How do individuals with congenital blindness form a conscious representation of a world they have never seen? What happens to visual-devoted brain structure in individuals who are born deprived of sight?
Object and Spatial Visual Pathways in the Human Brain Match-to-Sample Task (Haxby et al, J Neurosci, 1994)
Object Form Topography in Ventral Extrastriate Cortex
Are the topographically organized, category-related related patterns of neural response in the ventral visual pathway a representation of visual sensory images or a more abstract representation of object form that is not dependent on sensory modality?
One-back 2 minutes 2 minutes 2 minutes 15 sec 15 sec Simple exploration fmri: 3 Tesla GE scanner (GRE-EPI, TR= 2.5 sec, FOV= 24 cm, 3.5-mm sagittal slices, 8 runs of 174 volumes)
Cortical regions activated during visual and/or tactile discrimination tasks in the sighted individuals
Cortical areas activated during tactile discrimination in sighted and congenitally blind individuals Sighted subjects Blind subjects
Are patterns of neural activity elicited during tactile discrimination category-specific?
Is the category-specificity of patterns of neural response supramodal?
Object based representation in the ventral stream - Summary Ventral extrastriate cortex shows category-specific responses to the tactile exploration of objects during a discrimination task Cortical areas involved in visual discrimination of objects are also involved in tactile discrimination of the same objects Category-related related patterns of response also in blind individuals who have had no visual experience no merely due to visual-based imagery visual experience is not a pre-requisite requisite for development of category-related related patterns of neural response
Parieto-occipital occipital cortex Is this supramodal organization restricted to ventral extrastriate cortex, or does it extend to other visual areas?
Dorsal stream response to visual and tactile spatial location tasks Subjects: : 6 sighted and 4 congenitally blind right- handed healthy volunteers (mean age±sd sd= = 28±1.3) MRI: 1.5 Tesla GE scanner (GRE-EPI; EPI; SPGR) Tactile 10 sec of tactile exploration + 5sec ISI (16 matrices 2D or 3D) Visual 5 sec of visual exploration + 5 sec ISI (24 matrices 2D or 3D) Block Design One-back recognition task Randomized stimuli presentation
Dorsal extrastriate cortex is activated bothb by optic and tactile spatial detection in sighted and blind individuals Ricciardi et al., Neuroscience, 2006
The Mental Clock Task for investigating multimodal spatial imagery in sighted and congenitally blind individuals Tactile Task ST 4,5sec 30 sec 1sec 5sec 20 sec ISI ICI 15 sec Auditory Task Visual Task ST 2sec 30 sec 1sec 5sec ISI ICI 20 sec 10 sec Bonino et al., in preparation
Similar occipito-parietal parietal networks substain multimodal spatial imagery in sighted and congenitally blind individuals Sighted (n=10) Blind (n=1 (n=10) PPC/IPS Visual Tactile PPC/IPS Auditory x=-49 z= +41 x=-49 z= +41
Spatial localization in the dorsal stream - Summary These results demonstrate that dorsal extrastriate cortical areas are involved both in visual and tactile motion and spatial discrimination Thus, similarly to ventro-temporal cortical regions, the dorsal pathway shows supramodal features, and therefore can process localization data independently from the sensory modality through which information is acquired
Is this supramodal organization restricted to ventral extrastriate cortex, or does it extend to other visual areas? hmt+
Coherent changes in visual images caused by object or viewer movement are called optic flow: it provides information about object form, position, orientation, and movement, as well as self-motion within the environment Tactile exploration of the environment involves analogous changes in tactile images, or tactile flow : Object Form Position and movement Consistency
hmt+ responds to specific components of optic flow We investigated whether the extrastriate visual cortical region, hmt+, plays a role in supramodal representation of sensory motion that is not mediated by visual mental imagery We measured neural activity in sighted subjects during passive perception of optic and tactile motion in congenital and early blind subjects during perception of tactile motion
Subjects: 7 sighted and 4 congenitally blind healthy righthanded volunteers (26 ± 3 yr) fmri: 1.5 Tesla GE scanner GRE-EPI TR= 3.0 sec, FOV= 24 cm 22-26 5-mm axial slices, 10-12 runs of 100 volumes Rotational flow Translational flow
hmt+ is activated both by optic and tactile motion perception in sighted Optic Flow Tactile Flow Tactile/Optic Overlap
Tactile motion activates hmt+ in congenitally blind subjects Optic Flow Tactile Flow
