Differences in the nearpoint of convergence with target type.

PURPOSE: The purpose of this study was to determine the effect on measurements of the nearpoint of convergence (NPC) of different target types. In order to assess the influence of accommodation, the NPC was also measured under conditions of varying accommodative demand. METHODS: The NPC was measured to the nearest 0.5 cm using three targets: the RAF rule, the sharpened tip of a pencil and the tip of the examiner's index finger. All measurements were performed under the same conditions on two groups of asymptomatic subjects, a group of 14 presbyopic subjects and a group of 14, younger, non-presbyopic subjects. The influence of accommodative demand was assessed in the non-presbyopic group by measuring the NPC while subjects viewed the RAF rule target through +2.00 and -2.00 lenses held in front of their eyes. RESULTS: For the presbyopic group, the NPC (break) and NPC (recovery) were independent of target type. However, the NPC (break) was significantly less remote than the NPC (recovery). Comparative data for the non-presbyopic group showed that NPC (break) for the RAF target was less remote than for either the pencil tip or finger tip targets. In agreement with the results from the presbyopic group, the NPC (recovery) was independent of target type. CONCLUSION: For subjects with little or no accommodation, the NPC does not depend on the target used and is the same measured with the RAF rule, a pencil tip or finger tip. In non-presbyopic subjects there appears to be a small accommodative influence on the NPC, which is target dependent. However, the difference is probably not clinically important.

Spatial scale of visual analysis for vernier acuity does not vary over time.

The visual system filters spatial pattern through a range of narrowly tuned spatial mechanisms, but the rules by which the outputs of these mechanisms are combined across time to extract precise geometrical information are not yet clear. One hypothesis is that spatial analysis shifts over time from coarse to finer spatial scales, in order to extract fine spatial information. An alternative hypothesis is that thresholds are determined by the signal-to-noise ratio within an optimal spatial scale. In this study, we measured vernier acuity across exposure duration for equally visible long lines and short lines and found no improvement in spatial precision with time. Using a simultaneous spatial-masking paradigm, we determined the active spatial scales at 100 and 1000 ms. The results show no significant changes in spatial scale, or in the size-range of active scales, for the two exposure durations. Furthermore, whereas vernier thresholds vary markedly with line contrast, we find only modest shifts in spatial scale. Taken together, our results suggest that for vernier acuity, spatial scale is selected very early, and that vernier thresholds are predominantly limited by signal strength within that spatial scale.

Position acuity with opposite-contrast polarity features: evidence for a nonlinear collector mechanism for position acuity?

Vernier acuity for opposite-contrast polarity stimuli clearly poses problems for local contrast models of relative position processing. In Expt 1 we show that vernier thresholds for abutting, or closely separated features of opposite-contrast polarity, are degraded across a wide range of stimulus strengths and configurations; but for widely separated stimuli they are more or less independent of contrast polarity (confirming and extending previous work). In Expts 2 and 3 we use a one-dimensional spatial noise masking paradigm to investigate to what extent the same mechanisms masked by this noise contribute to the relative position processing of same and opposite polarity stimuli. The orientation tuning functions determined using this paradigm are quite different for same and opposite polarity targets, for both line vernier acuity, and closely spaced two-dot alignment. However, for widely separated targets (24 min arc or more), they are similar. Over a range of separations from 3 to 30 min arc, for same and opposite polarity dots, masking is strongest at a spatial frequency of about 10 c/deg. Our results are consistent with the notion that signals from early (and relatively high spatial frequency) linear filters are collected in a second-stage nonlinear mechanism, which collates information along an orientation trajectory. We suggest that different properties of the mechanisms at each level of processing, can constrain positional acuity at small and large separations.

Rod temporal channels.

Mechanisms underlying rod temporal contrast sensitivity have been considered in terms of a fast retinal signal predominating at mesopic levels and a slower retinal signal predominating at scotopic levels. Here we use a small signal masking method, which has previously been used to delineate the cone-mediated cortical temporal channels, to investigate their rod-mediated cortical counterparts. The results suggest that there are three different rod-mediated cortical temporal channels, one which is lowpass and two which are bandpass. These mechanisms co-exist at all light levels and their relative sensitivity depend on the stimulus spatio-temporal frequency.

Spatial alignment across gaps: contributions of orientation and spatial scale.

To assess the contributions of orientation and spatial scale to the processing of relative-position information for broadband spatial targets, we measure misalignment thresholds for dots separated by as much as 6 deg, in the presence of one-dimensional spatial noise. For all the dot separations, thresholds for misalignment are raised most when the mask is oriented at approximately 20 deg to either side of true alignment. This bimodal orientation tuning function appears to be fundamental to the alignment judgment, including abutting vernier acuity for equally visible lines [Vision Res. 33, 1619 (1993)]. With increasing dot separation the spatial frequency at which peak masking occurs becomes progressively lower, a finding that suggests that the spatial mechanisms important for processing this information become larger. However, the rate of increase in size of these putative mechanisms is insufficient to account for the increase in relative-position thresholds for increasingly separated stimuli (i.e., Weber's law for alignment). In addition, oriented masks placed between two target lines lead to threshold elevation, revealing that the collection of positional information between target features may be important for optimal processing of misalignment thresholds. The findings of this study suggest that, although shifts in spatial scale of the underlying low-level oriented mechanisms may contribute to increased misalignment thresholds with increasing separation, additional factors, such as positional uncertainty associated with eccentricity per se, are limiting.

