A possible neuronal account for the behavioural heterogeneity in congenital prosopagnosia

The extensive review paper by Geskin and Behrmann (Geskin & Behrmann, 2017) provides a survey of the behavioral literature of congenital prosopagnosia accumulated over the past years. The main goal of this survey is to address a longstanding debate in the cognitive neuroscience literature pertaining to the extent to which face and object processing share the same cognitive and neural mechanisms. Congenital prosopagnosia provides a unique testbed in which this issue can be addressed, given the well-documented core deficit in face processing and the absence of an apparent lesion, in contrast to cases of acquired prosopagnosia (Grill-Spector, Weiner, Kay, & Gomez, 2017). The main outcome of this review is that roughly two-thirds of the documented individuals diagnosed with CP also appear to have an impairment in object processing. This is somewhat surprising, given that despite the lack of formal behavioral and/or neural diagnostic criteria, most researchers would agree that CP is primarily considered a deficit in face processing. Yet, about a third of the reviewed cases exhibit a deficit which is confined to the domain of faces. Carving the behavioral variability associated with CP is of great importance as it enables deeper understanding of the mechanisms underlying the impairment. Moreover, these findings also have general implications for our understanding of normal face and object representation in the human brain. Beyond the cognitive level, the findings also raise questions regarding the neural models that could underlie such behavioral heterogeneity. Specifically, what would be the most parsimonious neural mechanism that could simultaneously account for such different behavioral profiles? A deficit affecting both face and object processing, as found in two-thirds of the CP participants, is compatible with a system associated with more general, high-level visual processing. In contrast, a deficit that is restricted to faces and does not extend to other visual domains, as observed in the remaining third, is more compatible with a modular dual system organization in which faces and objects are processed by separate systems. How could one reconcile these different behavioral profiles within a single neural model and a single disorder? Normal face recognition appears to be accomplished via the coordinated activity of multiple nodes of a distributed neural network (see GrillSpector et al., 2017, for a recent review). Hence, a number of studies in recent years, including our own (Rosenthal, Sporns, & Avidan, 2017), have begun to adopt a network methodology and framework to better characterize the functional aspects of this system. According to this approach, the totality of the network connections and interactions can be summarized and systematically analyzed using concepts of network science. In general, networks comprise a set of elements (i.e., nodes) and their dyadic (pairwise) connections (i.e., edges), which permit the characterization of each element’s connection pattern and topology (Bullmore & Sporns, 2009). Within this framework, the study of individuals with CP provides the opportunity to determine whether a disrupted function of one or more of the nodes or edges within this network leads to an altered