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Fitting physical screen parameters to the human eye
... When this function is used, however, a new visual frame does not always render each time an eye-tracking measurement is taken (or vice versa). This is a minor problem for visual cognition, as these logging/rendering frequencies are sufficiently high to begin with [79]. Regardless, for the ease of the methodology, implementation, and data evaluation, it is preferable to use hardware in which the display/eye-tracker frequencies match and synchronize. ...
Not all eye-tracking methodology and data processing are equal. While the use of eye-tracking is intricate because of its grounding in visual physiology, traditional 2D eye-tracking methods are supported by software, tools, and reference studies. This is not so true for eye-tracking methods applied in virtual reality (imaginary 3D environments). Previous research regarded the domain of eye-tracking in 3D virtual reality as an untamed realm with unaddressed issues. The present paper explores these issues, discusses possible solutions at a theoretical level, and offers example implementations. The paper also proposes a workflow and software architecture that encompasses an entire experimental scenario, including virtual scene preparation and operationalization of visual stimuli, experimental data collection and considerations for ambiguous visual stimuli, post-hoc data correction, data aggregation, and visualization. The paper is accompanied by examples of eye-tracking data collection and evaluation based on ongoing research of indoor evacuation behavior.
... The vertical addressability of most computer terminals today ranges between 9 × 10 -2 mm. Thus, current displays do not approach the addressability required to represent high-resolution characters without sampling distortions [8]. It also has been reported that the human eye can perceive improvements in resolution up to the equivalent of 1,600 scan lines [16]. ...
... The rise and decay seen in our phosphor triad (Fig. 1) is consistent with previous estimates derived from single phosphor activations (Farrell, 1991;Sperling, 1971b;Vingrys and King-Smith, 1986;Westheimer, 1993). The fast decay means that there is no chance of luminous carry over or phosphor persistence even with a frame rate of up to 200 Hz (5 ms). ...
We demonstrate that cathode-ray-tube (CRT) monitors commonly used as stimulus generators in visual neuroscience produce signal artefacts. This arises from two factors, one being the finite time needed for the raster scan of the CRT to cross the receptive field being stimulated, and the other being the restraint imposed by the impulse response of the phosphor itself. Together these factors result in smearing or blurring that manifests as high frequency noise, distorting the desired signal applied by the investigator. Our analysis identifies those conditions that promote these artefacts and we describe methods for their minimisation. We suggest that a monitor frame rate >/=100 Hz provides a reasonable trade-off between refresh and the generators of high frequency noise.
Recently a number of systems have been developed to implement and improve the visual communication over screen-camera links. In this paper we study an opposite problem: how to prevent unauthorized users from videotaping a video played on a screen, such as in a theater, while do not affect the viewing experience of legitimate audiences. We propose and develop a light-weight hardware-free system, called Kaleido, that ensures these properties by taking advantage of the limited disparities between the screen-eye channel and the screen-camera channel. Kaleido does not require any extra hardware and is purely based on re-encoding the original video frame into multiple frames used for displaying. We extensively test our system Kaleido using a variety of smartphone cameras. Our experiments confirm that Kaleido preserves the high-quality screen-eye channel while reducing the secondary screen-camera channel quality significantly.
Cathode ray tube (CRT) display and liquid crystal display (LCD) were compared for their suitability in visual tasks. For this purpose visual performance was assessed by means of a search task carried out using both displays with different levels of ambient light. In addition, suitability was rated subjectively by users of visual display units (VDUs). Error frequency for search tasks carried out using LCD were significantly smaller when compared to error frequency for tasks at CRT. LCD gave rise to 34% less errors than did CRT. Reaction time in search task was found to be significantly shorter using LCD when tasks were carried out in darkness. Subjective rated suitability of LCD was scored twice as high as suitability of CRT. Results indicate that LCD used in this experiment may give better viewing conditions in comparison to CRT display.
This study examines the effects of multimodal feedback on the performance of older adults with different visual abilities. Older adults possessing normal vision (n=29) and those who have been diagnosed with Age-Related Macular Degeneration (n=30) performed a series of drag-and-drop tasks under varying forms of feedback. User performance was assessed with measures of feedback exposure times and accuracy. Results indicated that for some cases, non-visual (e.g. auditory or haptic) and multimodal (bi- and trimodal) feedback forms demonstrated significant performance gains over the visual feedback form, for both AMD and normally sighted users. In addition to visual acuity, effects of manual dexterity and computer experience are considered.
This research focuses on characterizing visually impairedcomputer users performance on graphical user interfaces by linkingclinical assessments of low vision with visual icon identification.This was accomplished by evaluating user performance on basicidentification and selection tasks within a graphical userinterface, comparing partially sighted user performance with fullysighted user performance, and linking task performance to specificprofiles of visual impairment. Results indicate that visual acuity,contrast sensitivity, visual field and color perception weresignificant predictors of task performance. In addition, icon sizeand background color significantly influenced performance.Suggestions for future research are provided. Keywords
The augmentation of computer technologies with multimodal interfaces has great potential for improving interaction with these
technologies via the use of different sensory feedback. This may be of particular importance for individuals from divergent
user populations, who have varying interaction needs and abilities. This study examines the effects of a multimodal interface,
utilizing auditory, haptic, and visual feedback, on the performance of elderly users with varying levels of visual abilities.
