The effects of crowding on eye movement patterns in reading

Acta Psychologica 160 (2015) 23–34

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The effects of crowding on eye movement patterns in reading Emanuela Bricolo a,b,⁎, Carola Salvi a, Marialuisa Martelli c,d, Lisa S. Arduino e,f, Roberta Daini a,b a

Psychology Department, University of Milano-Bicocca, Milano, Italy Milan Center for Neuroscience, Milan, Italy c Psychology Department, University of Rome “La Sapienza”, Rome, Italy d IRCCS Fondazione Santa Lucia, Rome, Italy e Department of Human Sciences, University LUMSA, Rome, Italy f Institute of Cognitive Sciences and Technologies, ISTC-CNR, Rome, Italy b

a r t i c l e

i n f o

Article history: Received 17 April 2014 Received in revised form 7 June 2015 Accepted 8 June 2015 Available online xxxx Keywords: Crowding Reading Eye movements Space

a b s t r a c t Crowding is a phenomenon that characterizes normal periphery limiting letter identification when other letters surround the signal. We investigated the nature of the reading limitation of crowding by analyzing eyemovement patterns. The stimuli consisted of two items varying across trials for letter spacing (spaced, unspaced and increased size), lexicality (words or pseudowords), number of letters (4, 6, 8), and reading modality (oral and silent). In Experiments 1 and 2 (oral and silent reading, respectively) the results show that an increase in letter spacing induced an increase in the number of fixations and in gaze duration, but a reduction in the first fixation duration. More importantly, increasing letter size (Experiment 3) produced the same first fixation duration advantage as empty spacing, indicating that, as predicted by crowding, only center-to-center letter distance, and not spacing per se, matters. Moreover, when the letter size was enlarged the number of fixations did not increase as much as in the previous experiments, suggesting that this measure depends on visual acuity rather than on crowding. Finally, gaze duration, a measure of word recognition, did not change with the letter size enlargement. No qualitative differences were found between oral and silent reading experiments (1 and 2), indicating that the articulatory process did not influence the outcome. Finally, a facilitatory effect of lexicality was found in all conditions, indicating an interaction between perceptual and lexical processing. Overall, our results indicate that crowding influences normal word reading by means of an increase in first fixation duration, a measure of word encoding, which we interpret as a modulatory effect of attention on critical spacing. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The relationship between eye-movements and reading has been studied for a long time. At the beginning of the 20th Century, Huey calculated that, while reading a text, the eyes move across the page (saccadic eye movements) at a nearly constant rate and that fluent adult readers make about four fixations per second (Huey, 1908). As a consequence, the reading rate was thought to be the product of the number of fixations and the number of letters that could be acquired in each fixation (Woodworth, 1938). Subsequently, O'Regan (1980) suggested that the amplitude of saccades in reading should be expressed as a number of characters rather than as degrees of visual angle, and Morrison and Rayner (1981) showed that the average saccade amplitude remains constant at 5–6 characters with increasing character size. Recently, it has been shown that crowding, a decoding impairment limiting the number of letters that can be processed in parallel in a glimpse, predicts reading rate (Pelli, Tillman, Su, Berger, & Majaj, 2007). ⁎ Corresponding author at: Dipartimento di Psicologia, Università di Milano-Bicocca, Edificio U6, Piazza dell'Ateneo Nuovo 1, 20126 Milano, Italy. E-mail address: [email protected] (E. Bricolo).

http://dx.doi.org/10.1016/j.actpsy.2015.06.003 0001-6918/© 2015 Elsevier B.V. All rights reserved.

Crowding is a well-studied operationally defined psychophysical phenomenon, whereby a letter is hardly identified when surrounded by nearby letters. The aim of this study is to show the eye-movement marker of crowding in functional reading. 1.1. Crowding Beyond acuity, letter recognition is impaired by crowding (for a review see Pelli, Palomares, & Majaj, 2004; Levi, 2008; Whitney & Levi, 2011). This phenomenon, first named by Stuart and Burian (1962), has been explained in terms of the failure of the feature integration process within a spatial window (e.g. Parkes, Lund, Angelucci, Solomon, & Morgan, 2001; Pelli et al., 2004). This window has been variously termed recognition span, perceptual span, visual span or uncrowded window (Legge, Mansfield & Chung, 2001; O'Regan, 1990; Pelli et al., 2007; Rayner, 1986). Pelli et al. (2007) showed that the visual span (i.e., the number of letters that can be processed in a glimpse) corresponds to the size of the uncrowded window, namely, the letters that escape crowding at a given retinal eccentricity. The crowding effect is in fact related to the critical spacing between letters that is needed to restore

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E. Bricolo et al. / Acta Psychologica 160 (2015) 23–34

recognition. This spacing is roughly equal to half of the target viewing eccentricity (Bouma, 1970). Bouma's proportionality of critical spacing with eccentricity means that feature integration failure is present almost always in the periphery. In fact, for the identification of a foveal letter, the integration field extends only through a few minutes of arc, which is close to the acuity threshold (Latham & Whitaker, 1996), while the amplitude of the integration field increases together with eccentricity but independently from visual acuity. Critical spacing is not linked to letter size per se nor to empty spacing per se, but it is center-to-center letter distance which limits letter recognition in crowding (Arditi, Knoblauch, & Grunwald, 1990; Pelli et al., 2004; Strasburger, Harvey, & Rentschler, 1991). With this in mind, we examined whether the effect of interletter spacing on eye movements during reading could be attributed to crowding. Indeed, when reading a text, some letters fall in the fovea, but most letters are located in the periphery. Since critical spacing scales with eccentricity, there will be a point beyond which it will not be possible to identify the letters. The size of the uncrowded window for reading shrinks as it moves away from the foveal region (Chung, Mansfield, & Legge, 1998; Legge, Ahn, Klitz, & Luebker, 1997; Legge, Mansfield, & Chung, 2001; Legge et al., 2007; Pelli et al., 2007). In a fixed gaze condition, a proportional increase in spacing starting from fixation allows crowding to be avoided because the letters pushed further into the periphery have proportionally increasing spacing needs. On the other hand, this proportional increase in spacing starting from fixation is not feasible in an ecological reading context in which the eyes move continuously. Because of this, up to now, crowding has been studied almost exclusively with fixed gaze. We aimed to study the direct effect of crowding on the efficiency of reading by measuring eye movements in conditions of free viewing. In this condition, one possibility for partially reducing crowding is constant spacing. Indeed, while reading, some of the words will be seen parafoveally and increasing spacing at a constant rate would slightly move the crowding impairment towards the letters more in the periphery. Accordingly, it could be predicted that, in functional reading, when the eyes are free to move, an increase in letter spacing or letter size may similarly improve eye movement guidance by reducing the number of fixations or/and the fixation duration. Two studies suggested the involvement of crowding in the effect of spacing on eye movements measures. McDonald (2006) found that a reduction of letter spacing, keeping constant the spatial width of word stimuli, increased fixation duration. Hautala, Hyona, and Aro (2011) compared two different spacings given by proportional font and monospaced font. They found that the former, where an increase in the number of letters did not widen the word's spatial extent, induced an increase in fixation duration and gaze duration with respect to the latter. Although Hautala et al. (2011) attributed this effect to the number of letters, both studies suggested a role of crowding in fixation duration. 1.2. Visual span, perceptual span, and the lexicality status of the stimuli The visuo-spatial distribution of characters is relevant for the calculation and the programming of sacades, and the manipulation of both interletter and interword spacing greatly influences reading and saccadic eye movements (e.g., Paterson and Jordan, 2010; Pollatsek & Rayner, 1982). McConkie and Rayner (1975) elegantly demonstrated that the amount of information that is used by the observer to guide saccades while reading extends for up to 10 characters to the right of fixation. However, when random letters are used, the span size is considerably lower than McConkie and Rayner's (1975) estimate. O'Regan (1990) proposed a distinction between the perceptual span that is obtained with words and that might be influenced by the lexical knowledge of the stimuli, and the visual span that is obtained with random letters (see Rayner, 1986 but also Legge et al., 1997, 2001; Chung et al., 1998; Legge et al., 2007; Legge & Bigelow, 2011). This suggests an interaction between perceptual and lexical components, during eye-movement guidance in reading. The first step in reading aloud consists of the mapping of visual features onto

