Neuroplasticity refers to the brain’s incredible ability to adapt to stimuli from the changing environment. It contributes to rewiring in the brain that helps everyone, especially those with early-onset cognitive deficits, effectively navigate the multisensory world. This neural reorganization in these individuals translates into behavioral skills and performances that are normal or even better than those with intact sensory function (Pascual-Leone and Torres, 1993). Intending to harness the neuroplastic potential to improve future rehabilitative research, Ptito and colleagues investigated whether different portions of the ventral system are equally recruited to process non-visual information.
The visual system is subdivided into dorsal and ventral processing streams; the dorsal (“where”) stream participates in navigating the object’s spatial locations, whereas the ventral (“what”) stream is associated with object recognition and form representation (Goodale and Milner, 1992). When a blind person processes information, such as Braille reading, vibrotactile discrimination, and haptic (tactile) object exploration, the visually-deprived occipital cortex becomes activated (Pascual-Leone and Torres, 1993). Haptic object recognition refers to the ability to distinguish different objects through touch without visual inputs. The visual cortex, now “cortical free real estate,” is recruited to process tactile or auditory sensory tasks. Previous studies reported activation in portions of the ventral “where” stream during haptic object recognition in blind subjects. However, very few studies reported similar observations in the dorsal “what” stream, or when subjects process non-haptic object recognition tasks. Therefore, these implications motivated Ptito and his team to investigate whether information delivered non-haptically, and to a body part not primarily devoted to shaping recognition, can similarly activate the occipital cortex.
The research team employed the electro-tactile stimulation method with the 2-D tongue display unit (TDU) to train blind (experimental) and sighted (control) individuals to recognize shapes non-haptically. The apparatus consisting of the TDU is applied on the tongue to receive electrical pulses conveying information from four computer-generated basic geometric shapes, and subjects of both groups performed shape recognition tasks during whole-brain functional magnetic resonance imaging (fMRI). fMRI measures the blood oxygenation level-dependent (BOLD) signals. BOLD signals increase when blood releases more oxygen to supply to actively firing neurons, thus allowing fMRI to detect active regions of the brain at any given time (Raichle, 2010). By examining BOLD patterns, Ptito and colleagues discovered the inferotemporal cortex’s recruitment, including the lateral-occipital tactile-visual area (LOtv), when both experiment and control subjects detect non-tactile shapes from the electrotactile stimulation of the tongue. Area LOtv, a subregion in the lateral occipital cortex of the dorsal stream, is activated when 3-D stimuli from haptic or auditory modalities are processed (Amedi et al., 2007). However, Ptito and colleagues have confirmed that area LOtv is also activated in both blind and sighted subjects when 2-D non-tactile stimuli are processed. Therefore, this result expands on previous knowledge and confirms the hypothesis that the area within the dorsal stream is also recruited in visually-deficient subjects to process non-visual, non-tactile, abstract shape recognition information.
Besides discovering the occipital cortex’s supramodal functionality, the researchers also reported that blind subjects activated larger areas of occipital and occipitotemporal cortices compared to blindfolded subjects. This is seen in the significant BOLD increase in many subregions in the blind subject’s occipital cortex (e.g., cuneus, lingual gyrus, and the inferior, middle, and superior occipital gyri). Ptito and his team acknowledged that object familiarity, which modulates the relationship between visual object imagery and haptic shape perception, might also affect the non-haptic recognition (Lacey et al., 2010). However, they argued that object familiarity does not apply to their non-haptic shape perception results, since blind subjects refused to create visual imagery of objects during the pre-study training sessions. Moreover, the researchers also highlighted how those blind subjects never had any visual experiences in the first place. Both of their rebutting reasons seem less convincing and somewhat more subjective. Since the study’s participant pool is relatively small, the fact that subjects deny having visual imagery of the objects cannot be thoroughly confirmed and generalized to a larger population. Future investigation with a larger sample size, more careful pre-study assessment, and implementation of more complex geometric objects would provide a better assessment of this argument.
