Pupillometry: the Index of Cognitive Activity in a dual-task study

Pupillometry: the Index of Cognitive Activity in a dual-task study Vera Demberg (vera@coli.uni-saarland.de) Cluster of Excellence, Saarland University, Campus C7.4, 66123 Saarbr¨ucken, Germany Abstract This paper reports experimental results on the index of cogni- tive activity (ICA), a recent micro-level measure in pupillom- etry, which relates processing load to the frequency of rapid small dilations of the pupil. We collected pupil size during a tracking task which was cast in a simulated driving context, as well as for a dual task of simultaneous tracking and language processing. The present results are the first to evaluate the ICA measure on these tasks. We find that the ICA is sensitive both to the simulated driving and the language task, and that it is more responsive to our driving task than overall pupil dilation. Overall, the use of the ICA as opposed to traditional pupil- lometry seems promising, as our data provide initial evidence that the ICA may be more responsive, and a more fine-grained measure of cognitive load than traditional macro-scale pupil dilation measures. Keywords: Pupillometry, Index of Cognitive Activity, Dual Task, Language, Driving Introduction The size of the pupil has long been known to reflect arousal (Hess & Polt, 1960) and cognitive load in a variety of differ- ent tasks such as arithmetic problems (Hess & Polt, 1964), digit recall (Kahneman & Beatty, 1966), attention (Beatty, 1982) as well as language complexity (Schluroff, 1982; Just & Carpenter, 1993; Hy¨on¨a, Tommola, & Alaja, 1995; Zellin, Pannekamp, Toepel, & der Meer, 2011; Frank & Thompson, 2012), grammatical violations (Gutirrez & Shapiro, 2010) and context integration effects (Engelhardt, Ferreira, & Pat- senko, 2010). All of these studies have looked at the macro- level effect of the overall dilation of the pupil as response to a stimulus. Recently, another micro-level measure of pupil dilation has been proposed, called the “Index of Cognitive Activity” or ICA (Marshall, 2000, 2002, 2007), which does not relate processing load to the overall changes in size of the pupil, but instead counts the frequency of rapid small dilation, which are usually discarded as pupillary hippus (Beatty & Lucero-Wagoner, 2000). The ICA has been argued to be ro- bust to changes in ambient light and eye-movements, and can therefore be hoped to be more reliable and robust than over- all pupil dilation. Furthermore, as it does not use the overall dilation of the pupil which can vary as a function of lighting and individual, the frequency of the rapid pupil dilations is argued to be more comparable across tasks and subjects. If it reliably reflects processing load, the ICA would be a convenient method to assess processing load using an eye- tracker, in naturalistic environments, e.g. while driving a car, and could therefore usefully complement the range of exper- imental paradigms currently used. To our knowledge, the present paper is the first to test its response to a tracking task, and to analyze properties of the Index of Cognitive Activity such as its response delay to a stimulus. The application of the method in a realistic sce- nario (measuring linguistically induced cognitive load during driving) also bears relevance for practical applications. The Index of Cognitive Activity The Index of Cognitive Activity is a patented measure of cog- nitive load which has previously only been evaluated on a small range of tasks (Marshall, 2000, 2002, 2007; Schwalm, 2008; Schwalm, Keinath, & Zimmer, 2008) including digit span tasks, and a simulated driving task. Using the ICA as a measure of processing load is motivated by the finding that pupil size can be affected by two different processes: lighting conditions and cognitive activity. In the overall pupil dila- tion, these two effects are confounded, even in stable lighting because there is a so-called “light reflex”, meaning that the pupil oscillates irregularly and continually. Pupil dilation is controlled by two groups of muscles: circular muscles, which make the pupil contract and radial muscles, which make the pupil dilate. Furthermore, we know that the activation and inhibition patterns are different for reaction to light and re- action to cognitive activity (Marshall, 2000): dilations due to cognitive activity are very short and abrupt, while pupil size changes due to lighting are slower and larger. The ICA therefore tries to disentangle these patterns by performing a wavelet analysis on the pupil dilation record to remove all large oscillations and retain only the very short and rapid events (larger than a specified threshold), which are then at- tributed to the effect of cognitive activity. The ICA events (rapid small dilations) per second are counted, divided by the number of expected ICA events per second (30), and the resulting number is then transformed using the hyperbolic tangent function, in order to obtain a number between zero and one 1 . To obtain a continuous mea- sure, blinks are factored out by linear interpolation of adja- cent events. When using the EyeTracking.Inc software, an ICA value per second is produced. To obtain finer granularity, we also calculated a per-100-msec ICA value from the ICA events (i.e. the rapid dilation events). Due to the short time span, we could not interpolate for blinks (which take about 100msecs) and therefore simply excluded from our analysis time all frames during which a blink or partial blink occurred. Background on Pupillometry and the LC-NE area It has been observed that pupil dilation is strongly correlated with activity in the locus caeruleus (LC) region of the brain. 1 The method is patented, and the analysis program has to be licensed from EyeTracking, Inc., San Diego, CA. For details see (Marshall, 2000).