Christoph Strauch

Pupil Size Changes as an Active Information Channel for Biofeedback Applications

Pupil size is usually regarded as a passive information channel that provides insight into cognitive and affective states but defies any further control. However, in a recent study (Ehlers et al. 2015) we demonstrate that sympathetic activity indexed by pupil dynamics allows strategic interference by means of simple cognitive techniques. Utilizing positive/negative imaginings, subjects were able to expand pupil diameter beyond baseline variations; albeit with varying degrees of success and only over brief periods. The current study provides a comprehensive replication on the basis of considerable changes to the experimental set-up. Results show that stricter methodological conditions (controlled baseline settings and specified user instructions) strengthen the reported effect, whereas overall performance increases by one standard deviation. Effects are thereby not restricted to pupillary level. Parallel recordings of skin conductance changes prove a general enhancement of induced autonomic arousal. Considering the stability of the results across studies, we conclude that pupil size information exceeds affective monitoring and may constitute an active input channel in human–computer interaction. Furthermore, since variations in pupil diameter reliably display self-induced changes in sympathetic arousal, the relevance of this parameter is strongly indicated for future approaches in clinical biofeedback.

Towards Pupil-Assisted Target Selection in Natural Settings: Introducing an On-Screen Keyboard

Preliminary reports have shown the possibility to assist input commands in HCI via pupil dilation. Applicability of these findings is however subject to further investigations, since the specificity of changes in diameter is low, e.g. through variations in brightness. Investigating employability and shape of pupil size dynamics outside a strictly controllccced laboratory, we implemented the emulation of selection via an integrated mechanism of pupil dilation and constriction that could speed up a dwell time of 1.5 s. During the operation of an on-screen keyboard, 21 subjects were able to type via this mechanism, needing 1 s on average per keystroke and producing only slightly more than 1% false positive selections. Hereby, pupil dynamics were assessed. More than 90% of keystrokes could be accelerated under assistance of pupil variations. As suggested from basic research, pupil dilated when fixating later selected keys and constricted shortly afterwards. This finding was consistent between all subjects, however, pupil dynamics were shifted in regard to temporal occurrence and amplitude of diameter changes. Pupil-Assisted Target Selection shows potential in non-strictly controlled environments for computer input and may be further improved on the basis of this data. This might culminate in an integrated gaze-based object selection mechanism that could go beyond the benchmarking dwell time performance.

Pupil-Assisted Target Selection (PATS).

Physiological signals such as pupil size changes promise improvements for human-computer-interaction. However, the pupil response is known to be rather slow and unspecific. This hinders its application in target selection up to now. Nevertheless, there are indications for fast diameter changes accompanying cognitive processing already at early stages so that pupil effects can even precede psycho-motor activity. Building on these findings, we investigated the potential of short-latency pupil size changes for improving target selection in a search and select task. Pupil assisted target selection (PATS) was shown to be competitive to both, purely pupil-based and to dwell-time based selection modes in regard to selection times, but at the cost of more false positives than for a dwell-time approach in a search and select task. This demonstrates the usefulness of PATS as a means for target selection. The observed pupil dynamics correspond to early signal courses in basic research. Pupil dynamics also suggest room for further improvements of the integrated concept of pupil-assisted target selection.

Slow Trends

As of recently, research efforts are intensified to operationalize pupil dynamics for cognitive and affective classification in human-machine interaction. However, signal analysis of pupil diameter changes is problematic since the respective dynamics consist of three essential components that have to be disentangled: Very slow diameter changes, slow and high frequencies. The current paper discusses the amount of slow trends in pupillary signal courses and the effects on functional parameters of pupil dilations. Thereby we confront our data with linear detrending approaches and reveal various forms of trend progressions that differ over time and cannot be fixed with conventional linear procedures.

Pupil dilation but not microsaccade rate robustly reveals decision formation

Pupil dilation, an indicator of arousal that is generally regarded as unspecific, amongst others reflects decision formation and reveals choice. Employing letter selection in a Go/NoGo task, we show that choice can robustly be predicted by the pupillary signal, even under the presence of strong interfering factors such as changes in brightness or motor execution. In addition, a larger difference in pupil dilation between target and distractor conditions for NoGo compared to Go was demonstrated, underlining the particular appropriateness of the paradigm for decision research. Incorporating microsaccades, a variable that is suggested to covary with pupil diameter, we show that decision formation can only be observed in pupil diameter. However, microsaccade rate and pupil size covaried for motor execution and both reflected choice after key press with smaller effect size for microsaccade rate. We argue that combining pupil dilation and microsaccade rate may help dissociating decision-related changes in pupil diameter from interfering factors. Considering the interlinked main neural correlates of pupil dilation and microsaccade generation, these findings point to a selective role of locus coeruleus compared to superior colliculus in decision formation.

Training facilitates cognitive control on pupil dilation

Physiological responses are generally involuntary; however, real-time feedback enables, at least to a certain extent, to voluntary control automatic processes. Recently, it was demonstrated that even pupil dilation is subject to controlled interference. To address effects of training on the ability to exercise control on pupil dilation, the current study examines repeated exercise over seven successive days. Participants utilize self-induced changes in arousal to increase pupil diameter, real-time feedback was applied to evaluate and improve individual performance. We observe inter-individual differences with regard to responsiveness of the pupillary response: six of eight participants considerably increase pupil diameter already during the first session, two exhibit only slight changes, and all showed rather stable performance throughout training. There was a trend towards stronger peak amplitudes that tend to occur increasingly early across time. Hence, higher cognitive control on pupil dilations can be practiced by most users and may therefore provide an appropriate input mechanism in human-computer interaction.

A view to a click: Pupil size changes as input command in eyes-only human-computer interaction

Abstract In the field of Human-Computer Interaction (HCI), affective computation extends parameterization by including implicit user information, e.g. galvanic skin responses or heart rate variability. Physiological responses are generally involuntary and measurements primarily derive emotional features to differentiate cognitive and affective states. Biofeedback applications externalize covert physiological responses and allow interference by means of simple cognitive techniques. Here, signal responses do not necessarily indicate cognitive or affective dispositions but depict a users intention. As demonstrated recently, even pupil diameter is subject to explicit control, albeit with individually varying degrees of success. The current study explores to what extent users can transfer implicit events (here: pupil size changes) into explicit input requests. In Experiment 1 we carried out biofeedback sessions to determine individual strategies to achieve cognitive control on pupil dilation. In Experiment 2 we applied explicit pupil dilations as input commands in a visual search-and-select task. Implementation of varying thresholds ensured the involvement of different levels of cognitive effort to exceed selection criteria. Results show that a short period of freely exploring the mechanisms of action enabled most subjects to achieve cognitive control on pupil dilation, even during increased tonic activation provoked by high task load. Hence, the associated dynamics no longer define automatic events but indicate the degree of explicitness to be imposed upon implicit processing. During pupil-based object selection, a strict criterion was associated with a high cognitive load, prolonged selection times and a notable number of time-outs, whereas a soft criterion correlated with fast selections and low cognitive demands. Together with the outcome of a user survey, the current findings suggest explicit pupil size changes constitute an appropriate selection mechanism for HCI even in the context of high cognitive workload.