Despite being unable to confirm the exact mechanism, Ptito and colleagues proposed that these cross-modal responses observed between the non-haptic stimulation and activation of related occipital areas might be mediated by the unmasking and strengthening of existing cortico-cortical synaptic connections. This unmasking and strengthening, therefore, accounts for the larger activation of visual areas of the blind subjects. They proposed two potential hypotheses to explain this larger activation in the experimental group. The first proposal speculated that the cortico-cortical feedback pathway from the primary somatosensory cortex through the posterior parietal cortex might act as a mediator in integrating object feature processing. Drawing the speculation from previous macaque and human investigations, which showed stimulation of anterior and ventral intraparietal areas, Ptito and colleagues reasoned that blind subjects might rely on these similar multimodal areas to recruit the visual cortex. This speculation is plausible, as it is further bolstered by the result of their study that shows similarly strong activation of subregions in the posterior parietal cortex in both blind and sighted subjects. The second proposal is reasonable but requires more substantial evidence; the researchers suggested that there might be a newly developed subcortical projection that evokes activity in the visual cortex in the blind groups. Since blind subjects have experienced sensory deficiency for over 20 years— a time more than sufficient for new neurons to grow and extend their axons to new targets— new, aberrant connections might have been formed within the visual cortex (Cole et al., 2020). Moreover, there is increasing evidence that neurogenesis occurs in adulthood, which offers a new direction for future investigation of the neuronal influence on neuroplasticity (Cole et al., 2020).
Regardless, the study could have improved on some of the following limitations. As mentioned in the above analysis, recruiting more congenitally blind participants might help make the study results more reliable and generalizable. A larger sample size could settle the debate of whether visual imagery exists in early-onset blind individuals and allow the researchers to detect possible confounders (e.g., subjects are familiar with the objects during shape discrimination training). Larger sample size would also allow more rigorous statistical comparisons between the sighted and congenitally blind groups. However, the sample size limitation might be compromised if the authors instead manipulate other independent variables, such as the objects’ shapes. Additionally, what if the electro-tactile stimulation method replaces the TDU with an ear or elbow display unit? As these body parts are also not primarily involved in shape recognition, would similar areas in the occipital cortex be recruited to process cross-modal information from these body parts?
All in all, the researchers concluded that both the dorsal and ventral processing streams are preserved in early born cognitively deficient individuals. The findings substantiate their past research on information processing via the 2D TDU in blind individuals. It also expands on the fact that different cortical areas in the occipital cortex undergo reorganization to process a wide range of non-visual information that is not limited to somatosensation. The conclusion of Ptito et al. paves the way for future investigations in neural plasticity. First, it strongly acknowledges the nervous system’s remarkable ability to adapt to rapidly changing external inputs. Second, these cross-modal interactions can be further leveraged in future investigations to promote functional adaptation in patients suffering from sensory loss resulting from neurodegenerative diseases or disorders, such as spinal cord injury, amyotrophic lateral sclerosis, and stroke. It would be interesting to apply a similar method to investigate the cross-modal recruitment of the temporal cortex (parallel to how the occipital cortex is recruited in this study) in processing other types of stimuli in deaf individuals. This hypothetical data can then be used to attempt artificially rewiring these connections to enable patients to regain their lost sensations and body functions.
Regardless, Ptito and colleagues’ research highlights that there are still much-needed investigations to better understand brain plasticity mechanisms. Careful consideration and application of these potential mechanisms, perhaps based on an individual’s basic needs and different disease contexts, might elucidate more practical potentials in modulating how a plastic brain adapts and navigates in a multisensory world.
About the Author
Huong T. Le is a senior at Harvard College concentrating in Neuroscience.
