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Unmasking Saccadic Uncrowding UC Berkeley UC Berkeley Previously Published Works Title Unmasking saccadic uncrowding. Permalink https://escholarship.org/uc/item/52t3z95b Authors Ağaoğlu, Mehmet N Öğmen, Haluk Chung, Susana TL Publication Date 2016-10-01 DOI 10.1016/j.visres.2016.08.003 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Vision Research 127 (2016) 152–164 Contents lists available at ScienceDirect Vision Research journal homepage: www.elsevier.com/locate/visres Unmasking saccadic uncrowding ⇑ Mehmet N. Ag˘aog˘lu a, , Haluk Ög˘men b, Susana T.L. Chung a a School of Optometry, University of California, Berkeley, Berkeley, CA 94720-2020, United States b Department of Electrical & Computer Engineering, University of Denver, Denver, CO 80208, United States article info abstract Article history: Stimuli that are briefly presented around the time of saccades are often perceived with spatiotemporal Received 5 July 2016 distortions. These distortions do not always have deleterious effects on the visibility and identification Received in revised form 6 August 2016 of a stimulus. Recent studies reported that when a stimulus is the target of an intended saccade, it is Accepted 7 August 2016 released from both masking and crowding. Here, we investigated pre-saccadic changes in single and crowded letter recognition performance in the absence (Experiment 1) and the presence (Experiment 2) of backward masks to determine the extent to which saccadic ‘‘uncrowding” and ‘‘unmasking” mech- Keywords: anisms are similar. Our results show that pre-saccadic improvements in letter recognition performance Crowding are mostly due to the presence of masks and/or stimulus transients which occur after the target is pre- Masking Remapping sented. More importantly, we did not find any decrease in crowding strength before impending saccades. Perisaccadic perception A simplified version of a dual-channel neural model, originally proposed to explain masking phenomena, Neural modeling with several saccadic add-on mechanisms, could account for our results in Experiment 1. However, this model falls short in explaining how saccades drastically reduced the effect of backward masking (Experiment 2). The addition of a remapping mechanism that alters the relative spatial positions of stim- uli was needed to fully account for the improvements observed when backward masks followed the letter stimuli. Taken together, our results (i) are inconsistent with saccadic uncrowding, (ii) strongly support saccadic unmasking, and (iii) suggest that pre-saccadic letter recognition is modulated by multiple perisaccadic mechanisms with different time courses. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction factors, such as attention (Freeman & Pelli, 2007; Grubb et al., 2013; Yeshurun & Rashal, 2010) and perceptual grouping Objects that fall on the fovea can be easily recognized whereas (Manassi, Sayim, & Herzog, 2012, 2013) also affect crowding. they may be difficult to recognize in the periphery due to factors Crowding is a major bottleneck for object recognition and a funda- such as the reduced photoreceptor density of the peripheral retina. mental component of conscious spatial vision (Levi, 2008; Pelli & Recognition is even more difficult when objects are closely sur- Tillman, 2008; Whitney & Levi, 2011). rounded by other objects, a phenomenon called crowding While crowding impairs the recognition of an object, it leaves (Bouma, 1970; Chung, Levi, & Legge, 2001). The crowding strength the detectability of its features unaffected (e.g., Levi, Hariharan, & is generally defined as a reduction in recognition performance due Klein, 2002). Masking refers to the reduction in visibility of a (tar- to the presence of flanking objects (Levi, 2008; Whitney & Levi, get) stimulus when it is presented in close spatiotemporal proxim- 2011). There are many accounts of crowding, but one account sug- ity to another (mask) stimulus (Breitmeyer & Ogmen, 2006). gests that crowding results from the obligatory integration of fea- Masking and crowding are affected similarly by certain spatiotem- tures within a spatial window. The extent of this window, the poral properties of the stimuli such as stimulus onset asynchrony critical distance, scales with eccentricity (Bouma, 1970; Chung and duration (Chung et al., 2001; Lev & Polat, 2015); however, they et al., 2001). The crowding strength and the critical distance also have distinct characteristics (Chung et al., 2001; Pelli, depend on temporal properties of the stimuli (Chung, 2016; Palomares, & Majaj, 2004). Chung & Patel, 2011; Lev, Yehezkel, & Polat, 2014). Many other In this paper, we define flankers (i.e., stimuli crowding a target stimulus) as stimuli of the same object category (e.g., letters) as the target, although