Frequency-dependent modulation of foveal contrast sensitivity by fine-scale exogenously triggered attention

Visual spatial attention is a fundamental mechanism that enables both humans (Carrasco, 2011) and animals (Saban et al., 2017; Hahner and Nieder, 2025) to selectively process information from their environment. Often, shifts in spatial attention are accompanied by eye movements to focus on a specific location, a process known as overt spatial attention (Kaspar, 2013). However, covert spatial attention, the ability to shift attention independently of eye movements, is equally crucial in daily life. This ability enables us to monitor locations beyond our line of sight, such as when driving and keeping track of peripheral surroundings.

Covert spatial attention is typically categorized into two types: endogenous and exogenous attention. Endogenous attention refers to the voluntary allocation of processing resources to a specific location. While this shift occurs relatively slowly, taking approximately 200–300 ms to reach the target region, it can be sustained for an extended duration (Theeuwes, 1994; Findlay, 2003; Chica and Lupiáñez, 2009; Carrasco, 2011; Chica et al., 2013; Dugué et al., 2020). In contrast, exogenous attention is driven by salient stimuli that automatically capture attention (Theeuwes, 1994; Findlay, 2003; Chica and Lupiáñez, 2009; Carrasco, 2011; Chica et al., 2013; Dugué et al., 2020). This shift is rapid but transient, often followed by a phenomenon known as inhibition of return, moving attention away from the initially attended location (Klein, 2000; Chica and Lupiáñez, 2009). Compared to endogenous attention, exogenous attention is more automatic and less flexible (Corbetta and Shulman, 2002; Knudsen, 2007; Carrasco, 2011).

Until recently, research on the effects of covert attention on visual perception has focused primarily on extrafoveal vision. A vast body of literature has demonstrated that covert attention enhances visual contrast sensitivity (CS) (Carrasco et al., 2000; Martínez-Trujillo and Treue, 2002; Reynolds and Chelazzi, 2004; Pestilli and Carrasco, 2005; Li et al., 2008; Foster et al., 2021) and increases spatial resolution (Yeshurun and Carrasco, 1998; Carrasco et al., 2002; Carrasco et al., 2006; Jigo and Carrasco, 2018) at selectively cued locations in the extrafovea. In contrast, attention within the high-acuity 1° foveola has often been considered uniform and distributed evenly throughout this small region. Therefore, the effects of attention in the fovea are traditionally studied using large stimuli encompassing one or more degrees of visual angle (Miniussi et al., 2002; Jigo and Carrasco, 2020; Papaioannou and Luck, 2020). However, recent findings showed that even within the 1° foveola, both endogenous (Poletti et al., 2017) and exogenous (Guzhang et al., 2021) attention can be covertly allocated in a highly spatially selective manner. For both types of covert attention, observers were better able to discriminate the orientation of fine details at an attended location—cued endogenously or exogenously—compared to nearby uncued locations just 0.26° away. Although these results highlighted the strikingly fine grain of attentional control, they also raised new questions. In particular, it remains unknown which spatial frequencies (SFs) benefit from fine-grained attentional shifts within the foveola. While Guzhang et al., 2021 demonstrated visual enhancement from exogenous attention at the foveal scale, the orientation discrimination task used in the study was relatively coarse, requiring observers to determine whether a stimulus was tilted ±45°. Despite the small size of the stimulus, such a task does not require high SFs (e.g., >10 cycles per degree [CPD]); in fact, frequencies around 4–8 CPD should be sufficient to perform it effectively. Therefore, the perceptual enhancement observed in Guzhang et al., 2021 could be due to an enhancement of only lower or only higher SFs, or perhaps a broad range of SFs. The overall improvement in orientation discrimination of fine spatial stimuli is compatible with any of these scenarios.

