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At First Sight: a High-Level Pop out Effect for Faces

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At First Sight: a High-Level Pop out Effect for Faces View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Vision Research 45 (2005) 1707–1724 www.elsevier.com/locate/visres At first sight: A high-level pop out effect for faces Orit Hershler, Shaul Hochstein * Neurobiology Department, Institute of Life Sciences and Center for Neural Computation, Hebrew University, Jerusalem 91904, Israel Received 23 February 2004; received in revised form 9 December 2004 Abstract To determine the nature of face perception, several studies used the visual search paradigm, whereby subjects detect an odd target among distractors. When detection reaction time is set-size independent, the odd element is said to ‘‘pop out’’, reflecting a basic mechanism or map for the relevant feature. A number of previous studies suggested that schematic faces do not pop out. We show that natural face stimuli do pop out among assorted non-face objects. Animal faces, on the other hand, do not pop out from among the same assorted non-face objects. In addition, search for a face among distractors of another object category is easier than the reverse search, and face search is mediated by holistic face characteristics, rather than by face parts. Our results indicate that the association of pop out with elementary features and lower cortical areas may be incorrect. Instead, face search, and indeed all fea- ture search, may reflect high-level activity with generalization over spatial and other property details. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Visual; Search; Face; Detection; Asymmetry 1. Introduction & James, 1967). Numerous brain studies imply the exis- tence of specific face recognition units and areas within The human face is one of the most important object the brain, including single cell recordings in monkey categories that we recognize and numerous studies sug- (infero-temporal cortex; Perrett, Rolls, & Caan, 1982), gest a unique processing mechanism within our visual human fMRI brain scans (the fusiform face area, system. Newborn infants prefer faces to other stimuli FFA; Ishai, Ungerleider, Martin, & Haxby, 2000; (e.g. Haaf & Bell, 1967; Muir, Humphrey, & Humphrey, Kanwisher, McDermott, & Chun, 1997) and event re- 1994), possibly reflecting an innate face detection mech- lated potential studies (ERP; Carmel & Bentin, 2002). anism. Inverted faces are harder to recognize than up- Several of the above findings have been criticized. right ones, an inversion effect that is significantly The special processing abilities of the visual system for larger for faces than for other categories (Diamond & faces may derive from other factors, as for example Carey, 1986; Kanwisher, Tong, & Nakayama, 1998; Ta- the greater learned expertise for facial recognition or naka & Farah, 1991), suggesting that faces are processed the fact that faces are commonly recognized at the sub- holistically. The appearance of prosopagnosia, a neuro- ordinate level of categorization, whereas other objects psychological condition that impairs the ability to recog- are classified at the basic category level (Tarr & Gau- nize familiar faces whilst sparing normal object thier, 2000). More specifically, the face-inversion effect recognition implies a specific locus for face recognition may also exist for other categories in which the subject (Barton, Press, Keenan, & OÕConnor, 2002; Warrington has expertise, for example dogs (Diamond & Carey, 1986). Prosopagnosia may only appear to be limited to faces, as this is an extremely homogeneous class com- * Corresponding author. Tel.: +972 2 6585193; fax: +972 2 6586296. pared to other objects (Levine & Calvanio, 1989). As E-mail address: [email protected] (S. Hochstein). for the brain studies, both homogeneity and expertise 0042-6989/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.visres.2004.12.021 1708 O. Hershler, S. Hochstein / Vision Research 45 (2005) 1707–1724 seem to play a role in the recruitment of the FFA for face that violated both dimensions was least effective certain stimuli (Gauthier, Skudlarski, Gore, & Ander- as a distractor. When the distractor did not contain son, 2000), while the evidence for specific face recogni- any facial features, but was a globe in the shape of a tion units is also consistent with a range of other face, the upright face did pop out. possible explanations (Hasson, Levy, Behrmann, Hen- Other researchers have focused on questions of affect dler, & Malach, 2002; Kanwisher, 2001; Tarr & Gau- or identity. Hansen and Hansen (1988) reported that a thier, 2000). face with an angry expression pops out of an array of happy faces. However, Purcell, Stewart, and Skov 1.1. Visual search for faces (1996) argued that this pop out effect was the result of an artifact of extraneous dark areas in the angry faces, In order to try and clarify the case of face perception, which the subjects became aware of and used to detect several studies turned to the