Hemiface contributions to hemispheric dominance in visual speech perception.

Abstract

Lateralized displays are used widely to investigate hemispheric asymmetry in language perception. However, few studies have used lateralized displays to investigate hemispheric asymmetry in visual speech perception, and those that have yielded mixed results. This issue was investigated in the current study by presenting visual speech to either the left hemisphere (LH) or the right hemisphere (RH) using the face as recorded (normal), a mirror image of the normal face (reversed), and chimeric displays constructed by duplicating and reversing just one hemiface (left or right) to form symmetrical images (left-duplicated, right-duplicated). The projection of displays to each hemisphere was controlled precisely by an automated eye-tracking technique. Visual speech perception showed the same, clear LH advantage for normal and reversed displays, a greater LH advantage for right-duplicated displays, and no hemispheric difference for left-duplicated displays. Of particular note is that perception of LH displays was affected greatly by the presence of right-hemiface information, whereas perception of RH displays was unaffected by changes in hemiface content. Thus, when investigated under precise viewing conditions, the indications are not only that the dominant processes of visual speech perception are located in the LH but that these processes are uniquely sensitive to right-hemiface information.

When half a face is as good as a whole: Effects of simple substantial occlusion on visual and audiovisual speech perception

“…Campbell (1982Campbell ( , 1986 used static chimeric faces formed from the left and right hemifaces of a speaker, such that one half of each chimeric face showed one sound, while the other half showed another. In the present study, each image was naturally dynamic and contained no contradictory hemiface information, suggesting that the natural effectiveness of the two hemifaces for visual and audiovisual speech perception may actually be identical when faces are viewed normally (but see Jordan & Thomas, 2007). Moreover, in the study by Nicholls et al (2004), perception of information from the oral area was prevented by using a small patch to occlude just one half of the mouth (left or right) and no other part of the face.…”

“…However, the human face is not perfectly symmetrical (e.g., Bennett, Delmonico, & Bond, 1987;Carbary, Almierigi, & Harris, 1999;Gilbert & Bakan, 1973;Jordan & Thomas, 2007;Lindzey, Prince, & Wright, 1952;McCurdy, 1949;Seinen & Van der Werff, 1969;Wolff, 1933). Indeed, research measuring facial movements has revealed evidence of hemiface asymmetries in talkers' articulations, where the left side of the mouth (we refer to locations on the face as an observer sees them; e.g., left = left from an observer's point of view) opens sooner and wider during speaking, probably due to left hemisphere control over speech production (Graves, 1983;Graves, Goodglass, & Landis, 1982;Wolff, 1933;Wolf & Goodale, 1987).…”

“…Articulation of /ba/ and /bi/ involves a bilabial closure followed by an open mouth, and articulation of /va/ and /vi/ involves a labiodental closure followed by an open mouth, all of which are prominent and highly visible maneuvers when lower facial areas are unoccluded. In contrast, articulation of /ga/ and /gi/ involves less prominent visible movement (essentially, an open mouth throughout each articulation), and crucial articulatory maneuvers within the vocal tract (e.g., velar, alveolar) that are not normally 3 The temptation to invoke hemispheric projections to explain differences in hemiface processing for facial displays deserves comment (see also Jordan & Thomas, 2007). First, numerous studies have indicated that there is an overlap at the midline of up to 3º where visual information will project to both hemispheres (for recent reviews, see Jordan & Paterson, 2009.…”

Asymmetry and Symmetry in the Beauty of Human Faces

“…Indeed, physical features in human faces have been found to be naturally and normally asymmetrical in numerous studies, whether using X-rays [39] or threedimensional morphometric measurements [33,40], or direct measurements [29]. Functional asymmetries have also been reported, particularly for smiling [14,41], speaking [42,43], and the appearance of beauty [14]. Perfectly bilaterally symmetrical faces created on a computer (not morphed, averaged, or digitally smoothed) were judged significantly less attractive than normal faces [15].…”

Visual Speech Perception in Foveal and Extrafoveal Vision: Further Implications for Divisions in Hemispheric Projections

“…Consequently, visual speech encountered in locations to the left of fixation may project only to the right hemisphere (RH) and visual speech encountered in locations to the right of fixation may project only to the left hemisphere (LH), and this division is likely to have important consequences for how visual speech is processed. Indeed, several studies have shown that, when a talking face is observed, although visual speech ultimately produces activation in both hemispheres, activation is more extensive in the LH, in areas known to be involved in auditory speech perception, and this is consistent with the dominant role of the LH in processing language ([12]–[15], see also [16]). Thus, although the RH is implicated in many aspects of facial processing (for a review, see [17]), processes located in left cortex seem to dominate visual speech perception.…”