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Kenneth J.W. Craik (1914–1945), while discussing how the human visual system recognized objects, wrote: Now in mathematics it is legitimate to seek transformations through which certain quantities (such as the physical laws of nature and the velocity of light in relativity theory) remain invariant. In fact, the action of various physical devices which “recognize” or respond identically to certain simple objects can be treated in terms of such transformations. Thus the essential part of physical “recognizing” instruments is usually a filter—whether it be a mechanical sieve, an optical filter, or a tuned electrical circuit —which “passes” only quantities of the kind it is required to identify and rejects all others. Mathematically, the situation here is that, in a perfect filter, the transformation leaves the desired quantity unaltered, but reduces all others to zero.

The observations of the spectrum made by Newton, Young, Wollaston and Helmholte are approximated and accounted for. Increasing the number of delineations allows progressively more bands differing in colour to be perceived, in addition to the three blocks of colour seen in the undelineated spectrum. The rate at which further delineation permits more colours to be observed decreases, however, so that up to 30 colours can be perceived in the subdivided spectrum. The wavelength discrimination measurements agree well with previous data. Enhanced colour discrimination is shown to require luminance contrast transients containing only the first few Fourier harmonics.

Same day admissions for surgery represent a growing portion of the operating room workload. Vigilance during preanesthetic evaluation and standards of preoperative preparation must be maintained for patients who are undergoing elective procedures. This case study reports the preanesthetic evaluation and perioperative management of a Jehovah's Witness who bled to a hematocrit of 9.0% during a myomectomy. The discussion considers the need for preoperative preparation and conservation of circulating red blood cells. Perioperative management modalities to conserve red blood cells including deliberate, controlled hypotension; autotransfusion; and normovolemic and hypervolemic hemodilution, are presented.

The human visual system's response to luminance discontinuities on continuous image gradients is measured. It is argued that since quantization artifacts often appear on smooth gradients, the versatility of sinewave gratings in controlling gradient properties can be used to quantify worst-case situations. The threshold spatial dimensions of discrete steps, such as would appear in a staircase approximation to the grating, are determined under a variety of experimental conditions. The smallest threshold step size defines a critical sample size, which is used to calculate limits for artifact-free reconstruction of sinusoidal luminance gratings. Rather than employ ad hoc collections of test images, the methods and results described suggest a more general approach to achieving visually optimal allocation of imaging resources.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

During three years of drilling and sampling in the San Juan Basin, 524 coal core samples were collected from 149 drill sites on approximate 2 mi centers in the Fruitland, Menefee, Crevasse Canyon, and Moreno Hill Formations. Most of these drill sites were placed along the lines parallel to the Late Cretaceous shorelines in areas where thickest coals are at depths of about 200 ft. Eight drill sites in the Chacra Mesa field were drilled on a line perpendicular to the shorelines on 1 mi centers; the Moreno Hill Formation sites in the Salt Lake field and the Fruitland Formation in the Fruitland field were drilled on trends perpendicular to the Late Cretaceous shorelines.The Fruitland Formation coals were from the Fruitland, Bisti, and Star Lake fields; the upper coal member of the Menefee Formation coals were from the Chacra Mesa and La Ventana fields; the Cleary Coal Member of the Menefee Formation coals were from the La Ventana, Chacra Mesa, San Mateo, and Standing Rock fields; and the remaining Menefee coals analyzed were from the Moreno field. Cleary-Gibson, Gibson, and Dilco Coal Member coals of the Crevasse Canyon Formation were cored and analyzed from the Gallup and Crownpoint fields, and coals from the Moreno Hill Formation were collected and analyzed from the Salt Lake field.Significant aspects of chemistry, quality, coal rank, thickness trends, stratigraphic-depositional features, and petrographic composition are presented in this report. Petrographic analyses indicate a difference in maceral content between the Fruitland coals and the other older formations sampled. The Fruitland coals have a higher percentage of liptinite, in particular the resinite constituent of the liptinite group is much higher than for the Menefee and Crevasse Canyon coals. In general, the Fruitland coals have a higher hydrogen/volatile matter ratio which is in part influenced by the resinite content. The Moreno Hill Formation coals in the Salt Lake field have the highest inertinite content of all the formations sampled, indicating oxidation during coal development.The Fruitland coals tend to be the most economically viable because of their thickness, relative continuity, and overall quality, albeit having a high ash yield. The Menefee Formation coals do not have the same degree of bed continuity or thickness as the Fruitland coals, but the ash yield is lower and the rank is equivalent to that of the Fruitland coals in the Bisti and Star Lake fields. Of the Menefee Formation coal fields investigated, the San Mateo field has the best economic potential because of the thickness of the coals and the low sulfur and ash. The Cleary-Gibson coals in the Gallup area have the greatest economic potential of the Crevasse Canyon fields examined. This coal-bearing sequence has both multiple coal beds and relatively thick coals. The Cleary-Gibson coals also have low ash and sulfur, which enhance their economic potential. Of the remaining fields drilled, the upper coal member Menefee coals in the La Ventana field and the Crevasse Canyon coals in parts of the Crownpoint field may be economic. The La Ventana upper member coals are relatively thick and have a low ash yield. In the Crownpoint field the coals are thickest in the area northwest of the town of Crownpoint and near Borrego Pass.