visual search paradigm. In the target. In sum, none of the above studies have con- this paradigm subjects are asked to detect an odd ele- clusively found a pop out effect for visual search of faces ment, the target, in an array of distractors. When the on a background of inverted faces or other face like dis- reaction time for the detection of the odd element is tractors. Finally, Tong and Nakayama (1999) studied vi- independent of the number of distractors, the odd ele- sual search for subjectsÕ own faces versus unfamiliar ment is said to ‘‘pop out’’ (Treisman & Gelade, 1980). faces and found persistent processing advantages for This definition of pop out does not refer to the absolute the own face. reaction time, which may vary with task difficulty even The present experiments address the question of vi- for parallel searches (Santhi & Reeves, 2004). sual search for a face once again. We believe that the Nothdurft (1993) used schematic drawings of faces, failure to detect a clear pop out effect in the above stud- and failed to find pop out for all series except one. In ies may have been due to the use of largely schematic this last experiment, the target was an upright drawing drawings of faces, which were perhaps not sufficiently of a face with hair, while the distractors were these same face-like, and the choice of distractors, which were per- drawings but inverted. However, in a control experi- haps too similar to faces. In a recent study, faces on ment, in which only the facial features were removed, scene-like distractors did pop out (Lewis & Edmonds, but the hair remained, the target also popped out. The 2002). Our experiments present subjects with more real- pop out was thus explained by the presence of a visual istic line drawings or even photographs of faces, while cue independent of the faces (hair orientation). distractors are drawings or photographs of other ob- Brown, Huey, and Findlay (1997) used black-and- jects, and thus unlike the target faces. white photographs with the hair removed and set in ovoid templates in a special visual search paradigm, in 1.2. The visual search paradigm which the target and distractors are presented in the periphery, around a fixation point. The subjects were The visual search paradigm is associated with feature asked to move their eyes to the target as quickly as pos- integration theory (FIT; Treisman & Gelade, 1980; sible, and their eye movements were recorded and ana- Treisman & Souther, 1985). When reaction time (RT) lyzed in terms of accuracy and latency. Targets were is set-size independent, the distinctive feature is said upright or inverted faces with distractors being in the to be detected in parallel by a pre-attentive or spread- opposite orientation. The experiments in this study attention mechanism, called feature search or pop out, again failed to establish pop out for faces. The authors usually found when the odd element differs significantly did find a practice effect specific for upright, but not in- from all the distractors by the same elemental feature, verted, faces. Subjects trained on upright face targets such as color. When RT increases linearly with array improved markedly in latency and accuracy for upright size, visual search is said to be serial, requiring sequen- faces only, while those trained on inverted faces im- tial focused attention. This kind of search usually occurs proved only slightly for both upright and inverted faces when the target differs from the distractors by a conjunc- equally. The authors conclude that upright faces have a tion of two or more basic features or when the difference special status in tasks that require configural learning. is very small. Kuehn and Jolicoeur (1994) investigated the impact According to FIT, feature search is made possible by of the quality, orientation and similarity of the stimuli an explicit neuronal representation for the target fea- on the visual search for faces. Although they concluded ture, which does not overlap with the neuronal represen- that faces do not pop out on a background of distractors tation of the distractor features (Treisman & Souther, containing facial features, search for a face became 1985). Perceptual dimensions such as ‘‘color’’ or ‘‘orien- markedly easier when the distractors looked less like tation’’ are represented in separate continua of feature faces. In one experiment, distractors were created by maps. Each of these maps represents one feature, such scrambling the features of a face along the two dimen- as ‘‘red’’ or ‘‘green’’ in the color continuum of maps, sions of top-down order and symmetry. The scrambled or ‘‘vertical’’ or ‘‘horizontal’’ for orientation. Focused O. Hershler, S. Hochstein / Vision Research 45 (2005) 1707–1724 1709 attention is necessary to retrieve location information from these maps, but the presence and amount of activ- ity in any given map can be detected without focused attention. Feature search is explained by categorical detection of the presence of activity anywhere in the rel- evant feature map.
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