The effects of extrafoveal attention have been found to differ across SFs: while extrafoveal endogenous attention enhances a broad range of SFs (Lu and Dosher, 2004; Jigo and Carrasco, 2020), extrafoveal exogenous attention selectively enhances high SFs (Carrasco et al., 2006; Barbot et al., 2011; Barbot et al., 2012; Carretié et al., 2012; Jigo and Carrasco, 2020; Fernández et al., 2022), peaking just above the SF characterized by the highest sensitivity at a given eccentricity (Jigo and Carrasco, 2020). Whether fine spatial exogenous attention at the foveal scale modulates visual discrimination similarly is an open question. Generally, fine control of spatial attention at the foveal scale is required when examining fine spatial details, such as reading small text in a book or noticing subtle changes, like a traffic light switching or unexpected pedestrians from afar while driving (Figure 1A). In these tasks, precise allocation of attention likely helps distinguish and recognize individual letters and details. It is possible that in the foveola, exogenous attention modulates a narrow range of SFs, similar to how it operates extrafoveally. However, while humans can resolve SFs up to 30 CPD in the foveola (Intoy and Rucci, 2020; Clark et al., 2022), extrafoveally, SFs above 10 CPD cannot be resolved. Therefore, even if the perceptual enhancement driven by fine spatial attention is limited to a narrow range of lower SFs, the enhanced frequency range may shift toward higher SFs in the foveola compared to what happens extrafoveally (Figure 1B). Alternatively, attention at the foveal scale might preserve its enhancement of low SFs while extending it to high SFs, leading to a broad, rather than narrow, range of modulation (Figure 1C). Any of these scenarios could account for the improvement in orientation discrimination observed in Guzhang et al., 2021.

Figure 1

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In the current study, we addressed two main questions. First, does exogenous attention at the foveolar scale enhance visual processing across a narrow or a broader range of SFs? Second, if the enhancement operated within a narrow frequency band, which range of SFs benefits the most from such fine-grained shifts of attention? Addressing these questions is crucial because, while it is now established that covert attention can be selectively shifted even within the central fovea, it remains unclear whether it follows the same modus operandi foveally and extrafoveally. If a similar range of SFs is enhanced by exogenous attention in both the foveola and extrafovea, it would suggest that exogenous attention operates similarly across the visual field, regardless of the spatial resolution achievable at different eccentricities. In contrast, if the modulation of SFs differs, it would indicate that the mechanisms of exogenous attention are flexibly tuned in the foveola and adjusted based on the spatial resolution that can be achieved at this scale.

Studying attentional control in the foveola presents unique challenges. Continuous microscopic eye movements during fixation cause constant displacement of the retinal input (Martinez-Conde et al., 2004; Rucci and Poletti, 2015; Krauzlis et al., 2017), making it difficult to limit visual stimulation to the desired eccentricity at this scale. This poses a significant issue when investigating covert attention in the central foveola. To address these challenges, we employed high-precision eye-tracking (Wu et al., 2023) combined with gaze-contingent display control (Santini et al., 2007) to precisely monitor gaze position throughout each trial to ensure that any effects observed are solely due to covert exogenous attention and are not driven by fixational saccades (Hafed and Clark, 2002; Yuval-Greenberg et al., 2014; Shelchkova and Poletti, 2020; Guzhang et al., 2024).

To examine the effects of high-resolution exogenous attention within the fovea on visual discrimination of stimuli at different SFs, we employed a 2AFC visual discrimination task in which observers were asked to discriminate the orientation of a small Gabor patch (30′ × 30′ with an overlaying 5.4′ Gaussian window, tilted ±45°) 30′ from either left or right of the fixation marker when prompted by a response cue (Figure 2A). Note that the Gabor patches used in the current study were much smaller than those typically used in studies probing extrafoveal attention (Rossi and Paradiso, 1995; Gobell and Carrasco, 2005; Herrmann et al., 2010; Jigo and Carrasco, 2020). On each trial, the orientation and phase of the Gabor patch were randomized at each location independently. Eye movements were monitored at high resolution using a digital Dual Purkinje Image eye tracker (Wu et al., 2023) to ensure that observers maintained the center of gaze within a 10′ × 10′ window around the fixation point throughout the trial (Figure 2D). There were no systematic differences in fixation position between valid and neutral conditions, either horizontally (valid: 0.20′ ± 0.66′ SD; neutral: 0.21′ ± 0.63′ SD; p = 0.53) or vertically (valid: –0.52′ ± 0.84′ SD; neutral: –0.48′ ± 0.82′ SD; p = 0.34).

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The experiment included two cueing conditions, valid and neutral, that were randomly interleaved with equal probability within each experimental block. In the valid condition, shifts of exogenous attention were elicited by a small and brief white flash (exogenous cue) presented 100 ms before the target either on the left or the right of the visual field. The cue, with 100% validity, appeared just outside the upcoming target location. In the neutral condition, no exogenous cue was presented.

