Report-difference superiority 1 Title: The basis of report-difference superiority in delayed perceptual comparisons tasks

word count: 229 Report-difference superiority 4 Visual short-term memory (VSTM) is a capacity-limited store of recently viewed visual information (Cowan, 2001; Phillips, 1974). VSTM performs a role in mediating perception across consecutive glances or brief visual disruptions (Luck, 2006; Hollingworth, 2006). Consequently, VSTM is involved in tasks which require perceptual comparison of visual information which is temporally separated in some way or which is derived across successive views (Markman & Gentner, 2000). This role can be seen in tasks such as the detection of change (Luck & Vogel, 1998; Pashler, 1988), simultaneous comparisons (Scott-Brown, Baker & Orbach, 2010; Huang, 2010), matching-to-sample (Mangini, Villano & Crowell, 2010), probe-matching (Griffin & Nobre, 2003), and the detection of sameness/feature repetitions (Hyun, Woodman, Vogel, Hollingworth & Luck, 2009; Wilson & Goddard, 2011). There is a curious asymmetry in the efficiency with which VSTM comparisons are made. Specifically, participants tend to be markedly better at reporting a difference between two displays, i.e. reporting that a feature or object has changed, than they are at noticing a sameness between displays, i.e. reporting that a feature or object has not changed (Hyun et al., 2009; Pilling & Barrett, 2016). This finding of report-difference superiority (hereafter, RDS) was noted some time ago in work looking at perceptual comparisons (Egeth, 1966; Taylor, 1976; Farrell, 1985). However, this earlier work treated perceptual comparisons as something only involving current visual processing and did not consider the likely involvement of VSTM. Hyun and colleagues (Hyun, et al., 2009) were the first to directly explore this asymmetry from the perspective of VSTM. In a critical experiment they presented two displays, each containing four disks arranged as a notional square, in sequence interleaved by a blank interval. In their task the number of colours that differed across the two displays was parametrically varied, on some trials all the disks retained the same colour across the displays, in others one-tofour of the disks were given a different colour, one previously not present in the array. The experiment presented participants with two displays each coloured disks. Participants had to perform one of two types of comparison task, each of which required a binary decision. Under any difference instruction they had to report if any (i.e. one or more) of the colours was different between the two displays, under any sameness instruction they had to report if any (i.e. one or more) of the colours was the same across the displays. Hyun et al. found that performance was markedly better for the any difference task. This was most clearly exemplified in the condition where two of the colours were different and two the Report-difference superiority 5 same, a positive response was required under both task instructions and there were the same number of critical features present for each task instruction. Despite this, participants were faster and more accurate in detecting the difference than when required to detect the absence of a difference. Hyun et al. (2009) explained the RDS as a consequence of the manner in which information is compared between a VSTM-held representation and a current visual input. It was suggested that difference judgements are supported by the existence of a reflexive mechanism in the brain which automatically compares the locations of VSTM-held object with those derived from current input. In this explanation, the locations in the test array are automatically compared in parallel with the VSTM-held representation of the memory array. Where the features are different, a transient signal is generated which tends to attract focal attention and a subsequent confirmatory active comparison of the location contents. In the case of a sameness, the absence of a mismatch doesn’t generate any such transient in the initial reflexive comparison; the consequence of this is that sameness detection is wholly reliant on active serial comparisons between memory-held and viewed representations, meaning that performance tends to be poorer. Hyun et al. supported this interpretation with evidence from eye movements and EEG recordings as well as behavioural data, all of which indicated that there is automatic registration of a perceptual mismatch event and that this tends to influence internal attention. Others in the literature have similarly suggested that detection of change is influenced by an initial automatic comparison stage which guides a second confirmatory comparison process (Gilchrist & Cowan, 2014; Yin, Gao & Jin et al, 2012; Pilling & Barrett, 2018) The purpose of the current experiments was to evaluate whether this proposed mechanism is by itself sufficient explanation for the report-difference advantage. We suspected, a priori, that there may be at least one additional process underlying the RDS. Specifically, it is known that observers are sensitive to global statistical properties (Ariely, 2001; Chong & Treisman, 2003). It is known that such statistical information is generated automatically from viewed displays (Chong & Treisman, 2005), and that observers are sensitive to the changes in statistical properties across temporally separated displays (Brady & Alvarez, 2011; Haberman & Whitney, 2011). The original claims were that such global statistical information was extracted in an entirely pre-attentive manner, however more recent work suggests that this process utilises attention in the same way as does attending to object features individually (e.g. Huang, 2015; Jackson-Nielsen, Cohen, & Pitts, 2017) To reiterate, Hyun et al. propose that RDS is explained wholly as a consequence of a putative reflexive mechanism which makes local comparisons between spatial locations in VSTM-held Report-difference superiority 6 representations and those of current input. If this is the case then RDS should depend entirely on the compared items being ones which maintain their spatial configuration between memory and test. If the item locations are scrambled between memory and test then a local comparison mechanism could not provide any useful information about the presence or absence of a feature change. Consequently, under these conditions, RDS should no longer occur. If however, detection of difference is, in part, supported by comparison of global statistics (e.g. comparison of mean hue values) then the report-difference advantage might persist even when required to compare items with scrambled locations. Experiment 1 tested this possibility.