Ptito et al. (2005)

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Cross-Modal Plasticity Revealed by Electrotactile Stimulation of the Tongue in the Congenitally Blind (Ptito et al., 2005)

The experimenters in this study used PET scanning to study cross-modal plasticity in the congenitally blind, using electrotactile stimulation of the tongue. Before training with the tongue display unit (TDU), neither the blind nor the sighted control subjects showed any change in regional cerebral blood flow (rCBF) in the occipital cortex. After training, however, rCBF in the occipital cortex of the blind subjects increased dramatically, providing evidence of training-induced plasticity

Participants

Participants included 6 blind and 5 sighted blind-folded controls with a mean age of 29 years. MRI scans of the blind subjects were normal. Sighted control subjects also had normal neurological exams and normal vision.

Training

Participants were trained to use their tongue in a Snellen orientation detection task using the TDU. The training lasted for seven days for one-hour sessions. For training, observers projected a T onto the subject's tongue using the TDU. Subjects were allowed to orient the T how they pleased in order to get used to the machine. They were scanned before and after training.

Results

Before training, no significant changes in regional cerebral blood flow (rCBF) were observed in the occipital cortex of either group. After practice for the blind, however, activity in the occipital cortex increased. This increase in activity was not observed in the sighted participants, providing evidence for training-induced plasticity in the congenitally blind.

Interestingly, the rate of learning was actually equal among both groups of blind and sighted subjects.

An inter-regional correlation analysis showed that task-related rCBF changes in the left posterior parietal cortex were positively correlated with rCBF changes in the occipital area of the trained blind participants.

All six blind subjects showed activation in the occipital cortex following training with the TDU.

Conclusion

This data revealed that cross-modal plasticity in the blind develops rapidly and that the occipital cortex is part of a functional neural network for tactile discrimination in conjunction with the posterior parietal cortex. Data further showed that the tongue can act as a portal to convey somatosensory information to visual cortex.

Blind subjects showed activation of the occipital cortex while using the TDU after training while sighted control subjects did not despite an equal learning curve, showing that there is a functional role for the occipital lobe even in the congenitally blind.

In contrast, sighted subjects actually showed a deactivation of the visual cortex. However, this is in line with the observation that focusing on one sensory modality causes decreased activity in areas of the brain focusing on other modalities. Sighted controls showed, instead, activation of the somatosensory cortex.

It has been suggested that the occipital activation can be explained by the fact that it is adjacent to the superior parietal lobe, in which tactile information is processed. Thus the tactile information may be "leaking" into the occipital lobe due to it not being used for anything else.