We tested four SFs, 4, 8, 12, and 20 CPDs, (Figure 2C), ensuring at least one full cycle of modulation within the Gabor patch even at the lowest SF (4 CPD) (Howell and Hess, 1978). The highest SF (20 CPD) is close to the limit of visual resolution at the eccentricity tested here. For each SF tested, an initial threshold contrast was estimated by methods of constant stimuli, then discrimination accuracy was measured at four contrast levels around the initial threshold estimate, and one additional level at the maximum contrast to measure the asymptotic performance (AP; see Methods). The contrast level was kept constant within each experimental block but was randomized across blocks.

To examine how high-resolution exogenous attention influences performance for stimuli at different SFs, we fitted individual psychometric curves of contrast level vs. discrimination accuracy and estimated contrast thresholds for each cueing condition and SF for each observer (Figure 3A shows the psychometric curves for one example observer with the result of all psychometric curves included in Figure 3—figure supplement 1).

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We first examined the full factorial effects of SF and cueing on CS estimated from the observers’ psychometric functions by fitting a linear mixed-effects regression to the estimates of log-transformed CS at all 8 conditions (4 SF × 2 cueing) across observers. The CS captures the effects on the intercept and/or slope of the psychometric function.

Consistent with the literature (Howell and Hess, 1978; Bex et al., 2009; Lovegrove et al., 1980; Rovamo et al., 1992; Rovamo et al., 1993; Pointer and Hess, 1989), we observed a main effect of SF on CS (${\chi }^2(3)=65.3$, p < 0.001, Figure 3B; see Appendix 1—table 1 for full model output); CS was highest at 4 CPD and decreased as the SF increased (post hoc pairwise comparisons revealed several significant differences between SFs; post hoc pairwise comparisons of the estimated marginal means, ps ≤ 0.0001 for 4 vs. 8 CPD, 4 vs. 12 CPD, 4 vs. 20 CPD, 8 vs. 20 CPD, 12 vs. 20 CPD). We also observed a significant main effect of attention on CS (${\chi }^2(1)=7.3$, p < 0.001; ${\text{mean}}_{\text{valid}}=10.1\pm 7.56$ SD and ${\text{mean}}_{\text{neutral}}=9.03\pm 6.39$ SD): averaging across all SFs, the valid cueing condition resulted in higher CS (Figure 3B, C). These findings indicate that exogenous attention led to a contrast gain across SFs in the attended subfoveolar region.

Notably, the improvement in CS driven by fine-grained attention was not uniform across SFs (Figure 3D, also visible in Figure 3B, C). We observed a statistically significant interaction between SF and attention (${\chi }^2(3)=9.3$, p = 0.0258), indicating that contrast gains were selective. Specifically, CS exhibited a contrast gain in the valid condition compared to the neutral condition at lower SFs (4 and 8 CPD) (${\text{mean gain}}_{\text{4 CPD}}=2.62\pm 2.13$ SD, Cohen’s $d=0.79$ and ${\text{mean gain}}_{\text{8 CPD}}=1.36\pm 1.25$ SD, $d=0.75$, ps < 0.004; see Appendix 1—table 2 for all pairwise comparisons). However, the CS gains at higher SFs (12 and 20 CPD) were smaller and were not statistically significant (${\text{mean gain}}_{\text{12 CPD}}=0.09\pm 0.91$ SD and ${\text{mean gain}}_{\text{20 CPD}}=0.11\pm 0.13$ SD, ps > 0.5). In addition to examining the contrast gain within each SF, we also compared the amount of contrast gain within each pair of SFs (see Appendix 1—table 3 for all pairwise comparisons). Post hoc pairwise comparisons revealed that attention modulation did not differ between the two low-mid SFs (4 and 8 CPD, $p>0.8$) or between the two mid-high SFs (12 and 20 CPD, p > 0.7). Importantly, attention led to a larger enhancement of CS at the low-mid SFs (4 and 8 CPD) compared to 12 CPD ($\mathrm{\Delta }{\text{mean gain}}_{\text{4 CPD - 12 CPD}}=2.53\pm 2.16$ SD, $d=0.65$, and $\mathrm{\Delta }{\text{mean gain}}_{\text{8 CPD - 12 CPD}}=1.27\pm 0.99$ SD, $d=0.62$, ps < 0.025). When comparing the low-mid SFs (4 and 8 CPD) to the highest SF tested (20 CPD), differences in contrast gains were even larger, though these effects were not statistically significant ($\mathrm{\Delta }{\text{mean gain}}_{\text{4 CPD - 20 CPD}}=3.35\pm 1.96$ SD, p = 0.066 and $\mathrm{\Delta }{\text{mean gain}}_{\text{8 CPD - 20 CPD}}=1.45\pm 1.46$ SD, p = 0.084). As detailed in Methods, two observers did not have data for the 20 CPD condition. Statistical power might thus have been reduced for comparisons against this condition.

