Pascual-Leone et al. (1995)
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Contents |
Introduction
Pascula-Leone, Wassermann, Sadato, and Hallett (1995) conducted a study to examine the representation of the motor cortical outputs in relation to a preceding activity. This study emphasized the importance of the correlation between precise timing and skill acquisition. More specifically, the current study focused exclusively on six blind participants, who were proficient in reading Braille, which requires the use of the tip of the index finger in order to discriminate between differential patterns of raised dots. The finger is subject to side-to-side movements “at a constant amplitude and speed to enhance this sensory discrimination by the pressure receptors in the skin” (Pascula-Leone et al., 1995). In previous studies, it has been found that the sensorimotor representation of the reading finger is enlarged in these blind, proficient Braille readers when compared to the same finger of the opposite hand, or with either finger in normal, sighted individuals.
The extent to which this modulation is enlarged would very well question the size and stability of these cortical representations, for it seems unlikely that such an enlargement would inhibit a proficient Braille reader from the use of his/her other fingers. This dilemma has led to the hypothesis that if this transformation in the motor cortical output is taking place during skill acquisition that requires the use of a specific body part, it should also be expected to reduce to a baseline after learning of the relevant task has occurred. Thus, the cortical representation gives rise to a dynamic, flexible system, whose activation is dependent upon the previous activity (Pascula-Leone et al., 1995). The flexibility of this system led the researchers in the current study to further investigate the stability as well as the size of this motor cortical output representation in proficient Braille readers.
- The Braille Alphabet
Method
Six proficient Braille readers (four men and two women), with ages ranging from 44 to 57 years, participated in this study. These participants were all completely blind before the age of ten, learned to read Braille before the age of 13, and all used the right index finger for character recognition and the left index for line spacing. The experiment tested participants on two different Mondays, which were separated by one week. All Braille readers were tested two times per days (once in the morning and once in the evening). It should be noted that one of the days, in which the participant was tested, was considered to be a “work day,” where he/she read Braille for four to six hours. In contrast, the participants were required to request one of the two testing days off from work without notifying the experimenter, in which they read no Braille. This date was referred to as the “control day” and used as a means of comparison in statistical procedures following the experiment. Using the focal Transcranial Magnetic Stimulation (TMS), this instrument mapped the motor cortical outputs to the left first dorsal interosseous (FDI) as well as the right abductor digiti minimi (ADM) muscles (Pascula-Leone et al., 1995). Additionally, electrodes were connected to the participant’s finger muscles to evaluate the extent to which the brain areas connected to this cortical modulation were enlarged.
Results
The findings of the current study support the aforementioned hypothesis that the motor cortical representation is comprised of a dynamic and flexible system, whose organization is largely dependent upon the previous, relevant task. In other words, the current experiment was able to show significant changes in the motor cortical outputs that rapidly adjust to meet the demands and successful completion of the required task.
Discussion
This study highlights the developmental characteristic of plasticity in the brain. It shows that this phenomenon is ongoing and not limited to brain damaged individuals. For instance, skill acquisition requires the growth of new neurons that adapt to the relevant task. A case study reported in the article discussed a 54-year-old female, who was blind from birth, due to a rare eye condition called Retrolental Fibroplasia. This disease is most salient in infants and usually results from high concentrations of oxygen, which causes abnormal growth of the fibrous tissue behind the lens to take place (Pascula-Leone et al., 1995). Like the participants in the experimental design, she, too, was a proficient Braille reader, who showed an enlarged motor cortical representation of the right, reading hand (FDI) in contrast to that of her left FDI. After a period of nine months, this subject was tested again, and results from this experiment showed a significant reduction in the cortical output map of the right FDI. Stunned by this finding, researchers were notified of the participant’s recent vacation, in which she did not engage in any Braille reading. Consequently, she was asked to return to the laboratory at the end of work week and surprisingly, they found a “return” to the enlargement of the motor cortical output map that was documented in the first experiment.
Based on this reported case study, along with the findings of the current experiment, it can be concluded that skill acquisition relies on plastic changes in the neural network that must adapt to the demands of the new task. Proficiency in learning may very well rely on a rapid modulation of the cortical representation, which gives rises to a correlation between precision of time and skill acquisition. However, the development of this capacity has also been shown to consist of intracortical connections that become latent due to lack of exposure and practice of the relevant skill. Additionally, a point should be made about this latency, for on the days that did not involve Braille reading, these participants were still most likely engaging in tasks that required the use of similar body parts, and thus the motor cortical outputs were adjusted for these activities. This finding gives rise to the plastic component of the brain, in that the “rewiring” of this neural network results in a failure to rapidly respond after prolonged exposure to a task that requires the use of the same body parts (Pascula-Leone et al., 1995). While this study certainly highlights the underlying neural mechanisms of the plasticity phenomenon, studies involving non-proficient Braille readers should be investigated to assess the types of neural changes that take place following similarly delayed exposure to training.
References
Pascual-Leone, A., Wassermann, E. M., Sadato, N., Hallett, M. (1995). The role of reading activity on the modulation of motor cortical outputs to the reading hand in braille readers. Annals of Neurology, 38, 910- 915.