Past measurements of precorneal tear film thickness in rabbits and humans gave values of 4-7 microns. These have been the accepted values. However, measurements presented in this report are much higher. Earlier techniques may have changed film thickness or not have fully included the layer containing mucus. Little is known about the mucus component. Thickness was measured in freshly killed animals of 10 species using two independent optical techniques. Coherent light was reflected from eyes and thickness was determined from spacing of interference fringes. Clear images of tissue structure were obtained with confocal microscopy, the tear film was visually identified, and thickness measured. Measurements by the two methods were closely correlated in each animal. No film was detected in trout or carp. In other species, thickness ranged from 9 microns in frogs to 15 microns in gerbils. Values are significantly larger than earlier estimates. Film thickness in three living rabbits was not significantly different from that measured shortly after death. There was no variation in thickness at five positions across the cornea in three species. Mucus content of rat tear film was examined by measuring thinning after application of 4, 8, and 20% (weight/volume) acetylcysteine, a mucolytic agent. Thickness was considerably reduced compared to controls, and the film almost completely removed by the highest concentration. The authors propose that the film is largely composed of mucus, not free aqueous solution. Our methods include such layers, and measurements are correspondingly larger than earlier ones.

The question of how we can be unaware of the deficit in our (monocular) visual field, equivalent in size to 76 full moons, is examined. A new method of investigating the response to moving images on and near the blind spot has been found. The image of a computer-generated line, which is made to lengthen with time and pass over the blind spot, is seen as shorter than that of a similar, parallel line which passes outside the blind spot. This perceived difference in length corresponds to the actual width of the blind spot. Our unawareness of blind spots and scotomata is often described as involving some form of 'filling in' process. The rapid variation in cortical receptive field size, recently found to occur in response to stabilized images, may provide a general filling-in mechanism for the removal from perception of otherwise-distracting, stabilizing image regions (such as the shadows of blood vessels and clinical scotomata).

These experiments examined the extent to which low spatial frequencies are processed independently. The assessment was carried out with respect to both detection and discrimination performance. For simple sinusoidal gratings, pairs of stimuli could be discriminated when their contrasts reached threshold, if the ratio of their spatial frequencies was 3:1 or larger, suggesting idependent processing in separate channels. For smaller frequency ratios, slightly more contrast was required for discrimination than for detection, suggesting that stimuli were not processed by entirely separate channels. The detection and discrimination thresholds of complex grating stimuli fell within the ranges which would be expected if probability summation effects and summation of different closely spaced harmonic frequencies within single channels are considered, supporting the hypothesis of independent processing of low-spatial frequency information. The single exception to this involved discrimination of a square wave and a square wave with its fundamental component removed. In this case, discrimination required considerably more contrast than detection, even when factors of probability summation and within-channel summation of harmonics are considered.

