Journal of Vision (2009) 9(5):11, 1–9 http://journalofvision.org/9/5/11/ 1

Aging alters surround modulation of perceived

Department of Optometry and Vision Science, Renee Karas The University of Melbourne, VIC, Australia

Department of Optometry and Vision Science, Allison M. McKendrick The University of Melbourne, VIC, Australia

It is well established that many visual functions deteriorate with age. Perhaps counter-intuitively, a recent study revealed that older people actually require less time to discriminate the direction of motion of large, high contrast moving stimuli than young adults (L. R. Betts, C. P. Taylor, A. B. Sekuler, & P. J. Bennett, 2005). L. R. Betts et al. (2005) proposed their finding as evidence for a reduction of cortical inhibitory function within the aging . There is some neurophysiological support for this suggestion, as broadening of visual cortical neural tuning consistent with reduced inhibitory function has been observed in older animals. Here we explore the perceptual consequences of center-surround suppression within the healthy aging human visual system and report data from a center-surround contrast discrimination task (the Chubb contrast illusion). We predicted that older observers would demonstrate less center-surround suppression than younger subjects (consistent with reduced inhibition). Our data does not support this prediction as perceived contrast was altered more by surround modulation in the older than younger group (t(33) = 2.53, p = 0.02). A possible explanation for our findings is a decrease in perceptual brightness induction in the elderly group. Brightness induction relies on neural synchronization which might be disrupted by aging. Keywords: aging, contrast discrimination, surround modulation, contrast matching Citation: Karas, R., & McKendrick, A. M. (2009). Aging alters surround modulation of perceived contrast. Journal of Vision, 9(5):11, 1–9, http://journalofvision.org/9/5/11/, doi:10.1167/9.5.11.

Another example of cortical that relies Introduction on inhibitory function is surround suppression. There are a number of psychophysical visual tasks that enable the demonstration of surround suppression in the human Recent neurophysiological studies in primate (Leventhal, visual system (Chubb, Sperling, & Solomon, 1989; Wang, Pu, Zhou, & Ma, 2003; Schmolesky, Wang, Pu, & Petrov, Carandini, & McKee, 2005; Polat & Sagi, 1993; Leventhal, 2000) and cat (Hua et al., 2006) have demon- Tadin, Lappin, Gilroy, & Blake, 2003). Tadin et al. (2003) strated that the orientation and direction selectivity of neurons demonstrate surround suppression in the visual motion in primary (area V1) is altered in aging animals. processing pathways by establishing that increasing the In addition to demonstrating a significant reduction in size of a high contrast moving pattern counter-intuitively orientation and direction tuning, these studies also showed makes it more difficult to perceive its direction of motion. that older visual cortex shows increased spontaneous and Tadin et al. (2003) proposed surround inhibition as the visually evoked neural activity. The observation of reduced mechanism for this decrease in motion sensitivity for neural response tuning led Schmolesky et al. (2000)to larger higher contrast stimuli. hypothesize that age may negatively affect GABAergic The unusual perceptual phenomena described by Tadin connections in V1, leading to a reduction in inhibitory et al. (2003) motivated an investigation by Betts, Taylor, function in the visual cortex. Inhibition is central to a Sekuler, and Bennett (2005) into center-surround visual number of aspects of visual processing, including neural motion processing by elderly people. Betts et al. (2005) response selectivity for orientation and direction, hence a sought to ascertain whether surround inhibition deterio- reduction in inhibition could explain the reduced orienta- rated with age. If so, the performance of older people tion and direction tuning of aged visual neurons. This was would be expected to be less affected by large high further studied by Leventhal et al. (2003) who hypothesized contrast stimulation in the surround of neural receptive that the application of GABA and GABA agonists on fields than younger people. Indeed, their study showed individual V1 cells would restore orientation and direction that older people were able to discriminate the direction of tuning. In support of their hypothesis, they found that after motion of large, high contrast stimuli in shorter durations GABA administration some of the cells responded strongly than younger adults (Betts et al., 2005). From this finding, to a narrow range of preferred orientations and directions Betts et al. (2005) suggested that it was the reduction in and exhibited nearly no response to non-preferred orienta- efficacy of inhibitory mechanisms which produce sur- tions and directions (Leventhal et al., 2003). round suppression in neurons in the visual motion doi: 10.1167/9.5.11 Received December 18, 2008; published May 14, 2009 ISSN 1534-7362 * ARVO