Older adults who possess normal vision (n=29) and those who have been diagnosed with age-related macular degeneration (AMD)
(n=30), with different levels of visual acuity, were involved in the study. Participants were asked to complete a series of
‘drag-anddrop’ tasks under varying forms of feedback. User performance was assessed with the measure of final target highlight
time. The findings suggest that the addition of other feedback modalities, to the traditionally used visual feedback form,
can enhance the performance of users, particularly those with visual impairments.
This study examines effects of the most common cause of blindness in persons over the age of 55 in the United States, age-related macular degeneration (AMD), on the performance of older adults when completing a simple computer-based task. Older users with normal vision (n = 6) and with AMD (n = 6) performed a series of drag-and-drop tasks that incorporated a variety of different feedback modalities. The user groups were equivalent with respect to traditional visual function parameters (i.e., visual acuity, contrast sensitivity, and color vision) and measured subject cofactors, aside from the presence or absence of AMD (i.e., drusen and retinal pigment epithelial mottling). Task performance was assessed with measures of time (trial time and feedback exposure time) and accuracy (error frequency). Results indicate that users with AMD exhibited decreased performance with respect to required feedback exposure time, total trial time, and errors committed. Some nonvisual and multimodal feedback forms show potential as solutions for enhanced performance, for those with AMD as well as for visually healthy older adults.
This paper describes computer graphics techniques for presenting visual stimuli in a vernier format composed out of coloured texture patterns. Such stimuli can be used to investigate the performance at the task of localising boundaries mediated by changes in colour and/or texture. We summarise the contents as follows: (1) Techniques for presenting visual stimuli are reviewed with a view to how they might be used to present colour and texture verniers. (2) The design of the vernier stimuli for the localisation task is considered. (3) Significant elements of this design are: (a) the use of non-isoplanatic textures to avoid interference effects at boundaries, (b) the modulation of the texture patterns along axes in MacLeod-Boynton colour space so that relative retinal cone contributions are controlled, and (c) the use of double-buffering, colour map manipulation, and contrast randomisation techniques to avoid problems commonly encountered when presenting computer graphics stimuli on colour monitors. (4) Results of a psychophysical experiment that presents colour and texture verniers are reported elsewhere.
The design of low vision aids for partially sighted people, such as magnifiers, is evaluated theoretically in order to identify meaningful directions of research for improving these aids. Both reading with and without a magnifier involves alternating sequences of locating and recognizing textual information. Little is known, however, about these processes in magnifier reading. On the basis of an extensive review of the literature, two topics are identified that stand out as being in need of experimental investigation: (1) the relationship between the (typo)graphical characteristics of printed text and the location and recognition of textual information, and (2) the interplay of location and recognition processes in magnifier reading, including the role of non-visual factors, such as movements of the hand, trunk, head and eyes in this interplay. With regard to the first topic, it is expected that the visibility and, hence, the recognition of textual information by partially sighted people can be improved by matching the fundamental spatial frequencies of graphical structures with the spatial contrast sensitivity of partially sighted people. With regard to the second topic, it is argued that persistent problems in the design of the magnifiers, such as the optimal window size, can only be resolved by studying reading with a magnifier as a (multimodal) perceptual-motor activity.
As tactile feedback and degree of freedom for instrument movement are restricted in laparoscopic surgery, the video image plays the most crucial role in giving the surgeon information about the performance of the operation. The development of small, reliable, high-resolution imaging systems is essential for the surgeon's acquisition detailed information about the tissues being manipulated. Image quality depends on each component of the laparoscopic imaging unit. In this context, it is crucial for the surgeon to have an understanding of how the video signal is formed, transmitted, and displayed. Moreover, the surgeon also needs to have an idea about the basic principles and specifications of the surgical video systems (i.e. charge-coupled device (CCD) camera, monitors, and digitizers). This knowledge is essential for choosing pieces of equipment and knowing how to assemble them into a functional operating suite. The aim of this review is to provide the surgeon with the basics of video signaling, and to familiarize him or her with the technical principles of the surgical video systems. An insight into the future of laparoscopic video systems also is made, and practical tips for improving image quality and troubleshooting are given throughout the article.
Threshold sensitivity was measured for sinusoidal movement of bright 1-deg lines against a dark background as a function of
oscillation frequency and retinal location. Sensitivity was greatest in the fovea and at a frequency of 1–2 Hz. Peripheral
sensitivity was more narrowly tuned than foveal sensitivity. The presence of a stationary reference line affected mainly the
foveal sensitivity. The results are interpreted as evidence for both position- and velocity-sensitive mechanisms in the movement
detection system.
A film of an object in motion presents on the screen a sequence of static views, while the human observer sees the object moving smoothly across the screen. Questions related to the perceptual identity of continuous and stroboscopic displays are examined. Time-sampled moving images are considered along with the contrast distribution of continuous motion, the contrast distribution of stroboscopic motion, the frequency spectrum of continuous motion, the frequency spectrum of stroboscopic motion, the approximation of the limits of human visual sensitivity to spatial and temporal frequencies by a window of visibility, the critical sampling frequency, the contrast distribution of staircase motion and the frequency spectrum of this motion, and the spatial dependence of the critical sampling frequency. Attention is given to apparent motion, models of motion, image recording, and computer-generated imagery.
