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Godzilla in the corridor: The Ontario SARS crisis in historical perspective.

Ontario nurses were employed as the front-line workers when SARS descended upon Toronto in March 2003. Once the crisis had subsided, many nurses remarked that SARS had forever altered their chosen profession; employment, which they once viewed as relatively safe, had been transformed into potentially life-threatening. This discussion provides descriptions of these expressions through nurses who experienced the crisis and chose to go on the public record. Secondly, it compares the subjective perceptions of those nurses to those held by nurses who worked through historical epidemics of unknown or contested epidemiology. The historical literature on nursing in yellow fever, cholera and influenza epidemics has been employed to offer insight. The goal is to determine whether the SARS outbreak was a unique experience for nurses or whether similar experiences were shared by nurses in the past? In summary, the reactions of nurses when confronted with the possibility of contracting a deadly disease remain altogether human, not dissimilar in past or present. Nurses' responses to SARS can be usefully studied within a larger historical vision of crisis nursing, and information or impressions from earlier crises are potentially of interest to the nursing profession.

Attitude of Health Personnel↗

Evoked potential estimates of the time course of adaptation and recovery to counterphase gratings.

Scalp-recorded evoked potentials (VEP) were sequentially sampled in humans during adaptation to and recovery from prolonged viewing of counterphase sinusoidal grating targets. The sum of the power at the first and second harmonics of the Fourier-transformed VEP components was found to decrease during adaptation and increase during recovery. Time constants (T) for the adaptation and recovery processes as estimated from exponential functions ranged from 2.9 to 19 sec, varying non-monotonically with the spatial frequency and contrast of the stimulus. The observed T values are shorter than those reported in psychophysical studies of adaptation but overlap estimates derived from single cell studies. An unexpected finding was the occurrence of a 3-6 sec delay in the appearance of the maximum VEP response after the onset of the adaptation stimulus. The delay occurred in all subjects and at all spatial frequencies when moderate to high adapting contrasts (e.g. greater than 0.2) were used. The data support a feature-selective, multi-channel lateral inhibitory model of spatial vision and suggest the presence of tonic inhibition between the channels.

Adaptation, Ocular↗

Driving as night falls: the contribution of retinal flow and visual direction to the control of steering.

We have the ability to locomote at high speeds, and we usually negotiate bends safely, even when visual information is degraded, for example, when driving at night. There are three sources of visual information that could support successful steering. An observer fixating a steering target that is eccentric to the current heading must rotate their gaze. The gaze rotation may be detected by using head and eye movement signals (extra-retinal direction: ERD) or their retinal counterpart, visual direction (VD). The gaze rotation also transforms the global retinal flow (RF) field, which may enable direct steering judgments. In this study, we manipulate VD and RF to determine their contribution toward steering a curved path in the presence of ERD. The results suggest a model that uses a weighted combination of all three information sources, but results also suggest that this weighting may change in reduced visibility, such as in low-light conditions.

Automobile Driving↗

Computerized calculation scheme for toric intraocular lenses.

BACKGROUND AND PURPOSE: While a number of intraocular lens (IOL) power prediction formulae are well established for determination of spherical lenses, no common strategy has been published for the computation of toric IOLs. The purpose of this study is to describe a paraxial computing scheme for tracing an axial pencil of rays through the 'optical system eye' containing astigmatic refractive surfaces with their axes at random. The capabilities of this computing scheme are demonstrated with clinical examples. METHODS: Based on a schematic model eye with spherocylindric surfaces, we use two alternative notations for description of vergences or prescriptions: (1) standard notation (refraction in both cardinal meridians and axis), and (2) component notation (spherical equivalent and cylindric component in 0 degrees and 45 degrees. Refractive surfaces are added to the vergence in component notation, whereas the transformation of the vergence through media is performed in the standard notation for both cardinal meridians. For calculation of the toric lens implant, a pencil of rays is traced through the spectacle and the cornea to the estimated lens position as well as backwards from the retina to the estimated lens position. For calculation of residual spectacle refraction, a pencil of rays is traced backwards from the retina through the toric lens implant and the cornea to the spectacle plane. RESULTS: In example 1 we calculate a 'thin toric lens' for compensation of a corneal astigmatism to achieve a spherical target refraction. In example 2 we compute a 'thick toric lens', which has to compensate for an oblique corneal astigmatism and rotate the spectacle cylinder to the against the rule position to enhance near vision. In example 3 we estimate the residual refraction at the corneal plane after implantation of a thick toric lens, when the cylinder of the lens implant is compensating the corneal cylinder in part and the axis of implantation is not fully aligned with the axis of the corneal astigmatism. CONCLUSION: This novel mathematical concept for computation of toric IOLs or prediction of the refractive outcome with a toric implant in place is a straightforward, computer-based approach, which may substitute for more or less empirical methods of determining toric IOL implants.

