Estimating the statistical power to detect set size effects in contralateral delay activity

The contralateral delay activity (CDA) is an event-related potential component commonly used to examine the online processes of visual working memory. Here, we provide a robust analysis of the statistical power that is needed to achieve reliable and reproducible results with the CDA. Using two very large EEG datasets that examined the contrast between CDA amplitude with set sizes 2 and 6 items (Unsworth et al., 2015) and set sizes 2 and 4 items (Hakim et al., 2019), we present a subsampling analysis that estimates the statistical power achieved with varying numbers of subjects and trials based on the proportion of significant tests in 10,000 iterations. We also generated simulated data using Bayesian multilevel modelling to estimate power beyond the bounds of the original datasets. The number of trials and subjects required depends critically on the effect size. Detecting the presence of the CDA – a reliable difference between contralateral and ipsilateral electrodes during the memory period – required only 30-50 clean trials with a sample of 25 subjects to achieve approximately 80% statistical power. However, for detecting a difference in CDA amplitude between two set sizes, a substantially larger number of trials and subjects was required; approximately 400 clean trials with 25 subjects to achieve 80% power. Thus, to achieve robust tests of how CDA activity differs across conditions, it is essential to be mindful of the estimated effect size. We recommend researchers designing experiments to detect set size differences in the CDA collect substantially more trials per subject.

Search for Face Identity or Expression: Set Size Effects in Developmental Prosopagnosia

The set size effect during visual search indexes the effects of processing load and thus the efficiency of perceptual mechanisms. Our goal was to investigate whether individuals with developmental prosopagnosia show increased set size effects when searching faces for face identity and how this compares to search for face expression. We tested 29 healthy individuals and 13 individuals with developmental prosopagnosia. Participants were shown sets of three to seven faces to judge whether the identities or expressions of the faces were the same across all stimuli or if one differed. The set size effect was the slope of the linear regression between the number of faces in the array and the response time. Accuracy was similar in both controls and prosopagnosic participants. Developmental prosopagnosic participants displayed increased set size effects in face identity search but not in expression search. Single-participant analyses reveal that 11 developmental prosopagnosic participants showed a putative classical dissociation, with impairments in identity but not expression search. Signal detection theory analysis showed that identity set size effects were highly reliable in discriminating prosopagnosic participants from controls. Finally, the set size ratios of same to different trials were consistent with the predictions of self-terminated serial search models for control participants and prosopagnosic participants engaged in expression search but deviated from those predictions for identity search by the prosopagnosic cohort. We conclude that the face set size effect reveals a highly prevalent and selective perceptual inefficiency for processing face identity in developmental prosopagnosia.

Set-size effects in change detection depend on failures of retrieval and/or comparison and not on perception, encoding or storage

Set-size effects in change detection is often used to investigate the capacity limits of dividing attention. Such capacity limits have been attributed to a variety of processes including perception, memory encoding, memory storage, memory retrieval, comparison and decision. In this study, we investigated the locus of the effect of increasing set size from 1 to 2. To measure purely attentional effects and not other phenomena such as crowding, a precue was used to manipulate relevant set size and keep the display constant across conditions. The task was to detect a change in the orientation of 1 or 2 Gabor patterns. The locus of the capacity limits was determined by varying when observers were cued to the only stimulus that was relevant. We began by measuring the baseline set-size effect in an initial experiment. In the next experiment, a 100% valid postcue was added to test for an effect of decision. This postcue did not change the set-size effects. In the critical experiments, a 100% valid cue was provided during the retention interval between displays, or only one stimulus was presented in the second display (local recognition). For both of these conditions, there was little or no set-size effect. This pattern of results was found for both hard-to-discriminate stimuli typical of perception experiments and easy-to-discriminate stimuli typical of memory experiments. These results are consistent with capacity limits in memory retrieval, and/or comparison. For these set sizes, the results are not consistent with capacity limits in perception, memory encoding or memory storage.Significance SectionThe change detection paradigm is often used to demonstrate effects of divided attention. But it is not clear whether these effects are due to perception, memory, or judgment and decision. In this article, we present new evidence that the divided attention effect in change detection is due to limits in memory retrieval or comparison processes. These results are not consistent with limits in perception, memory encoding or memory storage.