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processing pathways that was responsible for the age less effect in older subjects than younger subjects differences in spatial suppression. A reduction in the (consistent with reduced inhibition), and hence that the inhibition of large and potentially uninteresting features of older participants would perceive the central target to be the visual system is unfortunately likely to be a dis- closer to its real contrast than the younger group. Some- advantage for real world visual tasks. what intriguingly, we found the complete opposite result Visual processing occurs via partially anatomically and to that we predicted. Our older group showed a greater functionally distinct pathways. Tadin et al. (2003) dem- alteration in perceived contrast of the target as a onstrated that their paradigm showed features consistent consequence of the surround than the younger group. with what is known about response properties and receptive field size of neurons in cortical area MT (a key area for visual motion processing in the extrastriate visual cortex). Given that evidence consistent with changes in Methods inhibitory function in primate is present for V1 neurons, it seems likely that perceptual alterations consistent with reduced center-surround cortical inhibitory function in V1 Participants would also be measurable in elderly humans. In this study we used a center-surround visual task (the Chubb contrast Eighteen younger adults (aged 18 to 30, mean = 24, illusion (Chubb et al., 1989)) to explore the perceptual stdev = 4.02) and 17 older adults (aged 60 to 72, mean = consequences of center-surround suppression in the 66, stdev = 4.47) participated in the study. Participants elderly. The Chubb illusion is illustrated in Figure 1 and were recruited by responding to a written advertisement involves the presentation of a small low contrast textured placed in electronic newsletters associated with the patch surrounded by an annulus of same texture but of University of Melbourne and in community newspapers. much higher contrast. The presence of the high contrast The study was approved by the Human Research Ethics surround, results in the central patch appearing to be of Committee of The University of Melbourne and all lower contrast than it truly is, that is, the contrast participants provided written informed consent prior to perception of the central patch is suppressed by the high taking part in the project. All volunteers underwent a full contrast surround. The experiments were designed to test ocular examination (including assessment of anterior and the prediction that the high contrast surround would have posterior ocular structures, and intraocular pressure).

Figure 1. Stimuli used in the surround contrast discrimination task. Stimuli were presented consecutively to participants who indicated which of the two intervals had the higher contrast. The target stimulus (front panel) was surrounded by an annulus of 95% contrast and had a radius of 4 degrees. The target was presented for 500 ms followed by a 500 ms interstimulus interval before a second stimulus was displayed (back panel). The smaller center stimulus had a radius of 0.67 degrees and seven different contrast levels were randomly presented. All targets were comprised of 4 c/deg bandpass filtered noise.