Aged↗

Receptors, photoreception and brain perception. New insights.

Once photons have activated photosensitive cell receptors, a biochemical process mediated by G-proteins transforms the initial signal into nerve potentials. The generated impulses transmit the information through ganglion cells, after a complex interaction with other neurons by means of different neurotransmitters. Since visual function is processed in parallel, ganglion cells are divided into M-neurons which are in charge of capturing large objects, P-neurons capable of analyzing fine details and colors, and non-M, non-P neurons which are sensitive to changes in light intensity. Retina, bipolar and ganglion cells share circular receptive fields with an antagonistic surround whereas the lateral geniculate nucleus possesses rectangular receptive fields. Thus, when central cones are stimulated, ON-center cells depolarize, while OFF-center cells hyperpolarize. At the brain cortex, the magnocellular layers lead to orientation and achromatic perception, the parvocellular layers perform color vision in the blobs and achromatic contrast and orientation in the interblobs, and eventually, binocular perception is the result of multiple disparities phenomenon. On these bases, patients with agnosia for form and pattern or for depth and movement have been described. Likewise, color blindness is another disease that could be the result of photoreceptor dysfunctions or brain perception defects.

Animals↗

[The clinical use of steady-state flash visual evoked potentials].

The authors applied 30Hz flash stimuli to record steady-state flash visual evoked potentials (FVEP) and extracted the amplitudes and phases of the first and second harmonics as the characteristic value by discrete Fourier transform (DFT). 46 normal controls (89 eyes) and 78 patients (109 eyes) were detected and analysed by this method. The result showed that the characteristic values of the normal controls are significantly different in three age groups and showed the positive in 35 eyes with the optic never diseases (82.9%), 25 eyes with age-related macular degeneration (92%), and 9 eyes with retinal detachment involved macular (100%). It indicates the method is a sensitive index to evaluate the macular visual function. After preliminary observation, the visual acuities of postoperation were found above 0.4 in cataracts (28 eyes) or opacities (12 eyes) if preoperative FVEP were normal and non-complicated, or less than 0.4 if preoperative FVEP were abnormal. This suggests the method is one of the effective index to detect the postoperative vision of the media opacities. The advantages, disadvantages and the clinical applications were discussed.

Adolescent↗

Learning pop-out detection: specificities to stimulus characteristics.

Training induces dramatic improvement in the performance of pop-out detection. In this study, we examined the specificities of this improvement to stimulus characteristics. We found that learning is specific within basic visual dimensions: orientation, size and position. Accordingly, following training with one set of orientations, rotating target and distractors by 30 deg or more substantially hampers performance. Furthermore, rotation of either target or distractors alone greatly increases threshold. Learning is not transferred to reduced-size stimuli. Position specificity near fixation may be finer than 0.7 deg. On the other hand, learning transfers to the untrained eye, to expanded images, to mirror image transformations and to homologous positions across the midline (near fixation). Thus, learning must occur at a processing level which is early enough to maintain fine separability along basic stimulus dimensions, yet sufficiently high to manifest the described generalizations. We suggest that the site of early perceptual learning is one of the cortical areas which receive input from primary visual cortex, V1, and where top-down attentional control is present.

Adolescent↗

Synaptic mechanisms of a tonic EPSP in crustacean visual interneurons: analysis and simulation.

