For letter identification, temporal sensitivity decreased as a function of increasing retinal eccentricity. To determine if the differences between these results and the previously cited findings of increased flicker sensitivity were due to the nature of the psychophysical task and/or the physical parameters of the stimuli, we next measured the temporal sensitivity of the peripheral field for the tasks of grating discrimination, letter detection, and symmetry detection.
Experiment II. Grating Orientation Discrimination
If the previous findings with letter identification were predominately based on the spatial frequency content of the letter elements, then grating discrimination using square-wave gratings having the same dominant frequencies as those contained in the letters used in Experiment I should show similar changes in temporal sensitivity with size and retinal eccentricity. If these previous findings were unique to the task of letter identification, then duration thresholds for discrimination of the orientation of extended gratings might show different interactions between spatial frequency and retinal eccentricity.
Grating Orientation Discrimination.
For each trial, the orientation of the gratings was randomly chosen to be either horizontal or vertical. a The subject was required to fixate on the LED, respond as to whether the LED flickered, and state the orientation of the grating. The subject was required to guess when the grating orientation could not be discriminated.
The median (N = 4) threshold duration (in frames) for grating discrimination are plotted as a function of position on the temporal retina in Fig. 5. For each point, the interquartile range was smaller than the size of the symbol. Each size is represented by a unique symbol. For all grating sizes, the duration needed to discriminate the orientation of the grating increased with increasing eccentricity. At all eccentricities, as grating size decreased, threshold duration increased; however, the rate of decrease in sensitivity with eccentricity was considerably less for the two larger target sizes. For the 40-min arc gratings, median threshold ranged from 1 frame at 3° to 10 frames at 22° eccentricity, whereas for the 46-min arc gratings, median threshold did not change appreciably over the range of eccentricities tested, ranging from one frame at 3° to two frames at 22° eccentricity. The data for each grating size were best fitted by Equation 1, and the r2 values of these fits ranged from 0.86 to 0.99. As in experiment I, Ao values decreased as grating size increased; however, unlike the findings for letter identification, A1 decreased with the larger grating sizes (Fig. 3 B).
Experiment III. Letter Detection
The differences between the findings of experiment I and experiment II could be due to either the local differences in the physical characteristics between letter stimuli and gratings or the differences in the perceptual/cognitive requirements of the psychophysical tasks. The first hypothesis is based on the observation that identification of the letters depended on the global configuration of the line segments, on line segment lengths that were shorter than the segments of corresponding gratings, and/or on obliquely oriented segments. To test these hypotheses, we presented the same physical letter stimuli used in experiment I but altered the subject’s task. In this experiment, subjects were required only to report detection of letters, not to identify the letter presented.
A set of nine Sloan letters (as in experiment I) was used in experiment III. Letter size ranged from 4.5 to 46 min arc, with stroke width of one-fifth of the overall size. After the warning tone, a letter (chosen randomly from the set of nine letters for each trial) was or was not presented. For each trial, the subject was required to state whether the LED flickered and whether a letter was present or absent. In this experiment, the subject was not required to identify the letter.
The median (N = 4) threshold durations (in frames) for letter detection are plotted as a function of position on the temporal retina in Fig. 6. For each point, the interquartile range was smaller than the size of the symbol. Each size is represented by a unique symbol. For each letter size, the duration required to detect a letter increased with increasing eccentricity. The rate of loss was much less for larger letters, ranging from a median threshold of 1.5 frames at the fovea to a median threshold of five frames at 22°. At all eccentricities, as letter size decreased, threshold duration increased. The r2 values of the fits of Equation 1 ranged from 0.90 to 0.94. Ao values decreased as letter size increased; the slopes of the curve (A1) describing letter detection threshold duration as a function of eccentricity were also dependent on letter size (Fig. 3 C). Therefore, for the task of letter detection, the rate of increase in threshold duration as a function of eccentricity was less for larger than for smaller letters. These findings were similar to those obtained for grating orientation discrimination and dissimilar to those observed for letter identification, even though the physical stimuli were the same.
Experiment IV. Symmetry Detection
The above experiments demonstrated that the target size independence of the rate of loss of duration sensitivity for the task of letter identification is not due to the spatial frequency content of the stimuli. Instead, these results may be due to the pattern identification processes involved in letter identification. We tested this hypothesis using symmetrically oriented line patterns. The stroke width of the segments in the symmetry patterns was similar to those used to construct the letter and grating stimuli. Only the spatial relationships among the segments were altered.
There were five different patterns of each type of symmetry, and for each trial, one of the 20 patterns was chosen randomly for presentation. Four staircases were run simultaneously, one for each type of symmetry and one for asymmetric targets. The subject was required to fixate on the LED, respond as to whether the LED flickered, and state the type of symmetry seen. The subject was required to guess when the symmetry could not be identified. The trial continued until all four staircases were completed.
