Irrespective of whether one has substantial perceptual expertise for a class of stimuli, an observer invariably encounters novel exemplars from this class. To understand how novel exemplars are represented, we examined the extent to which previous experience with a category constrains the acquisition and nature of representation of subsequent exemplars from that category. Participants completed a perceptual training paradigm with either novel other-race faces (category of experience) or novel computer-generated objects (YUFOs) that included pairwise similarity ratings at the beginning, middle, and end of training, and a 20-d visual search training task on a subset of category exemplars. Analyses of pairwise similarity ratings revealed multiple dissociations between the representational spaces for those learning faces and those learning YUFOs. First, representational distance changes were more selective for faces than YUFOs; trained faces exhibited greater magnitude in representational distance change relative to untrained faces, whereas this trained-untrained distance change was much smaller for YUFOs. Second, there was a difference in where the representational distance changes were observed; for faces, representations that were closer together before training exhibited a greater distance change relative to those that were farther apart before training. For YUFOs, however, the distance changes occurred more uniformly across representational space. Last, there was a decrease in dimensionality of the representational space after training on YUFOs, but not after training on faces. Together, these findings demonstrate how previous category experience governs representational patterns of exemplar learning as well as the underlying dimensionality of the representational space.

Although shape perception is primarily considered a function of the ventral visual pathway, previous research has shown that both dorsal and ventral pathways represent shape information. Here, we examine whether the shape-selective electrophysiological signals observed in dorsal cortex are a product of the connectivity to ventral cortex or are independently computed. We conducted multiple EEG studies in which we manipulated the input parameters of the stimuli so as to bias processing to either the dorsal or ventral visual pathway. Participants viewed displays of common objects with shape information parametrically degraded across five levels. We measured shape sensitivity by regressing the amplitude of the evoked signal against the degree of stimulus scrambling. Experiment 1, which included grayscale versions of the stimuli, served as a benchmark establishing the temporal pattern of shape processing during typical object perception. These stimuli evoked broad and sustained patterns of shape sensitivity beginning as early as 50 msec after stimulus onset. In Experiments 2 and 3, we calibrated the stimuli such that visual information was delivered primarily through parvocellular inputs, which mainly project to the ventral pathway, or through koniocellular inputs, which mainly project to the dorsal pathway. In the second and third experiments, shape sensitivity was observed, but in distinct spatio-temporal configurations from each other and from that elicited by grayscale inputs. Of particular interest, in the koniocellular condition, shape selectivity emerged earlier than in the parvocellular condition. These findings support the conclusion of distinct dorsal pathway computations of object shape, independent from the ventral pathway.

Humans recognize faces with ease, despite the complexity of the task and of the visual system which underlies it. Different spatial regions, including both the core and extended face processing networks, and distinct temporal stages of processing have been implicated in face recognition, but there is ongoing controversy regarding the extent to which the mechanisms for recognizing a familiar face differ from those for an unfamiliar face. Here, we used electroencephalogram (EEG) and flicker SSVEP, a high signal-to-noise approach, and searchlight decoding methods to elucidate the mechanisms mediating the processing of familiar and unfamiliar faces in the time domain. Familiar and unfamiliar faces were presented periodically at 15 Hz, 6 Hz and 3.75 Hz either upright or inverted in separate blocks, with the rationale that faster frequencies require shorter processing times per image and tap into fundamentally different levels of visual processing. The 15 Hz trials, likely to reflect early visual processing, exhibited enhanced neural responses for familiar over unfamiliar face trials, but only when the faces were upright. In contrast, decoding methods revealed similar classification accuracies for upright and inverted faces for both familiar and unfamiliar faces. For the 6 Hz frequency, familiar faces had lower amplitude responses than unfamiliar faces, and decoding familiarity was more accurate for upright compared with inverted faces. Finally, the 3.75 Hz frequency revealed no main effects of familiarity, but decoding showed significant correlations with behavioral ratings of face familiarity, suggesting that activity evoked by this slow presentation frequency reflected higher-level, cognitive aspects of familiarity processing. This three-way dissociation between frequencies reveals that fundamentally different stages of the visual hierarchy are modulated by face familiarity. The combination of experimental and analytical approaches used here represent a novel method for elucidating spatio-temporal characteristics within the visual system.