Light-evoked synaptic responses of identified visual interneurons, sustaining fibers (SFs), were quantitatively analyzed, and a neuronal cable model was used to calculate voltage attenuations and predict synaptic responses. The cable model is based on morphological measurements of SFs filled with Lucifer yellow and passive membrane properties assessed by current injection in the proximal portion of the dendritic arbor. The morphological and electrophysiological measurements were made in different preparations on homologues of the same identified interneurons. The excitatory postsynaptic potential (EPSP) elicited with high-intensity light consists of a transient phase (mean amplitude 33.1 mV) and a second phase (the plateau) that decays slowly relative to the membrane time constant (mean amplitude 24.4 mV). The mean extrapolated reversal potentials are -19.1 mV for the transient and -22.3 mV during the plateau. The change in input conductance associated with the plateau phase of the response showed a peak of 121% above the resting input conductance and decayed to approximately 50% above resting conductance over several seconds. Compartmental cable models (18, 19) were used to calculate voltage attenuations and local synaptic conductances within the SF dendritic tree. The dendrites are electrotonically compact, and voltage attenuations average 6% for current flowing distally from the recording site (injected) and 45% for current flowing proximally to the recording site. The steady-state EPSP is associated with a calculated 80% decrease in the net dendritic membrane resistivity. The synaptic response, calculated for an EPSP distributed throughout the dentritic tree (using this conductance change and the measured steady-state reversal potential) was 27.0 mV, compared with an observed mean value of 24.4 mV. The calculated relationship between steady-state EPSP amplitude and dendritic membrane resistivity (Rs) is a sigmoidal function that resembles the intensity/response function of the SF. We can therefore correctly predict the transformation from light intensity to compound EPSP amplitude by calculating the intervening synaptic membrane resistivity and voltage values. These functions are affected in a predictable manner by the passive membrane resistivity (Rm) and the EPSP reversal potentials. Tetrodotoxin (TTX) application abolished all SF spiking but left the EPSP essentially unchanged, suggesting that the neuronal pathway from photoreceptors to SFs is mainly or entirely comprised of nonspiking (i.e., TTX-insensitive) elements.

Animals↗

Localized effects of spatial frequency adaptation.

The spatially localized threshold-elevation aftereffect of spatial-frequency adaptation was measured by using localized, aperiodic test patterns that have bandpass Fourier transforms. At a given retinal location, the threshold-elevation curves are consistent with the fatigue of size-turned mechanisms with center-surround sensitivity profile. Only a few different sizes of such mechanisms were required to fit the local results. The local aftereffect was also measured as a function of eccentricity near the fovea. The results indicate that the threshold-elevation aftereffect of spatial-frequency adaptation is not spatially homogeneous.

Adaptation, Ocular↗

Horizontal cells of the primate retina: cone specificity without spectral opponency.

The chromatic dimensions of human color vision have a neural basis in the retina. Ganglion cells, the output neurons of the retina, exhibit spectral opponency; they are excited by some wavelengths and inhibited by others. The hypothesis that the opponent circuitry emerges from selective connections between horizontal cell interneurons and cone photoreceptors sensitive to long, middle, and short wavelengths (L-, M-, and S-cones) was tested by physiologically and anatomically characterizing cone connections of horizontal cell mosaics in macaque monkeys. H1 horizontal cells received input only from L- and M-cones, whereas H2 horizontal cells received a strong input from S-cones and a weaker input from L- and M-cones. All cone inputs were the same sign, and both horizontal cell types lacked opponency. Despite cone type selectivity, the horizontal cell cannot be the locus of an opponent transformation in primates, including humans.

Animals↗

The big brown eyes of Samuel Pepys.

Samuel Pepys (1633-1703) is known for writing the finest diary in the English language. He was a man of remarkable accomplishments who transformed the English Navy, was president of the Royal Society, and was a member of the British Parliament. He survived the Great Plague and imprisonment in the Tower of London. During the years when he was writing the diary, Pepys began to experience great pain in his eyes when reading and writing and from photophobia, which caused him to give up writing the diary. Pepys also had an ultimately unjustifiable fear of blindness.

Asthenopia↗

Seeing objects in motion.

This paper reports estimates of the conjoint spatiotemporal tuning functions of the neural mechanisms of the human vision system which detect image motion. The functions were derived from measurements of the minimum contrast necessary to detect the direction of drift of a sinusoidal grating, in the presence of phase-reversed masking gratings of various spatial and temporal frequencies. A mask of similar spatial and temporal frequencies to the test grating reduces sensitivity considerably, whereas one differing greatly in spatial or temporal frequency has little or no effect. The results show that for test gratings drifting at 8 Hz, the tuning function is bandpass in both space and time, peaked at the temporal and spatial frequency (SF) of the test (SFs were 0.1, 1 or 5 c deg-1; c represents cycles throughout). For a grating of 5 c deg-1 drifting at 0.3 Hz, the function is bandpass in space but lowpass in time. Fourier transform of the frequency results yields a function in space-time which we term the 'spatiotemporal receptive field'. For movement detectors (bandpass in space and time) the fields comprise alternating ridges of opposing polarity, elongated in space-time along the preferred velocity axis of the detector. We suggest that this organization explains how detectors analyse form and motion concurrently and accounts, at least in part, for a variety of perceptual phenomena, including summation, reduction of motion smear, metacontrast, stroboscopic motion and spatiotemporal interpolation.

