Visual recognition of faces, objects, and words using degraded stimuli: where and when it occurs

Abstract

We studied time course and cerebral localisation of word, object, and face recognition using event-related potentials (ERPs) and source localisation techniques. To compare activation rates of these three categories, we used degraded images that easily pop out without any change in the physical features of the stimuli, once the meaning is revealed. Comparisons before and after identification show additional periods of activation beginning at 100 msec for faces and at around 200 msec for objects and words. For faces, this activation occurs predominantly in right temporal areas, whereas for objects, the specific time period gives rise to bilateral posterior but right dominant foci. Finally, words show a maximum area of activation in the left temporooccipital area at their specific time period. These results provide unequivocal evidence that when effects of low-level visual features are circumvented, faces, objects, and words are not only distinct in terms of their anatomic routes, but also in terms of their times of processing.

Figure 1

Examples of the three categories of stimuli used during ERP recordings. Left: Visual stimuli as presented during the ERP measures. Right: Where in the visual scene the actual stimuli are hidden. A, B, C: Examples of faces, objects, and words, respectively.

Figure 2

Results of ERP map series segmentation. A: Segment maps (SMs) for the eight scalp potentials, explaining the four grand mean ERP map series, are illustrated with positive values in red and negative values in blue. SM2 (red) was specific to faces and was named SMF. SM5 (blue) was specific to objects and was named SMO. Finally, SM6 (green) was specific to words and named SMW. B: Grand mean ERP traces (all 125 channels superimposed) are shown for the unidentified condition (UC), identified faces (IF), identified objects (IO), and identified words (IW) from top to bottom, respectively. Arrows below ERPs indicate the periods during which each SM was present.

Figure 3

Source localisation. A, B, C: Estimations obtained using the distributed linear source localisation (LAURA) procedure for the segment maps specific to faces (SMF), objects (SMO), and words (SMW), respectively. The current density (green, low current density; red, high current density) maxima are shown on six horizontal slices of the average brain. SMF shows right temporal gyrus activation, with a small secondary source in left parietal region. SMO shows bilateral regions of activation, maximal in right lingual and middle occipital gyri and with a symmetrical secondary source on the left. For SMW, a maximum is found in posterior left lingual and middle occipital gyri, with a small secondary right source in posterior cingulum.