References
Amedi, A., Stern, W.M., Camprodon, J.A., Bermpohl, F., Merabet, L., Rotman, S., Pascual-Leone, A. 2007. Shape conveyed by visual-to-auditory sensory substitution activates the lateral occipital complex. Nature Neuroscience, 10(6), 687-689. https://doi-org.ezp-prod1.hul.harvard.edu/10.1038/nn1912
Cole, J., Espinueva, D.F., Seib, D.R., Ash, A.M., Cooke, M.B., Cahill, S.P., Snyder, J.S. 2020. Adult-Born Hippocampal Neurons Undergo Extended Development and Are Morphologically Distinct from Neonatally-Born Neurons. The Journal of Neuroscience, 40(30), 5740-5756. https://doi-org.ezp-prod1.hul.harvard.edu/10.1523/JNEUROSCI.1665-19.2020
Goodale, M.A., & Milner, A.D. 1992. Separate visual pathways for perception and action. Trends in Neurosciences (Regular Ed.), 15(1), 20-25. https://doi.org/10.1016/0166-2236(92)90344-8
Lacey, S., Flueckiger, P., Stilla, R., Lava, M., & Sathian, K. 2010. Object familiarity modulates the relationship between visual object imagery and haptic shape perception. NeuroImage (Orlando, Fla.), 49(3), 1977-1990. https://doi.org/10.1016/j.neuroimage.2009.10.081
Merabet, L.B., Rizzo, J.F., Amedi, A., Somers, D.C., & Pascual-Leone, A. 2005. What blindness can tell us about seeing again: Merging neuroplasticity and neuroprostheses. Nature Reviews. Neuroscience, 6(1), 71-77. https://doi-org.ezp-prod1.hul.harvard.edu/10.1038/nrn1586
Merabet, LB, & Pascual-Leone, A. 2009. Neural reorganization following sensory loss: The opportunity of change. Nature Reviews. Neuroscience, 11(1), 44-52. https://doi-org.ezp-prod1.hul.harvard.edu/10.1038/nrn2758
Raichle, M.E. 2006. The Brain's Dark Energy. Science (American Association for the Advancement of Science), 314(5803), 1249-1250. Retrieved December 7, 2020, from http://www.jstor.org/stable/20032872
Pascual-Leone, A., & Torres, F. (1993). Plasticity of the sensorimotor cortex representation of the reading finger in Braille readers. Brain : a journal of neurology, 116 ( Pt 1), 39–52. https://doi-org.ezp-prod1.hul.harvard.edu/10.1093/brain/116.1.39
Ptito, M., Matteau, I., Wang, A., Paulson, O.B., Siebner, H.R., & Kupers, R. 2012. Crossmodal Recruitment of the Ventral Visual Stream in Congenital Blindness. Neural Plasticity, 2012, 1-9. https://doi.org/10.1155/2012/304045.
The visual system is subdivided into dorsal and ventral processing streams; the dorsal (“where”) stream participates in navigating the object’s spatial locations, whereas the ventral (“what”) stream is associated with object recognition and form representation (Goodale and Milner, 1992). When a blind person processes information, such as Braille reading, vibrotactile discrimination, and haptic (tactile) object exploration, the visually-deprived occipital cortex becomes activated (Pascual-Leone and Torres, 1993). Haptic object recognition refers to the ability to distinguish different objects through touch without visual inputs. The visual cortex, now “cortical free real estate,” is recruited to process tactile or auditory sensory tasks. Previous studies reported activation in portions of the ventral “where” stream during haptic object recognition in blind subjects. However, very few studies reported similar observations in the dorsal “what” stream, or when subjects process non-haptic object recognition tasks. Therefore, these implications motivated Ptito and his team to investigate whether information delivered non-haptically, and to a body part not primarily devoted to shaping recognition, can similarly activate the occipital cortex.
The research team employed the electro-tactile stimulation method with the 2-D tongue display unit (TDU) to train blind (experimental) and sighted (control) individuals to recognize shapes non-haptically. The apparatus consisting of the TDU is applied on the tongue to receive electrical pulses conveying information from four computer-generated basic geometric shapes, and subjects of both groups performed shape recognition tasks during whole-brain functional magnetic resonance imaging (fMRI). fMRI measures the blood oxygenation level-dependent (BOLD) signals. BOLD signals increase when blood releases more oxygen to supply to actively firing neurons, thus allowing fMRI to detect active regions of the brain at any given time (Raichle, 2010). By examining BOLD patterns, Ptito and colleagues discovered the inferotemporal cortex’s recruitment, including the lateral-occipital tactile-visual area (LOtv), when both experiment and control subjects detect non-tactile shapes from the electrotactile stimulation of the tongue. Area LOtv, a subregion in the lateral occipital cortex of the dorsal stream, is activated when 3-D stimuli from haptic or auditory modalities are processed (Amedi et al., 2007). However, Ptito and colleagues have confirmed that area LOtv is also activated in both blind and sighted subjects when 2-D non-tactile stimuli are processed. Therefore, this result expands on previous knowledge and confirms the hypothesis that the area within the dorsal stream is also recruited in visually-deficient subjects to process non-visual, non-tactile, abstract shape recognition information.