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Inclusion criteria included: normal ocular health for age, inter-stimulus interval of 500 ms, a second patch of visual acuity of 6/7.5 or better, refractive error of no more variable contrast was presented, also for 500 ms. Observ- than T5.00 diopters spherical and T2.00D astigmatism. ers were instructed to indicate which of the two intervals Participants were not allowed to be taking medications they perceived as having the higher contrast. The contrast known to affect visual function, and were not permitted to of the patch presented in the second interval was varied to have systemic conditions known to affect visual function enable a psychometric function to be obtained using a (for example: diabetes). method of constant stimuli (MOCS). Seven contrast levels were randomly interleaved and were each presented 30 times. Each participant initially performed an abbre- Stimuli and procedures viated MOCS (13 levels, presented 4 times each) that was used to select the range of the final 7 contrasts used in The computerized psychophysics experiments were more extensive procedure. A similar procedure was used conducted using a personal computer with a gamma- for the second contrast discrimination task: the “surround” corrected Sony G520 21 inch CRT monitor (frame rate condition. In this case, the 40% contrast target patch 100 Hz, resolution 1264 947 pixels, maximum presented in the first interval was surrounded by a high luminance of 100 cd/m2). Custom software was written contrast textured annulus (as illustrated in Figure 1). All in Matlab 7.0 (Mathworks, Natick, MA, USA) and the other features of the task were identical to the “no stimuli were displayed using a VSG 2/5 graphics system surround” condition. (Cambridge Research Systems, Kent, UK). Participants Example psychometric functions from a single younger viewed the center of the screen from a distance of 50 cm observer are shown in Figure 2. From the psychometric with their chin and head stabilized using a chin and functions we obtained measures of the spread of the forehead rest. All measures were conducted within a function (an estimate of the precision of the observer in single test session of approximately 2 hours in duration. making a contrast discrimination judgment) and threshold The stimuli used for the experiments are shown in (enabling the calculation of bias from the reference Figure 1. The stimulus parameters were based on those contrast). Estimates of spread and threshold were obtained used by Dakin, Carlin, and Hemsley (2005) who used the by fitting the data with a modified Cumulative Gaussian Chubb illusion (Chubb et al., 1989) to explore for (Wichmann & Hill, 2001): psychophysical correlates of reduced cortical inhibition in people with schizophrenia. Our experimental design YðtÞ¼1jðFP þð1 j FP j FNÞGðt; 2; AÞÞ; ð1Þ was similar to that of Dakin et al. (2005), with the exception that we reduced the spatial frequency of the where G(t, 2, A) is the Cumulative Gaussian distribution stimulus from 8 to 4 c/deg. We reduced the spatial with mean 2 and standard deviation A for value t. FP and frequency to reduce differences in contrast sensitivity FN represent the false positive and negative rates between our older and younger groups as contrast respectively. This equation gives a psychometric function sensitivity is increasingly reduced as spatial frequency that accounts for false responses by assuming that a false increases in the elderly (Owsley, Sekuler, & Siemsen, response is made independently of the underlying Gaussian 1983). The central gray textured stimulus had a radius distribution of responses. Curves were fit using a bootstrap of 0.67 degrees and was comprised of 4 cycle per procedure (1000 repetitions) enabling 95% confidence limits degree band-pass filtered noise (1 octave bandwidth). In to be estimated for the threshold (2) and spread (A) some conditions, the target was embedded in an annulus parameters. For this experiment, the threshold (2)repre- (4 degrees in radius) of the same textured noise but of sents the contrast level for which the reference subjectively 95% contrast (front panel of Figure 1). appeared the same as the target patch (the point-of- subjective equality (PSE)). All participants received substantive training prior to Contrast discrimination the collection of formal data. Participants were random- ized to perform either the “surround” or “no surround” Participants performed two contrast discrimination condition first and this was counterbalanced between older tasks: one to determine their ability to discriminate a and younger groups to avoid order dependent effects of difference in contrast between two central target patches, learning or fatigue between groups. the other to determine how their performance changed when the target was presented within a different surround- ing visual context (the annulus, the task illustrated in Contrast detection Figure 1). Each task used a two-interval forced choice methodology. For the condition where the target was As it is well known that normal aging reduces contrast presented alone, in the first interval the target patch was sensitivity (Elliott, 1987; Morrison & McGrath, 1985; presented at a fixed contrast of 40% for 500 ms. After an Owsley et al., 1983; Sekuler & Hutman, 1980; Wright &

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Figure 2. Example psychometric function for one younger observer. Panel (a) shows the raw data for the no surround (filled symbols) and surround (unfilled symbols) conditions. Fitted curves are the mean cumulative Gaussian returned by the bootstrapping procedure. The curves have similar slopes but different points-of-subjective equality, with the surround causing the target patch to appear to be of lower contrast than the veridical contrast of 40% (dotted vertical line). Panels (b) and (c) show the fitted curves for the no-surround and surround separately with the shaded area illustrating the 95% confidence limits of the curve fits.