In this investigation contrast threshold measurements are compared with supra-threshold perception for a group of human amblyopes. The results indicate that human amblyopia involves, in some cases, not only loss of sensitivity but spatial distortion. Thus a new group of amblyopes can now be identified in which only distortion occurs. These results have important physiological implications for both the normal and abnormal visual systems. Neurophysiologists investigating visual loss from deprivation should assess whether similar distortions occur in animals. This question may hold the answer to whether the present animal models are relevant to the human condition.

Understanding of the processes and mechanisms of vision has advanced enormously in recent years. Despite this, there are only tenuous links between current lighting design practice and visual science. This is an unsatisfactory state of affairs since, at face value, lighting design should seek to optimise the performance of the visual system. This paper describes an experiment investigating changes in response time of subjects detecting movement of a pointer, with decreasing adaptation luminance. A physiological explanation of the results is suggested and the implications for lighting design are discussed.

Visual numerosity judgements were made for tachistoscopically presented linear arrays of dots or lines. The interelement interval (which could be specified in spatial frequency terms) was constant for a given array but varied across conditions. A clear limit in the accuracy of numerosity judgements was found to be set at 4 for regularly spaced elements with spatial frequencies below approximately 10 cycles/deg (element and interelement interval of 0·05 deg). This limit in terms of accuracy is accompanied by a fast and almost constant response time for arrays of 4 or less, compared to response times for arrays of more than 4 elements. The limit in accuracy falls to 2 elements rather than 4 for spacing narrower than 0·05 deg although with such spacing the elements are still easily resolved. The limit of 4 is found if the stimulus is a bright afterimage, lasting for approximately 60 s. This result suggests that the limit is independent of the time allowed for a single fixation and is a perceptual limit rather than a limit in some memory buffer. ‘Numerosity’ units are proposed to account for the results.

The publicity which has been given to Visual Display Terminals (VDTs) in recent years has been prompted by initial complaints from operators of visual and postural discomfort. The interest resulting from these complaints has led to much work being carried out on what appeared to be a potentially large problem. This work has precipitated recommendations and specifications from many quarters which in many cases are overcritical and rigid and do not take into account the great flexibility of the visual system and the need for mobility to maintain postural comfort. In this paper the VDT issue is considered with reference to: the known physiology of the visual and postural mechanisms; the psychological factors such as fatigue, boredom, stress and performance of operators; and the clinical aspects of ocular comfort.

1. The appearance of square gratings with some of their Fourier components missing has been investigated for both threshold and suprathreshold contrasts. 2. If high frequency components are removed from a square grating there is only a very small effect on the detection threshold, or suprathreshold appearance, unless the components are visible by themselves. 3. If the fundamental frequency is removed from a square-wave grating which has a spatial frequency lower than 1 cycle per degree (c/d) the contrast sensitivity is not altered. This is a generalisation of the Craik-Cornsweet illusion. If the contrast is raised above the detection threshold the grating is indistinguishable from a square grating, unless the contrast is high enough to see the fundamental when it is presented alone. 4. If the fundamental is removed from a square grating which has a spatial frequency higher than 1 c/d the contrast threshold and the appearance at all contrasts are changed. At threshold it appears as a sinusoidal grating of three times the fundamental frequency. The threshold is dictated solely by the amplitude of the third harmonic. If the contrast is further raised, so that the fifth harmonic also reaches threshold, the periodictiy of the fundamental is seen. 5. Therefore, gratings of many different luminance profiles (including the Craik-Cornsweet profile) all produce the perception of a square grating simply because those missing components which would be required in each case to produce a perfect square are by themselves undetectable. The visual system responds as though hardwired to detect square gratings and edges by means of quasi-Fourier analysis. 6. These results are analagous to the missing fundamental, or residue, effect in hearing.

