Personality Model of a Social Character Daniela M. Romano,
[email protected], Alvin Ka Lun Wong,
[email protected] The Department of Computer Science, Regent Court, 211 Portobello Street Sheffield, S1 4DP, UK
Abstract Virtual characters are increasingly becoming a common interface for communication between humans and computers, or for portraying human’s behaviour in leisure or educational setting (Louchart, Aylett, Pickering, Romano, 2004). If their creation aim is to portray humans, they should be inspired by the way humans express their feeling and interact with each other. We present here a personality model that attempts to emphasise the believability of a virtual character, by displaying personality and emotions in a social context. Such model enables the creation of characters that can account for both stability and variability of a real person in action and his/her behaviour across time and place. It models not only the person’s emotional status, but also the social tasks they are confronted with and construct over time, maintaining a history of the social interactions the character has held with other agents or real people in the virtual world. To meet the above requirements, elements from established personality models have been put together to form a framework of components that interacts in a consistent way to represent the personality of the character.
Architecture Overview The proposed personality model consists of four modules described below. The Emotion Module defines the emotional status of a character at a particular time. Such emotional status depends not only on the current contact between the two characters but also on their interaction history. The Memory Module stores the history of previous verbal and non-verbal interactions of the character with other characters in the virtual world in the form of emotional status. The Social-Cognitive Factors Module accounts for the relatively stable personality factors over time. These factors are connected with one to several emotional attributes of the Emotion Module. The Event Processing Module determines the gestures and actions/speech actions that a character should perform according to their emotional status. Figure 1 shows the relationships between these modules. The personality model receives a notification when an event, or external stimuli, occurs in the environment, and produces an event for the environment, which represents the behaviour of the person modelled in response to the event in input. The information passed to the personality model for the occurrence of an event (e.g. shake hands, nodding, etc) includes the intensity of the event, which is strength of the emotions involved the event, and a list of the social-cognitive(SC) personality factors that the event may have an effect on, the Social Context in which the character is in and other relevant Environment Information. To express the personality of the behaviour a list of gestures and actions/speech actions is available to the model to display the personality of the character. For each gesture different intensities are available. The personality model output is the event that occurs in the environment. The Emotions Module The OCC Emotion Model has often been used to describe the emotional state of a virtual character (Bartneck, 2002)(Kshirsagar, 2002). The original OCC Model (Ortony, Clore and Collins, 1988) is complex and probably too fine grained for the development of computerised believable characters A simplified version is proposed in Ortony (2003) where six positive and five negative categories are used to describe the emotions of the believable agent. The positive categories are joy, hope, relief, pride, gratitude and love,
while the negative categories are distress, fear, disappointment, anger and hate. Each of these categories is stored as a numerical value, and in our model has been given an upper and a lower bound to represent the extreme value of that emotional category (e.g. extremely happy/ extremely unhappy for the category “joy”).
Figure 1 Architecture of the Personality Model
The intensity of a emotion can go beyond the boundary value; however, the behaviour displayed of the character would not change once a boundary has been exceeded, unless the value drops below that boundary. Since every human has different limits, these boundary values differ between different virtual characters. Also each emotional category can have different boundary values. Furthermore a human often has different attitudes and opinions toward the people he/she interacts with. Therefore a set of emotional states representing these different attitudes has been stored. The way these emotional states are recorded is described in the next section. The Memory Module In personality models reviewed (Ortony et Al., 1988)(Cervone, 1999)(McCrae & Costa, 1996) the history of the previous interactions is never recorded. The authors of this paper believe that the events happened in the past influence to some extent one’s behaviour in the future, consequently a Memory module has been introduced in the personality model presented. The status of the last interaction with any of the “people” of the virtual world is store in the interaction history for each computer-controlled character. This represents the attitude towards that person when the two “people” meet again. The history of interactions affects the mood of a character, and consequently its emotional processing, with repercussions on the personality displayed. Also a “generic” emotional status is stored to represent the personality of a virtual character, and it is continuously modified as an aggregate of all the interactions in the virtual world. This is used when the character starts a conversation with a stranger. The Social-Cognitive Factors Module This module describes the character sense of self and it was modelled from the social-cognitive theory of Cervone (1999), which best describe the consistency and coherence of a functioning personality and the relatively stable patterns of behaviour which form the basis of an agent’s unique personality. The socialcognitive factors identified by Cervone (1999) are social knowledge, personal goals, personal standards, expectations about the world, reflections upon oneself, and affective experiences. They reflect the interpersonal, cultural and social behaviour that contribute a character recurrent style interpreting, planning, and responding to events. Social knowledge refers to individuals’ knowledge about the people living in their country or region, and the way these people live. Personal Goals and Standards refer to the goals an individual aims to achieve and the moral principles that one has. Expectation about the world is the belief that a certain consequence will follow certain actions. People usually focus their attention according to this expectation and behave differently as a result. Reflection upon oneself refers to the knowledge of one’s particular mental status, including their beliefs, desires, and sensations. Affective Experiences refers to the social experiences one had gone through, which have either a positive or negative impact to the individual. The personality social-cognitive factors function as interconnected components that varies across characters, guiding and constraining the activations of the particular action or emotion generated by the personality.