These results demonstrate that, similar to the selectivity in enhancements in visual periphery, micro-shifts of exogenous attention within the central fovea selectively enhanced CS primarily for low- to mid-range frequencies (4–8 CPD).

In addition to CS, we examined the effects of attention on AP. Research on extrafoveal vision has returned mixed results with respect to the effects of exogenous attention on AP. It has been found that exogenous attention can enhance both CS (contrast gain) but also AP, a phenomenon known as response gain (Morrone et al., 2002; Morrone et al., 2004; Ling and Carrasco, 2006; Pestilli et al., 2009; Reynolds and Heeger, 2009; Herrmann et al., 2010). However, some studies have found significant effects only on CS, with no notable impact on AP in extrafoveal vision (Cameron et al., 2002; Herrmann et al., 2010; Jigo and Carrasco, 2020). It has been argued that whether covert attention influences contrast gain, response gain, or both, depends on stimulus size, size of the attended area, as well as spatial uncertainty (Herrmann et al., 2010; Wu et al., 2021; Reynolds and Heeger, 2009).

To examine how AP was impacted by the fine-grained shifts of foveal exogenous attention across SFs, we fitted a generalized linear mixed-effects regression to the estimated AP from all 8 conditions of all observers (see Appendix 2—table 1 for full model output). Observers’ ability to discriminate the orientation at full contrast (AP) decreased with increasing frequency (Figure 4B, C), but this change was not significant (${\chi }^2(3)=7.6$, p = 0.0545) (post hoc pairwise comparisons revealed significant differences in AP only between 8 and 20 CPD (p = 0.015) as well as 12 and 20 CPD (p = 0.048)). The main effect of attention was significant, with overall higher AP in the valid condition compared to the neutral condition (${\chi }^2(1)=11.9$, p < 0.001; ${\text{mean}}_{\text{valid}}=0.97\pm 0.03$ SD and ${\text{mean}}_{\text{neutral}}=0.94\pm 0.04$ SD; see Figure 4A, C). These findings suggest that fine-grained attention resulted in a general response gain, enhancing the ability to discriminate the orientation of high-contrast stimuli.

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Unlike for CS, we did not observe a significant interaction between SF and attention on AP (Figure 4D, ${\chi }^2(3)=5.1$, p > 0.15) (post hoc pairwise comparisons between valid and neutral conditions within each SF revealed that attention significantly increased AP compared to the neutral condition at 8 and 12 CPD (mean gain_{8 CPD} = 0.05 ± 0.04 SD, p < 0.0001 and mean gain_{12 CPD} = 0.03 ± 0.03 SD, p = 0.0108) but not at 4 and 20 CPD (mean gain_{4 CPD} = 0.02 ± 0.03 SD, p = 0.0819 and Δmean gain_{20 CPD} = 0.03 ± 0.04 SD, p = 0.0501; see Appendix 2—table 2 for all pairwise comparisons). When comparing the attentional benefit on AP across pairs of SFs, we found a significant difference (Δmean gain) only between 4 and 8 CPD (p = 0.0240; see Appendix 2—table 3 for all pairwise comparisons)). Therefore, AP exhibited a significant effect of attention alone, with no detected significant effects of SF or of the interaction between SF and attention.

Whereas attention is often believed to be either uniformly allocated at the center of gaze or selectively shifted to locations outside of the central fovea, recent research has shown that humans are also capable of allocating attention within the central fovea in a spatially selective manner, enhancing our ability to perceive fine spatial stimuli (Poletti et al., 2017; Guzhang et al., 2021). Humans can focus processing resources on a specific region of the central fovea, enhancing visual processing within that small area while suppressing processing at other unattended locations just a few arcminutes away.