Certain numerical abilities appear to be relatively ubiquitous in the animal kingdom, including the ability to recognize and differentiate relative quantities. This skill is present in human adults and children, as well as in nonhuman primates and, perhaps surprisingly, is also demonstrated by lower species such as mosquitofish and spiders, despite the absence of cortical computation available to primates. This ubiquity of numerical competence suggests that representations that connect to numerical tasks are likely subserved by evolutionarily conserved regions of the nervous system. Here, we test the hypothesis that the evaluation of relative numerical quantities is subserved by lower-order brain structures in humans. Using a monocular/dichoptic paradigm, across four experiments, we show that the discrimination of displays, consisting of both large (5-80) and small (1-4) numbers of dots, is facilitated in the monocular, subcortical portions of the visual system. This is only the case, however, when observers evaluate larger ratios of 3:1 or 4:1, but not smaller ratios, closer to 1:1. This profile of competence matches closely the skill with which newborn infants and other species can discriminate numerical quantity. These findings suggest conservation of ontogenetically and phylogenetically lower-order systems in adults' numerical abilities. The involvement of subcortical structures in representing numerical quantities provokes a reconsideration of current theories of the neural basis of numerical cognition, inasmuch as it bolsters the cross-species continuity of the biological system for numerical abilities.

Certain numerical abilities appear to be relatively ubiquitous in the animal kingdom, including the ability to recognize and differentiate relative quantities. This skill is present in human adults and children, as well as in non-human primates. Perhaps surprisingly, numerosity abilities are also demonstrated by lower species such as mosquito fish and spiders, despite the absence of cortical computation available to primates. This ubiquity of numerical competence suggests that representations that connect to numerical tasks are likely subserved by evolutionarily conserved regions of the nervous system. Here, we test the hypothesis that the visual evaluation of relative numerical quantities is subserved by lower-order brain structures in humans. We use a paradigm (Wheatstone stereoscope) that permits presentation of a stimulus to a single eye and compare discrimination performance when targets are shown monocularly versus dichoptically. Across multiple experiments, we show that the discrimination of displays, consisting of both large (5-80) and small (1-4) numbers of dots, is facilitated in the monocular, subcortical portions of the visual system. This is only the case, however, when observers evaluate larger ratios of 3:1 or 4:1, but not smaller ratios, closer to 1:1. Additional analyses evaluating which of 9 continuous stimulus parameters (such as size, spacing, coverage, etc.) explained participants' performance revealed that number best accounted for behavior and did so with an effect size an order of magnitude larger than any other parameter. This profile of competence matches closely the skill with which newborn infants and other species can discriminate numerical quantity. These findings suggest conservation of ontogenetically and phylogenetically lower-order systems in adults' numerical abilities. The involvement of subcortical structures in representing numerical quantities provokes a reconsideration of current theories of the neural basis of numerical cognition, in as much as it bolsters the cross-species continuity of the biological system for numerical abilities. Meeting abstract presented at VSS 2017

A recent theoretical account posits that, during the acquisition of word recognition in childhood, the pressure to couple visual and language representations in the left hemisphere (LH) results in competition with the LH representation of faces, which consequently become largely, albeit not exclusively, lateralized to the right hemisphere (RH). We explore predictions from this hypothesis using a hemifield behavioural paradigm with words and faces as stimuli, with concurrent event-related potential (ERP) measurement, in a group of adults with developmental dyslexia (DD) or with congenital prosopagnosia (CP) and matched control participants. Behaviourally, the DD group exhibited clear deficits in both word and face processing relative to controls, while the CP group showed a specific deficit in face processing only. This pattern was mirrored in the ERP data too. The DD group evinced neither the normal ERP pattern of RH dominance for faces nor the LH dominance for words. In contrast, the CP group showed the typical ERP superiority for words in the LH but did not show the typical RH superiority for faces. These findings are consistent with the hypothesis that the typical hemispheric organization for words can develop in the absence of typical hemispheric organization for faces but not vice versa, supporting the account of interactive perceptual development.

A recent theoretical account (Behrmann & Plaut, 2011, 2015) and associated empirical data (Dundas et al., 2013) argues for the interdependence of the development of hemispheric organization for words with that of faces in typically developing children. Specifically, the claim is that, during the course of acquiring word recognition skills, the pressure to couple visual and language representations in the left hemisphere (LH) results in competition for representation of faces, and these latter representations become largely lateralized in the right hemisphere (RH). This account makes a specific prediction with regard to atypical development and that is that typical hemispheric organization for words can develop in the context of atypical development of face processing but not vice versa, and the latter prediction is upheld in recent findings from individuals with developmental dyslexia. We explore the first prediction using a visual hemifield presentation paradigm with words and faces, with concurrent ERP measurement, in a group of adults with congenital prosopagnosia (CP), a lifelong impairment in face recognition, and typically developed controls. In contrast with the controls, the CP group failed to produce the dominant RH response for faces, with equal magnitude N170 components elicited by faces and words. Both the controls and the CP adults, however, produced a dominant LH response for words, with greater N170 component in response to words than to faces. Differences between the groups in hemispheric organization were specific to the N170 and were not found in the P100 component. These findings are consistent with the account that typical hemispheric organization for words can develop in the absence of typical hemispheric organization for faces. Additionally, when considered with evidence from developmental dyslexia, the results here support the directionality of interdependent hemispheric development and the hypothesis that hemispheric organization for words may precede that for faces. Meeting abstract presented at VSS 2016