Text can be depicted by luminance contrast (i.e., differences in luminance between characters and background) or by color contrast (i.e., differences in chromaticity). We used a psychophysical method to measure the reading speeds of eight normal and ten low-vision subjects for text displayed on a color monitor. Reading speed was measured as a function of luminance contrast, color contrast (derived from mixtures of red and green), and combinations of the two. When color contrast is high, normal subjects can read as rapidly as with high luminance contrast (greater than 300 words/min). Curves of reading speed versus contrast have the same shape for the two forms of contrast and are superimposed when contrast is measured in multiples of a threshold value. When both color and luminance contrast are present, there is no sign of additive interaction, and performance is determined by the form of contrast yielding the highest reading rate. Our findings suggest that color contrast and luminance contrast are coded in similar ways in the visual system but that the neural signals used in letter recognition are carried by different pathways for color and luminance. We found no advantages of color contrast for low-vision reading. For text composed of 6 degrees characters, all low-vision subjects read better with luminance contrast than with color contrast.
The maximum flicker frequency was determined over a 5+6-log-unit range of retinal illuminance for a stimulus configuration designed to isolate the linear response from long-wavelength (R) cones. For a particular retinal location, the data conformed to the Ferry-Porter law and departed significantly from the predictions of the diffusion equation. The slope of the function was an invariant characteristic and was unaffected by stimulus intensity or area, modulation waveform, or modulation amplitude. However, the slope varied substantially with retinal locus, increasing by more than a factor of 2 between the foveola and 35 degrees eccentricity. This increase shows that the time constant of the linear, unadapted visual response decreases with increasing eccentricity. The difference between foveola and periphery remained at high spatial frequencies, implying that it was not attributable to lateral inhibitory effects.
Does the color of text influence its legibility? There are reasons why it may do so for specific groups of low-vision observers. We used psychophysical methods to measure the effects of wavelength on the reading performance of four normal observers, two dichromats, and twenty-five low-vision observers. Reading rates were measured for text scanned across the face of a television (TV) monitor. We compared performance under four luminance-matched conditions in which sets of neutral-density and Wratten color filters were placed in front of the TV screen--blue (lambda max = 430 nm), green (lambda max = 550 nm), red (lambda max = 650 nm), and gray. Under photopic conditions, the reading rates of normal subjects were independent of wavelength, with the exception of characters near the acuity limit. At lower luminances, wavelength effects could be explained by the shift from photopic to scotopic vision. It was hypothesized that light scatter or absorption in eyes with cloudy ocular media would result in depressed performance in the blue. Only one of seven subjects demonstrated this effect, which we traced to wavelength-specific absorption. Observers with advanced photoreceptor disorders tended to read blue text faster than red text. This could not be explained on the basis of photopic spectral sensitivities alone. Finally, the presence of central or peripheral field loss was not predictive of wavelength-specific effects in reading. On the whole, wavelength only occasionally plays a significant role in reading. When it does, performance tends to be depressed either in the red or the blue and to be nearly optimal for green or gray.
A method of producing red-green and blue-yellow sinusoidal chromatic gratings is used which permits the correction of all chromatic aberrations. A quantitative criterion is adopted to choose the intensity match of the two colours in the stimulus: this is the intensity ratio at which contrast sensitivity for the chromatic grating differs most from the contrast sensitivity for a monochromatic luminance grating. Results show that this intensity match varies with spatial frequency and does not necessarily correspond to a luminance match between the colours. Contrast sensitivities to the chromatic gratings at the criterion intensity match are measured as a function of spatial frequency, using field sizes ranging from 2 to 23 deg. Both blue-yellow and red-green contrast sensitivity functions have similar low-pass characteristics, with no low-frequency attenuation even at low frequencies below 0.1 cycles/deg. These functions indicate that the limiting acuities based on red-green and blue-yellow colour discriminations are similar at 11 or 12 cycles/deg. Comparisons between contrast sensitivity functions for the chromatic and monochromatic gratings are made at the same mean luminances. Results show that, at low spatial frequencies below 0.5 cycles/deg, contrast sensitivity is greater to the chromatic gratings, consisting of two monochromatic gratings added in antiphase, than to either monochromatic grating alone. Above 0.5 cycles/deg, contrast sensitivity is greater to monochromatic than to chromatic gratings.
Contrast masking was studied psychophysically. A two-alternative forced-choice procedure was used to measure contrast thresholds for 2.0 cpd sine-wave gratings in the presence of masking sine-wave gratings. Thresholds were measured for 11 masker contrasts spanning three log units, and seven masker frequencies ranging +/- one octave from the signal frequency. Corresponding measurements were made for gratings with horizontal widths of 0.75 degrees (narrow fields) and 6.0 degrees (wide fields). For high contrast maskers at all frequencies, signal thresholds were related to masking contrast by power functions with exponents near 0.6. For a range of low masking contrasts, signal thresholds were reduced by the masker. For the wide fields, high contrast masking tuning functions peaked at the signal frequency, were slightly asymmetric, and had approximately invariant half-maximum frequencies that lie 3/4 octave below and 1 octave above the signal frequency. The corresponding low contrast tuning functions exhibited peak threshold reduction at the signal frequency, with half-minimum frequencies at roughly +/- 0.25 octaves. For the narrow fields, the masking tuning functions were much broader at both low and high masking contrasts. A masking model is presented that encompasses contrast detection, discrimination, and masking phenomena. Central constructs of the model include a linear spatial frequency filter, a nonlinear transducer, and a process of spatial pooling that acts at low contrasts only.