These findings raise the question of what SFs are enhanced by fine-grained shifts of covert attention within the foveola. In our previous work (Guzhang et al., 2021), observers were asked to perform a coarse orientation (±45°) discrimination task. This experiment did not manipulate SF. Subjectively, however, the stimuli in this task could be distinguished as long as frequency information of more than 3 CPD was available to the observer. The attentional gain we observed in the discrimination task could therefore have resulted from foveal exogenous attention enhancing SFs anywhere above 3 CPD. Thus, it remains unclear which SFs are enhanced when exogenous attention is allocated at the fine scale within the foveola. Additionally, it is unclear whether fine-grained attention in the foveola is governed by the same principles and is modulated similarly to extrafoveal attention, especially considering the stark differences in spatial resolution between the foveola and the rest of the visual field. The high-acuity foveola can resolve SFs up to 30 CPD, whereas just 5° away from the center of gaze, this limit drops to around 10 CPD (Virsu and Rovamo, 1979). Therefore, rather than enhancing the same range of low SFs as for extrafoveal vision, fine-grained foveal attention may shift or extend its enhancement toward higher frequencies in the foveola.

Our findings indicate that fine-grained shifts of covert exogenous attention in the foveola enhance CS within a narrow range of SFs, peaking at low to mid frequencies (4–8 CPD). In particular, we found little or no attentional gain at higher SFs (12–20 CPD), which are closer to the limits of visual resolution at the eccentricity tested (0.3° from the preferred locus of fixation). Whereas enhancements in CS were relatively selective to a narrow band of SFs, overall AP increased as a result of exogenous attention, with no detected dependence on SF. However, it is worth noting that the statistical power to detect the interaction might differ between analyses of CS and analyses of AP, given that the latter tends to involve differences close to its bounds (Bicknell et al., 2025; Jaeger, 2008).

Prior work on coarse exogenous attention shifts between central and peripheral vision has shown that peak attentional benefits for CS occur around 2–4 CPD, with a sizable benefit occurring also at 8 CPD, for a large foveal stimulus (Jigo and Carrasco, 2020). In the present study, the largest attentional gains were observed at the lowest SF tested (4 and 8 CPD). Because of the small stimulus size required to probe fine-grained attention at the foveal scale, we prioritized testing of SFs that yielded at least one full cycle within the stimulus aperture. Nevertheless, we conducted a post hoc evaluation at 2 CPD using the same Gabor size (see Figure 3—figure supplement 2). Because this stimulus contained less than one full cycle, performance may have been compromised (Howell and Hess, 1978). With this caveat in mind, baseline CS was comparable at 2 and 4 CPD and declined at 8 CPD, suggesting a plateau between 2 and 4 CPD. Attention significantly improved CS also at 2 CPD, with contrast gain comparable to that observed at 4 and 8 CPD (see caption of Figure 3—figure supplement 2). Thus, the range of SFs enhanced by fine-grained, exogenously triggered attention closely mirrors that observed when attention is broadly distributed across the fovea. The magnitude of the contrast gain (≈20%) was likewise consistent with prior findings (Jigo and Carrasco, 2020) and slightly larger than that reported by Herrmann et al., 2010 (see Figure 3—figure supplement 4).

The dataset and analysis code associated with this work are available on Open Science Framework (OSF) https://doi.org/10.17605/OSF.IO/PG7HD.

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   Frequency-dependent modulation of foveal contrast sensitivity by fine-scale exogenously triggered attention.

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Author details

1. Yue Guzhang

   Department of Brain and Cognitive Sciences, University of Rochester, Rochester, United States

   Contribution

   Conceptualization, Data curation, Formal analysis, Validation, Investigation, Visualization, Methodology, Writing – original draft, Project administration, Writing – review and editing

   For correspondence

   yzh191@u.rochester.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0009-0008-6637-2284

2. T Florian Jaeger

   1. Department of Brain and Cognitive Sciences, University of Rochester, Rochester, United States
   2. Goergen Institute for Data Science and Artificial Intelligence, University of Rochester, Rochester, United States

   Contribution

   Resources, Formal analysis, Supervision, Visualization, Methodology, Writing – original draft, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-1158-7308

3. Martina Poletti

   1. Department of Brain and Cognitive Sciences, University of Rochester, Rochester, United States
   2. Center for Visual Science, University of Rochester, Rochester, United States
   3. Department of Neurosciences, University of Rochester, Rochester, United States

   Contribution

   Formal analysis, Supervision, Validation, Writing – review and editing

   For correspondence

   martina_poletti@urmc.rochester.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-4773-8745

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