representations through the computation of a set of letters that are displayed in a horizontal spatial orientation (McClelland & Rumelhart, 1981, Rumelhart & McClelland, 1982). This computation is probably achieved in parallel and represents a major challenge for word recognition models that need to incorporate visual limitations, such as crowding (e.g., Coltheart, Rastle, Perry, Langdon, & Ziegler, 2001; Plaut, McClelland, Seidenberg, & Patterson, 1996). In accordance to the dual route model of word recognition (DRC) proposed by Coltheart et al. (2001), while pseudowords are read via a slow grapheme-to-phoneme conversion strategy (GPC route), words can be read with both the GPC route and a less slow direct lexical matching (lexical route). According to the DRC model, reading aloud would be achieved in parallel using the lexical route and serially using the grapheme-to-phoneme conversion rule (but see Zorzi, Houghton, & Butterworth, 1998). The lexicality advantage may thus suggest that during reading, acquisition letter processing is optimized through a reduction in the size of the integration fields with a consequent increase in the uncrowded window size expressed by a reduction in number of fixation. However, if the perceptual limitation set by crowding constitutes a rigid bottleneck one might expect the same number of letters to be uncrowded when words and pseudowords are presented (Levi, 2008; Pelli & Tillman, 2008). In this case subjects may use a guessing strategy for words (e.g., Paap, Newsome, McDonald, & Schvaneveldt, 1982), producing different decoding times. In this vein, differences may be found in the fixation duration for these types of stimuli when crowding is relieved by increasing the spacing or size of letters. In the present study, we conducted three experiments in order to analyze the effects of interletter spacing, lexicality and number of characters on eye movements during reading. We developed a new two items reading task that allowed the testing of the effects of center-to-center letter distance (either by manipulating the letter spacing within a word or the font size). As in functional reading, in this task the reading pattern of the second item (the only one analyzed) is influenced by a previous similar item and not by a fixed starting point (as in a single item reading task). In the first experiment, we recorded eye movements in normal readers by manipulating spacing and stimulus length while observers read words and pseudowords aloud. Although investigation of the complexities of oral compared to silent reading is out of the scope of the present paper, in the second experiment, in order to exclude the interference of time consuming articulatory processes, which could have slowed visual scanning, we asked new participants to perform the same task reading silently. The third experiment used the same stimuli and procedures as Experiment 2, but manipulated character size rather than spacing. We hypothesized that if the observed changes are due to crowding and not to the insertion of empty interletter spacing per se, then manipulating size or spacing should lead to similar results. In particular, it has been shown that increasing spacing induces more fixations, reduces fixation duration and does not influence gaze duration (e.g., Slattery & Rayner, 2013). We hypothesized that the number of fixations depends mostly on the string spatial extension. Thus, we predicted obtaining similar results on this parameter by increasing the number of letters or the spaces between them. In contrast, we conjectured that the decrease in fixation duration may reflect encoding and may be due to a release from crowding. In this vein, we predicted the same reduction in fixation duration when increasing letter size or letter spacing. On the other hand, if the increase in the number of fixations and the decrease in fixation duration are due to spacing per se, the manipulation of size should not induce the same effects as the introduction of empty spacing. 2. Experiment 1: oral reading The first experiment was designed to study the effect of interletter spacing on eye movements. We required participants to read aloud.

E. Bricolo et al. / Acta Psychologica 160 (2015) 23–34

This allowed us to control for accuracy. We measured the effect of lexicality by comparing words and pseudowords. To maintain the exploratory behavior as close to functional reading as possible, we presented two-word stimuli in a row and analyzed only the second stimulus (target word). Finally, to allow correct identification of the last fixation on the target word, we added a secondary task consisting in the identification of a letter presented at the right edge of the screen. Observers were instructed to perform the letter identification task as fast as possible after reading the stimuli. 2.1. Methods 2.1.1. Participants Sixteen students at the University of Milano-Bicocca participated in the experiment (6 males and 10 females, mean age 22.4 ± 2.9 years). All participants had normal or corrected to normal vision (with contact lenses only), were native speakers of Italian and were skilled readers. Visual acuity was evaluated using the Lea SYMBOLS® charts (Hyvärinen, Näsänen, & Laurinen, 1980). 2.1.2. Apparatus Participants' eye movements were recorded using a monocular video-based eye tracking system (ASL MODEL 5000, Applied Science Laboratories Inc.). Horizontal and vertical coordinates of the eye line of gaze were recorded at a 60 Hz sampling rate and stored on a separate PC for offline analysis. Eye position was measured with a spatial resolution of about 0.5 deg. Saccades were defined as movements of the eyes between fixations. Fixations were defined as periods when the line of gaze remained within a 0.5 deg circle for at least 48 ms. Eye blinks were detected as any abrupt “loss” of the eye position signal. The stimuli were presented on a 19-in. Samsung SyncMaster 1200nf monitor with a 1024 × 768 pixel resolution. Participants sat with their head supported by a chin rest and a forehead rest that was 67 cm from the screen. Stimulus presentations and response recordings were controlled using a PC running E-Prime (Psychology Software Tools, Inc., version 1.2).

