Threshold exposure duration as a function of luminance for visual recognition

Threshold exposure duration was measured at set luminances for recognition at successively higher criteria of 1, 2, 3, 4, and 5 correct letters in nonsense words. Two sizes of letters were used and triads of Landolt “Cs” were compared with letters. Tachistoscopic procedures were the typical ascending seriesin the method of limits and pre- and postexposure light adaptation. Obtained log t vs log I functions were two-part straight line curves both describable by the equation, log t + log I^k = log C, but with k and C abruptly changing at a certain I. Reciprocity (k = 1) held only for low Is and the low criterion; as criterion was raised at low Is k became less negative (about ?.60 at five correct) for both sizesand for “Cs.” At higher Is, k shifted from ?.25 to ?.40 with increasing criterion for large letters, but variedaround ?.45 at all criteria for small. The lower the criterion, the lower the / and t at which k and C abruptly changed for large letters, but only t was lower for small. Changes in t with acuity requirement change, and from one criterion level to the next, were not uniform over all Is. Results may reflect obscuring procedural factors or basic properties of visual recognition processes. Some interpretations in terms of procedural variables are offered. It appears more likely that visual recognition involves central processes for which time is needed in excess of that for summation of luminances for mere detection by sensory processes, as expressed in Bloch s’ reciprocity law.     

Visual acuity in the pigeon II: effects of target distance and retinal lesions

Visual acuity thresholds for grating targets were determined for three pigeons at target distances ranging from 13 to 73 cm. These measurements were made both while the birds were wearing goggles restricting vision to the frontal field of view and when vision was unrestricted. Using a slightly different method, performance was also compared for target distances of 6 and 13 cm while the goggles were in place. For a second group of three pigeons, acuity data were obtained before and after laser lesions of the retina's foveal region. The findings suggested that acuity was relatively poor for targets at the intermediate distances and that it improved as distance both increased and decreased from these intermediate values. The acuity improvement with increasing distance did not occur, however, when the birds were wearing frontal goggles. The data appear to be consistent with Catania's 1964 suggestion that the pigeon has separate frontal and lateral visual systems that differ in their refractive characteristics. Foveal lesions did not appear to affect acuity for distant targets, and it was concluded that, while the fovea serves the lateral field of view, its presence is not necessary to the static acuity characteristics of this system.     

Fechner’s paradox reflects a nonmonotone relation between binocular brightness and luminance

Twenty subjects judged the brightness of binocularly fused targets whose monocular luminances were varied independently. On each trial, the left eye was presented with one of two relatively high luminances and the right eye was presented with one of 15 luminances from the range in which Fechner’s paradox is effective. The objective of the experiment was to determine whether the psychophysical function over this range was nonmonotonic and Ll-shaped, as implied by several models of binocular brightness, or monotone increasing, but discontinuous at zero right-eye luminance. The functions associated with both left-eye intensities were found to be nonmonotone. Both minima were near the upper bound of the mesopic range.     

The retinal threshold gradient in the presence of a high-luminance target and in total darkness

The retinal threshold was measured along the horizontal meridian for the dark-adapted eye and for the same retinal region light-adapted by stray light from a small foveally fixated high-luminance target. This was done to quantify the irradiation phenomenon in the foveal-parafoveal boundary region. Two target luminances were investigated (5,531 and 13,000 fL). Seven Ss made threshold settings using a variable-intensity test spot whose retinal image was located at various angular separations from the foveal center as far as 10 deg in the temporal field. Threshold slopes were steeper for test spots imaged from 15min to 1 deg of arc from the foveal center than for spots imaged from ldegl5min to 10 deg of arc from the foveal center for the illuminated target conditions. The angular distance between the fovea and the intersection of the steep component with the shallow component of each threshold curve tended to shift toward the target’s edge with an increase in target luminance. These data are interpreted as an indication that a dynamic neural mechanism is involved in producing the irradiation phenomenon.     

The imbalance of oculomotor capture in unilateral visual neglect

Visual neglect has been associated with an imbalance in the level of activity in the saccadic system: activity in the contralesional field is suppressed, which makes target selection unlikely. We recorded eye movements of a patient with hemispatial neglect and a group of healthy participants during an oculomotor distractor paradigm. Results showed that the interfering effects of a distractor were very strong when presented in her ipsilesional visual field. However, when the distractor was presented in her contralesional field, there were no interfering effects when the target was presented in her ipsilesional field. These findings could not be explained by the presence of a visual field defect as revealed by the results of two hemianopic patients. Our results are in line with an imbalance in the level of activity in the saccadic system in visual neglect because visual elements presented in the contralesional field did not compete for saccadic selection.     