The overall size needed to detect symmetry was larger than for letter identification, letter detection, or grating orientation discrimination at equivalent eccentricities. The median (N = 4) threshold durations (in frames) for detection of the three types of symmetry are plotted as a function of position on the temporal retina in Fig. 7. The thresholds for detection of asymmetrical targets (not shown) were always shorter than the thresholds for the detection of targets with symmetry, with the exception of the largest target size. This is because the subjects tended to guess “asymmetric” when they were unsure of the actual symmetry of the target and because at shorter durations and increasing eccentricity, all targets appeared asymmetric. For most median data points, the interquartile range was smaller than the size of the symbol. The data for each type of symmetry is represented by a unique symbol (□ for vertical; • for horizontal; and ◊ for double). For any size and any symmetry type, the duration needed to detect the symmetry of the target increased with increasing eccentricity. At any eccentricity for all types of symmetry, as size decreased, threshold duration increased. The r2 values of the fits of Equation 1 ranged from 0.91 to 0.99. Ao values decreased as target size increased, much like the change in A0 values observed for letter detection. A1 also decreased as a function of target size for all types of symmetry (Fig. 3 D).
We have demonstrated that the duration sensitivity for the tasks of letter identification, grating orientation discrimination, letter detection, and symmetry detection decreased with increasing retinal eccentricity. This is in contrast to the reports of increased temporal sensitivity in the peripheral retinal. Many of the studies that have reported increased temporal sensitivity of the peripheral visual field used experimental paradigms where the stimulus was a large spatially uniform, flickering stimulus and the psychophysical task was stimulus detection. 12–16,18–20,24 For example, studies have shown that the highest resolvable flicker frequency is not independent of visual field location when stimuli of constant size and uniform spatial luminance are tested. Rovamo and Raninen 25 have shown that the highest resolvable flicker frequency to a 9° sinusoidally modulated stimulus increased as a function of increasing eccentricity; they also observed increased peripheral sensitivity whether or not the overall stimulus size was M-scaled. Similar increases in psychophysical flicker sensitivity for targets presented in the peripheral visual field have been reported in many studies. 13,15,18,20,24–28 The increased temporal sensitivity of the peripheral retina also holds for pulsed stimuli. Hartmann et al. 26 have demonstrated an increased temporal sensitivity in the periphery over a light-to-dark ratio of 1:1 to 1:2 for a 1° target. These authors have also demonstrated that the same effect occurs for stimulus sizes from 0.5° to 3° and over a range of photopic luminances. The increase in temporal sensitivity of the peripheral retina has been attributed to the increased diameter of cone photoreceptors, 18 and it has previously been demonstrated that an increase in temporal sensitivity as a function of retinal eccentricity can be observed at the level of the outer retina. 24
In the current experiments, we did not find an increase in temporal sensitivity as a function of eccentricity for any task, even for the task of letter detection. Our findings may have been due to the spatial frequency content of our stimuli. It has been frequently reported that the resolution acuity of the peripheral retina decreases due to physiological and anatomical reductions in cone and ganglion cell densities and due to increases in ganglion cell and cortical cell receptive field size as a function of retinal eccentricity. 2,8,9,27,29–32 It has also been reported that contrast sensitivity for sine-wave gratings declines as a function of eccentricity and that the peak of the contrast sensitivity function shifts to lower spatial frequencies in the periphery. 2,9,32–35 With low cortical spatial frequency gratings, contrast sensitivity was independent of eccentricity for exposure durations of 100 to 1000 ms; whereas with high cortical spatial frequency gratings, contrast sensitivity increased as a function of exposure duration for all eccentricities studied. 5 There have been numerous reports concerning the spatio-temporal tuning of the magno- and parvocellular substreams. In the classic descriptions of the sensitivity of these visual streams, the magnocellular pathway is sensitive to large (lower spatial frequency) targets and higher temporal frequencies; whereas the parvocellular pathway is sensitive to color as well as higher spatial and lower temporal frequencies. This might explain the difference in eccentricity-dependent rates of loss between larger and smaller targets. However, this would imply that differences in the spatio-temporal tuning of the two pathways remained constant with eccentricity. Croner and Kaplan 36 have reported that the size of receptive fields for both pathways increases similarly as a function of retinal eccentricity. This results in each system shifting its spatial tuning to lower spatial frequencies in the periphery with a resulting gain in lower spatial frequency temporal sensitivity. We have also found that large target sizes (low spatial frequencies) have shorter duration thresholds in the peripheral retina than small targets for the tasks of letter detection, grating orientation discrimination, and symmetry detection. This was not the case for letter identification; the increase in duration thresholds with eccentricity was the same for large targets as it was for smaller targets even though the spatial frequency content of the stimuli was similar to those of the other psychophysical tasks.