With two electrical analogs of the brightness system it is shown that at a low luminance level (<5 photons) and in the low-frequency region (<2 cps), when under the special visual conditions no attenuation in the system occurs and with a symmetrical luminance variation such as a sinusoidal modulation or a square-wave modulation with a 1:1 on-off ratio, half the crest-to-trough value of the percentage variation at flicker fusion equals the internal threshold value r0; but with an asymmetrical variation such as a square-wave modulation with a 1:3 on-off ratio, it is the crest value of the periodical percentage variation above the mean luminance level which at flicker fusion equals the internal threshold value r0. At high luminance levels an overshoot in the low-frequency region occurs with both forms of square-wave modulation in accordance with the shape of the attenuation characteristic (AC) of the system under the experimental circumstances. At the steep slope of the ACs for the whole luminance range in cone vision, half the crest-to-trough value of the Fourier fundamental percentage variation at the site of the threshold mechanism located anywhere in the system, equals the internal threshold value r0 at flicker fusion.
Citation
HERBERT E. IVES, "A THEORY OF INTERMITTENT VISION," J. Opt. Soc. Am. 6, 343-359 (1922)
http://www.opticsinfobase.org/josa/abstract.cfm?URI=josa-6-4-343
Visual perception was investigated using spatial filters that are constrained by biological data. This report has four major parts: (1) a theoretical background to Fourier analysis; (2) a review of the literature relating to the spatial filtering characteristics of mammalian visual systems; (3) visual information processing in terms of spatial filtering; (4) the relating of contrast sensitivity to the identification of complex objects. The common denominator to all the investigations is spatial filtering. The major goal of the dissertation was the attempt to explain certain aspects of visual perception using a single concept: filtering. (Author)
Decreasing memory costs will soon allow grayscale displays in low-cost raster graphic terminals. Subtle shadings can be used to provide improvements in line quality and character flexibility which could allow raster displays to compete on better terms with the more expensive calligraphic displays. Algorithms for achieving smooth vectors and rotatable dot matrix characters are outlined and scan conversion is discussed. A discussion of the relation between image quality and number and distribution of gray levels follows, with concluding remarks on costs and other practical matters.
Character fonts on raster scanned display devices are usually represented by arrays of bits that are displayed as a matrix of black and white dots. This paper reviews a filtering and sampling method as applied to characters for building multiple bit per pixel arrays. These arrays can be used as alternative character representations for use on devices with gray scale capability. Discussed in this paper are both the filtering algorithms that are used to generate gray scale fonts and some consequences of using gray levels for the representation of fonts including:
1. The apparent resolution of the display is increased when using gray scale fonts allowing smaller fonts to be used with higher apparent positional accuracy and readability. This is especially important when using low resolution displays.
2. Fonts of any size and orientation can be generated automatically from suitable high precision representations. This automatic generation removes the tedious process of “bit tuning” fonts for a given display.
A method is described for predicting whether a visual display terminal (VDT) will appear to flicker given the display phosphor persistence, refresh frequency, luminance and other environmental factors such as the distance between the user and the VDT. Based on research on human temporal sensitivity (Kelly 1969), one can predict the maximum screen luminance and the minimum refresh frequency that will generate a flicker-free display for a theoretical standard observer. These predictions are tested across a range of refresh frequencies, screen luminance, screen phosphors and individual users.
Ten color combinations in addition to black on white were compared with respect to their effect on reading rate. Ten groups of 85 college students each were tested. Striking differences in rate of reading text material printed with various combinations of colored ink and paper were disclosed. The following list gives a rough idea of the findings: Providing good legibility: black on white, grass green on white, luster blue on white, and black on yellow. Providing fair legibility: tulip red on yellow, tulip red on white. Providing poor legibility: grass green on red, chromium orange on black, chromium orange on white, tulip red on green, black on purple. A maximum brightness-contrast between print and background is desirable for greatest legibility. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
The notion of a standard observer for flicker is described based on a theory of human flicker sensitivity. Based on this model and the data collected from four independent studies, an analytic method is proposed for predicting whether or not a particular display will be flicker-free given information about its phosphor, refresh rate and screen luminance. This method will provide display engineers with a way to evalute screen flicker during the design phase of a display and will be a satisfactory alternative to the present, rather cumbersome, empirical methods.
Nearly all books, magazines and newspapers are printed with black letters on a white background. As Fig. 1 illustrates, it is possible to display text as white letters on a black background. We refer to these alternatives - black-on-white or white-on-black - as the contrast polarity of the text. Although the choice of contrast polarity has little effect on the reading performance of normally sighted people, it is important for some individuals with low vision.
Two experiments measured thresholds for discriminating the movement direction of an isolated intensity edge. The luminance profile of the edge took the form of an integrated Gaussian. In the first experiment, displacement thresholds were measured as a function of edge blur width and contrast. In the second experiment, contrast thresholds were measured as a function of edge blur width and displacement. Using the estimated retinal profile of the edge (given the LSF of the display and of the optics of the eye), the data were found to collapse onto a single function relating the maximum spatial luminance gradient defined by the edge to the maximum temporal change in luminance generated by its displacement. There was a direct relationship between the two gradients at threshold, so that lower spatial gradients were paired with smaller temporal changes. Implications for current models of motion detection are examined.
It has long been thought that the foveal mosaic provides the anatomical basis for human visual resolution under optimal viewing conditions. Recent anatomical evidence shows that the foveal cone mosaic in the primate is a regular, triangular array of elements with only minor distortions. The sampling properties of such an array predict the existence of visible moiré patterns, or aliasing, produced when very fine gratings are imaged on the retina. Recent psychophysical experiments confirm this prediction, and allow the spacing between cones to be measured in the living eye. The cone array in the peripheral retina, unlike the fovea, is sparse and irregular, and it feeds an even sparser ganglion cell array. These arrays provide a new challenge to establish the link between the organization of the retina and the psychophysical limits of visual resolution.
