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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.

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.

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.

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.

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.

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.

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)

An apparatus has been developed in which high-contrast square-wave gratings are rotated in front of the amblyopic eye while the child is performing some task requiring visual concentration. In the course of three 7-minute treatments 73% of patients treated achieved 6/12 or better; 75% of these patients had previously undertaken conventional or minimal occlusion.

We investigated why we have no perception of a smeared image resulting from the reduction in contrast (grey-out) occurring at the retina during saccadic eye movements. By turning on light in the experimental room only during the eye movement, we were able to show that this grey-out was perceived as a smeared image of the visual scene. However, when the experimental room was illuminated before and/or after the saccade as well as during the saccade, perception of the grey-out was obliterated. During a period of fixation, perception of a blank image comparable in duration to an eye movement could also be eliminated by a preceding or following clear image. We conclude that lack of perception during saccadic eye movements made in normal contoured environments results primarily from the visual “masking” effect of a clear image before and/or after the eye movement acting on the grey-out during the eye movement. This “saccadic omission” is entirely a visual phenomenon and is far more powerful than the usually studied elevation of visual threshold for detection of a flash, “saccadic suppression.”

Eight patients with a subjective disorder of vision yet normal Snellen acuities after optic neuritis were shown to have an abnormal contrast sensitivity function in their affected eye. It appears that certain disorders of vision are associated with an abnormality of the contrast sensitivity function in spite of near normal visual acuity. Such an abnormality may affect pattern recognition without having an influence on Snellen acuity because of the high contrast of the latter and its predominant association with the higher spatial frequencies. Contrast sensitivity function is thus the only tool available to study those aspects of vision which have remained impervious to other subjective tests of visual function.

VERY few photons need to be captured by the receptors of the human eye to excite a sensation of light. The number required is certainly no greater than 100 and may be much less1,2. A single rod requires only one photon to signal light3. A question which therefore arises is why do we not see punctate photon absorptions when we look at a very dim patch of light? The retina catches incident photons only sporadically in time, and therefore in space, when a source is very dim. If very few photons are sufficient to detect light, it might be expected that we should see isolated flashes of light, so that a patch of light appears mottled or speckled. Indeed, if a dim object is viewed on the screen of a cascaded image intensifier, so that each quantum caught by the cathode is amplified, we do see quanta as individual flashes or specks on the display phosphor2. This apparent contradiction creates a physiological problem ignored in most standard textbooks on vision although the issue has been raised4,5. We report here a possible solution; we have observed point sources of light set up one at a time very rapidly and at random by computer within a square patch on the face of an oscilloscope and find that as the luminance of the point sources is decreased the number of specks seen increases markedly, and that the appearance of motion, due presumably in the first place to the effect of the chance sequences of specks on motion-sensitive neurones, becomes weaker. We propose that at or near the threshold, the number of specks seen increases to infinity so that the specks crowd together to appear uniform and so the motion ceases.

1. Using experimental curves relating the threshold contrast of sinusoidal grating patterns to their spatial frequency, the expected threshold contrast curves for three aperiodic patterns, viz. a single half-cycle sinusoid bar, a single full-cycle sinusoid bar, and the boundary between an extended sinusoidal grating and a 50% grey surround, are calculated. In this calculation the assumption is made that the system is linear near the threshold.2. Experiments are described in which the actual threshold contrast curves are determined for these aperiodic patterns by three observers. The patterns were generated on the face of an oscilloscope and could be varied in size and contrast.3. These experimental curves agree well with the predictions in the high frequency region (i.e. above about 10 c/deg), but below this various complicating factors restrict the validity of the calculations.4. Thus there is no reason to suppose that a linear theory cannot be used to predict visibility of aperiodic patterns near threshold.

1. Differences of threshold contrast are predicted from optical theory for a grating acuity target in monochromatic and white light. The greatest differences, up to 65%, are predicted for gratings of lower contrast and pitch than those normally used in measurements of visual acuity.2. Using three subjects, we measured contrast thresholds with 1.5 and 2.5 mm diameter artificial pupils for natural and paralysed accommodation, using a tungsten lamp and wave-lengths of 546 and 578 mm.3. Excellent agreement is obtained between predicted and measured differences.4. Results confirm that observed acuity and sensitivity differences between white and monochromatic lights are largely optical in origin, but involve at least two independent colour mechanisms as spectral weighting functions. Stiles's pi(4) and pi(5) sensitivities afford a much better fit to observed differences than the C.I.E. visibility curve.

1. The evoked potential in response to a grating alternating in phase at 8 c/s was recorded as a function of contrast from the occiput of man. 2. It was found that a linear relation exists between the log. of contrast and the amplitude of the evoked potential. 3. Extrapolation to zero amplitude voltage of the regression line between the amplitude of the evoked potential and log. contrast predicts the psychophysical threshold. This law was found to hold over the wide range of spatial frequencies tested. 4. Below 3 c/deg the results are best fitted with two regression lines; one of these is generated from the foveal and the other from the parafoveal representation in the cortex. 5. The slope of the regression lines was found to be almost independent of either the spatial frequency or the area of the stimulus grating. 6. The slope of the regression lines could be markedly increased by using as a stimulus either two different spatial frequencies, or two different orientations, presented simultaneously. 7. Using the evoked potential the selectivity to orientation was found to be so high that a channel was not influenced by another orientation 15° away. 8. The channels selectively sensitive to spatial frequency were highly selective and were not influenced by another spatial frequency one octave removed in spatial frequency. 9. It is concluded that in man there exist neurones highly selective to both orientation and spatial frequency.

IF two gratings with the same sinusoidal luminance profile, one vertically orientated and one horizontally orientated, are imaged by means of two projectors on a white screen, the appearance continuously changes. First one grating is seen clearly, then the other; in other words, the gratings are seen to alternate or rival. There are periods when both gratings are seen together, but there are never periods when both disappear1.

1. It was shown that the potentials evoked by using a grating pattern alternated in phase at 8 Hz is proportional to the logarithm of the supra-threshold contrast. Other functions were considered, but they did not describe the data so parsimoniously. 2. The same logarithmic function described the results when a grating was simply flashed on and off; therefore, the apparent movement accompanying the phase alternations is not necessary to evoke the potential. 3. The contrast at which the evoked potential reached the theoretically zero voltage (C0) was compared with the psychophysical contrast threshold, determined by means of proportion-of-time seen measurements; the C0 contrast corresponded to 50% time seen. 4. The potential, corrected for the proportion-of-time seen, was found linearly related to contrast.

1. The experiments were carried out on pretrigeminal cats. 2. By recording potentials evoked from the visual cortex by a grating stimulus, it was established that there was a linear relation between the voltage generated and the logarithm of the contrast of the grating. 3. The voltage evoked by the grating was independent of the orientation of the grating. 4. It has previously been shown in man that, if the contrast is determined by an extrapolation to the point at which a zero voltage occurs, this value corresponds to the psychophysical threshold. On the assumption that the threshold of the cat also occurs at zero voltage, thresholds for a number of spatial frequencies and orientations were determined. 5. When the threshold sensitivity function for the cat is compared with man it is found to be displaced to lower spatial frequencies by a factor of about ten. This means that while the cat cannot see such high spatial frequencies as man, it can see lower frequencies better than man.