Giulia Dormal

Impact of blindness onset on the functional organization and the connectivity of the occipital cortex

Contrasting the impact of congenital versus late-onset acquired blindness provides a unique model to probe how experience at different developmental periods shapes the functional organization of the occipital cortex. We used functional magnetic resonance imaging to characterize brain activations of congenitally blind, late-onset blind and two groups of sighted control individuals while they processed either the pitch or the spatial attributes of sounds. Whereas both blind groups recruited occipital regions for sound processing, activity in bilateral cuneus was only apparent in the congenitally blind, highlighting the existence of region-specific critical periods for crossmodal plasticity. Most importantly, the preferential activation of the right dorsal stream (middle occipital gyrus and cuneus) for the spatial processing of sounds was only observed in the congenitally blind. This demonstrates that vision has to be lost during an early sensitive period in order to transfer its functional specialization for space processing toward a non-visual modality. We then used a combination of dynamic causal modelling with Bayesian model selection to demonstrate that auditory-driven activity in primary visual cortex is better explained by direct connections with primary auditory cortex in the congenitally blind whereas it relies more on feedback inputs from parietal regions in the late-onset blind group. Taken together, these results demonstrate the crucial role of the developmental period of visual deprivation in (re)shaping the functional architecture and the connectivity of the occipital cortex. Such findings are clinically important now that a growing number of medical interventions may restore vision after a period of visual deprivation.

A common right fronto-parietal network for numerosity and duration processing: An fMRI study

Numerosity and duration processing have been modeled by a functional mechanism taking the form of an accumulator working under two different operative modes. Separate investigations of their cerebral substrates have revealed partly similar patterns of activation, mainly in parietal and frontal areas. However, the precise cerebral implementation of the accumulator model within these areas has not yet been directly assessed. In this study, we asked participants to categorize the numerosity of flashed dot sequences or the duration of single dot displays, and we used functional magnetic resonance imaging (fMRI) to examine the common neural correlates of these processes. The results reveal a large right‐lateralized fronto‐parietal network, including the intraparietal sulcus (IPS) and areas in the precentral, middle and superior frontal gyri, which is activated by both numerosity and duration processing. Complementary psychophysiological interaction (PPI) analyses show a functional connectivity between the right IPS and the frontal areas in both tasks, whereas the right IPS was functionally connected to the left IPS and the right precentral area in the numerosity categorization task only. We propose that the right IPS underlies a common magnitude processing system for both numerosity and duration, possibly corresponding to the encoding and accumulation stages of the accumulator model, whereas the frontal areas are involved in subsequent working‐memory storage and decision‐making processes. Hum Brain Mapp, 2011.

Embodied numbers: The role of vision in the development of number-space interactions

The strong association between numbers and space is found in the well-documented SNARC effect (Spatial Numerical Association of Response Codes), where responses on small/large numbers are faster in the left/right side of space, respectively. However, little is known about the developmental process through which numbers are mapped onto external physical space. Here we show that early blind individuals, but not late blind or sighted, demonstrate a reversed SNARC effect when performing a numerical comparison task with hands crossed over the body midline. Importantly, this reversed SNARC effect was not observed in any group of participants in a control parity judgment task. The present study therefore demonstrates that early visual experience drives the development of an external coordinate system for the visuo-spatial representation of numbers and further supports the idea that different types of spatial information are engaged in specific numerical tasks.

Functional selectivity in sensory-deprived cortices

In a recent study, Lomber, Meredith, and Kral (2010) investigated crossmodal reorganization in congenitally deaf cats. They demonstrated that specific regions of the auditory cortex are responsible for distinct supranormal visual performances following early auditory deprivation. These exciting results are considered in light of recently increasing research suggesting that crossmodal plasticity associated with early sensory deprivation follows organizational principles that maintain the functional specialization of the colonized brain regions.

Mode-dependent and mode-independent representations of numerosity in the right intraparietal sulcus

In humans, areas around the intraparietal sulcus (IPS) have been found to play a crucial role in coding nonsymbolic numerosities (i.e., number of elements in a collection). In the parietal cortex of monkeys, some populations of neurons were found to respond selectively to sequentially- or simultaneously-presented numerosities, whereas other populations showed similar activation in both modes of presentation. However, whether such mode-dependent and -independent representations of numerosity also exist in humans is still unknown. Here, we used fMRI to identify the areas involved in numerosity processing while participants classified linear arrays of dots (simultaneous stimuli) or flashed dot sequences (sequential stimuli). The processing of simultaneous numerosities induced activations bilaterally in several areas of the IPS, whereas activations during the processing of sequential numerosities were restricted to the right hemisphere. A conjunction analysis showed that only the right IPS and precentral gyrus showed overlapping activations during the judgement of sequential and simultaneous stimuli. Voxelwise correlations confirmed the highly similar pattern of activation found in these regions during both tasks. This pattern was weaker or absent in mode-dependent regions, like the right inferior frontal cortex and the lateral occipital complex. Finally, a close look at the right IPS revealed an anterior-to-posterior gradient of activation with selective activation for sequential and simultaneous stimuli in the anterior and posterior areas, respectively, and overlapping activations in-between. This study provides the first direct evidence that, in humans, the right IPS contains both mode-dependent and mode-independent representations of numerosity.