Very little is known about the effects of visual impairment on reading. We used psychophysical methods to study reading by 16 low-vision observers. Reading rates were measured for text scanned across the face of a TV monitor while varying parameters that are likely to be important in low vision: angular character size, number of characters in the field, number of dots composing each character, contrast polarity (white-on-black vs black-on-white text), and character spacing. Despite diverse pathologies and degrees of vision loss in our sample, several major generalizations emerged. There is a wide variation in peak reading rates among low-vision observers, but 64% of the variance can be accounted for by two major distinctions: intact central fields vs central-field loss and cloudy vs clear ocular media. Peak reading rates for observers with central-field loss were very low (median 25 words/minute), while peak reading rates for observers with intact central fields were at least 90 words/minute (median 130 words/minute). Most low-vision readers require magnification to obtain characters of optimal size. Sloan M acuity was a better predictor of optimal character size than Snellen acuity, accounting for 72% of the variance. Low-vision reading is similar to normal reading in several respects. For example, both show the same dependence on the number of characters in the field. Our results provide estimates of the best reading performance to be expected from low-vision observers with characteristic forms of vision loss, and the stimulus parameters necessary for optimal performance. These results will be useful in the development of clinical tests of low vision, and in the design of low-vision reading aids.
Recently several investigators have studied the problem of displaying text characters on grey level raster scan displays. Despite arguments suggesting that grey level displays are equivalent to very high resolution bitmaps, the performance of grey level displays has been disappointing. this paper will show that much of the problem can be traced to inappropriate antialiasing procedures. Instead of the classical (sin x)/x filter, the situation calls for a filter with characteristics matched both to the nature of display on CRTs and to the human visual system. We give examples to illustrate the problems of the existing methods and the advantages of the new methods. Although the techniques are described in terms of text, the results have application to the general antialiasing problem--at least in theory if not practice.
This research evaluated the sensitivity of four observer response measures to variations in the character size and dot luminance of a dot matrix display. Specifically, the research determined the sensitivity of recognition accuracy, response time, tachistoscopic recognition accuracy, and threshold visibility. Alphanumeric characters were presented to six subjects in noncontextual form on a variable-parameter CRT display programmed and driven by a minicomputer.
Recognition accuracy (percent correct response) was found to be the response measure that is most sensitive to the display parameters of character size and dot luminance. Character size, dot luminance, and viewing distance proved to have consistent and significant effects at viewing distances greater than 1.52 m. At lesser viewing distances, these parameters had little effect.
The results suggest that there is no major difference between the display requirements for computer-generated dot matrix displays versus those for conventional CRT displays.
Comparisons of the published data on the density D of receptive fields of retinal ganglion cells and on the cortical magnification factor M indicated that M2 is directly proportional to D in primates. Therefore, the human M can be estimated for the principal meridians of the visual field from the density-distribution of retinal ganglion cells and from the density of the centralmost cones. Using the previously published empirical data, we estimated the values of the human M and express the values in four simple equations that can be used for finding the value of M for any location of the visual field. The monocular values of M are not radially symmetric.
These analytically expressed values of M make it possible to predict contrast sensitivity and resolution for any location of the visual field. We measured contrast sensitivity functions at 25 different locations and found that the functions could be made similar by scaling the retinal dimensions of test gratings by the inverse values of M. Visual acuity and resolution could be predicted accurately for all retinal locations by means of a single constant multiplier of the estimated M.
The results indicate that the functional and structural properties of the visual system are very closely and similarly related across the whole retina. Visual acuity, e.g., bears the same optimal relation to the density of sampling executed by retinal ganglion cells at all locations of the visual field.
This study shows that photopic contrast sensitivity and resolution can be predicted by means of simple functions derived by using the cortical magnification factor M as a scale factor of mapping from the visual field into the striate cortex. We measured the minimum contrast required for discriminating the direction of movement or orientation of sinusoidal gratings, or for detecting them in central and peripheral vision. No qualitative differences were found between central and peripheral vision, and almost all quantitative differences observed could be removed by means of a size compensation derived from M.
The results indicated specificly that (1) visual patterns can be made equally visible if they are scaled so that their calculated cortical representations become equivalent; (2) contrast sensitivity follows the same power function of the cortical area stimulated by a grating at any eccentricity; (3) area and squared spatial frequency are reciprocally related as determinants of contrast sensitivity; and (4) acuity and resolution are directly proportional to M, and the minimum angle of resolution is directly proportional to M-1.
The power law of spatial summation expressed in (2) and (3) suggests the existence of a central integrator that pools the activity of cortical neurons. This summation mechanism makes the number of potentially activated visual cells the most important determinant of visibility and contrast sensitivity. The functional homogeneity of image processing across the visual field observed here agrees with the assumed anatomical and physiological uniformity of the visual cortex.