How important are eye movements to visual pattern analysis? Previous findings indicate that at least one visual task (counting) is seriously impaired without them. We asked whether a comparable limitation applies to pattern recognition. Subjects were presented with pairs of randomly generated arrays composed of black and white pixels. The subjects indicated whether the arrays were identical or differed by one pixel. In one experiment, they were instructed to use normal eye movements, in another they were required to fixate on a point between the arrays. When eye movements were permitted, subjects' performance showed evidence for a search in which the discrepant pixel was eventually found, given adequate inspection time. When fixation was required, search was less efficient and the discrepant pixel was sometimes not found, despite prolonged inspection time. These results were independent of target size over a wide range. Our findings indicate that eye movements play a crucial role in pattern analysis that is not related to resolution.

Frequency discrimination may represent one measure of the co-operative activity of spatial channels. In amblyopia, frequency discrimination is abnormal for higher spatial frequencies when temporal factors are restricted and subjective contrast cues eliminated. This result suggests, when considered in the light of the findings of Parts I and II, that in amblyopia, the co-operative activity of size selective channels is anomalous.

Fresnel prisms are often used in orthoptics for binocular anomalies and in low vision for visual field defects. These prisms are made of optical polyvinyl chloride (PVC) and this material increases chromatic dispersion and produces a loss of contrast. In this study, the effect of chromatic dispersion on contrast sensitivity is determined. Contrast thresholds over a wide range of spatial frequencies are obtained on a number of subjects with the use of Fresnel prisms ranging from 5 to 30 prism dioptres. As predicted there is a decrease in visual acuity and cut-off spatial frequency threshold as prism power increases. Similarly, a decrease in contrast sensitivity at high and medium spatial frequencies is recorded.

The cause of the low-frequency roll-off in the contrast sensitivity function of human vision was investigated. Evidence was found that low-frequency structure in the visual world is perceived not by harmonic analysis but by detection of contrast gradients. This hypothesis is supported by the thresholds of trapezoidal gratings, i.e. gratings which have regions of uniform luminance alternating with linear luminance ramps. Doubling the ramp slope halves the threshold at ramp widths of 0.5° and greater. Trapezoids with ramp widths less than 0.5° all have the same threshold which is equal to a square grating threshold. The visual world therefore is analyzed by two different mechanisms. Above the peak of the contrast sensitivity function harmonic analysis takes place and below it, contrast gradient analysis.

1. A simple but accurate method of measuring the depth of field of the eye is described. 2. At a fixed pupil size the hyperfocal distance was found to increase directly with the log of the background luminance. 3. If the retinal illumination is held constant and the pupil size is altered, the depth of field varies approximately inversely with the pupil diameter. With pupil diameters greater than 2.5 mm the observed deviation from theoretical expectation can be accounted for by the operation of the retinal direction effect of Stiles and Crawford. 4. Further evidence that the retinal direction effect modifies depth of field has been obtained by measurement with fields of different colour but similar luminance. 5. The correction of chromatic aberration in the eye by means of an achromatizing lens decreases depth of field. 6. The minimum estimate for depth of field obtained under optimum conditions was ± 0.3 D at a pupil diameter of 3 mm. 1. Description d'une methode simple mais exacte permettant de measurer ...

1. Optical quality of the eye was measured at eight pupil sizes between 1.5 and 6.6 mm diameter by recording the faint light emerging from the eye; this light was reflected from the bright image of a thin line on the fundus.2. The nature of the fundus reflexion was examined; it was found that the fundus acts very much like a perfect diffuser while retaining polarization.3. Using the result that the fundus acts like a diffuser, the recorded line images were Fourier analysed to provide modulation transfer functions. These functions indicate an optical quality considerably higher than that found in previous physical studies.4. Linespread profiles were then derived from the modulation transfer functions. These profiles are 40% narrower than those of previous physical studies for a 3.0 mm pupil. The narrowest profile occurred with a 2.4 mm pupil.5. Our results demonstrate that physical and psychophysical studies can yield similar estimates of optical quality. The influence of optical factors not common to both techniques is discussed. Evidence for the existence of neural ;image sharpening' mechanisms is reviewed.