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(see Fig. 1). The 432 words were selected from the LEXVAR database (Barca, Burani, & Arduino, 2002). Pseudowords were constructed from words and at least two letters were changed in random positions to preserve pronunciation and minimize word similarity. Pseudowords were matched with words in character spacing and number of letters. Words and pseudowords were composed of 4, 6 and 8 letters with 144 items for each length. The stimuli were presented in two conditions: unspaced (standard) spacing (0 pixel between characters; 21 pixels i.e. 0.70 deg center-to-center distance) and spaced (21 pixels between characters; 42 pixels, i.e. 1.40 deg center-to-center distance). Stimuli with the same lexicality, length and spacing were paired in the same trial. The word and pseudoword stimuli were presented in different blocks and the order of the two blocks was counterbalanced across subjects. Spacing and length conditions were randomized in the two lists. Forty practice trials (20 for each lexicality condition) were included at the beginning of each list and these trials were not considered in the analyses. 2.1.4. Procedure Participants were informed that they would have to read words or pseudoword stimuli aloud and perform an identification task (secondary task) of a letter that appeared at the right border of the screen and was vertically aligned with the stimuli (see Fig. 1). Prior to the beginning of the experiment, a 9-point calibration procedure was performed. Calibration was checked before each trial and was repeated if necessary to reduce possible eye position measurement errors due to subject repositioning movements. After a successful calibration, the trials were presented. Subjects were instructed to fixate on the middle-left point of the calibration screen that was colored in red. As soon as their gaze was on this point, the trial began with the appearance of the stimuli. When participants completed reading, they were required to identify a letter, either an M or an N that was positioned at the extreme right side of the screen, by pressing one of two buttons. No response time limit was given to the participants. The experiment took approximately 25 min to perform. 2.2. Results

2.1.3. Materials and design Stimuli were drawn in white on a black background (see Fig. 1). In each trial the stimuli consisted of two items (both words or both pseudowords) rendered in Courier font with each character having a fixed size of 21 × 21 pixels (0.70 × 0.70 deg). The two items were separated by a 1.71 deg empty space and presented horizontally in the center of the screen so that one word was displayed on the right side and the other word was displayed on the left side of the screen

Fig. 1. Example of the stimuli used in Experiments 1, 2 and 3: Small unspaced stimuli above, small spaced stimuli in the middle and larger stimuli and pseudowords below.

Our focus was on the saccades falling on the second word of each stimulus pair. First fixation durations and landing coordinates for all fixations on the second word were extracted using an ad-hoc program in MATLAB. All trials with a loss of signal or a blink (12.6%; mean 27.3; SD 14), with m–n identification errors (1.5%; mean 3.2; SD 2.4) or with refixations on the first word (2.8%; mean 6.1; SD 3.4) were discarded. The distribution of such missing data was comparable across experimental conditions. Reading errors were very few (3%; mean 6.6; SD 3.8). Overall, 93.3% (SD 2.97) of the errors were made while participants were reading non-words. In this condition no significant differences were found for unspaced (45.7%; SD 1.85) and spaced (47.6%; SD 1.84) non-words. Likewise no significant difference was found for spaced (3.8%; SD .59) and unspaced (2.9%; SD .41) words. Number of errors increased proportionally to the number of letters only in the nonword condition (respectively for the unspaced condition: 3.8%; SD .58 for 4 letter words, 13.3%; SD .81 for 6 letter words and 28.6%; SD 1.41 for 8 letter words; for the spaced condition: 4.8%; SD .6 for 4 letter words, 12.4%; SD 1.28 for 6 letter words and 30.5%; SD 1.63 for 8 letter words). None of the comparisons across spaced and unspaced number of errors turn out significant. The number of fixations, the first fixation duration and gaze duration for the second word were used as dependent variables. We chose, as a measure of word encoding, the first fixation duration because it is comparable across different word lengths. 2.2.1. Number of fixations For each subject, the number of fixations was computed and entered into a repeated measures ANOVA with spacing (unspaced and spaced),

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lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The number of fixations is reported in Table 1 and plotted in Fig. 2. All the main effects were significant. The main effect of spacing [F(1,15) = 113.27, p b 0.001] showed a significantly larger number of fixations for spaced (mean 2.61) compared to unspaced stimuli (mean 1.90). The main effect of lexicality [F(1,15) = 174.34, p b 0.001] showed more fixations for pseudowords compared to words (mean 2.68 and 1.83, respectively). The main effect of item length [F(2,30) = 120.65, p b 0.001] showed an increase in the number of fixations as the number of letters increased (4 letters: mean 1.54; 6 letters: mean 2.30; 8 letters: mean 2.93). All two-way interactions were also significant. The interaction between spacing and lexicality [F(1,15) = 40.78, p b 0.01] emerged because spacing caused a larger increase in fixations for pseudowords (unspaced: mean 2.19; spaced: mean 3.17) compared to words (1.61 compared to 2.06, unspaced and spaced, respectively). The interaction between spacing and item length [F(2,30) = 22.05, p b 0.001], according to Scheffé post-hoc comparisons, emerged because spacing caused a larger increase in fixations for 6 (p b 0.001) and 8 (p b 0.001) letter words compared to 4 (p b 0.001) letter words (spaced 4, 6, 8 items: 1.74, 2.75, 3.35, respectively; unspaced 4, 6, 8 items: 1.35, 1.85, 2.50, respectively). The interaction between lexicality and item length [F(2,30) = 65.90, p b 0.001] was also significant and reflected the higher number of fixations as item length increased for pseudowords compared to words. Scheffé post-hoc comparisons indicated that the number of fixations was higher for pseudowords compared to words for each length (p b 0.005). No other interactions were significant. 2.2.2. First fixation durations For each subject, the average first fixation duration for each stimulus was entered into a repeated measures 2 × 2 × 3 ANOVA with spacing (unspaced and spaced), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The first fixation duration is reported in Table 2 and plotted in Fig. 3. A significant main effect of spacing [F(1,15) = 31.64, p b 0.001] was found reflecting the shorter fixations for the spaced stimuli (188 ms) compared to the unspaced stimuli (220 ms). A significant main effect of lexicality [F(1,15) = 50.53, p b 0.001] was also found, which showed longer fixations for pseudowords (217 ms) than for words (192 ms). No other factors or interactions reached significance. 2.2.3. Gaze duration For each subject, gaze duration (the sum of the duration of all fixations on the target word before the eyes leave that word) was computed and entered into a repeated measures 2 × 2 × 3 ANOVA with spacing (unspaced and spaced), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The gaze duration is reported in Table 3 and plotted in Fig. 4. All the main effects were significant. The main effect of spacing [F(1,15) = 29.47, p b 0.001] showed a significantly longer gaze duration

Fig. 2. Experiment 1: The number of fixations (±1 SE) as a function of item length for the reading aloud of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

for spaced (mean 470.92 ms) compared to unspaced stimuli (mean 411.1 ms). The main effect of lexicality [F(1,15) = 167.38, p b 0.001] showed longer gaze duration for pseudowords compared to words (mean 543 ms and 339.01 ms, respectively). The main effect of item length [F(2,30) = 85.06, p b 0.001] showed an increase in the gaze duration as the number of letters increased (4 letters: mean 305.34 ms; 6 letters: mean 440.98 ms; 8 letters: mean 576.71 ms). All two-way interactions were also significant. The interaction between spacing and lexicality [F(1,15) = 31.48, p b 0.001], according to Scheffé posthoc comparisons, emerged because spacing induced longer gaze duration for pseudowords (unspaced: mean 493.47 ms; spaced: mean 592.53 ms; p b 0.001), but not for words (328.72 ms compared to 349.31 ms, unspaced and spaced, respectively; p = 0.27). The interaction between spacing and item length [F(2,30) = 5.31, p b 0.005], according to Scheffé post-hoc comparisons, emerged because spacing caused a longer gaze duration for 6 (p b 0.001) and 8 (p b 0.005) but not 4 (p = 0.23) letter stimuli compared to unspaced stimuli (spaced 4, 6, 8 items: 321.61 ms, 484.94 ms, 606.21 ms, respectively; unspaced 4, 6, 8 items: 289.01 ms, 397.02 ms, 547.21 ms, respectively). The interaction between lexicality and item length [F(2,30) = 52.29, p b 0.001] was also significant and reflected, according to Scheffé post-hoc