Long-Lasting Crossmodal Cortical Reorganization Triggered by Brief Postnatal Visual Deprivation

Animal and human studies have demonstrated that transient visual deprivation early in life, even for a very short period, permanently alters the response properties of neurons in the visual cortex and leads to corresponding behavioral visual deficits. While it is acknowledged that early-onset and longstanding blindness leads the occipital cortex to respond to non-visual stimulation, it remains unknown whether a short and transient period of postnatal visual deprivation is sufficient to trigger crossmodal reorganization that persists after years of visual experience. In the present study, we characterized brain responses to auditory stimuli in 11 adults who had been deprived of all patterned vision at birth by congenital cataracts in both eyes until they were treated at 9 to 238 days of age. When compared to controls with typical visual experience, the cataract-reversal group showed enhanced auditory-driven activity in focal visual regions. A combination of dynamic causal modeling with Bayesian model selection indicated that this auditory-driven activity in the occipital cortex was better explained by direct cortico-cortical connections with the primary auditory cortex than by subcortical connections. Thus, a short and transient period of visual deprivation early in life leads to enduring large-scale crossmodal reorganization of the brain circuitry typically dedicated to vision.

Plasticity of the Dorsal “Spatial” Stream in Visually Deprived Individuals

Studies on visually deprived individuals provide one of the most striking demonstrations that the brain is highly plastic and is able to rewire as a function of the sensory input it receives from the environment. In the current paper, we focus on spatial abilities that are typically related to the dorsal visual pathway (i.e., spatial/motion processing). Bringing together evidence from cataract-reversal individuals, early- and late-blind individuals and sight-recovery cases of long-standing blindness, we suggest that the dorsal “spatial” pathway is mostly plastic early in life and is then more resistant to subsequent experience once it is set, highlighting some limits of neuroplasticity.

Attentional shifts induced by uninformative number symbols modulate neural activity in human occipital cortex

Number processing interacts with space encoding in a wide variety of experimental paradigms. Most intriguingly, the passive viewing of uninformative number symbols can shift visuo-spatial attention to different target locations according to the number magnitude, i.e., small/large numbers facilitate processing of left/right targets, respectively. The brain architecture dedicated to these attention shifts associated with numbers remains unknown. Evoked potential recordings indicate that both early and late stages are involved in this spatio-numerical interaction, but the neuro-functional anatomy needs to be specified. Here we use, for the first time, functional magnetic resonance imaging (fMRI) to investigate attentional orienting following uninformative Arabic digits. We show that BOLD response in occipital visual regions is modulated by the congruency between digit magnitude (small/large) and target side (left/right). Additionally, we report higher BOLD responses following large (8, 9) compared to small (1, 2) digits in two bilateral parietal regions, yielding a significant effect of digit magnitude. We propose and discuss the view that encoding of semantic representations related to number symbols in parietal cortex leads to shifts in visuo-spatial attention and enhances visual processing in the occipital cortex according to number-space congruency rules.

Tracking the evolution of crossmodal plasticity and visual functions before and after sight restoration

Visual deprivation leads to massive reorganization in both the structure and function of the occipital cortex, raising crucial challenges for sight restoration. We tracked the behavioral, structural, and neurofunctional changes occurring in an early and severely visually impaired patient before and 1.5 and 7 mo after sight restoration with magnetic resonance imaging. Robust presurgical auditory responses were found in occipital cortex despite residual preoperative vision. In primary visual cortex, crossmodal auditory responses overlapped with visual responses and remained elevated even 7 mo after surgery. However, these crossmodal responses decreased in extrastriate occipital regions after surgery, together with improved behavioral vision and with increases in both gray matter density and neural activation in low-level visual regions. Selective responses in high-level visual regions involved in motion and face processing were observable even before surgery and did not evolve after surgery. Taken together, these findings demonstrate that structural and functional reorganization of occipital regions are present in an individual with a long-standing history of severe visual impairment and that such reorganizations can be partially reversed by visual restoration in adulthood.

Functional Preference for Object Sounds and Voices in the Brain of Early Blind and Sighted Individuals

Sounds activate occipital regions in early blind individuals. However, how different sound categories map onto specific regions of the occipital cortex remains a matter of debate. We used fMRI to characterize brain responses of early blind and sighted individuals to familiar object sounds, human voices, and their respective low-level control sounds. In addition, sighted participants were tested while viewing pictures of faces, objects, and phase-scrambled control pictures. In both early blind and sighted, a double dissociation was evidenced in bilateral auditory cortices between responses to voices and object sounds: Voices elicited categorical responses in bilateral superior temporal sulci, whereas object sounds elicited categorical responses along the lateral fissure bilaterally, including the primary auditory cortex and planum temporale. Outside the auditory regions, object sounds also elicited categorical responses in the left lateral and in the ventral occipitotemporal regions in both groups. These regions also showed response preference for images of objects in the sighted group, thus suggesting a functional specialization that is independent of sensory input and visual experience. Between-group comparisons revealed that, only in the blind group, categorical responses to object sounds extended more posteriorly into the occipital cortex. Functional connectivity analyses evidenced a selective increase in the functional coupling between these reorganized regions and regions of the ventral occipitotemporal cortex in the blind group. In contrast, vocal sounds did not elicit preferential responses in the occipital cortex in either group. Nevertheless, enhanced voice-selective connectivity between the left temporal voice area and the right fusiform gyrus were found in the blind group. Altogether, these findings suggest that, in the absence of developmental vision, separate auditory categories are not equipotent in driving selective auditory recruitment of occipitotemporal regions and highlight the presence of domain-selective constraints on the expression of cross-modal plasticity.