Cortical Organization of Auditory Discrimination and Localization in the Blind
Alexander A. Stevens, Ph.D.
Oregon Health & Science University , Portland, OR
David Mahoney Neuroimaging Program
June 1999, for 3 years
Alexander A. Stevens, Ph.D.
Assistant Professor, Oregon Health & Science University
Blindness results in reorganization of posterior cortical areas that are typically involved in vision as well as enlargement of primary and secondary auditory cortical areas in the temporal lobe. Changes in these areas will be associated with enhanced auditory perceptual abilities in the blind.
To determine how visual loss influences the neural organization of auditory perceptual functions in the occipital temporal and parietal cortices of congenitally blind, adventitiously blind and sighted controls. By examining auditory discrimination and sound localization we hope to determine how the organization of these pathways changes as a function of visual experience, age of onset of blindness. The long-term goal of this work is to elucidate how mechanisms of normal brain organization, development and visual deprivation influence the functional organization of posterior cortical areas in the blind.
This study employs functional magnetic resonance imaging (fMRI) as well as psychoacoustics, and cognitive testing. Functional MRI allows us to map changes in metabolic activity in the brain with millimeter resolution while subjects perform different auditory tasks. These tasks include discriminating between different auditory sounds, as well as discriminating the location of sounds. These techniques have revealed auditory dependent brain activity in areas of the brain normally associated with visual processing.
Generally, the fMRI studies of auditory object discrimination and spatial discrimination tasks revealed substantial changes in posterior cortical areas in the congenitally blind individuals, while the late blind resembled more the sighted subjects. Specifically, in both the auditory object discrimination and spatial discrimination fMRI study found substantial reorganization of medial and ventral occipital areas in the congenitally blind in that these areas responded to auditory stimuli. During auditory object discrimination, the adventitiously blind produced results similar to the sighted subjects with activity confined mainly to auditory cortical areas and little activity in the medial occipital area. However, the occipital activity was significantly negatively correlated with the age of blindness onset, indicating that the earlier blindness occurred, the more activity was present in visual cortex.
In contrast, activity in the planum temporale, an auditory association area, was positively associated with age of blindness onset, suggesting that this area, rather than occipital areas, is recruited more when blindness occurred later in life. It appears that compensatory changes occur in different ways at different stages of development in the blind. Congenital and early-onset blindness leads to substantial functional reorganization of posterior cortical areas, while later, post-pubertal onset leads to changes within the auditory cortical areas systems. Importantly, the auditory spatial discrimination study also revealed similar patterns of activity in all three groups, although again, there was evidence of greater recruitment of occipital areas, and significantly less activity in the auditory cortical areas greater in the congenitally blind compared to the late blind and sighted controls. One possible explanation for the results is that auditory speech and the spatial properties of sounds may drive the organization of posterior areas in the congenitally blind, because immediately after birth,speech properties are highly salient to infants, and infants quickly begin orienting to the location of sounds.