On the effect of subliminal priming on subjective perception of images: a machine learning approach Parmod Kumar 1 , Faisal Mahmood 2 , Dhanya Menoth Mohan 3 , Ken Wong 4 , Abhishek Agrawal 3 , Mohamed Elgendi 5 , Rohit Shukla 6 , Justin Dauwels 3* , and Alice H.D. Chan 3* Abstract— The research presented in this article investigates the influence of subliminal prime words on peoples’ judgment about images, through electroencephalograms (EEGs). In this cross domain priming paradigm, the participants are asked to rate how much they like the stimulus images, on a 7-point Likert scale, after being subliminally exposed to masked lexical prime words, with EEG recorded simultaneously. Statistical analysis tools are used to analyze the effect of priming on behavior, and machine learning techniques to infer the primes from EEGs. The experiment reveals strong effects of subliminal priming on the participants’ explicit rating of images. The subjective judgment affected by the priming makes visible change in event- related potentials (ERPs); results show larger ERP amplitude for the negative primes compared with positive and neutral primes. In addition, Support Vector Machine (SVM) based classifiers are proposed to infer the prime types from the average ERPs, which yields a classification rate of 70%. I. INTRODUCTION Priming is a nonconscious form of memory, which is concerned with perceptual identification of words and objects [1]. It refers to activating particular representation or associa- tion in memory just before carrying out an action or task. For example, if a person is exposed to the word ‘lung-cancer’, and later had a choice to smoke, there is a greater probability that the person choose not to smoke if the word is perceived negatively. The term ‘subliminal’ means ‘below a limen’ (sensory threshold). In other words, subliminal priming is not perceived by the conscious mind but has an influence on the unconscious mind. In the following, we summarize prior conceptual frame- works relevant to priming. The Lexical Decision Task (LDT) is a widely used task in cognitive psychology, which has An abstract of this paper was submitted at the 35 th annual international conference of the IEEE Engineering in Medicine and Biology Society, Osaka, Japan, 2013. * Corresponding author 1 P.K., 2 F.M., 4 K.W., 5 M.E., and 6 R.S. were with the Nanyang Technological University, School of Electrical and Electronic Engineering, Singapore 639798. P.K. is now with INRIA Sophia Antipolis-M´ editerran´ ee (e-mail:kumaparm@isu.edu). F.M. is now with the Okinawa Institute of Science and Technology, Okinawa, Japan 904-0495 (e-mail:faisal.mahmood@oist.jp). K.W. is now with Temasek Laboratories, National University of Singapore, 117411 Singapore (e- mail:kianwong@nus.edu.sg). M.E. is now with the University of Alberta Edmonton, AB, Canada (e-mail:moe.elgendi@gmail.com). R.S. is now with the University of Wisconsin-Madison, Madison, Wisconsin 53706 (email:rshukla3@wisc.edu). D.M.M., A.A., and J.D. are with the Nanyang Technological Univer- sity, School of Electrical and Electronic Engineering, Singapore 639798 (e-mail: dhanya001@e.ntu.edu.sg, abhishekagrawal.bits@gmail.com, and jdauwels@ntu.edu.sg). A.H.D.C. is with the Nanyang Technological Uni- versity, School of Humanities and Social Sciences, Singapore 639798 (e- mail:alice@ntu.edu.sg). shown that lexical decision responses are made more quickly for semantic priming, i.e., when the target word is related to the previous word presented [2]. A similar study on the lexical priming [3] demonstrates the occurrence of N250 and N400 component at Pz channel. Holcomb and Grainger addressed similar effects in their masked repetition priming paradigm [4]. Recently, Event-Related Potential (ERP) based studies have received increased attention in assessing the effect of affective priming on rating emotional pictures. In an earlier study, pleasant and unpleasant pictures are associated with enlarged early posterior negativity (EPN) as well as late positive potential (LPP) amplitudes compared to neutral pictures [5]. The processing of emotional pictures influences the ERP amplitude at separate latencies, reportedly found to occur at short latencies of 100-300 ms [6], [7]. ERP studies with picture priming mismatch have shown that a late posterior component between 225 ms and 500 ms can differentiate between priming conditions [8]. In a previous cross-domain study [9], a similar task to LDT is employed, where the lexical prime is replaced with a masked picture. The results show similarity with the LDT based study. This suggests that semantic processing of information can be achieved through subliminal presentation of pictures to affect successive processing in the memory. In a subsequent study [10], masked lexical primes are presented before participants are asked to rate how much they like certain pictures on a scale of 1-4. This paradigm uses cross- domain priming (word primes combined with target images) and demonstrates that subliminal affective priming can affect decision making. A more positive late signal following positive primes is evoked at the right-hemisphere electrodes. In the present study, a similar method as in [10] is devised aiming to examine the subliminal affective priming on image rating. The prime words used in this research exhibit positive, negative, and neutral connotations with respect to the pic- tures. We hypothesize that the prime will cause participants to respond differently to the pictures. We also expect to see a late ERP component, between 300-500 ms, in the posterior region, reflecting different priming conditions. In addition, this difference would be more pronounced in the right hemisphere than the left hemisphere. We also hypothesize that we would see the N400 effect at Pz reflecting the lexical prime effect. The contributions of this paper are as follows. We will show that the present experimental results replicate the previous accounts of ERP effects in a similar paradigm, and