The discrimination of heading from optic flow is not retinally invariant.

Three experiments were conducted to determine whether the discrimination of heading from optic flow is retinally invariant and to determine the importance of acuity in accounting for heading eccentricity effects. In the first experiment, observers were presented with radial flow fields simulating forward translation through a three-dimensional volume of dots. The flow fields subtended 10 degrees of visual angle and were presented at 0 degree, 10 degrees, 20 degrees, and 40 degrees of retinal eccentricity. The observers were asked to indicate whether the simulated movement was to the right or the left of a target that appeared at the end of the display sequence. Eye movements were monitored with an electrooculogram apparatus. In a second experiment, static acuity thresholds were derived for each of the observers at the same retinal eccentricities. There was a significant increase in heading detection thresholds with retinal eccentricity (from 0.92 degree at 0 degree retinal eccentricity to 3.47 degrees at 40 degrees). An analysis of covariance indicated that the variation in sensitivity to radial flow, as a function of retinal eccentricity, is independent of acuity. Similar results were obtained when the Vernier acuity of observers was measured. These results suggest that the discrimination of heading from radial flow is not retinally invariant.     

Experimental measures and theoretical account of hue scaling as a function of luminance

Functions relating the magnitude of chromatic response to stimulus luminance were determined at the unique green and yellow wavelengths. These measured functions were used to account for the shift from a preponderance of green at low stimulus luminances to a preponderance of yellow at higher luminances, for a stimulus wavelength of 550 nm. The two hue magnitude functions were found to have the same power-law exponents, and the hue shift was due to the difference in thresholds between the two opponent-color systems.     

The discrimination of heading from optic flow is not retinally invariant

Three experiments were conducted to determine whether the discrimination of heading from optic flow is retinally invariant and to determine the importance of acuity in accounting for heading eccentricity effects. In the first experiment, observers were presented with radial flow fields simulating forward translation through a three-dimensional volume of dots. The flow fields subtended 10° of visual angle and were presented at 0°, 10°, 20°, and 40° of retinal eccentricity. The observers were asked to indicate whether the simulated movement was to the right or the left of a target that appeared at the end of the display sequence. Eye movements were monitored with an electrooculogram apparatus. In a second experiment, static acuity thresholds were derived for each of the observers at the same retinal eccentricities. There was a significant increase in heading detection thresholds with retinal eccentricity (from 0.92° at 0° retinal eccentricity to 3.47° at 40°). An analysis of covariance indicated that the variation in sensitivity to radial flow, as a function of retinal eccentricity, is independent of acuity. Similar results were obtained when the Vernier acuity of observers was measured. These results suggest that the discrimination of heading from radial flow is not retinally invariant.     

Near-field visual acuity of pigeons: effects of head location and stimulus luminance.

Two pigeons were trained to discriminate a grating stimulus from a blank stimulus of equivalent luminance in a three-key chamber. The stimuli and blanks were presented behind a transparent center key. The procedure was a conditional discrimination in which pecks on the left key were reinforced if the blank had been present behind the center key and pecks on the right key were reinforced if the grating had been present behind the center key. The spatial frequency of the stimuli was varied in each session from four to 29.5 lines per millimeter in accordance with a variation of the method of constant stimuli. The number of lines per millimeter that the subjects could discriminate at threshold was determined from psychometric functions. Data were collected at five values of stimulus luminance ranging from--0.07 to 3.29 log cd/m2. The distance from the stimulus to the anterior nodal point of the eye, which was determined from measurements taken from high-speed motion-picture photographs of three additional pigeons and published intraocular measurements, was 62.0 mm. This distance and the grating detection thresholds were used to calculate the visual acuity of the birds at each level of luminance. Acuity improved with increasing luminance to a peak value of 0.52, which corresponds to a visual angle of 1.92 min, at a luminance of 2.33 log cd/m2. Further increase in luminance produced a small decline in acuity.     

The reaction-time/luminance relationship for pigeons to lights of different spectral compositions

Pigeons learned to peck a key when it was illuminated during a 2-sec trial. A white-noise ready signal preceded the onset of the light; a response terminated the trial and occasionally produced reinforcement. For every trial, reaction time was recorded as the temporal interval between light onset and keypeck response. The initial experiment used “white” light; subsequent experiments used monochromatic lights of 525 and 625 nm. Within each session, the luminance of the light stimulus varied randomly over a three-log-unit range. For white light, overlapping ranges were used to extend the total luminance variation to six log units. Resulting reaction-time/luminance functions for white light were decreasing over most of the range. However, a rise in reaction time with increasing luminance was seen in the midluminance region and again at very high values. At 625 nm, the function decreased rapidly at low luminances and then leveled off or rose; at 525 nm, it was relatively flat at low luminances, where reaction times were lower than they were to photopically matched 625-nm values. Sensory and nonsensory factors might contribute to the shapes of these functions, which may be too complex to be used for psychophysical scaling.     