THE area of visual cortex devoted to the analysis of a con-slant-size region in the visual field diminishes progressively for more peripheral locations. The change is described quantitatively by the cortical magnification factor, which indicates the linear extent of cortex in mm corresponding to one degree of visual angle at various eccentricities (angular distances from the middle fovea). The human cortical magnification factor has been estimated by Cowey and Rolls1 from the data of Brindley and Lewin2, who mapped the phosphenes (sensations of light) caused in the lower nasal visual field by electrical stimulation of the human visual cortex. Building on these results, we have studied the spatial contrast sensitivity functions in man at various eccentricities. We used two methods: in one the retinal image sizes of the test gratings were kept similar at different eccentricities and in the other, the calculated sizes of cortical projections of the test gratings were made similar at different locations. Our results indicate that peripheral contrast sensitivity and acuity are inferior to foveal performance, because of the diminished cortical projection area.
Retinal sensitivity to spatial patterns depends on the spatial distribution of receptive fields. Their natural distribution is neither perfectly random nor perfectly regular; its effects vary with the visual task involved. A model of the sustained ganglion cell in man is used to make quantitative predictions of the sine-wave contrast sensitivity for various hypothetical receptive-field distributions. In the spatial frequency domain, partial coherence among ganglion-cell responses can produce narrow bands of sensitivity to sinusoidal gratings. Thus receptive-field coherence may account for various spatial frequency effects previously thought to require a cortical mechanism.
This article presents a model of character recognition and the experiments used to develop and test it. The model applies to foveal viewing of blurred or unblurred characters and to tactile sensing of raised characters using the fingerpad. The primary goal of the model is to account for variations in legibility across character sets; a secondary goal is to account for variations in the cell entries of the confusion matrix for a given character set. The model consists of two distinct processing stages. The first involves transformation of each stimulus into an internal representation; this transformation consists of linear low-pass spatial filtering followed by nonlinear compression of stimulus intensity. The second stage involves both template matching of the transformed test stimulus with each of the stored internal representations of the characters within the set and response selection, which is assumed to conform to the unbiased choice model of Luce (1963). Though purely stimulus driven, the model accounts quite well for differences in the legibility of character sets differing in character type, size of character, and number of characters within the set; it is somewhat less successful in accounting for the details of each confusion matrix.
We evaluated a metric for predicting the discriminability of different digitized versions of alphanumeric characters. The metric is based on the assumption that there exists a visual filter such that discriminability is monotonic with the contrast energy in the visually filtered difference between stimuli. To test this hypothesis, we presented two same or different digital versions of a master character and asked subjects to indicate whether the characters were the same or different in a forced-choice procedure with feedback. The filtered contrast energy difference was calculated by convolving the difference between stimulus pairs with filters derived from published human contrast sensitivity functions, following an initial nonlinear transformation of stimulus intensity, and summing the squared result. For some types of stimulus difference, such as contrast quantization errors and Gaussian blurring, performance on discrimination tasks is monotonically related to the contrast energy of the filtered difference vector. The results are consistent with the hypothesis that there exists a single psychometric function that can predict the discriminability of different digitized versions of characters when displayed on various devices.
The effect of retinal eccentricity on stimulus identification was investigated by measuring observers' ability to identify digits of various sizes presented briefly at different locations in their visual fields. The stimuli were either presented one at a time or presented in pairs with each member of the pair at a different eccentricity and having a different size. Stimulus-response confusion matrices were used to calculate the information transmitted by each character as a function of stimulus size and retinal eccentricity. Linearly scaling stimulus size with retinal eccentricity yields equal information transmission across each subject's visual field. There was no difference between the amounts of information transmitted when the targets were presented alone and when they were transmitted in pairs. For small target sets, therefore, the amount of information transmitted from the screen to the observer can be approximated as the sum of the information at the separate locations.
Like the modulation transfer function of man-made imaging devices, the contrast sensitivity of the human eye can be measured with sinusoidal grating targets of various spatial frequencies. Criterion-free psychophysical methods permit us to regard the contrast sensitivity as a direct measure of the subject's visual performance, independent of subjective factors. Under these conditions, not only the shape of the contrast-sensitivity function but also its absolute values show good agreement among normal subjects. However, the most interesting properties of this function cannot be attributed to the optics of the eye, but must be understood in terms of the image-processing activities of the visual pathways. The contrast-sensitivity function varies with the size, brightness, motion and flicker of the test target, with the adaptive state of the subject's retina, and with his eye-movements. Most of these effects can be explained in terms of known neurophysiology.
The visual representation of spatial patterns begins with a series of linear transformations: the stimulus is blurred by the optics, spatially sampled by the photoreceptor array, spatially pooled by the ganglion-cell receptive fields, and so forth. Models of human spatial-pattern vision commonly summarize the initial transformations by a single linear transformation that maps the stimulus into an array of sensor responses. Some components of the initial linear transformations (e.g., lens blurring, photoreceptor sampling) have been estimated empirically; others have not. A computable model must include some assumptions concerning the unknown components of the initial linear encoding. Even a modest sketch of the initial visual encoding requires the specification of a large number of sensors, making the calculations required for performance predictions quite large. We describe procedures for reducing the computational burden of current models of spatial vision that ensure that the simplifications are consistent with the predictions of the complete model. We also describe a method for using pattern-sensitivity measurements to estimate the initial linear transformation. The method is based on the assumption that detection performance is monotonic with the vector length of the sensor responses. We show how contrast-threshold data can be used to estimate the linear transformation needed to characterize threshold performance.
