Abstract
Previous studies show that the primate and human visual system automatically generates a common and invariant representation from a visual object image and its mirror reflection. For humans, however, this mirror-image generalization seems to be partially suppressed through literacy acquisition, since literate adults have greater difficulty in recognizing mirror images of letters than those of other visual objects. At the neural level, such category-specific effect on mirror-image processing has been associated with the left occpitotemporal cortex (L-OTC), but it remains unclear whether the apparent “inhibition” on mirror letters is mediated by suppressing mirror-image representations covertly generated from normal letter stimuli. Using transcranial magnetic stimulation (TMS), we examined how transient disruption of the L-OTC affects mirror-image recognition during a same-different judgment task, while varying the semantic category (letters and non-letter objects), identity (same or different), and orientation (same or mirror-reversed) of the first and second stimuli. We found that magnetic stimulation of the L-OTC produced a significant delay in mirror-image recognition for letter-strings but not for other objects. By contrast, this category specific impact was not observed when TMS was applied to other control sites, including the right homologous area and vertex. These results thus demonstrate a causal link between the L-OTC and mirror-image discrimination in literate people. We further suggest that left-right sensitivity for letters is not achieved by a local inhibitory mechanism in the L-OTC but probably relies on the inter-regional coupling with other orientation-sensitive occipito-parietal regions.
Highlights
The human and primate ventral visual system is known to spontaneously generate a common and invariant representation from a visual object image and its mirror reflection, irrespective of their left-right orientation (Eger et al, 2004; Vuilleumier et al, 2005; Dehaene et al, 2010b; Freiwald and Tsao, 2010)
Mirror-image discrimination is likely to rely on such experience-dependent process occurring in the higher-order visual system during literacy development
Whereas the human ventral visual area involved in object recognition is generally known to represent visual objects and their mirror reversals as being the same (Eger et al, 2004; Vuilleumier et al, 2005; Dehaene et al, 2010b), the intrinsic propensity for mirror-image generalization should be partially suppressed through literacy training, since many writing systems include minimal pairs of mirror-image letters, such as “b” vs. “d” and “p” vs.”q” (Dehaene et al, 2005)
Summary
The human and primate ventral visual system is known to spontaneously generate a common and invariant representation from a visual object image and its mirror reflection, irrespective of their left-right orientation (Eger et al, 2004; Vuilleumier et al, 2005; Dehaene et al, 2010b; Freiwald and Tsao, 2010). Recent functional magnetic resonance imaging (fMRI) data show that such category-specific sensitivity in mirror-image processing relies on the left visual word-form area (VWFA) in the left fusiform gyrus (Dehaene et al, 2010a,b; Pegado et al, 2011) It remains unclear how the strong behavioral sensitivity to letter/word orientation is achieved in this and adjacent left occpitotemporal cortex (L-OTC). If the L-OTC comprises a category-specific inhibitory circuit for left-right discrimination, the visual recognition of mirror-reversed words should be facilitated when this region is disrupted by TMS. Magnetic stimulation of the region would disrupt the orientation-invariant representations of stimulus identity, and thereby induce a delay in same-different judgment about mirror images These effects should be strictly category-specific, i.e., detectable only for word stimuli and not for other visual objects
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