The relationship between search efficiency and crowding

The role of target salience in crowding has remained controversial largely because salience usually escapes objective measurement. Here we address this problem using search efficiency as a measure of target salience. In separate experiments observers determined whether parafoveal arrays of vertical Gabor patterns contained targets having a unique colour, a unique direction of motion, and a unique temporal frequency. We analysed search efficiency in the conventional manner using reaction-time gradients (in seconds per item). We also considered accuracy gradients (in percent-correct per item). Crowding is typically quantified by comparing the acuity for a target within an array to the acuity for a target presented alone. We measured orientation acuity for determining whether a slightly tilted target was clockwise or anticlockwise of vertical. Targets with a unique colour or direction of motion were found to pop out, ie (with one exception) reaction-time and accuracy gradients were insignificantly different from zero. Acuity for these targets was significantly greater than acuity for targets whose neighbours had the same colour and direction of motion. Manipulation of temporal frequency produced a wide range of search efficiencies. For three of four observers we found a linear relationship between acuity and the accuracy gradient, shallow gradients being associated with high acuity. In general, we find that crowding is weakened when observers can find a parafoveally presented target quickly and accurately.     

The role of the magnocellular visual pathway in the attentional blink

Visual attention has temporal limitations. In the attentional blink (AB) a stream of stimuli such as letters or digits are presented to a participant on a computer monitor at a rapid rate. Embedded in the stream are two targets that the participant must try to identify. Identification of the second target is severely impaired if it is presented within approximately 500ms of the first target. This is the 'blink' in visual attention. In this study we examined the role of the magnocellular visual pathway in the AB. This fast conducting pathway has high temporal resolution and contrast sensitivity. It is also insensitive to the direction of chromatic contrast, and this attribute was exploited in order to isolate its contributions to temporal attention. Colour defined, luminance noise masked AB streams were compared to AB streams of varying achromatic contrast. The four observers, (2F and 2M) aged between 21 and 35years, had normal visual acuity and colour vision. The colour stimuli produced a similar blink to the moderate contrast achromatic stimuli. This indicates that the magnocellular pathway does not have a privileged role in the attentional blink. We provide an explanation of previous apparently contradictory findings in terms of the role of different types of visual masking in the attentional blink.     

Memory and learning for visual signals in time and space

Vision is often characterized as a spatial sense, but what does that characterization imply about the relative ease of processing visual information distributed over time rather than over space? Three experiments addressed this question, using stimuli comprising random luminances. For some stimuli, individual items were presented sequentially, at 8 Hz; for other stimuli, individual items were presented simultaneously, as horizontal spatial arrays. For temporal sequences, subjects judged whether each of the last four luminances matched the corresponding luminance in the first four; for spatial arrays, they judged whether each of the right-hand four luminances matched the corresponding left-hand luminance. Overall, performance was far better with spatial presentations, even when the entire spatial array was presented for just tens of milliseconds. Experiment 2 demonstrated that there was no gain in performance from combining spatial and temporal information within a single stimulus. In a final experiment, particular spatial arrays or temporal sequences were made to recur intermittently, interspersed among, non-recurring stimuli. Performance improved steadily as particular stimulus exemplars recurred, with spatial and temporal stimuli being learned at equivalent rates. Logistic regression identified several shortcut strategies that subjects may have exploited while performing our task.     

Visual field size necessary for length comparison

Influence of visual field size upon acuity for comparing lengths of two lines was investigated. The visual field was effectively narrowed to any desired size by use of a TV display system. The TV screen presented a portion of the test stimulus, the position of which was controlled by the subject’s eye movement so that his fixation point always coincided with the center of the portion. The test stimulus was composed of two lines arranged horizontally, one of which had a length of 81 mm and the other a slightly different value. Sizes of the narrowed visual field were 20, 40, 70, 100, 150, and 340 mm wide. The comparison acuity dropped rapidly as the visual field was narrowed to and below 70 mm which was about the same extent as one of the two lines. The implication is that our excellent ability for length comparison is only possible when we can observe the whole line at one time.     

