The word-length effect in acquired alexia, and real and virtual hemianopia

A word-length effect is often described in pure alexia, with reading time proportional to the number of letters in a word. Given the frequent association of right hemianopia with pure alexia, it is uncertain whether and how much of the word-length effect may be attributable to the hemifield loss. To isolate the contribution of the visual field defect, we simulated hemianopia in healthy subjects with a gaze-contingent paradigm during an eye-tracking experiment. We found a minimal word-length effect of 14 ms/letter for full-field viewing, which increased to 38 ms/letter in right hemianopia and to 31 ms/letter in left hemianopia. We found a correlation between mean reading time and the slope of the word-length effect in hemianopic conditions. The 95% upper prediction limits for the word-length effect were 51 ms/letter in subjects with full visual fields and 161 ms/letter with simulated right hemianopia. These limits, which can be considered diagnostic criteria for an alexic word-length effect, were consistent with the reading performance of six patients with diagnoses based independently on perimetric and imaging data: two patients with probable hemianopic dyslexia, and four with alexia and lesions of the left fusiform gyrus, two with and two without hemianopia. Two of these patients also showed reduction of the word-length effect over months, one with and one without a reading rehabilitation program. Our findings clarify the magnitude of the word-length effect that originates from hemianopia alone, and show that the criteria for a word-length effect indicative of alexia differ according to the degree of associated hemifield loss

Holistic processing of shape cues in face identification: Evidence from face inversion, composite faces, and acquired prosopagnosia

Face recognition is based on two main sources of information: Three-dimensional (3-D) shape and two-dimensional surface reflectance (colour and texture). The respective contribution of these two sources of information in face identity matching task is usually equal, suggesting that there is no functional dissociation. However, there is recent evidence from electrophysiology and neuroimaging that contribution of shape and surface reflectance can be dissociated in time and neural localization. To understand the nature of a potential functional dissociation between shape and surface information during face individualization, we used a 3-D morphable model (Blanz & Vetter 1999) to generate pairs of face stimuli that differed selectively in shape, reflectance, or both. In three experiments, we provided evidence that the processing of shape and surface reflectance can be functionally dissociated. First, participants performed a delayed face matching task, in which discrimination between the sample and distractor faces with the same orientation (either upright or inverted) was possible based on shape information alone, reflectance information alone, or both. Inversion decreased performance for all conditions, but the effect was significantly larger when discrimination was based on shape information alone. Second, we found that participants’ composite face effect, a marker of holistic processing, was caused primarily by the presence of interfering shape cues, with little interference from surface reflectance cues. Finally, contrary to normal observers, a well-known patient with acquired prosopagnosia suffering from holistic face perception impairment performed significantly better when discriminating faces based on reflectance than on shape cues. Altogether, these observations support the view that the diagnosticity of shape information for individualizing faces depends relatively more on holistic face processing than that of surface reflectance cues