In a generalized form, the cortical magnification theory of peripheral vision predicts that the thresholds of any visual stimuli are similar across the whole visual field if the cortical stimulus representations calculated by means of the cortical magnification factor are similar independently of eccentricity. Failures of the theory in spatial vision were analyzed, and the theory was tested with five visual acuity tasks and two hyperacuity tasks. Almost all increases in thresholds with eccentricity were explained by the theory in five of these tasks, which included the two-dot vernier hyperacuity test, the measurement of visual acuities with gratings, the Snellen E test, and two acuity tests that required either separation between dots or discrimination between two mirror-symmetric forms. The two-dot vernier thresholds could be explained as a special case of orientation discrimination, and orientation discrimination at different eccentricities was in agreement with the cortical magnification theory. The increase of thresholds in peripheral vision was larger than predicted by the theory in the Landolt visual acuity and bisection hyperacuity tests, possibly because of retinal undersampling.
How does contrast affect reading rate? What is the role of contrast sensitivity? We measured reading rate as a function of the contrast and character size of text for subjects with normal vision. Reading rates were highest (about 350 words/min) for letters ranging in size from 0.25 degree to 2 degrees. Within this range, reading was very tolerant to contrast reduction--for 1 degree letters, reading rate decreased by less than a factor of two for a tenfold reduction in contrast. The results were very similar for white-on-black and black-on-white text. Reading rate declined more rapidly for very small (less than 0.25 degree) and very large (greater than 2 degrees) letters. People with low vision usually require large characters to read, so high contrast is particularly important for them. Taking 35 words/min to be a threshold for reading, we constructed a contrast-sensitivity function (CSF) for reading. We were able to relate the shape of this CSF to the shape of sine-wave grating CSFs.
This article proposes that adaptation to the fundamental spatial frequency of lines of text on a video display terminal (VDT) provides a single explanation for a wide variety of reports of visual fatigue by VDT users. Reliable contrast threshold elevations at spatial frequencies of 2, 3, and 5 cycles/deg were found after subjects read single-spaced text on a VDT. This adaptation also reduces sensitivity to spatial frequencies in the range (2 to 6 cycles/deg) largely responsible for the reflexive accommodative response (Bour, 1981. Charman and Heron, 1979), and therefore could account for objective optometric measures of disturbed accommodation as well as some subjective effects of VDT viewing. Implications for video display design and for further research are discussed.
This paper is about the visual requirements for reading with normal vision. It is the first in a series devoted to the psychophysics of reading with normal and low vision. We have measured reading rates for text scanned across the face of a TV monitor while varying parameters that are important in current theories of pattern vision. Our results provide estimates of the stimulus parameters required for optimal reading of scanned text. We have found that maximum reading rates are achieved for characters subtending 0.3 degree to 2 degrees. Contrast polarity (black-on-white vs white-on-black text) has no effect. Reading rate increases with field size, but only up to 4 characters, independent of character size. When text is low-pass spatial-frequency filtered, reading rate increases with bandwidth, but only up to two cycles/character, independent of character size. When text is matrix sampled, reading rate increases with sample density, but only up to a critical sample density which depends on character size. The critical sample density increases from about 4 X 4 samples/character for 0.1 degree characters to more than 20 X 20 samples/character for 24 degrees characters. We suggest that one spatial-frequency channel suffices for reading.
Bisection thresholds were measured as a function of the separation of the lines. For separations of less than 1.5 min, the addition of flanking lines facilitates bisection so that thresholds of less than 1 sec for discriminating the direction of offset could be reliably obtained. For larger separations an interval could be bisected to an accuracy of 1 part in 60. Experiments varying the length, luminance, and overlap of the lines suggest that different cues are used in these two regimes. A dual space-size analysis is presented that can account for these bisection thresholds over a wide range of experimental conditions. This quantitative analysis produces viewprints of the stimuli (analogous to the voiceprint of audition). Each viewprint shows the output of many spatial filters of different positions and sizes. A new filter shape is introduced that has advantages for modeling the visual system. The sensitivity of each filter is fixed by the contrast-response function. The analysis further shows that the limiting factors in spatial hyperacuity are both the contrast-response function and the spatial grain.
Photopic flicker data are explained in terms of a theoretical model of two retinal processes. The first is a linear diffusion process (presumably in the receptors), with a large dynamic range (~105). The second is a nonlinear inhibiting network (neural feedback at the synapses of the plexiform layers) that adaptively controls the sensitivity and time constants of the model. The magnitude of its transfer function fits the flicker data quantitatively at all frequencies, over a wide range of adaptation levels. The corresponding small-signal impulse responses are also calculated: their latencies and leading edges (associated with receptor activity) are invariant with adaptation level; the remaining phases of these transient waveforms (associated with the graded potentials of secondary neurons) adapt strongly, in accord with current histology and micro-electrode findings.
The contrast thresholds of equiluminous chromaticity-modulated gratings are measured for various waveforms (sine-, square-, and triangular-wave gratings). The expectation is expressed that only the fundamental Fourier component is of significance in the threshold visibility of colored gratings. In addition, the Fourier transformation is applied to the chromatic spatial-sensitivity curves. The transformed functions illustrate clearly the spatial organization of the contrast mechanisms. For 160 trolands, the summation area of the red–green chromatic activity extends over 10′, whereas the yellow–blue activity integrates over about 25′. A comparison of the Fourier transforms of a luminance- and chromatic-threshold contrast curve shows (1) inhibitory qualities and (2) the greater spatial sensitivity of the luminous function. The assumption is made that the visual resolution for differences of brightness, as well as chromaticity, is limited by the diffraction of light by the pupil. The visual acuity for differences of hue as a function of the background wavelength is thus predicted for a 30-cpd grating and compared with an empirical function. There is good agreement.
