Our study showed different mechanisms of SDEs between subitizing and estimation. Experiment 1, with an SOA of 200 ms, showed significant SDE only when the current magnitude was smaller than the previous magnitude (smaller oddball), but not when it was larger (larger oddball). In contrast, symmetrical SDEs were observed in estimation. Experiment 2 showed that SDE was eliminated for subitizing and reduced for estimation when perceptual interference from the immediately preceding RSVP item was greatly reduced by prolonging SOA to 494 ms. Experiment 4, on the other hand, showed that SDE was intact for subitizing and substantially reduced for estimation when post-perceptual interference to the first and second RSVP items was greatly reduced by removing RSVP items after the third one. These independent manipulations suggest that subitizing-related SDE is likely due to perceptual processing, while estimation-related SDE is due to a combination of perceptual and post-perceptual processing. Additionally, asymmetrical SDEs for subitizing and symmetrical ones for estimation were still observed even when non-numerical, continuous magnitude variables were controlled in Experiment 3 and when processing between the first and the second RSVP items was matched by adding a forward-mask in Experiment 5.
Different mechanisms of SDE between small and large numerosities
The present study suggested that the SDE mechanisms seem to be different between the two numerosity ranges. In estimation, over- and under-estimations of the mean numerical response between the first and the second oddball positions were accompanied by either a constant processing precision (in Experiment 1 & 3) or a very small increase of processing precision (in Experiment 4 & 5, negligible relative to the decreased processing precision in subitizing) across oddball positions. This dissociation between accuracy and precision for SDE on large numerosities is consistent with previous numerical literature indicating that SDE in estimation essentially introduces bias, with little or no influence on the precision of numerical discriminability (Fornaciai & Park, 2018a; Fornaciai & Park, 2019b). More generally, this is also consistent with the consensus view in the current SDE literature that SDE is a mechanism to achieve processing continuity and stability by integrating information over time (Cicchini et al., 2021; Fischer & Whitney, 2014). However, in subitizing, all experiments with 200ms SOA revealed similar patterns of changes in accuracy and precision, i.e., different from that in estimation. This finding reveals a novel type of SDE, whereby perceptual appearance drifts towards recent information along with decreased processing precision. Our findings thus reveal the diverse nature of SDE mechanisms. We identified at least two qualitatively different SDE types: one characterized by decreased processing precision, prioritizing processing continuity at the expense of processing stability, and the other characterized by stable processing precision, enabling the concurrent achievement of processing continuity and processing stability. These two types of SDEs manifest differentially in the small and large numerosity ranges, respectively.
There is currently a lively debate in the literature regarding the underlying mechanisms of SDE (Ceylan et al., 2021; Cicchini et al., 2017; Fischer & Whitney, 2014; Fornaciai & Park, 2018a). Initially, SDE was thought to originate from and act on early perceptual processing (Fischer & Whitney, 2014). However, recent studies have shown that SDE can instead originate from a high-level, post-perceptual processing of the previous trial and act on a low-level, perceptual processing of the current trial (Fornaciai & Park, 2019b, 2022; Schwiedrzik et al., 2014; St John-Saaltink et al., 2016). For example, Fornaciai and Park (2022) found that abstract information (symbolic numerosity) successfully yielded attractive serial dependence biases, and that external information at the level of judgment (by providing feedback of responses) significantly modulated the magnitude of SDEs. These findings align with our proposal that SDE of estimation may involve a combination of both perceptual and post-perceptual processing. Further studies are required to elucidate at which stage (origin or target) and how estimation-based SDEs combine perceptual and post-perceptual processing.
It remains unclear whether and how the neural substrates differ between subitizing- and estimation-based SDEs. Prior research by Fornaciai and Park (2018a) reported a neural signature of attractive SDEs emerging early in the visual stream in estimation. Recent studies (Fornaciai & Park, 2020a; Fornaciai et al., 2023) confirmed this early neural signature and demonstrated that this biased neural representation was preserved for a relatively long period and can be reactivated by reactivating hidden memory states (Wolff et al., 2015). Our study showed that subitizing-based SDEs primarily involve perceptual components, lacking post-perceptual components. Therefore, an interesting question arises as to whether the neural signature of subitizing-based SDE is limited to early stages and cannot be retained as a hidden memory state. This hypothesis merits further investigation.
