Changes in human short-wavelength-sensitive and achromatic resolution acuity with retinal eccentricity and meridian

Association between Topographic Features of the Retinal Nerve Fiber Bundle and Good Visual Acuity in Patients with Glaucoma

Purpose: Maintaining visual acuity in glaucoma patients is an important part of preventing the deterioration of quality of vision. We identified specific areas of the papillomacular bundle (PMB) that were strongly associated with visual acuity, based on en-face images derived from optical coherence topography (OCT) wide scans.Methods: The study recruited 23 eyes of 21 glaucoma patients (age: 61.3 ± 13.0 years, M: F = 9:12, Humphrey field analyzer-measured mean deviation: -19.9 ± 6.5 dB) with good best-corrected visual acuity (20/20 or more) and a remaining PMB with a maximum width no more than half that of the vertical disc diameter. En-face images were derived from 12 × 9 mm wide-scan images made with DRI-OCT (Triton, Topcon). Averaged en-face images were created by identifying the disc center and fovea line (DFL) and aligning it between images. We then measured the frequency of remaining PMB at 10 µm intervals along a vertical line intersecting the DFL at its midpoint. Finally, we used a logistic analysis in a much larger group of patients to identify cases of glaucoma with low BCVA (<20/20).Results: In the averaged en-face image, the residual PMB area appeared as a high-intensity region above the DFL. Analysis showed that residual PMB was most common in an area 830-870 µm above the DFL. The correlation coefficient of residual PMB in this area to BCVA was -0.57 (p < .01), and among OCT parameters in this residual PMB area, the AUC to identify decreased BCVA was highest for ganglion cell complex thickness (0.85, p < .01), with a cutoff of 87.5 µm.Conclusions: This study identified specific areas of the PMB that were associated with BCVA in wide-scan, en-face OCT images from glaucoma patients. This suggests that it may be possible to identify visual impairment during glaucoma treatment by measuring this area.

Connectomic Identification and Three-Dimensional Color Tuning of S-OFF Midget Ganglion Cells in the Primate Retina

In the trichromatic primate retina, the "midget" retinal ganglion cell is the classical substrate for red-green color signaling, with a circuitry that enables antagonistic responses between long (L)- and medium (M)-wavelength-sensitive cone inputs. Previous physiological studies showed that some OFF midget ganglion cells may receive sparse input from short (S)-wavelength-sensitive cones, but the effect of S-cone inputs on the chromatic tuning properties of such cells has not been explored. Moreover, anatomical evidence for a synaptic pathway from S cones to OFF midget ganglion cells through OFF midget bipolar cells remains ambiguous. In this study, we address both questions for the macaque monkey retina. First, we used serial block-face electron microscopy to show that every S cone in the parafoveal retina synapses principally with a single OFF midget bipolar cell, which in turn forms a private-line connection with an OFF midget ganglion cell. Second, we used patch electrophysiology to characterize the chromatic tuning of OFF midget ganglion cells in the near peripheral retina that receive combined input from L, M, and S cones. These "S-OFF" midget cells have a characteristic S-cone spatial signature, but demonstrate heterogeneous color properties due to the variable strength of L, M, and S cone input across the receptive field. Together, these findings strongly support the hypothesis that the OFF midget pathway is the major conduit for S-OFF signals in primate retina and redefines the pathway as a chromatically complex substrate that encodes color signals beyond the classically recognized L versus M and S versus L+M cardinal mechanisms.SIGNIFICANCE STATEMENT The first step of color processing in the visual pathway of primates occurs when signals from short (S)-, middle (M)-, and long (L)-wavelength-sensitive cone types interact antagonistically within the retinal circuitry to create color-opponent pathways. The midget (L versus M or "red-green") and small bistratified (S vs L+M, or "blue-yellow") ganglion cell pathways appear to provide the physiological origin of the cardinal axes of human color vision. Here we confirm the presence of an additional S-OFF midget circuit in the macaque monkey fovea with scanning block-face electron microscopy and show physiologically that a subpopulation of S-OFF midget cells combine S, L, and M cone inputs along noncardinal directions of color space, expanding the retinal role in color coding.