Choice of Stimulus Duration
Many of the published studies on the sensitivity of the peripheral retina have not manipulated duration parametrically, but have instead chosen either relatively long presentations (1–2 s, Rovamo and Raninen;37 500–1500 ms, Rovamo et al. 4 and Virsu and Rovamo 27) or relatively short presentations (100 ms, Strasburger et al.;38 8 Hz, Regan and Beverley;22 20 ms, Rovamo et al. 5). Our current findings demonstrate that the choice of stimulus duration will influence measured sensitivity and that the magnitude of this influence will depend on an interaction between the nature of the physical stimulus and the task requirements. This is especially true for the task of letter identification, where threshold durations can be on the order of a few seconds under certain stimulus and eccentricity conditions. Past studies have reported improvement of acuity thresholds with increasing exposure durations for centrally presented targets. 39,40 Visual acuity improved with duration up to a 500-ms exposure duration, depending on stimulus conditions. 40 It had been previously demonstrated that integration over longer durations for foveal viewing was not dependent on eye movements. Keesey 41 measured acuity under stabilized viewing conditions and found that threshold improvement with exposure duration was similar to that under nonstabilized viewing conditions, ruling out the effects of eye movements on the exposure-dependent improvement in acuity and suggesting a role for neural mechanisms. In the peripheral retina, we found that acuity improved with increasing duration up to a few seconds of duration. Given the sparser sampling density of retinal elements in the peripheral retina, it is possible that small eye movements occurring over the longer presentation durations may play a more important role than for foveal viewing. We previously demonstrated that when sparsely sample letter optotypes were presented, small movements of the optotypes improved letter identification accuracy. 42
These findings, coupled with the results of the present study, emphasize that stimulus duration should be carefully selected when designing psychophysical assays of visual function in the peripheral visual field or when choosing stimuli for rehabilitation training.
Detection, Discrimination, and Identification
We found that for stimuli with overall target sizes ≥23 min arc (or approximately ≤6.5 cpd), the threshold durations required for detection, discrimination, and identification were equivalent at the fovea (at least within the resolution of the duration of a single frame). Similarly, other published work has demonstrated no difference among localization, detection, and identification when targets are presented foveally. 24,43,44 In our study, threshold durations for detection or discrimination of targets presented in the peripheral retina were shorter than those for identification or symmetry detection. The difference between the thresholds for detection or discrimination and the thresholds for identification or symmetry detection increased with decreasing target size and increased with increasing eccentricity. For example, thresholds for identification of a 9-min arc letter at the fovea required a stimulus duration about five times longer than for either letter detection or grating orientation discrimination. Johnson et al. 45 examined luminance thresholds for detection and resolution of simple shapes. They found that both increased stimulus eccentricity and decreased target size elevated thresholds for detection and recognition. In agreement with our results, the elevation of thresholds for recognition was much greater as a function of eccentricity than that for detection. Recently, Busey 43 measured two-pulse and temporal contrast sensitivity functions for numbers presented foveally and at 6 deg. He found that in the fovea, localization and identification thresholds were determined by the same temporal frequencies. In the periphery, localization tasks were determined by higher temporal frequencies than identification tasks.
We examined the temporal aspects of letter identification and grating orientation discrimination in the peripheral field and compared these findings to temporal aspects of detection. Although we found that letter identification, letter detection, grating discrimination, and symmetry detection always required longer duration stimuli in the periphery than in the central visual field, spatially uniform flickering targets can be detected at shorter durations in the periphery than in the central visual field. At equivalent target sizes, letter identification demonstrated the steepest decline in duration sensitivity. This might be the case if the final stages of the identification process are rate limiting. That is, regardless of the interactions of the spatial content of the stimulus and the temporal tuning, the rate of eccentricity-dependent duration sensitivity loss for the task of identification was greater than for the tasks of detection and discrimination.
This work was supported by grants from the U.S. Department of Veterans Affairs, The Allene Reuss Foundation, The Helen Hoffritz Foundation, and a Center grant from The Foundation Fighting Blindness, Inc.
Received July 26, 2000; revision received December 6, 2000.
Department of Ophthalmology
New York University School of Medicine
New York, New York 10016
a Because of the differences in the number of choices used in the separate experiments reported in this paper, it is important to test for the equivalence of threshold points. The shape of the psychometric function and therefore the value obtained at threshold is influenced by the slope of the function and by the probability of guessing (number of alternative choices). An increase in the slope of the psychometric function would result in a lower stimulus value at a given threshold criteria (79% in our experiments). To test whether slope changes as a function of stimulus and eccentricity, we repeated these experiments using a method of constant stimulation. The average slope of the functions was 1.6 ± 0.52, and slope did not change systematically with stimulus condition. A change in the number of alternatives with no change in slope will simply increase by a constant the stimulus values for equivalent detection probabilities. In the experiments reported here, the differences in threshold temporal durations for a criterion of 79% correct will be an increase of 0.11 log units for nine choices (0.11 probability) and a 0.08 log unit increase for four choices (0.25 probability) over the value obtained for two choices (0.5 probability). This would result in a small and equal increase in threshold values for all conditions and eccentricities. It would not yield the pattern of results found in our experiments.
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Keywords:© 2001 American Academy of Optometry
peripheral visual field,; letter identification,; duration thresholds