1. Visual resolving power is known to be poorer for objects oriented obliquely as compared with horizontal and vertical orientations. Experiments were designed to evaluate the optical and neurophysiological factors involved. 2. Gratings with a sinusoidal light distribution were generated on the face of an oscilloscope. Spatial frequency and contrast could be varied while keeping the mean luminance of the grating constant. 3. Using a homatropinized eye with an artificial pupil and carefully corrected refraction, high resolution in the vertical and horizontal meridians as compared with the oblique meridians was found for gratings ranging in spatial frequency from 1 to 35 c/deg. 4. It is concluded from the similar behaviour of low and high frequency gratings that neither focus errors nor optical aniseikonia can account for these findings. 5. Additional proof that optical factors cannot significantly account for these preferred directions of resolution was obtained by forming interference fringes directly on the retina using a neon—helium laser as a coherent light source. 6. Similar orientational changes in resolution were found by by-passing the dioptrics with interference fringes. It is concluded that the effect is due to some orientational inequality in the visual nervous system.

1. The contrast thresholds of a variety of grating patterns have been measured over a wide range of spatial frequencies.2. Contrast thresholds for the detection of gratings whose luminance profiles are sine, square, rectangular or saw-tooth waves can be simply related using Fourier theory.3. Over a wide range of spatial frequencies the contrast threshold of a grating is determined only by the amplitude of the fundamental Fourier component of its wave form.4. Gratings of complex wave form cannot be distinguished from sine-wave gratings until their contrast has been raised to a level at which the higher harmonic components reach their independent threshold.5. These findings can be explained by the existence within the nervous system of linearly operating independent mechanisms selectively sensitive to limited ranges of spatial frequencies.

The effect of change in spatial frequency on the alternation rate of two crossed gratings was measured. The rate was found to decrease with increase in spatial frequency, but to change only little with contrast. Low alternation rate was observed for crossed square-wave gratings compared to crossed sine-wave gratings; here the rate of rivalry is largely dependent upon the presence or absence of the first three harmonic components rather than the higher harmonics which contribute to the sharp edges of the square wave. The results are compared with those for some ambiguous figures.

1. Grating patterns were used to obtain a quantitative description of cells in the visual cortex of the cat whose response amplitude depended critically upon the orientation of the moving grating. 2. In all such cells the impulse frequency was found to decrease linearly with angle on either side of an optimum angle (the preferred angle) until the response fell to zero or to a base frequency. The angular rate of change of response varied between cells and was expressed as the half-width at half amplitude (the angular selectivity). 3. The angular selectivity of thirty-five cells was determined and more than half (nineteen) of these fell within the range 14-26°. 4. Fourteen cells responded optimally only when the grating was moved in one direction. Twenty-one cells responded optimally to two directions of movement 180° apart, but the response in the two directions was not always equal. 5. No significant correlation was found between the response amplitude at the optimum angle and the angular selectivity. 6. The distribution of preferred angles did not show any difference between the oblique orientations and the vertical and horizontal orientations. 7. These results are compared with a previous psychophysical estimate of angular selectivity.