Target-object integration, attention distribution, and object orientation interactively modulate object-based selection

The representational basis of attentional selection can be object-based. Various studies have suggested, however, that object-based selection is less robust than spatial selection across experimental paradigms. We sought to examine the manner by which the following factors might explain this variation: Target-Object Integration (targets 'on' vs. part 'of' an object), Attention Distribution (narrow vs. wide), and Object Orientation (horizontal vs. vertical). In Experiment 1, participants discriminated between two targets presented 'on' an object in one session, or presented as a change 'of' an object in another session. There was no spatial cue-thus, attention was initially focused widely-and the objects were horizontal or vertical. We found evidence of object-based selection only when targets constituted a change 'of' an object. Additionally, object orientation modulated the sign of object-based selection: We observed a same-object advantage for horizontal objects, but a same-object cost for vertical objects. In Experiment 2, an informative cue preceded a single target presented 'on' an object or as a change 'of' an object (thus, attention was initially focused narrowly). Unlike in Experiment 1, we found evidence of object-based selection independent of target-object integration. We again found that the sign of selection was modulated by the objects' orientation. This result may reflect a meridian effect, which emerged due to anisotropies in the cortical representations when attention is oriented endogenously. Experiment 3 revealed that object orientation did not modulate object-based selection when attention was oriented exogenously. Our findings suggest that target-object integration, attention distribution, and object orientation modulate object-based selection, but only in combination.

Choice of Grating Orientation for Evaluation of Peripheral Vision

Purpose

Peripheral resolution acuity depends on the orientation of the stimuli. However, it is uncertain if such a meridional effect also exists for peripheral detection tasks because they are affected by optical errors. Knowledge of the quantitative differences in acuity for different grating orientations is crucial for choosing the appropriate stimuli for evaluations of peripheral resolution and detection tasks. We assessed resolution and detection thresholds for different grating orientations in the peripheral visual field.

Methods

Resolution and detection thresholds were evaluated for gratings of four different orientations in eight different visual field meridians in the 20-deg visual field in white light. Detection measurements in monochromatic light (543 nm; bandwidth, 10 nm) were also performed to evaluate the effects of chromatic aberration on the meridional effect. A combination of trial lenses and adaptive optics system was used to correct the monochromatic lower- and higher-order aberrations.

Results

For both resolution and detection tasks, gratings parallel to the visual field meridian had better threshold compared with the perpendicular gratings, whereas the two oblique gratings had similar thresholds. The parallel and perpendicular grating acuity differences for resolution and detection tasks were 0.16 logMAR and 0.11 logMAD, respectively. Elimination of chromatic errors did not affect the meridional preference in detection acuity.

Conclusions

Similar to peripheral resolution, detection also shows a meridional effect that appears to have a neural origin. The threshold difference seen for parallel and perpendicular gratings suggests the use of two oblique gratings as stimuli in alternative forced-choice procedures for peripheral vision evaluation to reduce measurement variation.

Neural bandwidth of veridical perception across the visual field

Neural undersampling of the retinal image limits the range of spatial frequencies that can be represented veridically by the array of retinal ganglion cells conveying visual information from eye to brain. Our goal was to demarcate the neural bandwidth and local anisotropy of veridical perception, unencumbered by optical imperfections of the eye, and to test competing hypotheses that might account for the results. Using monochromatic interference fringes to stimulate the retina with high-contrast sinusoidal gratings, we measured sampling-limited visual resolution along eight meridians from 0° to 50° of eccentricity. The resulting isoacuity contour maps revealed all of the expected features of the human array of retinal ganglion cells. Contours in the radial fringe maps are elongated horizontally, revealing the functional equivalent of the anatomical visual streak, and are extended into nasal retina and superior retina, indicating higher resolution along those meridians. Contours are larger in diameter for radial gratings compared to tangential or oblique gratings, indicating local anisotropy with highest bandwidth for radially oriented gratings. Comparison of these results to anatomical predictions indicates acuity is proportional to the sampling density of retinal ganglion cells everywhere in the retina. These results support the long-standing hypothesis that "pixel density" of the discrete neural image carried by the human optic nerve limits the spatial bandwidth of veridical perception at all retinal locations.

Interhemispheric integration in visual search

The search task of Luck, Hillyard, Mangun and Gazzaniga (1989) was optimised to test for the presence of a bilateral field advantage in the visual search capabilities of normal subjects. The modified design used geometrically regular arrays of 2, 4 or 8 items restricted to hemifields delineated by the vertical or horizontal meridian; the target, if present, appeared at one of two fixed positions per quadrant at an eccentricity of 11 deg. Group and individual performance data were analysed in terms of the slope of response time against display-size functions (‘RT slope’). Averaging performance across all conditions save display mode (bilateral vs. unilateral) revealed a significant bilateral advantage in the form of a 21% increase in apparent item scanning speed for target detection; in the absence of a target, bilateral displays gave a 5% increase in speed that was not significant. Factor analysis by ANOVA confirmed this main effect of display mode, and also revealed several higher order interactions with display geometry, indicating that the bilateral advantage was masked at certain target positions by a crowding-like effect.