Tactile motion: summary Sighted Optic Flow Blind Optic Flow Sighted Tactile Flow Tactile flow perception in sighted subjects activated the more anterior part of hmt+ but deactivated the more posterior part By contrast, tactile flow perception in blind subjects activated the full extent of hmt+, including the more posterior part that is deactivated by tactile flow in sighted subjects
Supramodal sensory motion Poirier C et al., Neuroimage, 2006 Auditory Flow Tactile Flow Optic Flow Beuchamp MS et al., J Neurosci, 2007
rtms-mediated functional suppression of hmt+ impairs tactile flow discrimination in humans Control condition: 0, -76, 30 mm Accuracy of response 100 90 80 70 60 50 40 30 20 10 0 no-tms Control-TMS hmt-tms 1 2 3 4 mean GAP BETWEEN VELOCITIES TMS Experimental condition (hmt+): -49, -62, 5 mm Gap: χ 2 (3)= 283.71, p.<0.01 TMS: χ 2 (2)= 46.21, p.<0.01
Tactile motion: summary Experiment 2 These results demonstrate that activation of hmt+ by tactile motion is not mediated by visual mental imagery and that the role of hmt+ in tactile motion perception can develop with no visual experience Moreover, visual experience appears to lead to a segregation of hmt+ into an anterior subregion (MST?) that is involved in the representation of both optic and tactile motion and a posterior subregion that is involved only in the representation of optic motion, possibly due to competitive interactions between visual and tactile inputs in normal development
Conclusions -1 Overall, the results of these studies indicate that the cortex of the ventral and dorsal visual pathways are able to process information acquired through different sensory modalities, and is therefore organized in a more abstract supramodal fashion This supramodal organization may also help to explain how individuals who have had no visual experience are able to acquire normal knowledge about objects and spatial representation, and interact effectively with the external world
Supramodal representation and plastic reorganization in the visual pathways in congenitally blind subjects Action Recognition Imagery Working Memory where dorsal stream Tactile/Auditory Spatial Localization what ventral stream Tactile Object Shape Recognition Tactile/ Auditory Motion Perception Tactile recognition, linguistic, nonvisual sensory and cognitive processes, such as for example Braille reading or verbal memory tasks
If the extrastriate visual cortex is organized in a more abstract supramodal fashion, what could we expect from higher-order cognitive functions that rely on the sensory processing of these brain areas, such as working memory or mental imagery?
Is visual experience necessary for the development of the mirror neuron system in the human brain? Mirror neurons are a particular class of visuomotor neurons, originally discovered in area F5 of the monkey premotor cortex, that discharge both when the monkey does a particular action and when it observes another individual
Experimental Paradigm Stimuli Action (sounds/movies) Environmental (sounds/movies) Motor Pantomime Task Odd-ball motor execution and sound recognition (for sighted also visual) Upon completion of scanning, aurally presented actions were subdivided into motor familiar and motor unfamiliar, based on ratings provided by the two distinct groups
Is MNS activated by sounds of familiar actions independently from visual experience? Familiar Action Sounds p<0.05 corrected
Is MNS response to aurally presented actions modulated by the degree of motor familiarity? p<0.05 uncorrected
The effect of motor familiarity varies within the MNS regions Blind Sighted
Motor familiarity modulates magnitude of MNS response to aurally presented actions Summary & Conclusions Sighted Blind MNS response to aurally presented actions does not depend on visual imagery or experience Effect of familiarity varies within the distinct regions of the MNS
Conclusions -2 Both congenitally blind and sighted individuals show overlapping networks subserving perceptual and highercognitive function Visual experience is not a necessary prerequisite for the development of the functional architecture of distinct cognitive abilities supramodal organization These findings further expand previous data indicating that representation of the external world relies on supramodal cortical association areas and may contribute to explain why individuals who have had no visual experience interact effectively with the surrounding environment
Conclusions -3 The study of the blind brain has shed a bright light on many questions regarding not only plastic rearrangements but also functional organization of the sighted brain itself The blind brain should not be considered as a disabled brain but rather as a truly differentially able brain