Drasdo, 1985) contrast detection thresholds were also from statistical normality. The non-parametric Mann- measured for the central target stimulus to establish the Whitney Rank Sum Test was used for distributions where magnitude of this effect for our stimulus. A yes-no data departed from a normal distribution. procedure was used where observers had to nominate if they saw the target stimulus appear by pressing a button. The stimulus was displayed for 500 ms and observers had 1.5 sec to respond. Contrast thresholds were measured Results using two interleaved staircases (3 down, 1 up) each of 6 reversals. The mean of the last four reversals was calculated as the result of a single staircase. This Contrast discrimination thresholds procedure was performed twice and the mean of the results of all staircases was returned as the final Figure 3 shows group mean performance (T95% threshold estimate. confidence limits for the mean) for the contrast discrim- ination tasks. Panel 3a shows the best estimates of the point-of-subjective equality (PSE) returned from the curve Statistics fitting. When the target was not surrounded by the annulus, the groups performed similarly with both Statistical analysis was performed using SigmaStat younger and older groups matching the 40% contrast 3.0 (SPSS, Chicago, IL, USA). All data sets were tested target patch with a patch that was close to 40% contrast for conformation to statistical normality (Kolmogorov- (no significant difference between groups, t(33) = 0.127, Smirnov test) and equal variance (Levene Median test). p = 0.90). When the surround was present (right hand side To compare performance between elderly and younger of panel 3a), both groups showed a bias in their perception groups, parametric tests (t-tests or ANOVA where appro- of the target’s contrast, with the patch appearing to be priate) were used for data distributions that did not depart significantly less than 40% contrast. The mean PSE for the

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Figure 3. Performance of the younger and older groups on the contrast discrimination task as returned from the fitted psychometric functions. Panels (a) and (b) show means T95% confidence intervals for the mean. Panels (c) and (d) show medians and 95th percentiles of the 95% confidence limit of the PSE estimate and spread estimates obtained from the individual curve fits.

older group was significantly less than that for the younger Figure 3d as medians and 95th percentiles. The mean group (t(33) = 2.53, p = 0.02), indicating that contrast spread of the psychometric function did not significantly perception was more biased by the presence of the annulus differ between groups (no surround: t(33) = 0.906, p = 0.37; for the older than the younger group. This finding was surround: t(33) = j0.70, p = 0.49) nor did the median 95% opposite to our hypothesis. Panel 3c shows the distribution confidence limit of the spread estimates (Mann-Whitney of the width of the 95% confidence limit of the PSE as Rank Sum Test; no surround: p = 0.31, surround: p =0.58). determined from the bootstrapping procedure (see Figure 2). Hence the data shown in Figures 3b and 3d demonstrates Data is shown as the median and 95th percentile. There that the older and younger groups were equally able to was no difference between groups for this measure for discriminate between stimuli of different contrasts. Several either the no surround (Mann-Whitney Rank Sum Test, previous studies have shown equivalent contrast discrim- p = 0.30) or surround condition (Mann-Whitney Rank Sum ination in older and younger groups (Beard, Yager, & Test, p = 0.88), indicating that the confidence in the Neufeld, 1994; Mei, Leat, & Hovis, 2007). estimates returned from the psychometric functions was thesameinthetwogroups. Group mean data for the spread of the psychometric Contrast detection thresholds function (a measure of the ability to discriminate between different contrast targets) is shown in Figure 3b,withthe It is well established that contrast sensitivity declines distribution of the 95% confidence limits of the spread with aging (Elliott, 1987; Morrison & McGrath, 1985; estimates from the individual curve fits being shown in Owsley, Gardner, Sekuler, & Lieberman, 1985; Owsley

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et al., 1983; Sekuler & Hutman, 1980). Consequently, while Figure 3 shows that the ability to discriminate between different contrasts was equivalent between groups, it should be expected that our older participants would, on average, have poorer contrast sensitivity (elevated contrast detection thresholds). As shown in Figure 4, group mean threshold was elevated for the older participants relative to the younger group (t-test on the log contrast threshold data: t(33) = j6.31, p G 0.001).

