Small Saccades and Image Complexity during Free Viewing of Natural Images in Schizophrenia

Abstract

In schizophrenia, patients display dysfunctions during the execution of simple visual tasks such as antisaccade or smooth pursuit. In more ecological scenarios, such as free viewing of natural images, patients appear to make fewer and longer visual fixations and display shorter scanpaths. It is not clear whether these measurements reflect alterations in their proficiency to perform basic eye movements, such as saccades and fixations, or are related to high-level mechanisms, such as exploration or attention. We utilized free exploration of natural images of different complexities as a model of an ecological context where normally operative mechanisms of visual control can be accurately measured. We quantified visual exploration as Euclidean distance, scanpaths, saccades, and visual fixation, using the standard SR-Research eye tracker algorithm (SR). We then compared this result with a computation that includes microsaccades (EM). We evaluated eight schizophrenia patients and corresponding healthy controls (HC). Next, we tested whether the decrement in the number of saccades and fixations, as well as their increment in duration reported previously in schizophrenia patients, resulted from the increasing occurrence of undetected microsaccades. We found that when utilizing the standard SR algorithm, patients displayed shorter scanpaths as well as fewer and shorter saccades and fixations. When we employed the EM algorithm, the differences in these parameters between patients and HC were no longer significant. On the other hand, we found that image complexity plays an important role in exploratory behaviors, demonstrating that this factor explains most of differences between eye-movement behaviors in schizophrenia patients. These results help elucidate the mechanisms of visual motor control that are affected in schizophrenia and contribute to the finding of adequate markers for diagnosis and treatment for this condition.

Eye movements of patients with schizophrenia in a natural environment

“…During free gaze and without any additional behavioral task, patients fixated significantly longer and less often than healthy controls. This is in line with a recent study by Egaña et al [ 9 ], investigating free viewing of natural images in the laboratory. In our study, median fixation duration was correlated with the PANSS item grandiosity in schizophrenia patients, indicating that some patients might have drifted off in their imagination during this task.…”

“…For specific oculomotor parameters, such as fixation duration and frequency, we found significant differences between the groups. Some of these differences resembled those reported under laboratory conditions, such as decreased exploratory eye movements [ 8 , 9 ], while others (e.g., tracking eye movement gain) seem to reach normal performance during natural vision. This might be a characteristic feature of eye-movement behavior in real-life environments which possibly trigger as of yet unknown compensatory mechanisms.…”

“…A large number of different eye-movement abnormalities have been described. Examples include decreased smooth-pursuit gain [ 2 – 4 ], increased anti-saccade error rates and latencies [ 5 ], changes in saccade dynamics [ 6 , 7 ], or different fixation patterns when viewing static pictures [ 8 , 9 ].…”

Influence of scene structure and content on visual search strategies

“…Therefore, it would be interesting to measure the parameters l and du over these trajectories and study how they change from one group to the other. Based on the reported trajectories in [49], we believe that the average l should change for the healthy control group, whereas for the schizophrenic group it should remain constant along the different levels of image complexity. Within this simple framework, we would be able to quantify behaviours that have been qualitatively observed.…”

“…Our method is simple enough to be used as a tool to further understand or diagnose mental disorders. For instance, a recent study investigated how the visual patterns, while performing different free viewing tasks, change for healthy control versus schizophrenic patients [49]. The authors presented a set of scenes, with different levels of image complexity, for the participants to freely explore and recorded the position of their eyes while doing so.…”