Development and Plasticity of Dorsal and Ventral Stream Processing

(Perceptual Development Lab)

I am particularly interested in cross-modal plasticity, and one focus of my lab is how auditory deprivation, or deafness, affects visual functioning.   The research projects I describe below test the hypothesis that deafness specifically affects the development of one subcomponent of the visual system - the dorsal stream.  This “dorsal stream” is an anatomically and functionally distinct subsystem of the visual system that projects to dorsal temporal and parietal cortex and is highly responsive to motion.  A second subsystem, the “ventral stream”, projects to ventral temporal cortex and is highly responsive to color and form, and is posited to be relatively unaffected by the absence of auditory input. 

Adults

My colleagues and I recorded ERPs (electroencephalography, or “brainwaves”) to motion, designed to activate the dorsal stream, and color, designed to activate the ventral stream.   As predicted by the hypothesis described above, amplitudes and scalp distributions of ERPs to the color stimuli were very similar for deaf and hearing adults.  On the other hand, striking group differences were observed in the ERPs recorded in response to motion stimuli.  The figure below shows that deaf adults produced reliably larger amplitude ERPs to motion stimuli than normally hearing adults, suggesting that visual motion evokes more neural activity in deaf than hearing individuals.  Further, motion-related ERPs in deaf adults were distributed more anteriorly than in hearing adults, suggesting that the enhanced activity may come from additional brain areas.  Together, these data provide the first evidence that auditory deprivation affects the dorsal visual stream, but not the ventral. 

ERPs are limited in their spatial resolution and often our hypotheses require localization of differences between deaf and hearing populations to specific brain regions.  The functional magnetic resonance imaging technique (fMRI) provides this kind of spatial resolution and is well suited for testing hypotheses about localization of function.  To take advantage of this feature, my colleagues and I designed an fMRI study to test the hypothesis that deaf adults produce significantly more activity in the posterior middle temporal gyrus (area MT), an area known to be highly responsive to visual motion that also increases activation with attention to motion.  Brain activity elicited by a moving field of dots was compared to activity elicited by a static field of dots.  Analyses showed that deaf participants produced a larger region of activation within MT than did normally hearing participants, but only when attending to the periphery.  Path analyses of the data suggested that increased activity between area MT and the posterior parietal cortex was the source of the population difference.  These data provide converging evidence that activity within the dorsal pathway in the deaf increases specifically when attention is directed to the periphery. 

A second fMRI study was designed to complement the ERP work described above.  In this study, object/form processing, a ventral stream function, was assessed in deaf and hearing adults with the hypothesis that this function would be unaffected by auditory deprivation.  Brain activation in response to photographs of everyday objects was compared to brain activation in response to scrambled images.  In response to object images, within-group analyses showed that both deaf and hearing participants activated the typical inferior occipitotemporal brain areas that are known to be important in processing visual form.  These preliminary results provide additional support for the hypothesis that auditory deprivation does not modify the ventral visual pathway. 

Development

In order to understand how the population effects described above emerge, it is important to understand whether these population effects are due, at least in part, to differences in the intrinsic maturational timetables of the dorsal and ventral streams.  To address this latter issue, I adapted the ERP paradigm used in the first study described above for use with normally hearing children between the ages of 6 and 10.  The hypothesis was that responses to color would reach adult-like levels earlier than responses to motion because of faster maturation in the ventral stream.  As shown in the figure below, responses to color in even the youngest participants displayed sharp, well-defined peaks with latencies similar to those recorded in adults.  On the other hand, responses to motion displayed less well-defined peaks in the youngest participants and greater differences in latencies between children and adults.  These results strongly support the hypothesis that the ventral stream matures earlier than the dorsal stream in normally hearing children. 

It is also important to establish when in development the differences between deaf and hearing populations emerge.  To address this issue, I am also collecting data on this ERP paradigm from deaf children ages

We are interested in continuing this research and extending it to other visual functions and other experimental techniques.  We will explore dorsal and ventral stream functions using fMRI to further characterize the spatial reorganization of visual functions that result from deafness.  We will extend our research to include other visual skills, such as spatial processing and form processing, and continue to describe the developmental trends of these skills in the deaf and typically developing populations.   Findings from this research will be important in shedding light on what mechanisms underlie cross-modal plasticity and how those mechanisms function in development to produce the typical organization of neurocognitive systems that we see in adults. 

Armstrong, B., Hillyard, S. A., Neville, H. J.,  & Mitchell, T. V. (2002).  Auditory deprivation affects processing of motion, but not color.  Cognitive Brain Research (14), 422-434.

Bavelier, D., Tomann, A., Hutton, C., Mitchell, T., Corina, D., & Neville, H.  (2000).  Visual attention to the periphery is enhanced in congenitally deaf individuals.  Journal of Neuroscience, 20(17):  RC93, 1-6.

Bavelier, D., Brozinsky, C., Tomann, A., Mitchell, T., Neville, H., & Liu, G. (2001).  Impact of early deafness and early exposure to sign language on the cerebral organization for motion processing.  Journal of Neuroscience, 21 (22):  8931-8942.

Mitchell, T., Tomann, A., Bavelier, D., Muray, S., Corina, D., Hutton, C., Liu, G. and Neville, H.  (November, 1998).  Cortical Re-organization for Visual Functions in Congenitally Deaf Subjects:  Part II. Object and Face Processing.  Society for Neuroscience.

Mitchell, T. V., & Neville, H. J. (in preparation).  Asynchrony in the development of electrophysiological responses to motion and color.