Table 1 The table reports means ± SE for the number of fixations for all experimental conditions for all experiments. Pseudowords 4

Words 6

8

4

Experiment 1: Spacing aloud Crowded 225.3 ± 8.9 Uncrowded 194.4 ± 9.2

229.6 ± 10.0 202.6 ± 9.0

247.7 ± 12.4 202.4 ± 10.5

196. 6 ± 9.0 180.1 ± 8.9

208.6 ± 10.6 175.6 ± 7.6

213.7 ± 10.0 174.9 ± 6. 7

Experiment 2: Spacing silent Crowded 222.5 ± 7.9 Uncrowded 181.1 ± 7.6

231.3 ± 8.7 169.2 ± 5.9

212.1 ± 9.4 167.2 ± 5.5

193.9 ± 7.2 176.9 ± 6.3

193.0 ± 6.8 170.7 ± 4.7

197.2 ± 7.4 165.5 ± 6.0

Experiment 3: Size silent Crowded Uncrowded

238.5 ± 13.8 180.2 ± 9.1

225.3 ± 11.3 171.7 ± 8.9

179.3 ± 7.0 173.4 ± 8.3

188.89 ± 7.0 169.1 ± 6.6

195.0 ± 9.5 157.2 ± 7.4

220.7 ± 13.2 197.4 ± 11.1

6

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Table 2 The table reports mean ± SE first fixation duration for all conditions for all experiments. Pseudowords 4

Words 6

8

4

6

8

Experiment 1: Spacing aloud Crowded 225.3 ± 8.8 Uncrowded 194.4 ± 9.2

229.6 ± 10.0 202.6 ± 9.0

247.6 ± 12.4 202.4 ± 10.5

196.6 ± 9.0 180.1 ± 8.9

208.6 ± 10.7 175.5 ± 7.6

213.7 ± 10.0 174.9 ± 6.7

Experiment 2: Spacing silent Crowded 222.5 ± 7.9 Uncrowded 181.1 ± 7.6

231.2 ± 8.7 169.1 ± 5.9

212.1 ± 9.4 167.2 ± 5.5

193.9 ± 7.2 176.9 ± 6.3

193.0 ± 6.7 170.7 ± 4.7

197.2 ± 7.4 165.5 ± 6.0

Experiment 3: Size silent Crowded Uncrowded

238.5 ± 13.8 180.2 ± 9.0

225.3 ± 11.3 171.7 ± 8.9

179.3 ± 7.0 173.4 ± 8.3

188.9 ± 7.0 169.1 ± 6.6

195.0 ± 9.5 157.2 ± 7.4

220.7 ± 13.2 197.4 ± 11.1

comparisons, that the increase in gaze duration as item length increased was higher for pseudowords compared to words for each length (p b 0.001). No other interactions were significant. 3. Discussion The results show that increasing spacing affected each one of the variables considered in the study: the number of fixations and the gaze duration increased, while the first fixation duration decreased. The number of fixations increased as a function of stimulus length both in terms of the number of letters and spacing. The gaze duration followed the number of fixations. The first fixation duration decreased with the increase in spacing, to a value of around 20–25% less than the usual fixation duration obtained in most eye movement studies. This latter finding, other than being consistent with recent findings on the effect of spacing on reading (Bai, Yan, Liversedge, Zang, & Rayner, 2008, Kohsom & Gobet, 1997; Paterson & Jordan, 2010; Sainio, Hyönä, Bingushi, & Bertram, 2007), could be explained by the fact that spacing induces a release in crowding, which is responsible for a reduction in the decoding time. An alternative explanation for the decrease in first fixation duration in the spaced condition could be that this variable is not independent from the number of saccades. If this was the case, we would have

observed complementary effects on the two measures in all conditions and no gaze duration increase. However, the two measures behaved differently from each other in different conditions: the number of fixations was sensitive to the item length while first fixation duration was not. Both measures were sensitive to lexicality in the same direction (both increasing for pseudowords), while they are affected by spacing in opposite directions. Our results are in agreement with the idea that an increase in fixation time may, at least in part, be due to greater task difficulty, which is reflected in the oculomotor programming of saccades (Kowler & Anton, 1987). The longer first fixation duration for unspaced stimuli is consistent with the idea that in this case encoding is limited by crowding. Finally, the increase in the number of fixations, as in gaze duration, with the increase in item length was higher for pseudowords than for words. The present results may represent the outcome of two different types of processing, similar to the dual route model proposed by Coltheart et al. (2001). In this case, the engagement of the nonlexical route, which is considered to be serial, would activate a different oculomotor behavior to the parallel analysis that is involved in the use of the lexical route (see also Paap et al., 1982). The absence of an interaction between lexicality and spacing in first fixation duration suggests that the mechanisms underlying the two effects are independent. Words may be encoded faster because fewer features are needed to identify the letters in words; first fixation duration may be shorter for spaced compared to unspaced letters because the features are relieved from crowding. The lack of interaction may imply that crowding imposes a hard limit on the visual content that is available for higher level processing, irrespective of stimulus type. 4. Experiment 2: silent reading In Experiment 1, subjects read the stimuli aloud and the results show that they were quite accurate. Nevertheless, it is possible that this modality lowered reading speed and altered the eye movement pattern. To exclude the articulatory component and make the task more similar to everyday reading, we replicated the conditions but asked the subjects to read silently. Previous research has found that reading silently and reading aloud differ quantitatively but not qualitatively in terms of parafoveal information facilitatory effects (Ashby, Yang, Evans, & Rayner, 2012). Accordingly, we expected quantitative, but no qualitative effects, on relevant variables such as lexicality and spacing.