The present study contributes to the ongoing debate on whether subitizing and estimation involve the same mechanisms (Dantzig, 1967; Dehaene, 1997). In terms of sensitivity and dependence on contextual information, it appears that small and large numerosities operate via distinct mechanisms. Critically, our findings demonstrated the multifaceted nature of SDE mechanisms in supporting continuity and/or stability of visual processing (Cicchini et al., 2021; Fischer & Whitney, 2014). As an active process integrating past and present information, SDE can be differentially engaged with cognitive modules subserving different functionalities, such as cognitive scenarios closely-related with early object recognition in subitizing (Trick & Pylyshyn, 1993) and ensemble numerosity extraction in estimation (Dantzig, 1967; Dehaene, 1997), respectively.
An object individuation-based 'temporal hysteresis' account for SDE in subitizing
We provide evidence for the first time that SDE can occur in subitizing. The SDE in small numerosities shows an asymmetrical pattern, different from the SDEs in large numerosities (Fornaciai & Park, 2018b, 2022). The asymmetrical SDEs in subitizing are unlikely be explained by several well-established effects such as difference of signal strength (Pascucci et al., 2019), persistence of vision (Coltheart, 1980), postdiction (Eagleman & Sejnowski, 2000), central tendency (Anobile et al., 2012), or ensemble representation (Manassi et al., 2017) (more details see the SI). Here, we propose a 'temporal hysteresis' account based on object individuation (Pylyshyn, 1989; Trick & Pylyshyn, 1993; Xu & Chun, 2009) to explain SDE in subitizing. Specifically, we propose that the asymmetrical SDE in subitizing reflects the disposition of temporal hysteresis (a delayed offset) in object individuation (Wutz et al., 2012; Wutz & Melcher, 2013, 2014). That is, object individuation resource has a tendency to be easy to open but difficult to close. In other words, within its limited capacity, the allocation of new object individuation resource is rapid and efficient. However, the reclamation of previously-occupied object individuation resource is relatively slow and inefficient. Therefore, the SDE observed in the smaller oddball condition (i.e., standard numerosity > oddball numerosity) of subitizing, that is, numerical overestimation of the position 2 relative to the position 1, is likely due to the residual activity of the net object individuation resource elicited by the previous standard numerosity. These resources are no longer required by the current oddball numerosity and cannot be immediately reclaimed by the object individuation system. In contrast, in the larger oddball condition (i.e., standard numerosity < oddball numerosity) of subitizing, there is no net object individuation resource elicited by the previous standard numerosity. Instead, the extra object individuation resource required by the current oddball numerosity can be efficiently opened and allocated by the object individuation system, leaving no bias in numerical processing between the first and the second positions, i.e., no SDE.
The object individuation-based 'temporal hysteresis' account received further support from the analysis of CoV, an index of the precision of underlying numerical processing (Revkin et al., 2008; Véronique et al., 2008). All experiments with a 200ms SOA showed an asymmetrical pattern of CoV in subitizing, where CoVs significantly increased with oddball positions in the smaller oddball conditions but remained stable and constant in the larger oddball conditions. These findings were consistent with the 'temporal hysteresis' account, in that the residual activity of the net object individuation resource, which was only present in the smaller oddball but not the larger oddball conditions, contributed to numerical processing in two ways: leading to an inflation of mean numerical response and a deterioration of processing precision by introducing extra noise into the object individuation system.
Hysteresis-like effects have been frequently observed across various aspects of human cognition, including motion direction perception (Williams et al., 1986), apparent motion (Hock et al., 1993), occluded object recognition (Ernst et al., 2021), spatial frequency integration during active perception (Brady & Oliva, 2012), contrast change-driven visual letter recognition (Kleinschmidt et al., 2002), and judgment of auditory pitch shift (Chambers & Pressnitzer, 2014; Giangrand et al., 2003). The present study provides new evidence for the ubiquity of hysteresis-like effects in human cognition. In light of recent research (Ernst et al., 2021) proposing that recurrent connectivity in the brain and its associated feedback/re-entrant processing may be key to understanding perceptual hysteresis for both auditory and visual stimuli (Buckthought et al., 2008; You et al., 2011), future studies investigating the role of feedback/re-entrant processing in the hysteresis-like SDE for subitizing would be of great interest. In particular, it would be intriguing to examine whether ’the residual activity of the net object individuation resource’ acting on current stimuli in the object individuation-based 'temporal hysteresis' account is linked with feedback/re-entrant processing (Ernst et al., 2021) of immediately preceding stimuli.
In conclusion, employing a novel continuous numerical oddball paradigm, we unveiled two distinct types of SDEs in numerical cognition: subitizing-based and estimation-based SDEs. The divergent patterns and underlying mechanisms of these SDEs imply that the continuity and stability of numerical processing can be dissociable in dynamic situations where numerical information is integrated over time. Our findings also provide additional empirical support for the functional dissociation between two numerical ranges: subitizing, likely linked to early object recognition, and estimation, likely associated with ensemble numerosity extraction.