Differential Threshold↗

Comparative study of electrophysiological and psychophysical measurement of the contrast sensitivity function in humans.

We measured the human contrast sensitivity function (CSF) both electrophysiologically with the steady-state visual evoked potential (VEP) and psychophysically using a method of ascending limits. VEP contrast thresholds were determined using a rapid recording technique in which the contrast of a counterphase modulated sine wave grating was swept logarithmically from 0.5 to 40% over a period of 10 s. For this pattern reversal stimulus the amplitude and phase of the second harmonic response as a function of contrast were measured using a discrete Fourier transform (DFT). Psychophysical thresholds were determined on the same trials used to record the VEP. Near threshold the VEP amplitude vs. contrast function was approximately linear and VEP contrast thresholds were estimated by a linear extrapolation to zero amplitude. The contrast thresholds obtained by the two methods correlated at 0.914 for 5 observers, with a mean discrepancy of only 12%. At higher contrasts, the VEP amplitude vs. contrast function often became nonmonotonic, sometimes showing two amplitude peaks.

Electrophysiology↗

[Fluorescence of rhodopsins and its relation to primary processes of light energy transformation].

A new area of the investigation of visual and bacterial rhodopsins--fluorescence spectroscopy of the pigments is discussed. Fluorescence properties are considered in relation to photochemical transformations of the pigments at low temperatures. A number of fluorescent states of the pigments are described. It is shown that the excited states of bacteriorhodopsin and visual rhodopsin are characterized by a series of common features. The analysis of general properties of the pigments excited states allows a conclusion that the singlet excited states take part in the photoreaction. The photoreaction scheme is discussed in which structural changes of the chromophore take place already in the excited state.

Animals↗

Computational problems in the analysis of eye movement signals in the determination of vestibulo-ocular reflex.

The vestibulo-ocular reflex preserves clear vision when the head is moving fast. Measurement of the vestibulo-ocular reflex during the head autorotation test is usually calibrated using a dynamic technique in which the initial part of the eye movement signal and the corresponding part of the stimulating head movement signal are matched. Dynamic techniques can, however, be prone to generate excessively small or large calibration coefficients if a calibration segment of such a signal is corrupted by saccades or artifacts of large amplitudes. We have implemented an effective improvement for computation of difficult signals, not infrequent in clinical data, by limiting influence of distorting parts in the eye movement signal. We also discuss pitfalls of parameter accuracy concerning gain and phase parameters of the vestibulo-ocular reflex that are used to differentiate abnormal results from the normal. Furthermore, we call the ubiquituous Fast Fourier Transform method into question with regard to these signals.

Adult↗

Geometric and induced effects in binocular stereopsis and motion parallax.

This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.

Depth Perception↗

Geometric transformations of pictured space.

In general, a picture can represent a specific environment of scene only when the picture is seen from a unique viewing point. The determination of this unique point and of the distortions that occur when the picture is viewed from other points is crucial to all aspects of pictorial perception. To clarify the effects of the point of observation on pictorial space, the present paper discusses how the correct point may be calculated, provides a geometric analysis of the effects of altering the viewing point, and briefly reviews the effects of such alterations on space perception.

Depth Perception↗

Nonlinear, near photo-realistic caricatures using a parametric facial appearance model.

A mathematical model previously developed for use in computer vision applications is presented as an empirical model for face space. The term appearance space is used to distinguish this from previous models. Appearance space is a linear vector space that is dimensionally optimal, enables us to model and describe any human facial appearance, and possesses characteristics that are plausible for the representation of psychological face space. Randomly sampling from a multivariate distribution for a location in appearance space produces entirely plausible faces, and manipulation of a small set of defining parameters enables the automatic generation of photo-realistic caricatures. The appearance space model leads us to the new concept of nonlinear caricatures, and we show that the accepted linear method for caricature is only a special case of a more general paradigm. Nonlinear methods are also viable, and we present examples of photographic quality caricatures, using a number of different transformation functions. Results of a simple experiment are presented that suggest that nonlinear transformations can accurately capture key aspects of the caricature effect. Finally, we discuss the relationship between appearance space, caricature, and facial distinctiveness. On the basis of our new theoretical framework, we suggest an experimental approach that can yield new evidence for the plausibility of face space and its ability to explain processes of recognition.

Artificial Intelligence↗