Reduced effective contrast stimuli in younger observers

Due to the elevated contrast thresholds of the older group (as shown in Figure 4) the stimuli used for the contrast discrimination task would have been closer to contrast threshold for the older group than the younger group. However, for all participants, the 40% target stimulus was substantially suprathreshold (the most Figure 5. The effect of manipulating contrast on the point-of- elevated contrast threshold in the older group was subjective equality for the center-surround contrast discrimination 11.8%). To explore whether the difference in contrast task where a) shows raw data and b) the bias in contrast sensitivity was likely to explain the enhanced shift of the perception as a ratio of target contrast. Data for three observers PSE experienced by the older group, we repeated the from the young adult group is shown. contrast discrimination task on a subset of three younger observers for target stimuli of 30% and 20% contrast. The need to be observed as the contrast of the stimuli was surround conditions for these targets were 71.25% and reduced. Figure 5 shows the performance of the three 47.5% contrast respectively to keep the ratio of center to younger observers for the contrast discrimination task for surround contrast consistent across all conditions. The reduced contrast targets. For each contrast condition and purpose of this additional experiment was to determine each observer, the magnitude of the perception bias was whether manipulating the target and surround stimuli to be determined (the difference between the surround and no closer to the observer’s contrast threshold changed the surround conditions). As the contrast of the target was magnitude of the perceived bias. In order to explain the reduced, so too was the magnitude of the bias. A repeated older observers’ performance, an elevation in bias would measured ANOVA revealed this effect to be statistically significant (p = 0.02), with post-hoc testing (Holm-Sidak method) showing that the bias at 20% contrast was less than that at either 30 or 40% contrast. Figure 5b shows the bias plotted as a ratio of target contrast, and in this case, there is no significant difference between contrast levels (RM-ANOVA, p = 0.32) as consistent with Weber like behavior. We recognize that this does not truly simulate the effect of reduced contrast sensitivity in older people. Nevertheless, making the target and surround closer to the subject’s contrast threshold shifted performance in the opposite direction to that necessary to explain the differ- ence between older and younger subjects for the 40% contrast condition (Figure 3). Our older observers would need better contrast sensitivity than the younger cohort for an on-average larger bias to be found in the older group.

Discussion

Figure 4. Contrast threshold performance for younger and older participants. Data is shown is group means (T95% confidence Our experiment produced results contrary to those we intervals of the mean). predicted. We hypothesized that the presence of the