The processing of temporal modulation at different levels of retinal illuminance.

How does our temporal vision change as the mean illuminance reduces? We have examined the processing of near-threshold temporal information for a range of illuminance values (2850--0.15 phot td). At high illuminance, the modulation transfer function can be shown to be mediated via three underlying temporal filters that vary in sensitivity with spatial frequency. As the mean illuminance decreases these channels appear to change their sensitivity. Even at the lowest (scotopic) illuminance levels we were able to find evidence for at least two channels mediating detection threshold. There are also changes in the tuning properties of these channels such that the processing of high temporal frequencies is differentially compromised, resulting in a reduction in the flicker fusion limit of each channel, and a shift in the peak of the band-pass channel. The slope of the fall-off in sensitivity at high temporal frequencies is unaffected by test spatial frequency at each illuminance level, suggesting its limiting factor is one that is insensitive to spatial frequency. We propose that the changes in the tuning of the temporal filters occur because of an early (e.g. photoreceptor) change in the response dynamics, or by interactions between photoreceptors, rather than changes at or beyond the level of the channel response.

Spatial scale shifts in amblyopia.

We used a masking paradigm to uncover the properties of the mechanisms engaged by the amblyopic visual system for vernier acuity and line detection. Line vernier and line detection thresholds were measured in the presence of one-dimensional noise masks varying in orientation, spatial frequency content or contrast. Our results reveal that in both normal and amblyopic eyes, there is a bimodal orientation tuning function for vernier acuity, i.e. vernier acuity is most strongly masked by mask orientations approx. +/- 10 deg on either side of the target lines. In contrast, in both normal and amblyopic eyes, line detection is most strongly masked when the mask and line target have the same orientation. In the normal fovea, the spatial frequency tuning is bandpass, with a peak spatial frequency of about 10 c/deg. In the amblyopic eyes, the spatial tuning is similar in specificity; however the peak is shifted to lower spatial frequencies, suggesting a shift in the scale of spatial processing of line stimuli. For all of the amblyopic eyes, the increased line detection thresholds are approximately proportional to the shift in spatial scale. In anisometropic amblyopes, the (unmasked) vernier threshold is elevated in proportion to the shift in spatial scale; however in some amblyopes with constant strabismus the shift in spatial scale is not sufficient to account for the degraded vernier acuity. The "extra" increase in vernier thresholds associated with strabismus may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.

Spatial scale shifts in peripheral vernier acuity.

Abutting line vernier acuity thresholds are markedly degraded in peripheral vision, while line detection thresholds are elevated to a much lesser extent. To study the spatial and orientation tuning properties of the mechanisms underlying peripheral line vernier acuity, abutting vernier thresholds were measured in the presence of one-dimensional band-limited spatial noise masks varying in orientation and spatial frequency. To examine the effects of these masks on target visibility, line detection thresholds were also measured. We find that in both the fovea and the periphery, noise masking produces marked elevations of vernier thresholds, which are tuned to both spatial frequency and orientation. (i) Spatial frequency tuning: in the fovea, the spatial frequency tuning is bandpass, with a bandwidth of approximately 2.5 octaves, and a peak spatial frequency of about 10 c/deg. In the periphery the spatial tuning is similar in bandwidth, however the peak shifts systematically to lower spatial frequencies with increasing eccentricity, implying that thresholds are mediated by spatial mechanisms tuned to progressively larger spatial scales with eccentricity. (ii) Orientation tuning: at all eccentricities there is a bimodal orientation tuning function for vernier acuity, consistent with the hypothesis that the responses of at least two filters, whose orientations straddle the target lines, are combined to extract vernier offset information. In contrast, at all eccentricities, line detection is most strongly masked when the mask and line target have the same orientation. For both the line detection and line vernier tasks, the scale of the most sensitive spatial mechanisms shifts systematically with eccentricity. The change in line detection threshold with eccentricity is approximately proportional to the variation in spatial scale; however this shift in spatial scale is not sufficient to account for the degraded peripheral vernier acuity. The extra increase in peripheral vernier thresholds may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.

Suprathreshold temporal-frequency discrimination in the fovea and the periphery.

To address the question of whether temporal-frequency information in the fovea and the periphery is processed in fundamentally different ways we measured temporal-frequency-discrimination thresholds for spatiotemporally narrow-band stimuli presented at suprathreshold contrast. Temporal-frequency-discrimination thresholds are similar (within a factor of 2) at the fovea and at 30 degrees in the periphery. We use a line-element approach and three spatiotemporally separable temporal mechanisms to model foveal and peripheral data with the same degree of fidelity. These findings suggest that not only are the front-end temporal mechanisms in the fovea and periphery likely to be similar but also the way in which their outputs are combined at more central sites is the same.

Orientation, masking, and vernier acuity for line targets.