1. Micro-electrode recordings have been made from single units in the visual cortex of the cat, during stimulation by moving grating patterns generated on a cathode ray tube. 2. The responses of the units have been measured in terms of either the frequency of impulses, or the contrast sensitivity, and expressed as a function of the spatial frequency of the grating pattern. 3. The amplitude of the responses recorded from cortical cell bodies was dependent upon the orientation of the moving grating, and for these units the stimulus was always presented at the preferred orientation. The response amplitude of other units did not depend upon the orientation of the grating stimulus, and these are believed to be the terminations of geniculate fibres. 4. The high spatial frequency end of the response function measured in terms of the contrast sensitivity could be well fitted by an exponential function. Subtraction of the blank count from the impulse frequency data gave a curve fitted by the same exponential function. The low frequency end was less consistent. 5. The spatial frequency at which this exponential function had fallen by one log. unit was taken to specify the position of the unit's response in the spatial frequency spectrum. For all units these values cover a range of four octaves of spatial frequency (from 0·18 to 3·8 cycles per degree). 6. It is suggested that these neurophysiological results support psychophysical evidence for the existence in the visual system of channels, each selectively sensitive to a narrow band of spatial frequencies.

La performance visuelle des batteurs de cricket est simulee sous un eclairage ambiant variable. Lorsque l'eclairage baisse jusqu'au niveau auquel on arrete habituellement de jouer, une augmentation rapide du temps de reaction apparait. C'est egalement le niveau auquel les lumieres des bureaux et des voitures sont allumees: c'est la sensation «d'obscurite»

The reaction-time for movement was found to be around 150 ms and was compared with the time required to take action while two moving lines are on a collision course. Subjects were trained to either avoid or maximize collision. They did so accurately. The action-time was about one-third the reaction-time.

If two gratings with a sinusoidal luminance profile are crossed and if each has a different colour then they are seen to alternate; first one dominates and then the other. Part of the time both are seen. The rate of this monocular rivalry varies with the angle between the two gratings. If the two are lined up in parallel the percept is quite stable and remains so until their orientation is about 15° to 20° apart. The rate of alternation then increases rapidly reaching a maximum rate of 30 per minute. The orientational selectivity of the rivalry suggests that the orientational properties of the neurones in the visual cortex discovered by Hubel and Wiesel are involved. Acknowledgements. We wish to thank Professor Peter M. Milner for his keen interest in these experiments and for acting as a subject. F.W.C. is supported by a grant from the Medical Research Council. The work of A.S.G. during her visit to Cambridge was supported in part by a research grant GB-31029 from the National Science Foundation. E.R.H. was awarded a Wellcome Trust Fellowship while in Cambridge.

DENTON1 has suggested that a possible function of the pupil light reflex might be to provide apertures optimum for visual acuity at different light intensities. At low levels of luminance visual acuity is probably limited by the rate at which light quanta activate the retinal receptors2, and thus a large pupil admitting the maximum amount of light would appear to be advantageous. As the luminance is increased into the photopic range, loss of image contrast due to optical aberrations, diffraction, and stray light also tend to limit visual acuity, although the fineness of the retinal cone mosaic may ultimately become the limiting factor. The decrease in pupil size which normally accompanies increase of illumination should reduce to some extent the deleterious effects on visual acuity of the optical aberrations. However, it has never been shown that the actual response of the pupil reflex to a given steady luminance does in fact result in an aperture giving maximum spatial resolution at this luminance. It was therefore decided to determine experimentally whether an optimum aperture existed at each luminance-level, by measuring the relations between pupil size, luminance, and visual acuity over a wide range.

Previous work has shown that anomalies in the contrast sensitivity functions of amblyopes are often insufficient to explain the degree of visual deficit in more complex tasks. Our stimuli were compound gratings composed of a fundamental and its third harmonic, added in either square-wave or triangle-wave phase. At medium to high spatial frequencies we find that many amblyopes, unlike normal observers, are unable to distinguish between such gratings which have identical power spectra, but different phase spectra. In this frequency range they can, however, easily distinguish between a compound grating and its fundamental component alone. It seems that in amblyopia visual processing occurs over a more truncated frequency range than is implied by detection experiments. Various explanations of this observation are considered.

