Perceptual and Category Processing of the Uncanny Valley Hypothesis' Dimension of Human Likeness: Some Methodological Issues

Abstract

Mori's Uncanny Valley Hypothesis 1,2 proposes that the perception of humanlike characters such as robots and, by extension, avatars (computergenerated characters) can evoke negative or positive affect (valence) depending on the object's degree of visual and behavioral realism along a dimension of human likeness (DHL) (Figure 1). But studies of affective valence of subjective responses to variously realistic non-human characters have produced inconsistent findings 3,4,5,6 . One of a number of reasons for this is that human likeness is not perceived as the hypothesis assumes. While the DHL can be defined following Mori's description as a smooth linear change in the degree of physical humanlike similarity, subjective perception of objects along the DHL can be understood in terms of the psychological effects of categorical perception (CP) 7 . Further behavioral and neuroimaging investigations of category processing and CP along the DHL and of the potential influence of the dimension's underlying category structure on affective experience are needed. This protocol therefore focuses on the DHL and allows examination of CP. Based on the protocol presented in the video as an example, issues surrounding the methodology in the protocol and the use in "uncanny" research of stimuli drawn from morph continua to represent the DHL are discussed in the article that accompanies the video. The use of neuroimaging and morph stimuli to represent the DHL in order to disentangle brain regions neurally responsive to physical human-like similarity from those responsive to category change and category processing is briefly illustrated.

A review of empirical evidence on different uncanny valley hypotheses: support for perceptual mismatch as one road to the valley of eeriness

“…Although the risk of image morphing artifacts can be diminished considerably by following the guidelines of Cheetham and Jäncke (2013), it is nevertheless possible that all confounding factors would not be avoided. Specifically, some ghosting for subtle facial features that are present in only one of the original images and slight blurring of contours generated by color interpolation could be unavoidable.…”

“…Image morphing artifacts are a threat to validity because they are likely to coincide with intermediate levels of human-likeness (i.e., the most processed images). Cheetham and Jäncke (2013) have published a detailed guideline for applying morphing to facial images in uncanny valley studies. We have adopted the following criteria from their guideline: (i) several morphed continua should be used, (ii) selected endpoint images should be similar to each other (i.e., the faces should have similar geometries, have neutral facial expressions, and represent individuals of similar ages), (iii) alignment disparities should be avoided, and (iv) any external features should be masked (i.e., hair and ears, jewelry, and other external features).…”

“…However, in this study, only one pair of images had been selected for creating the human-likeness continuum, the selected cartoon and human face were very dissimilar from each other, and no masking had been used (cf. Section Article Selection; and Cheetham and Jäncke, 2013). Hence, it is possible that the lower pleasantness ratings for intermediate morphs could have resulted from morphing artifacts rather than intermediate human-likeness level.…”

Category Processing and the human likeness dimension of the Uncanny Valley Hypothesis: Eye-Tracking Data

“…One likely reason for this inconsistency is the uncertainty surrounding the vague terminology used to describe the hypothesis’ valence dimension (e.g., Bartneck et al, 2007; Seyama and Nagayama, 2007; MacDorman et al, 2009a; Dill et al, 2012). Another reason might be related to the fact that the hypothesis’ dimension of human likeness ( DHL ), defined in terms of a linear change in the degree of physical humanlike similarity, is not subjectively perceived as a simple linear change in humanlike similarity (Cheetham et al, 2011; Cheetham and Jancke, 2013). Given the aim of uncanny research to understand subjective experience of nonhuman characters in terms of human likeness, this recent evidence suggests that a better understanding of how human likeness is really perceived is needed.…”

“…The DHL was represented using morph continua (e.g., Hanson, 2006; Seyama and Nagayama, 2007; Ho et al, 2008), the advantage of which is that they allow careful experimental control of differences in humanlike appearance and the exclusion of confounding perceptual dimensions (for a critical overview of the use of morph continua in uncanny research, see Cheetham and Jancke, 2013). The morph continua were generated from avatar and human parent faces and the morphs presented in a forced choice categorization task.…”