In a numerical model of search efficiency (i.e. RT slope), bilateral advantage was parameterised by an interhemispheric ‘transfer factor’ (T) that governs the strength of the ipsilateral representation of distractors, and modifies the level of intrahemispheric competition with the target. The factor T was found to be higher in superior field than inferior field; this result held for the modelled data of each individual subject, as well as the group, representing a uniform tendency for the bilateral advantage to be more prominent in inferior field. In fact statistical analysis and modelling of search efficiency showed that the geometrical display factors (target polar and quadrantic location, and associated crowding effects) were all remarkably consistent across subjects. Greater variability was inferred within a fixed, decisional component of response time, with individual subjects capable of opposite hemifield biases.

The results are interpretable by a guided search model of spatial attention – a first, parallel stage guiding selection by a second, serial stage – with the proviso that the first stage is relatively insular within each hemisphere. The bilateral advantage in search efficiency can then be attributed to a relative gain in target weight within the initial parallel stage, owing to a reduction in distractor competition mediated specifically by intrahemispheric circuitry. In the absence of a target there is no effective guidance, and hence no basis for a bilateral advantage to enhance search efficiency; the equivalence of scanning speed for the two display modes (bilateral and unilateral) implies a unitary second-stage process mediated via efficient interhemispheric integration.

The spatial and temporal deployment of voluntary attention across the visual field

Several studies have addressed the question of the time it takes for attention to shift from one position in space to another. Here we present a behavioural paradigm which offers a direct access to an estimate of voluntary shift time by comparing, in the same task, a situation in which subjects are required to re-engage their attention at the same spatial location with a situation in which they need to shift their attention to another location, all other sensory, cognitive and motor parameters being equal. We show that spatial attention takes on average 55 ms to voluntarily shift from one hemifield to the other and 38 ms to shift within the same hemifield. In addition, we show that across and within hemifields attentional processes are different. In particular, attentional spotlight division appears to be more difficult to operate within than across hemifields.

Spatial and temporal dynamics of attentional guidance during inefficient visual search

Spotting a prey or a predator is crucial in the natural environment and relies on the ability to extract quickly pertinent visual information. The experimental counterpart of this behavior is visual search (VS) where subjects have to identify a target amongst several distractors. In difficult VS tasks, it has been found that the reaction time (RT) is influenced by salience factors, such as the target-distractor similarity, and this finding is usually regarded as evidence for a guidance of attention by preattentive mechanisms. However, the use of RT measurements, a parameter which depends on multiple factors, allows only very indirect inferences about the underlying attentional mechanisms. The purpose of the present study was to determine the influence of salience factors on attentional guidance during VS, by measuring directly attentional allocation. We studied attention allocation by using a dual covert VS task in subjects who had 1) to detect a target amongst different items and 2) to report letters briefly flashed inside those items at different delays. As predicted, we showed that parallel processes guide attention towards the most relevant item by virtue of both goal-directed and stimulus-driven factors, and we demonstrated that this attentional selection is a prerequisite for target detection. In addition, we show that when the target is characterized by two features (conjunction VS), the goal-directed effects of both features are initially combined into a unique salience value, but at a later stage, grouping phenomena interact with the salience computation, and lead to the selection of a whole group of items. These results, in line with Guided Search Theory, show that efficient and rapid preattentive processes guide attention towards the most salient item, allowing to reduce the number of attentional shifts needed to find the target.

Independent variation of retinal S and M cone photoreceptor topographies: A survey of four families of mammals

In mammals, cone photoreceptor subtypes are thought to establish topographies that reflect the species-relevant properties of the visual environment. Middle- to long-wavelength-sensitive (M) cones are the dominant population and in most species they form an area centralis at the visual axis. Short-wavelength-sensitive (S) cone topographies do not always match this pattern. We here correlate the interrelationship of S and M cone topographies in representatives of several mammalian orders with different visual ecology, including man, cheetah, cat, Eurasian lynx, African lion, wild hog, roe deer, and red deer. Retinas were labeled with opsin antisera and S and M cone distributions as well as S/M cone ratios were mapped. We find that species inhabiting open environments show M cone horizontal streaks (cheetah, pig, deer). Species living in structured habitats (tiger, lynx, red deer) have increased S cone densities along the retinal margin. In species with active vision (cheetah, bear, tiger, man), S cone distributions are more likely to follow the centripetal M cone gradients. Small species show a ventral bias of peak S cone density which either matches the peak of M cone density in a temporal area centralis (diurnal sciurid rodents, tree shrews) or not (cat, manul, roe deer). Thus, in addition to habitat structure, physical size and specific lifestyle patterns (e.g. food acquisition) appear to underlie the independent variations of M and S cone topographies.