The displacement threshold is defined to be the smallest instantaneous target displacement that can be detected. Properties of the displacement threshold for a small, luminous spot were measured psychophysically. In a structureless field, the displacement threshold was near 1.5′, subject to individual variation. The effects of pattern were studied by measuring displacement thresholds at the centers of a set of annuli ranging from 2.85′–728′ dia. Displacement thresholds were reduced by the presence of the annuli and were as low as 0.3′. This threshold reduction could not be fully attributed to processes of relative spatial localization because displacement thresholds were lower than spatial localization (bull'seye) thresholds for annulus diameters greater than 20′. The displacement threshold is virtually independent of orientation and pupil size. It increased about 75% with a three log unit decrease in photopic target luminance. Displacement detection appears to depend upon the motion sense rather than the position sense. It may be limited by fixation accuracy.

It is shown that Vernier acuity is nearly proportional to line length (in the range 5-20 arc min), provided that the orientation of the target is randomized from trial to trial and provided that no gap interrupts the line. The effect of a pair of gaps, one in each of the Vernier bars leads to the conclusion that line terminations dissect the target into independent parts prior to the visual analysis of shape and orientation cues, which are only assessed between such features.

Temporary epicardial pacing leads are routinely placed in patients after cardiac surgery, but the positioning of ventricular leads and the use of atrial leads is not uniform. We examined the effect of the epicardial pacing site on ventricular function in 18 patients undergoing coronary surgery. Pacing wires were sutured in the right atrium, left ventricular apex, right ventricular apex, and right ventricular outflow tract before cardiopulmonary bypass. After atrial pacing, eight patients were ventricularly paced (group I) and 10 were atrioventricular (AV) sequentially (PR = 0.12 sec) paced (group II) at 100/minute from the three ventricular sites. Comparison of the groups showed that the addition of atrial activation during ventricular pacing resulted in higher cardiac indexes (2.54 +/- 0.61 vs 1.67 +/- 0.45 liters/min/m2;p less than .00005), higher systolic blood pressures (121 +/- 24 vs 89 +/- 26 mm Hg; p = .006), lower central venous pressures (5.5 +/- 3.2 vs 10.2 +/- 2.2 mm Hg; p = .048), and similar pulmonary arterial pressures (19.5 +/- 7.6/10.8 +/- 6.7 vs 24.7 +/- 3.5/15.4 +/- 3.4 mm Hg; p = NS). Cardiac index did not differ among group I patients during pacing from the different ventricular sites. In group II, cardiac index during pacing from the right ventricular apex was higher than during pacing from the right ventricular outflow tract or the left ventricular apex (2.62 +/- 0.57 vs 2.49 +/- 0.54 and 2.51 +/- 0.76 liters/min/m2, respectively; p = .03). Right ventricular outflow tract pacing resulted in higher cardiac indexes than left ventricular apical pacing in patients with stenosis of the left anterior descending coronary artery of 90% or more, while left ventricular apical pacing produced higher cardiac indexes in the absence of such lesions (p = .006).(ABSTRACT TRUNCATED AT 250 WORDS)

Low-resolution optical systems are more tolerant to defocus than are high-resolution systems. We wished to determine whether this principle applies to human vision. We used psychophysical methods to measure the effects of defocus in normal eyes under low-resolution conditions. Modulation transfer of sine-wave gratings was measured as a function of dioptric defocus at low and medium spatial frequencies. We defined the depth of focus at a given spatial frequency to be the dioptric range for which the modulation transfer exceeds 50% of its peak value. For dilated pupils, depth of focus increased from about 2.5 diopters (D) at 3.5 cycles/deg to about 17 D at 0.25 cycles/deg. From our results we predicted that tasks requiring only low spatial frequencies will be more tolerant to defocus than tasks requiring higher spatial frequencies. This prediction was confirmed in a letter-recognition experiment. The increasing tolerance to defocus at low spatial frequencies also implies that individuals with low acuity will be more tolerant to defocus than people with normal vision. We confirmed this prediction by measuring tolerance to defocus in 30 low-vision eyes.