Classical and inverted White's effects

In classical White's effect, intermediate-luminance targets appear lighter when they interrupt the dark stripes of a grating and darker when they interrupt the light stripes. The effect is reversed when targets are of double-increment or double-decrement luminance, relative to the luminances of grating stripes. To find a common explanation for classical and inverted effects, we ran two experiments. In experiment 1, we utilised intermediate-target displays to show that perceived transparency dominates over occlusion only when the target luminance is close to the luminances of top regions. This result weakens transparency-based accounts of White's effect. In experiment 2, we varied grating contrast and target luminance to measure the classical effect in seven intermediate-target cases, as well as the inverted effect in four double-increment and four double-decrement cases. Both types of effect are explained by a common model, based on assimilation to the top region and contrast with the interrupted region, weighted by adjacency along the luminance continuum.     

Visual acuity improvement following fading and feedback training—II. Relationship to changes in refractive error

The present study evaluated the effects of behavioral training on visual acuity and refractive error in myopic adults. Twenty volunteers were matched on the basis of refractive error and one member of each matched pair was randomly assigned to either Fading and Feedback training or a no-treatment control condition. Visual acuity was measured with both recognition and resolution measures. Refractive error was measured subjectively using the LaserSpec optometer, and objectively by two retinoscopic examinations performed by an ophthalmologist with and without cycloplegia. The results indicated significant increases in recognition visual acuity, but not resolution visual acuity for trained subjects compared to the no-treatment controls. Trained subjects did not show improvement in refractive error on any measure, and refractive error changes were not correlated with changes in either measure of visual acuity. These data suggest that the visual acuity improvements typically found with behavioral training are not associated with changes in refractive error.     

Mesopic lightness,brightness, and brightness contrast

At mesopic mean luminances, a fixed luminance contrast produces less brightness contrast than it does at photopic luminances. This suggests that lightnesses of surfaces might also be altered at low luminances. I measured lightness, brightness, and brightness contrast in CRT simulations of achromatic paper patchworks. The illuminance of the standard pattern was fixed, producing 0.12,1.2, or 12 cd/m². The illuminance on the test pattern was varied in a lightness constancy paradigm. Constant brightness contrast required more luminance contrast at lower mean luminances. Failures of lightness constancy occurred at the lowest mean luminances, but they were minor in comparison with the loss of brightness contrast in the same pattern. These results have implications for imaging applications. Often, image content falls in both the photopic and the mesopic ranges. Our results indicate that brightness contrast may decrease substantially in low-luminance regions without large changes of surface lightness.     

Visual field differences for clockwise and counterclockwise mental rotation

Geometric line drawings were presented to normal subjects in the left visual field (LVF) or right visual field (RVF) at various degrees of rotation from a centrally presented vertical standard. The task of the subject was to indicate with a reaction time (RT) response whether the laterally presented stimulus could be rotated into the vertical standard or if it was a rotated mirror image of the standard. In Study 1, an overall right hemisphere superiority was found for RT and accuracy on match trials. Most interestingly, interactions between Visual Field and Rotation Angle for the match accuracy data and between Visual Field and Direction of Rotation (clockwise or counterclockwise) for the match RT slopes were found. These interactions suggested that clockwise rotations were more readily performed in the LVF and counterclockwise rotations in the RVF, consistent with other literature for mental rotation. The purpose of Study 2 was to replicate this finding of visual field differences for rotation direction using a design in which direction and degree of rotation were varied orthogonally. No main effect of Visual Field was found. However, significant interactions between Visual Field and Rotation Angle were found for both RT and accuracy, confirming the presence of visual field differences for rotation direction in a new sample of subjects. These differences were discussed in terms of the possibly greater relevance of medially directed stimuli and a possible hemispheric bias for rotation direction, and in terms of interhemispheric transmission factors.     

Genetic and environmental factors in orientation anisotropy: a field study in the British Isles

Visual acuity for the detection of gratings at four orientations was measured for groups of ten boys and ten girls aged five to seven years, from the following four populations: Scots in Glasgow, Pakistanis in Glasgow, Gaels in Stornoway (Outer Hebrides) and East Anglians in Littleport (Cambridgeshire fenlands). The Glaswegians, both Scottish and Pakistani, showed the normal pattern of anisotropy, with poorest acuity for oblique orientations; the East Anglians showed no significant anisotropy; while the Gaels were unusual in showing poorest horizontal acuity. A group of fourteen Pakistani children in Stornoway differed slightly from a matched group of Gaels. The group differences bore little relation to the visual environments, and were probably due to genetic or cultural factors. The relatively poor horizontal acuity of the Gaels was not correlated with astigmatism. Sex differences were also found, with the boys showing higher mean acuity and a higher ratio between vertical and oblique acuity.