Across two main work packages, we used psychophysical, electroencephalography (EEG), and brain stimulation techniques (TMS) to address the processes of perceptual stability in magnitude perception.
The first research work package (WP2) was based on psychophysics and EEG. First, we investigated how the brain exploits information over time across multiple magnitude dimensions (size, duration, and numerosity) in the service of perceptual stability (Exp. 1). To this aim, we measured the psychophysical “serial dependence” effect, a perceptual bias whereby a stimulus that we are currently seeing appears more similar to what we saw in the recent past, while concurrently recording the EEG responses to the visual stimuli. Moreover, we also measured the neural signature of stability using a passive-viewing approach during EEG recording (Exp. 2), which allows to capture the perceptual aspects of stability in the absence of decision-making. In three subsequent studies (Exp. 3, Exp. 4, Exp. 5), we further addressed the specificity of the serial dependence effect for the task-relevance of different magnitude dimensions (i.e. whether they are actively judged or not), the interaction between perceptual stability and magnitude integration, and the nature of the magnitude integration effect itself. Magnitude integration is indeed an important feature of perception, and defines how we perceive a given dimension when multiple magnitudes are concurrently modulated. However, its brain mechanisms and relation with perceptual stability are unclear. Finally, in Exp. 6 and Exp. 7, we addressed whether stability in magnitude perception involves a generalized mechanism encompassing different perceptual dimensions and sensory modalities, or a series of distinct mechanisms. Overall, our results show that the neural and behavioural signatures of stability can be dissociated: the first seems to more genuinely capture perceptual processing, the second also involves decision-making processes. Our results also showed that magnitude integration is mediated by an active mechanism “binding” the different dimensions of an object, and not by a trivial interference, although it does not directly interact with stability. Finally, our results also suggest that the brain possesses different and potentially independent mechanisms mediating stability in different domains and sensory modalities, following different rules and showing different patterns of brain activity. The work in this WP has been presented at the ESCoP 2019 meeting, at the CyPi 2019 workshop, at the EWCN 2020, and will be presented at the ECVP meeting in 2021.
In the second research WP (WP3), we used TMS to further address the mechanisms of stability in magnitude perception. TMS involves delivering a magnetic pulse disrupting brain activity in a specific cortical area at a specific time, with a high degree of spatial and temporal precision. In this study, we focused on the serial dependence effect in numerosity and time perception, considering it as a correlate of perceptual stability (i.e. the consequence of stability on perception), and delivered TMS to the primary visual cortex (V1) at different times while participants judged the numerosity or duration of visual stimuli. Our results show that TMS to area V1 can successfully disrupt serial dependence, abolishing the effect, and crucially it does so in a time-sensitive fashion, depending on which brain processing stage gets disrupted. With this study, we have provided for the first time causal evidence for the involvement of V1 in perceptual stability, and demonstrated the timing of its involvement.