we note that flankers that belong to different ⇑ Corresponding author at: 360 Minor Hall, University of California, Berkeley, object categories can also induce crowding (Chanceaux, Mathot, Berkeley, CA 94720-2020, United States. & Grainger, 2014; He & Tjan, 2004). Masks are defined as stimuli E-mail address: [email protected] (M.N. Ag˘aog˘lu). http://dx.doi.org/10.1016/j.visres.2016.08.003 0042-6989/Ó 2016 Elsevier Ltd. All rights reserved. M.N. Ag˘aog˘lu et al. / Vision Research 127 (2016) 152–164 153 that do not have structures or features as those of the target (e.g., either during fixation (with the letters presented in the peripheral noise masks). Crowding can occur in the absence of masking, and retina), or after a saccadic eye movement following the offset of a vice versa. When flankers and masks are presented in combination, cue located at the location of the target letter. To quantify the the net effect may not be equal to the sum of the individual effects. changes in crowding strength due to impending saccades, the tar- Certain types of masks reduce flankers’ visibility, which in turn get letter was presented either alone or with two horizontally reduces crowding (Chakravarthi & Cavanagh, 2009). When a target flanking letters. In Experiment 1, we specifically tested the is crowded and weakly masked, the combined deleterious effect is hypotheses that (i) saccade targets are released from crowding, larger than the sum of their individual effects, known as ‘‘super- and (ii) perisaccadic mechanisms interact with the temporal order crowding” (Vickery, Shim, Chakravarthi, Jiang, & Luedeman, of the stimuli. In Experiment 2, we specifically tested the hypoth- 2009). Crowding the ‘‘crowders” (i.e., flankers) or masking the esis that impending saccades reduce masking. By comparing the masks can restore the recognizability or the visibility of a target results from both experiments, we also tested the hypothesis that (Manassi et al., 2013; Ogmen, Breitmeyer, Todd, & Mardon, remapping (in the form of perceptual displacements) affects pre- 2006). The evidence reviewed here suggests that, although they saccadic masking but not crowding. can be dissociated in some settings, masking and crowding might Saccades have been shown to result in increased detection share common neural mechanisms under certain conditions. More- thresholds (known as saccadic suppression) and enhanced discrim- over, most stimuli used to study crowding and masking activate ination performance (e.g., Bridgeman, Hendry, & Stark, 1975; both mechanisms, therefore, it is difficult to disentangle their indi- Deubel & Schneider, 1996). In order to understand how mecha- vidual contributions to the final percept. nisms with such opposite effects might play a role in pre- A powerful paradigm to study these processes is the pre- saccadic object recognition, we used a simplified version of a saccadic ‘‘uncrowding” and ‘‘unmasking”, in which the presenta- dual-channel neural model of visual masking to account for the tion of the stimuli (i.e., target-mask or target-flanker displays) data in both experiments. This model could account for letter prior to an impending saccade reduces the impairments due to recognition performance during fixation, suggesting that masking crowding and masking (De Pisapia, Kaunitz, & Melcher, 2010; and crowding might share common low-level mechanisms. Finally, Harrison, Mattingley, & Remington, 2013). Saccades constitute a by adding several independent saccadic mechanisms to this model, fundamental aspect of normal vision; hence, it is essential to we sought to tease apart different components of pre-saccadic understand how visual processes operate under pre-saccadic con- modulations in letter recognition. ditions. As a consequence of saccades, retinal images are frequently displaced, yet we perceive a stable and coherent world. How does 2. Methods the visual system achieve perceptual stability? Among many pro- posals, much attention is received by those based on updating a 2.1. Overview highly detailed retinotopic map via shifts of neuronal receptive fields (RF) or attentional resources (Burr, Ross, Binda, & Morrone, In Experiment 1, we aimed to determine whether or not, and 2010; Cavanagh, Hunt, Afraz, & Rolfs, 2010; Melcher & Colby, how pre-saccadic letter recognition is affected by the temporal 2008; Wurtz, 2008). Neurons which shift their RFs in parallel to order of presentation of a target and its flankers. The target letter the direction of an impending saccade toward the retinal locations was presented either alone (unflanked) or accompanied by two that they will occupy (‘‘future field”) after a saccade, commonly horizontally flanking letters with a varying flanker-target onset referred to as ‘‘remapping”, have been found in
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