Besides discovering the occipital cortex’s supramodal functionality, the researchers also reported that blind subjects activated larger areas of occipital and occipitotemporal cortices compared to blindfolded subjects. This is seen in the significant BOLD increase in many subregions in the blind subject’s occipital cortex (e.g., cuneus, lingual gyrus, and the inferior, middle, and superior occipital gyri). Ptito and his team acknowledged that object familiarity, which modulates the relationship between visual object imagery and haptic shape perception, might also affect the non-haptic recognition (Lacey et al., 2010). However, they argued that object familiarity does not apply to their non-haptic shape perception results, since blind subjects refused to create visual imagery of objects during the pre-study training sessions. Moreover, the researchers also highlighted how those blind subjects never had any visual experiences in the first place. Both of their rebutting reasons seem less convincing and somewhat more subjective. Since the study’s participant pool is relatively small, the fact that subjects deny having visual imagery of the objects cannot be thoroughly confirmed and generalized to a larger population. Future investigation with a larger sample size, more careful pre-study assessment, and implementation of more complex geometric objects would provide a better assessment of this argument.
Despite being unable to confirm the exact mechanism, Ptito and colleagues proposed that these cross-modal responses observed between the non-haptic stimulation and activation of related occipital areas might be mediated by the unmasking and strengthening of existing cortico-cortical synaptic connections. This unmasking and strengthening, therefore, accounts for the larger activation of visual areas of the blind subjects. They proposed two potential hypotheses to explain this larger activation in the experimental group. The first proposal speculated that the cortico-cortical feedback pathway from the primary somatosensory cortex through the posterior parietal cortex might act as a mediator in integrating object feature processing. Drawing the speculation from previous macaque and human investigations, which showed stimulation of anterior and ventral intraparietal areas, Ptito and colleagues reasoned that blind subjects might rely on these similar multimodal areas to recruit the visual cortex. This speculation is plausible, as it is further bolstered by the result of their study that shows similarly strong activation of subregions in the posterior parietal cortex in both blind and sighted subjects. The second proposal is reasonable but requires more substantial evidence; the researchers suggested that there might be a newly developed subcortical projection that evokes activity in the visual cortex in the blind groups. Since blind subjects have experienced sensory deficiency for over 20 years— a time more than sufficient for new neurons to grow and extend their axons to new targets— new, aberrant connections might have been formed within the visual cortex (Cole et al., 2020). Moreover, there is increasing evidence that neurogenesis occurs in adulthood, which offers a new direction for future investigation of the neuronal influence on neuroplasticity (Cole et al., 2020).
Regardless, the study could have improved on some of the following limitations. As mentioned in the above analysis, recruiting more congenitally blind participants might help make the study results more reliable and generalizable. A larger sample size could settle the debate of whether visual imagery exists in early-onset blind individuals and allow the researchers to detect possible confounders (e.g., subjects are familiar with the objects during shape discrimination training). Larger sample size would also allow more rigorous statistical comparisons between the sighted and congenitally blind groups. However, the sample size limitation might be compromised if the authors instead manipulate other independent variables, such as the objects’ shapes. Additionally, what if the electro-tactile stimulation method replaces the TDU with an ear or elbow display unit? As these body parts are also not primarily involved in shape recognition, would similar areas in the occipital cortex be recruited to process cross-modal information from these body parts?