Fig. 3. Experiment 1: The mean first fixation duration (±1 SE) as a function of item length for the reading aloud of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.1. Methods 4.1.1. Participants Sixteen students at the University of Milano-Bicocca participated in the present experiment (2 males and 14 females, mean age

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Table 3 The table reports mean ± SE gaze duration for all conditions for all experiments. Pseudowords 4

Words 6

8

4

6

8

Experiment 1: Spacing aloud Crowded 326.3 ± 30.5 Uncrowded 378.6 ± 29.3

469.4 ± 32.0 596.0 ± 24.6

684.7 ± 36.5 803.0 ± 35.9

251.8 ± 20.4 264.6 ± 19.8

324.7 ± 26.6 373.9 ± 22.6

409.7 ± 27.6 409.4 ± 19.7

Experiment 2: Spacing silent Crowded 263.8 ± 16.3 Uncrowded 284.6 ± 18.9

372.6 ± 25.4 443.0 ± 34.6

483.0 ± 38.2 553.4 ± 41.2

215.2 ± 10.3 241.5 ± 13.8

257.4 ± 16.7 321.7 ± 19.8

318.1 ± 25.5 343.9 ± 14.1

Experiment 3: Size silent Crowded Uncrowded

429.7 ± 41.6 381.3 ± 33.5

483.4 ± 48.6 482.7 ± 48.6

190.4 ± 8.0 200.9 ± 12.6

236.4 ± 14.2 242.4 ± 18.9

294.2 ± 27.7 303.4 ± 24.9

270.1 ± 19.0 281.6 ± 25.1

22.6 ± 4.5 years). All participants had normal or corrected to normal vision (with contact lenses only), were native speakers of Italian and were skilled readers. Visual acuity was evaluated using the Lea SYMBOLS® charts (Hyvärinen, Näsänen & Laurinen, 1980). 4.1.2. Apparatus The same apparatus as in Experiment 1 was used. 4.1.3. Materials and design The stimuli were identical to those used in Experiment 1. 4.1.4. Procedure The only procedural difference from Experiment 1 was that participants were required to perform the reading task silently. 4.2. Results All trials with a loss of signal or a blink (10.4%; mean 22.4; SD 13.2), in which subjects made m–n identification errors (1.1%, mean 2.4; SD 2.9), or with refixations on the first word (2.4%; mean 5.1; SD 6), were discarded. The distribution of such errors was comparable across experimental conditions. All remaining trials were considered valid and were used for all subsequent analyses. The number of fixations and the first fixation duration for the second word were used as dependent variables.

Fig. 4. Experiment 1: The mean gaze duration (±1 SE) as a function of item length for the reading aloud of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2.1. Number of fixations For each subject, the number of fixations on the second item of each stimulus was computed and entered into a repeated measures 2 × 2 × 3 ANOVA with spacing (unspaced and spaced), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The number of fixations is plotted in Fig. 5. All the main effects were significant. The main effect of spacing [F(1,15) = 115.11, p b 0.001] showed a significantly larger number of fixations for spaced (2.10) compared to unspaced stimuli (1.55). The main effect of lexicality [F(1,15) = 24.10, p b 0.001] showed more fixations for pseudowords than for words (2.06 compared to 1.59, respectively). The main effect of item length [F(2,30) = 104.83, p b 0.001] showed an increase in the number of fixations as the number of letters increased (1.33, 1.84, 2.30 for 4, 6 and 8 letter items, respectively). All two-way interactions were significant. The interaction between spacing and lexicality [F(1,15) = 12.10, p b 0.005] emerged because spacing caused a larger increase in fixations for pseudowords (unspaced: 1.71 spaced: 2.41) compared to words (unspaced: 1.38; spaced: 1.80). The interaction between spacing and item length [F(2,30) = 13.82, p b 0.001], according to Scheffé post-hoc comparisons,

Fig. 5. Experiment 2: The number of fixations (±1 SE) as a function of item length for the silent reading of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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emerged because spacing caused an increase in fixations as item length increased (unspaced: 1.16, 1.50, 1.98, for 4, 6 and 8 letter items, respectively; spaced: 1.51, 2.18, 2.63 for 4, 6 and 8 letter items, respectively). All Scheffé post-hoc comparisons were significant at p b 0.001. The interaction between lexicality and item length [F(2,30) = 21.70, p b 0.001] was also significant and reflected a larger number of fixations on pseudowords compared to words at item lengths 6 (pseudowords: 2.06; words: 1.62; Scheffé p b 0.001) and 8 (pseudowords: 2.70; words: 1.90; Scheffé p b 0.001), but not at an item length of 4 (pseudowords: 1.41; words: 1.25; Scheffé post-hoc p = 0.32). The three-way interaction was not significant. 4.2.2. First fixation durations For each subject, the average fixation duration on the second item of each stimulus was computed and entered into a repeated measures 2 × 2 × 3 ANOVA with spacing (unspaced and spaced), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The mean first fixation duration is plotted in Fig. 6. As expected, a significant main effect of lexicality [F(1,15) = 10.82, p b 0.005] was found and showed longer fixations for pseudowords (197 ms) than for words (183 ms). More interestingly, a significant main effect of spacing [F(1,15) = 47.67, p b 0.001] was also found, which reflected longer fixations on unspaced text (208 ms) than on spaced stimuli (172 ms). The interaction between spacing and lexicality also reached significance [F(1,15) = 41.27, p b 0.001] and emerged because the spacing of letters had a bigger effect on the 1st fixation duration of pseudowords (unspaced: 222 ms; spaced: 172 ms) than of words (unspaced: 197 ms; spaced: 171 ms), so that pseudowords, but not words, differed from each other. Neither the main factor of item length nor its interaction with the other factors reached statistical significance. 4.2.3. Gaze duration For each subject, gaze duration was computed as in Experiment 1 and entered into a repeated measures 2 × 2 × 3 ANOVA with spacing (unspaced and spaced), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The gaze duration is reported in Table 3 and plotted in Fig. 7.

Fig. 7. Experiment 2: The mean gaze duration (±1 SE) as a function of item length for the reading aloud of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

All the main effects were significant. The main effect of spacing [F(1,15) = 29.67, p b 0.001] showed a significantly longer gaze duration for spaced (mean 364.68 ms) compared to unspaced stimuli (mean 318.35 ms). The main effect of lexicality [F(1,15) = 26.73, p b 0.001] showed longer gaze duration for pseudowords compared to words (mean 400.05 ms and 282.98 ms, respectively). The main effect of item length [F(2,30) = 83.17, p b 0.001] showed an increase in the gaze duration as the number of letters increased (4 letters: mean 251.27 ms; 6 letters: mean 348.67 ms; 8 letters: mean 424.56 ms). The two-way interaction between spacing and item length tended to significance [F(2,30) = 3.14, p = 0.057] and this, according to Scheffé post-hoc comparisons, was because spacing caused a longer gaze duration for 6 (p b 0.001) and 8 (p b 0.005) but not 4 (p = 0.61) letter stimuli compared to unspaced stimuli (spaced 4, 6, 8 items: 263.3 ms, 382.37 ms, 448.65 ms, respectively; unspaced 4, 6, 8 items: 239.5 ms, 314.98 ms, 400.55 ms, respectively). The interaction between lexicality and item length [F(2,30) = 20.09, p b 0.001] was significant and reflected, according to Scheffé post-hoc comparisons, that the increase in gaze duration as item length increased was higher for pseudowords compared to words for each length (p b 0.001). No other interactions were significant.