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high contrast surround would have less effect on the to demonstrate reduced surround suppression when com- perceived contrast of the central target for the older pared to younger adults. This predicted finding would be adults, consistent with a reduction in inhibitory function consistent with our original hypothesis, however is within the visual cortex. This expectation was derived opposite to that supported by the data. from the results of Betts et al. (2005) who found that With that in mind, these findings are likely to reflect older people were better able to perceive large, high differences in neural function, but what might these contrast moving stimuli than were younger people, differences be? As demonstrated in this experiment and consistent with a reduction in inhibitory surround strength in many previous studies, a stimulus’ perceived contrast is (Betts et al., 2005). Intriguingly, our results were in the influenced by surrounding stimuli. It is well known that opposite direction to our initial prediction. While this grating-type stimuli with surrounds of the same orienta- study was primarily interested in changes in visual tion (referred to as iso-oriented) are suppressed due to performance in the elderly that are presumed to arise (Cannon & Fullenkamp, 1991, 1996). from cortical mechanisms, the well-known deterioration However, when stimuli are surrounded with either a cross- of optical quality with age also needs consideration. oriented surround or a surround out of phase with the One important factor is decreased retinal illuminance, center stimulus (Cannon & Fullenkamp, 1991; Yu, Klein, primarily due to senile miosis (age-related decrease in & Levi, 2001) the opposite result is obtained, that is, there size) (Winn, Whitaker, Elliott, & Phillips, 1994). is little suppression, and sometimes stimulus contrast We did not experimentally investigate the effect of enhancement. One mechanism used to explain such retinal illuminance on our methodology because of the enhancement in perceived contrast is referred to as pre-existence of substantial evidence that retinal illumi- brightness induction which results from local luminance nance does not explain the deterioration of contrast contrast at the edge of the central target (Biederlack et al., processing in older adults. Indeed, many studies which 2006; Yu et al., 2001). Our random texture stimulus was have looked at contrast sensitivity and contrast related not continuous between the center and the surround of the tasks have eliminated retinal illuminance as the major stimulus, hence may have resulted in some degree of cause of changes seen in older observers (Betts, brightness induction similar to a phase-shifted grating. We Sekuler, & Bennett, 2007; Betts et al., 2005; Elliott, chose a non-continuous border in order to minimize the Whitaker, & MacVeigh, 1990; Sloane, Owsley, & effect of contextual modulation believed to result from Alvarez, 1988). Similar to Betts et al. (2005), we could feedback from higher centers (Lotto & Purves, 2001). If have ran an additional control experiment involving present, brightness induction due to phase differences at neutral density filters on our younger participants in order the border of the center and surround of the stimulus to emulate the retinal illuminance reduction of the older would act to partially negate the impact of the inhibitory participants. Alternately, we could have dilated the surround (that is: brightness induction would make the of the older participants so that they would appear more central target appear higher contrast, whereas the presence like those of the younger participants. However, with the of the high contrast annulus would result in surround above-mentioned literature eliminating retinal illuminance suppression reducing the perceived contrast of the central as a cause of difference on other contrast processing stimulus). measures, we consider it unlikely that retinal illuminance While somewhat speculative, it is interesting to con- is the major cause of the differences seen in this present sider whether our results could be explained by a study. reduction in the magnitude of brightness enhancement in Due to their reduced contrast sensitivity the experimen- our elderly group. A recent physiological study has found tal stimuli were a reduced effective contrast for the older that brightness induction is related to neuronal synchroni- group. Spatial summation area varies with stimulus zation (Biederlack et al., 2006). Their study found that contrast: for example; the preferred stimulus size of a when brightness enhancement is induced by phase offset V1 neuron increases with decreases in stimulus contrast of gratings between center and surround, the synchroniza- (Tailby, Solomon, Peirce, & Metha, 2007). Consequently, tion increases between neurons responding to the center a reduced effective contrast for the older participants grating, while rates of firing remain unaffected. As would predict integration over a larger spatial area. Betts, previously mentioned, there is considerable evidence for Sekuler, and Bennett (2009) have recently explored spatial an increase in spontaneous and visually evoked neuronal summation in older and younger participants for center- firing in older animals (Hua et al., 2006; Leventhal et al., surround motion stimuli and conclude that alterations in 2003; Schmolesky et al., 2000). Furthermore, psycho- spatial summation predicted by reduced effective contrast physical studies have found some evidence consistent with sensitivity in older observers are insufficient to explain an increase in intrinsic neural noise in older humans (Betts their results. In our study, given that the experimental et al., 2007). If there is, in fact, an increase in spontaneous target was of fixed size, an increase in central spatial neuronal firing in older people, then that could decrease summation area would presumably result in a reduced synchronization of neuronal firing and, therefore, sub- area of surround, hence potentially less inhibitory sequently decrease the ability to produce brightness strength. Such a situation would predict the older group enhancement.

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