Abstract
The great majority of head and face discrimination studies have utilized either frontal or left/right rotated views of the head, while virtually none have examined the up/down direction or interactions between these two dimensions. A principal component (PC) analysis of head shapes defined relative to the bridge of the nose suggests that just three components can encode head shape across ±40° horizontally and ±20° vertically. Furthermore, these PCs suggest that horizontal and vertical head orientations may be represented orthogonally. To test these hypotheses, discrimination thresholds for head orientation were measured for left/right discrimination among head shapes that were either oriented up, frontal, or down in the vertical dimension. Thresholds were found to be independent of vertical orientation. A control experiment randomized the vertical orientation from trial to trial and showed that left/right orientation discrimination was unaffected. An analogous result was obtained for discrimination of head orientation in the up/down direction with randomization across left/right orientations. Thus, discrimination of vertical head orientations is independent of horizontal head orientation, a result consistent with the PC analysis. In further studies we are using head orientation aftereffects following adaptation (Fang & He, Neuron, 45, 7930800, 2005) to characterize receptive fields responsible for the representation of left/right and up/down head orientations. These results imply that the visual system can estimate head orientation independently in two dimensions, and this may greatly simplify the process of individual face encoding and recognition.
CIHR grant #172103 & CIHR Training Grant in Vision Health Research.