SHORTER LATENCIES OF COMPONENTS OF MIDDLE LATENCY AUDITORY EVOKED POTENTIALS IN CONGENITALLY  BLIND COMPARED TO NORMAL SIGHTED SUBJECTS

N. K. Manjunath, R. Srinivas, K. S. Nirmala,
H. R. Nagendra, Arun Kumar and Shirley Telles*

Vivekananda Kendra Yoga Research Foundation, Bangalore, India and
Department ]of Neurology, M.S. Ramaiah Medical Teaching Hospital, Bangalore, India

(Received 14 April 1998)

A previous study which reported shorter latencies of the Nb component of AEP-MLRS in congenitally blind compared to normal sighted subjects, formed the basis for the present study. The blind subjects had received a rehabilitation program from the age of 4 years onwards, which may have influenced auditory function. Hence the present study was designed to compare the AEP MLRs of normal sighted subjects with age-matched blind subjects who had not undergone early rehabilitation. Auditory evoked potentials (0 to 100 ms. range) were recorded in 10 congenitally blind subjects (average age = 22.4 + 4.9 yrs.) and an equal number of age matched subjects with normal vision. There were two repetitions per subject. The peak latencies of both the Pa (maximum positive peak between Na and 35 ms.) and Nb (maximum negative peak between 38 and 52 ms.) waves was significantly shorter in congenitally blind compared to normal sighted subjects. Since the Pa and Nb waves are believed to be generated by the superior temporal cortex (Heschl's gyrus), it appears that processing at this neural level occurs more efficiently in the blind. Also, in spite of the absence of an early rehabilitation program the present subjects showed the same auditory changes as those reported earlier.

Key words: Middle latency auditory evoked potentials; congenitally blind; normal-sighted; primary auditory area; auditory inputs for rehabilitation.

 Impaired vision during early development has been shown to give rise to compensatory changes in the auditory system. Auditory perceptual sensitivity was modified in the congenitally blind, enabling them to use echoes to perceive spatial positions of objects (Strelow and Brabyn, 1982). Development of such abilities may be related to changes in auditory processing. For example, the long latency event related potential waves (NI, P2, and P3) were found to have shorter latencies and higher amplitudes in early blind humans compared to those with normal vision (Niemeyer and Starlinger, 1981). The Nb component of AEP-MLR (average peak latency 44.3ms) had a significantly shorter latency in congenitally blind persons (Naveen, Srinivas, Nirmala, Nagendra and Telles, 1997). This suggested that auditory information processing at the level of the posteromedial part of the primary auditory cortex, the known generator of the Nb wave (Liegeois-Chauvel, Musolino, Badier, Marquis and Chauvel, 1994) was more efficient in the blind.

Based on investigations of somatosensory information in the blind it was thought that adaptive changes may be enhanced by experience, such as Braille reading (Sadato, Pascual-Leone, Grafman, Ibanez, Deiber, Dold and Hallett, 1996). In the study on AEP-MLRs cited above (Naveen et al., 1997), the subjects were all participating in a systematic education and rehabilitation program from the age of four years till the time of the study, when the group average age was 14.3 + 1.4 years. The program included learning based on Braille reading and auditory inputs. This gave rise to the speculation that the rehabilitation program may have contributed to the adaptations seen in auditory functioning.

Hence the present study was designed to determine the changes in auditory function (measured through middle latency auditory evoked responses, AEP-MLRS) in congenitally blind subjects who had minimal, or no formal rehabilitation.