Surround suppression effects on working memory performance in the general population and in people with schizophrenia : behavioural and ERPs evidence

Visual Working memory (WM) is a cognitive ability that allows to retain and manipulate information for a short period of time. WM is fundamental for mental functions and it supports several everyday activities such as learning, reasoning and language comprehension. In fact, impairments in WM, which are established in clinical conditions such as schizophrenia, have been related to poor quality of life factors, such as work/education status. Despite a large number of studies investigating WM, its underlying mechanisms are still a matter of debate both in the general population and in schizophrenia. A number of landmark studies have shown that early visual areas are active during the maintenance of information in WM, which emphasizes the importance of low-level visual processes in higher-level cognition. However, few studies have examined the basic visual processes underlying encoding into WM. For example, surround suppression (SS), in which the perception of a target is altered by the context in which it is embedded, is a largely known basic perceptual mechanism. However, it has not been explored whether SS can also impact WM representations. In three experiments, this project investigated how individual variations in the SS sensitivity affect WM in typical participants (Experiment 1), in patients with schizophrenia (Experiment 2) and in interaction with attention (Experiment 3). Stimuli that differentially triggered the strength of SS activity in early visual areas were used in a contrast matching (CM) task, an orientation discrimination (OD) task and in a WM task. In the WM task, participants viewed 1 to 3 sequentially presented gratings with different orientations surrounded by either orthogonal or parallel circular regions. They then judged whether the orientation of a subsequent probe (without a surround) matched any of the targets. ERPs signals were also measured during the WM task. In Experiment 1, in the CM task, 18 participants confirmed that a central target grating appeared to have less contrast in the context of a co-oriented surround compared to an orthogonally-oriented surround. WM performance decreased with the increment of load. Moreover, it was also decreased in the parallel compared to the orthogonal surround but only for Load 1, but not throughout all WM loads. During WM encoding, posterior P2 amplitudes were significantly higher in the orthogonal compared to the parallel condition, suggesting that posterior P2 respond to SS mechanisms. Experiment 2 tested 19 patients with schizophrenia and 20 matched controls. Confirming previous studies, patients contrast perception was not affected by the SS. In addition, the OD threshold was significantly higher in patients compared to controls and it negatively correlated with WM performance, suggesting that basic visual skills can relate to higher cognitive processing. Overall WM accuracy was lower in patients compared to controls. However, in contrast to controls, patients’ WM accuracy was not affected by SS. During encoding, posterior P2 amplitudes were decreased with stronger SS only in controls but not in patients. However, both in Exp. 1 and Exp. 2, no direct correlations were found between P2 and WM performance. Experiment 3 tested 20 participants on a modified version of the WM task in order to test whether LI interferes with attention. Here, a cue highlighted which item had to be memorised, over a list of three. Only behavioural data were collected. For hit rate, the position of the item to remember influenced performance only for the parallel, but not for the orthogonal surround. Overall, Experiment 3 seems to suggest that the focus of attention might be subjective to perceptual interference triggered by SS. Overall, this project successfully confirmed SS effects on perceived contrast in typical participants and the lack of SS in patients with schizophrenia. In addition, the difference in surround conditions was reflected in P2 in typical participants (Exp 1) but not in patients (Exp 2), suggesting that encoding processes in schizophrenia might not occur in the same time window as controls. Moreover, these results showed that lower 29 basic perceptual skills (such as OD) in schizophrenia are associated with decreased WM performance. However, in this project a direct relationship between stronger SS and WM was not found both in healthy and in schizophrenia populations. Future studies will need to clarify whether overall SS mechanisms (regardless of the strength of the effect) have an influence on WM performance compared to conditions in which SS is absent by the use, for example, of a “no surround” condition.