All in all, the researchers concluded that both the dorsal and ventral processing streams are preserved in early born cognitively deficient individuals. The findings substantiate their past research on information processing via the 2D TDU in blind individuals. It also expands on the fact that different cortical areas in the occipital cortex undergo reorganization to process a wide range of non-visual information that is not limited to somatosensation. The conclusion of Ptito et al. paves the way for future investigations in neural plasticity. First, it strongly acknowledges the nervous system’s remarkable ability to adapt to rapidly changing external inputs. Second, these cross-modal interactions can be further leveraged in future investigations to promote functional adaptation in patients suffering from sensory loss resulting from neurodegenerative diseases or disorders, such as spinal cord injury, amyotrophic lateral sclerosis, and stroke. It would be interesting to apply a similar method to investigate the cross-modal recruitment of the temporal cortex (parallel to how the occipital cortex is recruited in this study) in processing other types of stimuli in deaf individuals. This hypothetical data can then be used to attempt artificially rewiring these connections to enable patients to regain their lost sensations and body functions.
Regardless, Ptito and colleagues’ research highlights that there are still much-needed investigations to better understand brain plasticity mechanisms. Careful consideration and application of these potential mechanisms, perhaps based on an individual’s basic needs and different disease contexts, might elucidate more practical potentials in modulating how a plastic brain adapts and navigates in a multisensory world.
About the Author
Huong T. Le is a senior at Harvard College concentrating in Neuroscience.
References
Amedi, A., Stern, W.M., Camprodon, J.A., Bermpohl, F., Merabet, L., Rotman, S., Pascual-Leone, A. 2007. Shape conveyed by visual-to-auditory sensory substitution activates the lateral occipital complex. Nature Neuroscience, 10(6), 687-689. https://doi-org.ezp-prod1.hul.harvard.edu/10.1038/nn1912
Cole, J., Espinueva, D.F., Seib, D.R., Ash, A.M., Cooke, M.B., Cahill, S.P., Snyder, J.S. 2020. Adult-Born Hippocampal Neurons Undergo Extended Development and Are Morphologically Distinct from Neonatally-Born Neurons. The Journal of Neuroscience, 40(30), 5740-5756. https://doi-org.ezp-prod1.hul.harvard.edu/10.1523/JNEUROSCI.1665-19.2020
Goodale, M.A., & Milner, A.D. 1992. Separate visual pathways for perception and action. Trends in Neurosciences (Regular Ed.), 15(1), 20-25. https://doi.org/10.1016/0166-2236(92)90344-8
Lacey, S., Flueckiger, P., Stilla, R., Lava, M., & Sathian, K. 2010. Object familiarity modulates the relationship between visual object imagery and haptic shape perception. NeuroImage (Orlando, Fla.), 49(3), 1977-1990. https://doi.org/10.1016/j.neuroimage.2009.10.081
Merabet, L.B., Rizzo, J.F., Amedi, A., Somers, D.C., & Pascual-Leone, A. 2005. What blindness can tell us about seeing again: Merging neuroplasticity and neuroprostheses. Nature Reviews. Neuroscience, 6(1), 71-77. https://doi-org.ezp-prod1.hul.harvard.edu/10.1038/nrn1586
Merabet, LB, & Pascual-Leone, A. 2009. Neural reorganization following sensory loss: The opportunity of change. Nature Reviews. Neuroscience, 11(1), 44-52. https://doi-org.ezp-prod1.hul.harvard.edu/10.1038/nrn2758
Raichle, M.E. 2006. The Brain's Dark Energy. Science (American Association for the Advancement of Science), 314(5803), 1249-1250. Retrieved December 7, 2020, from http://www.jstor.org/stable/20032872
Pascual-Leone, A., & Torres, F. (1993). Plasticity of the sensorimotor cortex representation of the reading finger in Braille readers. Brain : a journal of neurology, 116 ( Pt 1), 39–52. https://doi-org.ezp-prod1.hul.harvard.edu/10.1093/brain/116.1.39
Ptito, M., Matteau, I., Wang, A., Paulson, O.B., Siebner, H.R., & Kupers, R. 2012. Crossmodal Recruitment of the Ventral Visual Stream in Congenital Blindness. Neural Plasticity, 2012, 1-9. https://doi.org/10.1155/2012/304045.
