Seiple, William PhD; Holopigian, Karen PhD; Shnayder, Yelizaveta MD; Szlyk, Janet P. PhD
Many aspects of visual performance decrease as a function of stimulus position in the peripheral visual field. For example, acuity and contrast sensitivity decrease as a function of retinal eccentricity. 1–6 These psychophysically measured deficits have been correlated with reductions of cone and ganglion cell densities, a reduction in the retinal ganglion cell to cone ratio, and a reduction in the cortical representation of the peripheral visual field. 2,7–11
Temporal sensitivity also exhibits change with increasing eccentricity; however, temporal sensitivity increases in the peripheral visual field. Pulse detection thresholds are shorter and flicker fusion frequencies are higher in the peripheral visual field than in the center whether or not the size of the stimulus is scaled with eccentricity. 12–16 In addition, motion sensitivity is tuned to higher velocities in the peripheral visual field than in the central visual field. 17 Measured electrophysiologically and psychophysically, the peak of the temporal modulation transfer function shifts to higher temporal frequencies with increasing eccentricity. 18–20
To further examine the interactions between spatial and temporal stimulus properties for targets presented in the peripheral visual field, we measured duration thresholds for letter identification, letter detection, grating discrimination, and symmetry detection. The purpose of this research was to assess whether the duration sensitivity of these psychophysical tasks benefited from the increased temporal sensitivity in the periphery. It was hoped that by quantifying duration sensitivity of the peripheral retina, more appropriate temporal stimuli could be used in rehabilitation programs that train reading using eccentric preferred retina loci in patients with advanced macular disease.
Four normally sighted, healthy adults (aged 24, 28, 39, and 48 years) participated in this series of experiments. All subjects were refracted to ≥20/20 central acuity at each of the viewing distances used in these experiments. Each subject was tested monocularly using the right eye. Targets were presented in the nasal visual field on the horizontal meridian at eccentricities ranging from 0° to 22°. After explanation of the experimental procedures, each subject gave informed consent to participate.
Targets were presented on an Apple 12-inch monochrome monitor (67-Hz frame rate). Mean background luminance was 46 cd/m2, and the target luminance was 73 cd/m2 (Weber contrast = 0.59). A white foam board surrounded the monitor and was illuminated to the same mean luminance as the cathode ray tube. By changing viewing distances (75 to 130 cm), fixation eccentricity (0 to 50 cm), and target size (3.4 to 15 mm), the following target sizes were available for testing: 4.5, 7, 9, 15, 23, 32, 40, 46, 55, and 64 min arc. In experiments I and III, letter optotypes ranging in overall size from 4.5 to 64 min arc were presented. In experiment III, square-wave gratings were presented within windows that ranged in overall size from 9 to 46 min arc. Two and one-half cycles of the grating (Michelson contrast = 63% contrast) were presented at each target size. Each half-cycle of the grating contained stroke widths that corresponded to those used to construct the letter targets. In experiment IV, patterns of symmetrically organized line segments with stroke widths similar to those used to construct the letter and grating targets were presented; symmetry targets ranged in overall size from 23 to 92 min arc. Four types of symmetrically oriented patterns were presented: double, horizontal, vertical, and asymmetrical. Examples of these targets are presented in Fig. 1.
The subject’s left eye was patched and his/her head was placed in a forehead and chin rest to minimize movement. Peripheral testing is a difficult task because subjects tend to saccade toward briefly presented peripheral targets. To reduce large eye movements during target presentation, the subject was instructed to fixate on an illuminated LED (mean luminance = 50 cd/m2). During each trial, the LED was or was not flickered (25 Hz, 50% modulation) simultaneously with the presentation of a peripheral target. The subject was required to respond whether the LED flickered and then to identify the peripheral target. A similar method to insure fixation was used by Regan and Beverley. 22 The position of the pupil was also monitored during stimulus presentation using a CCD camera. Saccades of a few degrees were readily apparent, and only trials with steady fixation and correct LED (fixation) responses were accepted to ensure that the targets always fell at the intended eccentricity. A “white” noise screen that physically controlled the duration of the stimulus followed the target presentation.
The order of testing was: (1) a 500-ms warning tone; (2) the centrally fixated LED was either flickered or not, and a target was simultaneously presented to the peripheral retina for a duration determined by the staircase procedure; (3) a “white” noise screen was presented for 750 ms; and (4) the subject was required to state whether the LED flickered and to identify the target seen in the periphery. The subject was required to guess when uncertain and was not given verbal feedback concerning the accuracy of his/her responses.
In all of these experiments, the dependent variable was target duration. Duration was controlled using a “3 down/1 up” staircase procedure: after three consecutive correct responses, target duration was decreased, and after one incorrect response, target duration was increased. Our software controlled the duration step size: for the first two reversals, the step size was 25 frames; for the next three reversals, the step size was ten frames; and for the final four reversals, the step size was one frame. The trial continued until nine reversals of the staircase were obtained. The durations of the final four reversals of the staircase were then averaged to determine the temporal threshold. If the duration exceeded 267 frames for three consecutive presentations, the test was stopped, and the threshold was entered as nonrecordable. Because a cathode ray tube was used to present the stimuli, all of the threshold results are presented as duration in number of frames (at a frame rate of 67 Hz, each frame was nominally 15 ms).
All of the duration threshold data when plotted as a function of eccentricity were best fitted by an exponential :
where Ao determines the vertical position of the function (the threshold at the fovea) and A1 determines the slopes of the curve (i.e., rate of sensitivity loss as a function of eccentricity (x)).
Experiment I. Letter Identification
A set of nine Sloan letters was chosen (D, H, K, N, O, R, S, V, and Z). Overall target size ranged from 9 × 9 to 64 × 64 min arc. Letters were drawn using a stroke width of one-fifth of the overall target window. Parish and Sperling 23 have demonstrated that discrimination frequency for letter identification peaks near 2.0 cycles per object spatial frequency. Each letter size was tested in an individual run. For each trial, a letter chosen randomly from the set of nine letters was presented, and the test continued until the staircase was completed.
The median (N = 4) threshold durations (in frames) for letter identification are plotted as a function of position on the temporal retina in Fig. 2. For most points, the interquartile range (i.e., data points lying between the 25th and 75th quartile) was smaller than the size of the symbol. The data for each letter size are represented by a unique symbol. For all letter sizes, the duration required to identify a letter increased with increasing eccentricity. At all eccentricities, as letter size decreased, threshold duration increased. The median data for each letter size were best fitted by Equation 1. The r2 values of these fits ranged from 0.93 to 0.99. Values of Ao decreased as letter size increased. Values of A1, which determine the slopes of the curve (the rates of sensitivity loss as a function of eccentricity), were similar for all letter sizes (Fig. 3 A). This is further illustrated in Fig. 4, where all of the data have been shifted horizontally (by their Ao) and are fit by a single function with A1 = 0.23.
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. Cited Here...
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