4.3. Discussion

Fig. 6. Experiment 2: The mean first fixation duration (±1 SE) as a function of item length for the silent reading of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Overall, the use of silent reading in this experiment reduced the number of fixations (mean: 1.82) relative to reading aloud (mean: 2.26) of Experiment 1 (t(30) = 3.37; p b 0.005), the gaze duration (mean: 341.5 compared to 441.0; t(30) = − 5.034; p b 0.001) and the first fixation duration (mean: 190.05 compared to 204.28; t(30) = − 3.639; p b 0.005). A direct comparison between oral and silent reading is out of the scope of the present paper, and several factors may potentially influence the difference across these two conditions with respect to the processing needed for speech production or the complex mechanisms involved in the eye-voice span (e.g. Morton, 1964; for a review see Carver, 1990). Nonetheless, with respect to the effects of the experimental manipulations, Experiment 2 replicated the results of Experiment 1, which suggests that qualitatively the articulatory component had no effect on eye movements and that silent reading was as reliable as reading aloud both in terms of accuracy and oculomotor behavior. For this reason, in the following experiment we asked participants to read silently.

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5. Experiment 3: silent reading and size manipulation The previous two experiments showed that an increase in spacing reduces fixation time, while the number of saccades and their amplitude grow. In this third experiment, we manipulated the center-tocenter letter spacing by increasing the text size up to a spacing that was equal to the spacing used in Experiments 1 and 2 (Fig. 1). Only crowding matters in the comparison between the small and large size conditions. It is well known that increasing font size increases energy (i.e. squared contrast times the letter area, that depends on the size of the tip marker, see Pelli & Farell, 1999). Nevertheless, efficiency for single letter identification is largely independent of size in the size range used here, thus we can rule out letter visibility (Pelli, Burns, Farell, & Moore-Page, 2006). Additionally, our text is high contrast, while reading time, fixation time, and number of saccades increase only when contrast is reduced below 10% (Legge et al., 1997). Finally, we can rule out masking since it extends beyond the target a distance of only 1.4 times acuity, and in our case even the small size condition around fixation is about 6 times above acuity (Song, Levi, & Pelli, 2014). Thus, crowding and not visibility nor masking should affect the difference across the two size conditions. On the basis of the crowding phenomenon we would predict size and spacing to produce similar effects on eye movement guidance. 5.1. Methods 5.1.1. Participants Sixteen students at the University of Milano-Bicocca participated in the experiment (5 males and 11 females, mean age 22.6 ± 2.7 years). All participants had normal or corrected to normal vision, were native speakers of Italian and were skilled readers. Visual acuity was evaluated using the Lea SYMBOLS® charts (Hyvärinen et al., 1980). 5.1.2. Apparatus The apparatus was identical to the apparatus that was used in Experiments 1 and 2.

5.2.1. Number of fixations For each subject, the number of fixations on the second item of each stimulus was computed and entered into a repeated measures 2 × 2 × 3 ANOVA with font size (small or large), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The number of fixations are reported in Table 1 and plotted in Fig. 8. The main effect of size [F(1,15) = 102.18, p b 0.001] showed a significantly larger number of fixations for large-sized compared to smallsized stimuli (1.80 compared to 1.56, respectively). The main effect of lexicality [F(1,15) = 50.82, p b 0.001] showed more fixations for pseudowords than for words (1.95 compared to 1.40, respectively) and the main effect of item length [F(2,30) = 76.79, p b 0.001] showed that the number of fixations increased with the number of letters (1.23, 1.71, 2.11, for 4, 6 and 8 letter items, respectively). We found a significant interaction between size and lexicality [F(1,15) = 9.27, p b 0.01] because a large size caused a larger increase in fixations for pseudowords (small: 1.81; large: 2.10) than for words (small: 1.32; large: 1.50). The interaction between size and item length [F(2,30) = 4.85, p b 0.05] indicated that an increase in the number of letters increased the effect of size (small-sized: 1.56, 1.61, 1.93, for 4, 6 and 8 letter items, respectively; large-sized: 1.30, 1.82, 2.28 for 4, 6 and 8 letter items, respectively). Scheffé post-hoc comparisons showed a significant difference between large and small stimuli at item lengths 6 and 8 (p b 0.001), but not at an item length of 4 (p = .14). The interaction between lexicality and item length [F(2,30) = 29.27, p b 0.001] was also significant and reflected the higher number of fixations as item length increased in pseudowords (1.34, 2.04, 2.49 for 4, 6 and 8 letter items, respectively) compared to words (1.11, 1.38, 1.72 for 4, 6 and 8 letter items, respectively). All Scheffé post-hoc comparisons were significant [p b 0.001]. The three-way interaction was not significant. 5.2.2. First fixation durations For each subject, the duration of each fixation on the second item of each stimulus was computed and the average duration of each initial fixation per word was calculated. This data was entered into a repeated measures ANOVA with the same factors used above: size (small or

5.1.3. Materials and design The same materials and design as in Experiments 1 and 2 were used. The only difference was that Experiment 3 used size manipulation rather than spacing manipulation. Stimuli were presented in two conditions: small font size (the same standard font size and spacing of Experiments 1 and 2) and large size (35 × 35 pixels, i.e. 1.16 × 1.16 deg; the space between characters was 7 pixels, i.e. 0.23 deg; the center-to-center distance was 42 pixels, i.e. 1.40 deg). The size used was within the plateau range for maximum reading speed, but the reading rate decreases for larger sizes (see Introduction). The center-to-center distance between the characters in the spaced conditions of Experiments 1 and 2 was the same as the large-sized stimuli in this experiment (see Fig. 1). 5.1.4. Procedure This experiment used the same procedure as Experiment 2. 5.2. Results All trials with a loss of signal, a blink (19.8%; mean 42.7; SD 29.2), in which subjects made m–n identification errors (1.4%; mean 2.9, SD 3), or with refixations on the first word (1.5%; mean 3.3; SD 3.3) were discarded. The distribution of such missing data was comparable across experimental conditions. All remaining trials were considered valid and were used for all subsequent analyses. The number of fixations, the first fixation duration and gaze duration for the second word were used as dependent variables.

Fig. 8. Experiment 3: The number of fixations (±1 SE) as a function of item length for the silent reading of words (black squares) and pseudowords (red circles). Results for the small letter size condition are shown as open symbols; results for the large letter size are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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large), lexicality (words and pseudowords) and item length (4, 6 and 8 letters). The mean first fixation duration is plotted in Fig. 9. A significant main effect of size [F(1,15) = 103.58, p b 0.001] was found and reflected longer fixations for small stimuli (208 ms) compared to enlarged stimuli (175 ms). A significant main effect of lexicality [F(1,15) = 35.25, p b 0.001] showed longer fixations for pseudowords (206 ms) than for words (177 ms). The interaction of lexicality by size [F(1,15) = 18.14, p b 0.001] was significant and reflected a bigger difference in 1st fixation duration for pseudowords (large-sized: 228; smallsized: 183 ms) than for words (large-sized: 188; small-sized: 166 ms) as character size increased. The interaction between size and item length [F(2,30) = 8.94, p b 0.001] was also significant and Scheffé post-hoc comparisons indicated that the two sizes were different at 6 and 8 letter items (both p b 0.001), but not at 4 letter items. However, comparisons showed a significant difference only in the large print between 4 and 8 letter items (185 ms at 4 letter items compared to 164 at 8 letter items; p b 0.05). None of the other comparisons were significant. Neither the main factor of item length nor the triple interaction reached statistical significance.