Recent cascaded-integrator models do not fit the sine-wave flicker thresholds as well as we might wish, but neither does the Ferry-Porter law. In fact, the Ferry-Porter function is not physically realizable as a linear model. By modifying it to yield realizable responses like those of the cascaded integrator, we obtain a much simpler model, which appears to be a special case of the photochemical diffusion mechanism proposed by Ives and more recently by Veringa. This model is a good fit, not only to the flicker data, but also to human phase-shift measurements obtained by the phosphene method. We infer that receptor-cell properties probably control the high-frequency linear filtering of flicker waveforms.
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.
Black, uppercase letters, subtending 6.0' of arc in height, were presented tachistoscopically to 6 subjects. An exposure duration was chosen to keep the subject's identification performance at about 50% correct. On each trial a single letter was presented, and the subject was required to identify the letter by verbal response. The resulting 26 X 26 confusion matrix was based on 3,900 trials (150 trials per letter). Several models of visual processing were used to generate predicted confusions among letter pairs. Models based on template overlap, geometric features, and two-dimensional spatial frequency content (Fourier transforms) were tested. The highest correlation (.70) between actual and predicted confusions was attained by the model based on the Fourier transformed letters filtered by the human contrast sensitivity function. These results demonstrate that the spatial frequency content of visual patterns can provide a valuable metric for predicting their psychological similarity. The results further suggest that spatial frequency models of visual processing are competitive with feature analysis models.
Moving the retinal image of a sinusoidal grating at a constant velocity (compensated for eye movements) provides controlled spatial and temporal frequencies at every point in the stimulus field. Using this controlled-velocity technique, we have measured the detection threshold for isoluminance, red/green gratings as a function of their spatial and temporal frequencies. The chromatic contrast-threshold surface obtained in this way is analogous to the achromatic contrast-threshold surface measured previously, but the results are quite different. For very low temporal frequencies (below 0.2 Hz), the chromatic sensitivity decreases steadily with decreasing temporal frequency. Below 0.01 Hz, chromatic patterns disappear completely even at maximum contrast (although achromatic or homochromatic patterns do not). In the region above 0.2 Hz, both achromatic and chromatic thresholds can be explained by the same receptive-field-like model. When the center and the surround components of this model are additively combined, they form the chromatic threshold surface; when the sign of either component is reversed, they form the achromatic one.
Contrast increment thresholds were measured as a function of the background contrast of suprathreshold sine wave gratings at 2 and 8 c/deg. The resulting contrast discrimination functions obey power laws with exponents near 0.6 at 2 c/deg, and 0.7 at 8 c/deg. These exponents are influenced only slightly by pattern adaptation, gated vs continuous background gratings, and the psychophysical method. Weber's Law does not hold for contrast discrimination under any of the conditions studied.
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.
Displacement thresholds were determined for unidirectional linear movement (A-B motion) and oscillatory movement (A-B-A motion) for various periods of motion. Both functions exhibited similar increases in motion thresholds for durations of movement greater than 800 msec, and nearly constant motion thresholds for durations of movement between 300 and 800 msec. At shorter durations, A-B motion thresholds remained essentially constant, whereas A-B-A (oscillatory) motion thresholds displayed a dramatic increase. The introduction of a pause at point B of the oscillatory movement decreased motion thresholds at short durations, supporting the hypothesis that temporal integration of the stimulus luminance at point B was a critical factor underlying the degraded performance. The temporal integration hypothesis was also supported by a subsequent experiment in which luminancetime trade-offs were demonstrated for motion thresholds at short durations of movement. These findings indicate that stimulus displacement is a primary determinant of movement detection sensitivity.
A photoelectric analog of the visual system is constructed in conformance with anatomical data. The analog has the form of a color television camera chain feeding electrical signals to a “computer” (the brain). Evaluation of characteristics is limited to elements preceding the computer, and particularly to the “luminance channel” of the color system.
The general psychophysical law is that equal stimulus ratios produce equal subjective ratios. A first-order approximation is a power function whose exponent varies from 0.3 (loudness) to 2.0 (visual flash rate). This holds for Class I (prothetic) or quantitative continua, distinguishable by 4 criteria: "the j.n.d. increases in subjective size as psychological magnitude increases, category rating-scales are concave downwards when plotted against psychological magnitude, comparative judgments exhibit a time-order error… , and equisection experiments exhibit hystersis" (a lagging behind of apparent sense differences). Class II (metathetic) or qualitative continua are lacking in these 4 criteria. Psychological scales based on direct ratio methods are better than Fechnerian methods, e.g., method of paired comparisons. 75 references. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
With sinusoidal modulation of the radiance of the stimulus as a function of time, amplitude thresholds are measured instead of the repetition-rate thresholds usually obtained in flicker-fusion experiments. Controlling the modulation amplitude independently of the time-average radiance provides an additional degree of freedom, so that the observer's adaptation level can be held constant while his amplitude sensitivity is measured as a function of the modulation frequency. With an "edgeless" flickering field, these amplitude sensitivity curves show a broad peak of maximum visual response, in the region from 10 to 20 cps at high photopic levels. Such classic relationships as the Ferry-Porter, Talbot-Plateau, and Weber-Fechner laws are derivable from the present results, as descriptions of the behavior of certain parts of the amplitude sensitivity curves as functions of adaptation level.