) as well as a classification of the residual spectrum and automorphic representations in analogy with results proved by Moeglin-Waldspurger and Jacquet-Shalika for GL(n).Comment: 49 pages; Appendix by N. Grba

Dynamical HII Region Evolution in Turbulent Molecular Clouds

We present numerical radiation-hydrodynamic simulations of the evolution of HII regions formed in an inhomogeneous medium resulting from turbulence simulations. We find that the filamentary structure of the underlying density distribution produces a highly irregular shape for the ionized region, in which the ionization front escapes to large distances in some directions within 80,000 years. In other directions, on the other hand, neutral gas in the form of dense globules persists within 1 parsec of the central star for the full duration of our simulation (400,000 years). Divergent photoablation flows from these globules maintain a root-mean-squared velocity in the ionized gas that is close to the ionized sound speed. Simulated images in optical emission lines show morphologies that are in strikingly detailed agreement with those observed in real HII regions.Comment: Minor changes to sync with accepted version. 7 pages, ApJ in press. Accompanying video available at http://ifront.org/wiki/Turbulent_Hii_Regions/Paper

A First- and Second-Order Motion Energy Analysis of Peripheral Motion Illusions Leads to Further Evidence of “Feature Blur” in Peripheral Vision

Anatomical and physiological differences between the central and peripheral visual systems are well documented. Recent findings have suggested that vision in the periphery is not just a scaled version of foveal vision, but rather is relatively poor at representing spatial and temporal phase and other visual features. Shapiro, Lu, Huang, Knight, and Ennis (2010) have recently examined a motion stimulus (the “curveball illusion”) in which the shift from foveal to peripheral viewing results in a dramatic spatial/temporal discontinuity. Here, we apply a similar analysis to a range of other spatial/temporal configurations that create perceptual conflict between foveal and peripheral vision.To elucidate how the differences between foveal and peripheral vision affect super-threshold vision, we created a series of complex visual displays that contain opposing sources of motion information. The displays (referred to as the peripheral escalator illusion, peripheral acceleration and deceleration illusions, rotating reversals illusion, and disappearing squares illusion) create dramatically different perceptions when viewed foveally versus peripherally. We compute the first-order and second-order directional motion energy available in the displays using a three-dimensional Fourier analysis in the (x, y, t) space. The peripheral escalator, acceleration and deceleration illusions and rotating reversals illusion all show a similar trend: in the fovea, the first-order motion energy and second-order motion energy can be perceptually separated from each other; in the periphery, the perception seems to correspond to a combination of the multiple sources of motion information. The disappearing squares illusion shows that the ability to assemble the features of Kanisza squares becomes slower in the periphery.The results lead us to hypothesize “feature blur” in the periphery (i.e., the peripheral visual system combines features that the foveal visual system can separate). Feature blur is of general importance because humans are frequently bringing the information in the periphery to the fovea and vice versa

Al Qaeda at the bar: coordinating ideologues and mercenaries in terrorist organizations

Most terrorist groups have limited lifespans. A number of scholars and casual observers have noted that terrorist organizations often are comprised of two types of participants: ideologues or "true believers" dedicated to the group's cause, and mercenaries, who are adept at raising money through illegal means. The latter are interested primarily in their personal gains and have relatively little ideological commitment. Terrorist groups need both participants in order to function effectively. The purpose of the study is to understand the impact of communication on the compositions of terrorist groups. Three experimental treatments consider a coordination problem, and focus on the behavior of the mercenaries. Participants choose whether or not to participate in a terrorist attack. Payoffs are U-shaped in the number of participants, and increase with the number of successful attacks. The treatments allow communication between a leader and frontline fighters ("leader" treatment) or among the frontline fighters themselves ("communication" treatment). In the first treatment, a group leader can post messages to the members, which has a 19 % coordination success rate. For the communication treatment, all participants can post messages anonymously to each other, which yields a 27 % coordination success rate. By contrast, the baseline ("no communication" treatment) shows a success rate of 11 %. We conclude from our experimental evidence that disrupting communications among the frontline fighters is more effective in terminating terrorist organizations