5.2.3. Gaze duration For each subject, gaze duration was computed as in previous experiments and entered into a 2 × 2 × 3 repeated measures ANOVA with font size (small and large), lexicality (words and pseudowords) and item length (4, 6 and 8 letters) as the main factors. The gaze duration is reported in Table 3 and plotted in Fig. 10. The main effect of lexicality [F(1,15) = 46.60, p b 0.001] showed longer gaze duration for pseudowords compared to words (mean 388.14 ms and 244.6 ms, respectively). The main effect of item length [F(2,30) = 43.03, p b 0.001] showed an increase in the gaze duration as the number of letters increased (4 letters: mean 235.76 ms; 6 letters: mean 322.44 ms; 8 letters: mean 390.92 ms). No effect of font size emerged, either as main effect or in the twoway interactions. The interaction between lexicality and item length [F(2,30) = 19.28, p b 0.001] was significant and reflected, according to Scheffé post-hoc comparisons, the fact that the increase in gaze duration as item length increased was higher for pseudowords compared to

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Fig. 10. Experiment 3: The mean gaze duration (±1 SE) as a function of item length for the reading aloud of words (black squares) and pseudowords (red circles). Results for the standard spacing condition are shown as open symbols; results for the spaced condition are shown as filled symbols. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

words for each length (p b 0.001), No other interactions were significant. 5.3. Discussion Experiment 3 replicated the results obtained in Experiment 2 (and Experiment 1), in terms of the number of fixations and the first fixation duration, but not in terms of gaze duration. Our observation that the increase in letter distance (by the introduction of spaces or an enhancement of text size) reduced the first fixation duration and increased the number of saccades confirms that the two manipulations are equivalent. The direction of the first effect is coherent with the idea that closely placed letters are jumbled and hard to identify. An increase in center-tocenter letter spacing reduced fixation duration that we ascribe to a reduction in crowding. However, our manipulation also caused an increase in stimulus spatial extension, which may be responsible for the increase in the number of fixations. Finally, the font size enlargement did not change gaze duration and this result suggests either that such a measure is not completely driven by the number of fixations or that it is due to the global processing of word recognition (see General discussion). 5.4. A comparison between letter spacing and letter size

Fig. 9. Experiment 3: The mean first fixation duration (±1 SE) as a function of item length for the silent reading of words (black squares) and pseudowords (red circles) for the small and large letter size (open and filled symbols, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The figures from Experiments 2 and 3 reveal a quantitative difference between letter spacing and character size manipulations in terms of the number of fixations (Figs. 5 and 8, respectively), but not for the first fixation duration (Figs. 6 and 9, respectively). A possible explanation of this difference could relate to visual acuity. In particular, in the case of letter spacing, small eccentric characters more easily fall below the acuity threshold, whereas in the case of large letters, size compensates for poor visual acuity. It follows that different predictions can be made, based on crowding and visual acuity: the former implying the same result for the two manipulations, the latter not. We added two analyses in order to show that the first fixation duration result follows the prediction of the crowding hypothesis, while the number of fixations follows the acuity hypothesis. In order to test our hypothesis we confined our analysis to the 8letter condition. From an acuity stand point, 8-letter strings fall further in the periphery, more so in the spaced condition, and the last letters may be close to acuity when size is small. As for crowding, 8-letter strings, relative to 4-letter strings, more likely exceed the size of the

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uncrowded window around fixation. However, since changing size or spacing is equivalent for crowding (i.e. the only relevant variable is center-to-center letter spacing) the two manipulations should affect crowding and, according to our results, first fixation durations similarly. We performed two ANOVAs, one on each variable, directly comparing the performance of the two different groups in the two experiments (2 and 3) at the greater length (8 letters), which is the length at which the effects might be maximized (see Fig. 11). The number of fixations was entered into a mixed ANOVA with two repeated measures: center-to-center distance (crowded or uncrowded) and lexicality (words and pseudowords) and a between factor: manipulation (space or size). Significant main effects for center-to-center distance [F(1,30) = 105.88, p b 0.001] and lexicality [F(1,30) = 66.12, p b 0.001] and for the interaction between the two repeated measures [F(1,30) = 14.38, p b 0.05] were found, as in the analysis of fixation duration. Additionally, the interaction between manipulation and center-to-center distance was significant [F(1,30) = 9.22, p b 0.005], suggesting that size and spacing are not comparable for this dependent measure. In particular, in the standard (crowded) condition (which was exactly the same in Experiments 2 and 3), the parameter did not differ between the two groups of subjects (1.672 compared to 1.575 for words and 2.285 compared to 2.288 for pseudowords), while the number of fixations in Experiment 2 (space manipulation) was higher than that in Experiment 3 (size manipulation), both for words (2.135 compared to 1.865), and pseudowords (3.115 compared to 2.701). This result is coherent with the acuity hypothesis which predicts such a difference and its direction. No other differences reached significance. The first fixation duration was also entered into a mixed ANOVA with two repeated measures: center-to-center distance (crowded or uncrowded) and lexicality (words and pseudowords) and a between factor: manipulation (space or size). We found a significant main effect for center-to-center distance [F(1,30) = 125.68, p b 0.001] and lexicality [F(1,30) = 43.63, p b 0.001], and a significant interaction between the two repeated measures [F(1,30) = 51.86, p b 0.001], as in the two separate experiments; A significant interaction between lexicality and manipulation was also found [F(1,30) = 4.75, p b 0.05]. Scheffé post-hoc comparisons indicated that the difference between pseudowords and words was present in both manipulations, but the one of size was bigger than that of space manipulation (206 ms 177 ms, p b 0.001; and 197 ms vs 183 ms, p b 0.05, respectively). No significant interaction between center-to-center distance and manipulations emerged, suggesting that what matters for fixation duration is center-to-center letter spacing, not size per se or spacing per se.