Cervone (1999) approach states that these interconnections are dynamic and may change across situations. Such dynamism has not been implemented in the program created to test the presented personality model, and with be the object of future research, as Cervone (1999) does not specify how, when and in what manner the interconnections change with the change of context or situation. In our model each social-cognitive personality factor has been mapped to one positive and a negative category of the emotional module. These mappings are fixed and the same across characters and derived empirically, based on experiments and observations in the evaluation conducted. Table1 shows the mapping. Personal Goals Joy, pride Disappointment, distress Personal Standards Gratitude Anger Social Knowledge Love Anger, hate Expectations about the world Hope Disappointment Reflections upon oneself Relief, pride Fear Affective Experience Joy Anger Table1 Connection between Personality and Emotional factors
It has to be noted that this mapping should be dynamic, given consideration to the fact that each individual may have different reaction to a particular event. Again this will be the object of future work. Event Processing and Emotion Formation An event in input to the personality model specifies the social-cognitive factors that are influenced by such event. The event processing procedure of the personality model is based on the OCC Event Process Model (Ortony, 1988), but it influences the social-cognitive factors, which in turn influence the emotional categories, an it is split into four stages: Classification, Quantification, Interaction and Expression. The Classification stage determines the personality factors influenced by an event, here the factors are assumed to be in input to the personality model, consequently this stage is not performed. In the Quantification stage, the model calculates how the intensities of the emotional categories vary, as a result of the variation of the affected social-cognitive factors in response to the event intensity in input. Since humans appear to react to the same event in different manners over time, to increase the characters believability and lower their predictability, we assume that a character reacts to the same intensity of the same type of event in input, following the normal distribution. For example, if an event is expected to produce a moderate anger in the character, the probability that the character will react with an expression such as: “not angry”, “neutral”, “angry”, “furious”, “very furious” is for example about 3%, 13%, 68%, 13% and 3% respectively following the standard normal distribution values. In other words, there is a high chance that the person will react to an event in the “typical” way, a small chance that the person react more intensely or neutrally to the event, and a even smaller chance that the person react in an extreme way to the event. Each character has a set of bell-shaped curves for each of the emotional categories. This set of curves models the variation of behaviour and varies amongst characters. The value for each of the emotional categories involved is calculated based on the intensity of the event in input and an adjustment factor drawn from a probability function that follows the normal distribution. A virtual human behaves in a different way from another, as they do not share the same set of curves, resulting in a personality difference, as for real people. In the interaction stage, an adjustment of the all the affected the emotional categories is performed to the “generic” emotional state. In this case the adjustment reflects a change of personality of a character over time as consequence of what happened in the past. This adjustment is considered to be small and there fore is calculated as a fraction of the adjustment made in the quantification stage. For each of the affect “generic” emotional state category the following calculation is performed: emotion category X = emotion category X + adjustment made in quantification stage * fraction()
Where fraction() is a function returning a random value ranging from 0 to 0.1.
In the expression stage, the emotional state of the model is expressed through the display of a verbal and/or non-verbal behaviour. The expressed behaviour may influence the behaviour of other “people” in the world. Since there are various ways to express the same emotional status, the personality model chooses the behaviour to express from a set using probability. Initially all emotion categories are classified as dominant and less dominant. This classification varies with the assigned background culture of character. Dominant emotion categories have a higher probability to be expressed, and less dominant emotion categories are given a lower probability to occur. When an emotion category exceeds a boundary, showing that one is an extreme state of emotion, the exceeded emotion becomes dominant, since it is likely that a human would stay in the same mood unless the intensity of such emotion has considerably lowered, moving within the boundaries. This prevents the creation of unrealistic situation in which an extremely angry character suddenly shows a depressed behaviour. Evaluation Given that the model described has made several assumptions, it is important to investigate whether the realism modelled has been achieved. An evaluation can be done by conducting a study on real human’s perception of the expressiveness of the character. So far only a quantitative evaluation of the actual values of the emotional categories has been done. 3D characters have been created and tried out in a series of situations and scenario to expose how the interaction between social-cognitive factors and the emotion categories takes place and changes the emotional status of the character in question, according to an event in the world. This allowed a validation of the particular behaviour chosen in response to variations of the status of the character. A pair of virtual humans with vastly different personality has been made interact to see how one’s behaviour influenced the other. See figure2.
Figure 2 Greeting and subsequent chat of two 3D “people”. The female is computer-generated and the male is user driven
References Bartneck, C. (2002), Integrating the OCC Model of Emotions in Embodied Characters, Workshop on Virtual Conversational Characters: Applications, Methods, and Research Challenges, Melbourne. Cervone, D (1999), Social-Cognitive Theories and the Coherence of Personality. In D. Cervone & Y. Shoda (Eds.), The coherence of personality: Socialcognitive bases of consistency, variability, and organization, pp. 3-37. Charles Annis, 2004, “Central Limit Theorem”, http://www.statisticalengineering.com/central_limit_theorem.htm David M. Lane, 2003, ”HyperStat Online Textbook”. http://davidmlane.com/hyperstat/normal_distribution.html Eric W. Weisstein, 1999. "Normal Distribution." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/NormalDistribution.html Kshirsagar, S (2002), Virtual Humans Personified, Proceedings Autonomous Agents Conference (AAMAS) 2002, pp 356-359. Louchart S., Aylett R., Pickering J., Romano D.M.(2004). Don' t hit me! - combining language and action in FearNot. AISB 2004 Convention: Motion, Emotion and Cognition, University of Leeds, UK, 29 March - 1 April 2004 Symposium on Language, Speech and Gesture for Expressive Characters University of Leeds, UK, 29 March - 1 April 2004 McCrae, R, Costa, P (1996) The five factor model of personality: Theoretical Perspective. Guilford Press Ortony, A., Clore, G., & Collins, A. (1988). The Cognitive Structure of Emotions. Cambridge University Press. Ortony. 2003. On making believable emotional agents believable. Trapple, R. P. ed.: Emotions in humans and artefacts, pp. MIT Press, Cambridge, USA.