In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two abutting lines, we investigated the effects of one-dimensional band-limited spatial noise masks superimposed with the target, on vernier thresholds. Unidirectional vernier thresholds were measured in the presence of masks varying in orientation, spatial frequency content and luminance modulation. Because of the dependence of vernier thresholds on target visibility, the effects of these masks on target detection thresholds were also measured. In accordance with the results of Findlay [(1973) Nature, 241, 135-137] but contrary to an hypothesis that the direction of the vernier offset is mediated by the differential output of spatial filters of a single orientation, our results reveal a bimodal orientation tuning function for vernier acuity. We propose that, for offset line targets, the differential responses of at least two filters with orientations which straddle the target lines are combined to extract relative position information. The spatial frequency tuning characteristics of the optimal mechanisms for mediating vernier information are similar to those optimal for detecting the target lines themselves, except that they are tuned to a slightly higher spatial frequency and have a slightly narrower bandwidth. The spatial mechanisms most sensitive to the vernier offset and to target detection exhibit similar responses to increases in mask modulation. This finding suggests that these tasks are limited by the same source of noise, and explains why under a variety of experimental manipulations, equally visible vernier targets result in similar vernier thresholds.

Visibility and vernier acuity for separated targets.

The purpose of this study was to investigate the roles of the putative "spatial filter" and "local sign" mechanisms in determining line vernier thresholds for a range of target separations, using stimulus contrast or visibility as a tool. In Expt 1, the effects of varying target contrast and exposure duration on vernier thresholds for lines separated by 90 min arc, where the reference line was fixated, were measured. Contrast thresholds for the nonfixated test line were also measured, so that the role of its visibility in limiting vernier thresholds could be assessed. Vernier thresholds decreased almost proportionally with increasing contrast only until the visibility of the test line reached about 3 times the contrast detection threshold, regardless of exposure duration. At higher visibility levels, vernier thresholds were virtually independent of target contrast. In Expt 2, the effects of varying target contrast on vernier thresholds for a range of target separations (2-90 min arc) were measured using a 250 msec exposure duration. Vernier thresholds for abutting lines and for those separated by 2 min arc, decreased with target contrast until about 30 times the test line's contrast detection threshold. However for lines separated by 4 min arc or more, they were only weakly dependent on target contrast at much lower visibility levels. We propose that for very close separations vernier thresholds are limited by the contrast response properties of spatial filters. For separations of 4 min arc or more, thresholds appear to be limited by the positional uncertainty of the test line, which increases with eccentricity.

Visibility, timing and vernier acuity.

To investigate the relationship between contrast detection and vernier acuity for abutting targets, the effects of varying target exposure duration (12-2000 msec) on vernier and contrast detection thresholds for long, thin lines and sinusoidal gratings (1 and 8 c/deg), were measured. Vernier thresholds decreased with both increasing exposure duration and increasing target contrast. Predictions made for equally visible targets show that the effect of exposure duration on vernier thresholds is almost completely accounted for by its effect on target visibility. Vernier thresholds and contrast detection thresholds for line targets were also measured in the presence of a spatiotemporal mask, for different exposure durations. Again, once the effect of this mask on target visibility was accounted for, there was virtually no remaining effect of exposure duration on vernier thresholds. The results of these experiments suggest that similar spatial mechanisms mediate both contrast detection thresholds and vernier thresholds for abutting targets; and that the processes involved in target detection and the extraction of relative position information are limited by the same factors.

Visibility, luminance and vernier acuity.

To assess the role of quantal limitations and target visibility on vernier acuity, we measured line detection thresholds and vernier thresholds for abutting dark line targets on a uniform background, for a range of retinal illuminance levels and target contrasts. Measurements were made for stimuli presented at the fovea, and at a retinal eccentricity of 2.5 deg. Although the relationship is truly curvilinear, line detection thresholds and vernier thresholds using targets of a fixed contrast, follow a square-root dependence on retinal illuminance over a significant portion of the illuminance range. Once the effect of retinal illuminance on line detection thresholds has been accounted for, there is little further effect of retinal illuminance on vernier thresholds, at least for visibility levels of up to five times the contrast detection threshold. This finding suggests that the spatial mechanisms which mediate changes in the detection threshold and vernier thresholds for abutting targets are similar, and are limited by the same sources of noise. Vernier thresholds for these thin line targets are approximately inversely proportional to target contrast for both retinal loci, and at all retinal illuminance levels tested. However, vernier thresholds for a constant visibility target are always lower for foveal than for eccentric stimuli, even after the effects of increased spatial pooling with increasing eccentricity are accounted for.

Sensitivity to temporal luminance modulation in congenital nystagmus.

Uniform field temporal contrast sensitivity functions were compared for 10 subjects with congenital nystagmus (seven idiopathic, three with albinism) and 10 normal observers. Sensitivity to luminance modulation did not differ significantly from normal at any temporal frequency tested except 0.5 Hz, at which the subjects with nystagmus had slightly higher sensitivity. In conjunction with other recent findings, their essentially normal temporal contrast sensitivity suggests that persons with nystagmus process retinal information continuously, rather than selectively during only only one phase of the nystagmus.