6. General discussion We performed three experiments to contrast the effects of letter spacing, letter size, lexicality and stimulus length in reading silently and reading aloud. Our experiments, all together, showed that either the introduction of spaces or the enlargement in the text determined a higher number of fixations and a reduction in the first fixation duration, whereas gaze duration was affected by our manipulations only in Experiments 1 and 2. The main effect of spacing was in line with the literature about text reading, confirming that our paradigm is reliable. Indeed, when readers' eye movements were recorded, previous investigators (Bai et al., 2008; Paterson & Jordan, 2010) have found a difference between normal unspaced text and text with spaces between characters. In particular, spaces between characters induced more fixations of shorter duration in both Chinese (Bai et al., 2008) and English (Paterson & Jordan, 2010). We found the same results in the Italian language and we replicated these results by increasing font size, which affected the center-to-center letter distance without disrupting the Gestalt of the word by abnormally segregating the letters in the word. Overall, the novelty of this study is linking the pattern of eye movements during reading to predictions based on crowding, in particular, contrasting the effect of stimulus spatial extension (number of letters) and its size. The center-to-center distance has been shown to be the only variable that is important for crowding (Pelli et al., 2007). We showed that fixation duration was not dependent on the introduction of empty space between letters per se since an increase in letter size produced the same decrease in fixation duration. On the other hand, the number of fixations depended on the portion of the visual field that was covered by the stimulus, and was affected similarly whether this area was generated by an increase in interletter spacing, text size or item length. Can our pattern of results be explained by a trade-off between the number of fixations and their duration? It is unlikely, since our experimental manipulations affected the number and the duration differently across the different conditions. The number of fixations and the first fixation duration were both sensitive to the lexicality of the stimulus, increasing for pseudowords relative to words. Nevertheless, while increasing the spatial extension by adding letters affected the number but not the duration of fixations, adding spaces or increasing size generated opposite effects on the two variables. Moreover, direct comparison between Experiments 2 and 3 showed that while the first fixation duration was exactly the same, independent of the type of manipulation (spacing or increasing size), the number of fixations was affected by the visual differences of the two manipulations, probably due to changes in visual acuity.

4

number of fixations

3,5 3 2,5 2 1,5 1 0,5

words crowded pseudow owords uncrowded pseudo crowded words uncrowded words

0 space

size nipulation Man

Fig. 11. Mean number of fixations (±1 SE) and fixations duration (±1 SE) for silent reading of crowded (open symbol) and uncrowded (filled symbols), pseudowords (red square) and words (black circles), in Experiment 2 (space manipulation) and 3 (size manipulation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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This evidence is contrary to the hypothesis that the shorter first fixation duration observed with uncrowded items could be a mere consequence of more frequent fixations. We propose that the first fixation duration reduction observed in the spaced/large-sized condition is due to relief from crowding. Conversely, regarding the increase in the number of fixations for the spaced/large words we believe that it is likely to be due to the larger spatial extent of the target words and is not a consequence of crowding (i.e., the effects on number and duration are independent). Thus, releasing from crowding by increasing interletter spacing or enlarging the stimulus reduces the first fixation duration, while increasing the spatial extent by adding letters increases the number of fixations with no effect on duration. Fig. 12 shows that the number of fixations increased as a function of the region of the visual field that was covered by the words and pseudowords by considering both the number of letters and the interletter distance (Experiments 2 and 3). The difference between Experiments 2 (circles) and 3 (squares), particularly for bigger extensions, could be due to visual acuity. An increase in the extent of the stimulus by the addition of spaces or letters increased the required number of fixations. Gaze duration was affected by spacing as much as the number of fixations, but it was not affected by font size enlargement, suggesting that it is not due to extension per se. Spacing, aside from extending the stimulus in the periphery, breaks up the perceptual grouping of letters, whereas increasing font size does not. Vinckier, Qiao, Pallier, Dehaene, and Cohen (2011) found that spacing letters, but not increasing font size, slows down reading and, according to the local combination detectors model (Dehaene, Cohen, Sigman, & Vinckier, 2005), they suggested that spacing disrupt the parallel processing of letters and letter grouping. Our eye movement results are coherent with this hypothesis and with the interpretation of gaze duration in terms of a marker for word recognition (e.g. Reichle, Pollatsek, & Rayner, 2006). The first fixation duration on a word, the number of fixations and gaze duration depended on lexicality and word predictability, which are both central factors in lexical processing (Kliegl, Grabner,

Fig. 12. Results from Experiments 2 (circles) and 3 (squares). The figure shows the number of fixations as a function of the extent of the visual field in degrees (deg) that was covered by the stimulus (words: black, open; pseudowords: red, filled). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Rolfs, & Engbert, 2004). Rayner, Fischer, and Pollatsek (1998) showed the same differences in the oculomotor behavior by comparing lowfrequency and high-frequency words. The lexicality effects on the first fixation duration may imply that words are decoded faster than pseudowords because fewer features are needed to identify letters. The higher rate of increase in the number of fixations with stimulus extension suggests that eye guidance is influenced by lexicality, which interacts with the size of the visual span. 7. Conclusion We conclude that the number of fixations on a word in reading depends on the spatial extension of the stimulus, which is due in part to its physical distance (e.g., a spaced 4-letter stimulus covers a larger part of the visual field than an unspaced 4-letter stimulus) and in part to the number of letters in the string (e.g., 8-letter stimuli cover more space than 4-letter stimuli). Moreover, this measure is sensitive to visual acuity (i.e. spaced 8-letter pseudowords requires more fixations than a large 8-letter pseudowords). The number of fixations also varies according to the type of processing triggered by the reading task (serially read pseudowords are parsed in smaller chunks relative to words). Gaze duration is not affected by crowding or stimulus extension but is sensitive to interletter spatial distance, corroborating the hypothesis that gaze duration reflects more global processing underlying word recognition. The encoding time, as expressed by the first fixation duration, is influenced by integration field limitations. These limitations correspond to the letter visibility impaired by crowding. Overall our study suggests that eye movement guidance reflects crowding limitations through the first fixation duration. McDonald (2006) suggested the same relationship between crowding and the effect of space reduction on fixation duration, although he had not disentangled the effect of spacing per se from crowding. A speculative explanation of how longer fixation duration can help reading when letters are crowded can be proposed in terms of attention. Some authors, in fact, have suggested that critical spacing can be influenced and modulated by top-down mechanisms, such as spatial attention (e.g., Intriligator & Cavanagh, 2001; Yeshurun & Rashal, 2010). Specifically, while focusing on a smaller area of visual space would require more time, it could increase the spatial resolution of the feature integration process, at least up to a structural limit. On the basis of this, we might predict that an attentional deficit could affect reading by improving crowding. The same prediction might be applied to conditions other than neglect, which is what studies on patients with posterior cortical atrophy (PCA) seem to suggest. PCA patients show reading deficits associated with the alteration of both perceptual and attentional mechanisms (e.g., Mendez, Shapira, & Clark, 2007; Saffran & Coslett, 1996). Indeed, most PCA patients develop both visual agnosia and simultagnosia (Mendez & Perryman, 2002). Recently, crowding has been taken into account in explaining the nature of the reading deficits in PCA patients. Crutch and Warrington (2007, 2009) state that the reading deficit in their PCA patients is caused by an abnormal and pathological crowding, which induces an altered integration of the perceptual characteristics of verbal stimuli (letters) presented simultaneously. “Confusability” of characteristics would make it impossible to correctly recognize stimuli, not only in the periphery but also in the fovea. Although Crutch and Warrington's interpretation of crowding and of the reading disorder in PCA patients had been given as purely bottom-up, an attentional interpretation is quite likely. Further evidence should be collected in order to support this interpretation. In conclusion we found an effect in first fixation duration during normal reading that we interpreted as due to crowding, being independent from stimulus extension at least in term of number of letters and equally affected by interletter distance and letter size. We suggested that the increase in first fixation duration is due to the role of attention

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