Tactile Spatial Acuity Enhancement in Blindness: Evidence for Experience-Dependent Mechanisms

Abstract

Tactile spatial acuity is enhanced in blindness, according to several studies, but the cause of this enhancement has been controversial. Two competing hypotheses are the tactile experience hypothesis (reliance on the sense of touch drives tactile-acuity enhancement) and the visual deprivation hypothesis (the absence of vision itself drives tactile-acuity enhancement). Here, we performed experiments to distinguish between these two hypotheses. We used force-controlled grating orientation tasks to compare the passive (finger stationary) tactile spatial acuity of 28 profoundly blind and 55 normally sighted humans on the index, middle, and ring fingers of each hand, and on the lips. The tactile experience hypothesis predicted that blind participants would outperform the sighted on the fingers, and that Braille reading would correlate with tactile acuity. The visual deprivation hypothesis predicted that blind participants would outperform the sighted on fingers and lips. Consistent with the tactile experience hypothesis, the blind significantly outperformed the sighted on all fingers, but not on the lips. Additionally, among blind participants, proficient Braille readers on their preferred reading index finger outperformed nonreaders. Finally, proficient Braille readers performed better with their preferred reading index finger than with the opposite index finger, and their acuity on the preferred reading finger correlated with their weekly reading time. These results clearly implicate reliance on the sense of touch as the trigger for tactile spatial acuity enhancement in the blind, and suggest the action of underlying experience-dependent neural mechanisms such as somatosensory and/or cross-modal cortical plasticity.

Superior Tactile Abilities in the Blind: Is Blindness Required?: Figure 1.

“…For instance, prior studies have shown that tactile finger acuity can increase in normal sighted individuals through intensive training (Sathian and Zangaladze, 1997;Harris et al, 2001), suggesting that blindness may only act as a facilitator in the development of increased tactile acuity, because it encourages training of the nonvisual senses to compensate for visual deprivation. Moreover, it was shown in the current study (Wong et al, 2011) that the ability of blind nonreaders was not statistically significantly better than sighted subjects, even though these blind nonreaders may have used their finger tactile sense for other purposes, such as object exploration and recognition, suggesting that blindness alone is not sufficient for the development of heightened sensory abilities. There is reason to believe, however, that visual deprivation can also directly lead to behavioral enhancements.…”

“…The findings of Wong et al (2011), along with those of previous research, raise interesting and important questions. Notably, is blindness a necessary prerequisite for the development of heightened sensory abilities?…”

“…A final point worth noting is the finding that early-and late-onset blind individuals showed comparable tactile spatial thresholds (Wong et al, 2011). This finding argues against the importance of the early onset of visual deprivation for the improvement of sensory abilities as well as to benefit from cross-modal recruitment of the deafferented occipital cortex, which is in marked contrast with many results in the literature (Cohen et al, 1999;Sadato et al, 2002).…”

“…Several results obtained by Wong et al (2011) highlight the importance of tactile experience in the development of heightened tactile spatial acuity observed in blind individuals (Goldreich and Kanics, 2003). Blind participants had smaller tactile spatial thresholds on their fingertips than did their agematched sighted counterparts.…”

“…In their recent article published in The Journal of Neuroscience, Wong et al (2011) examined whether superior ability of blind individuals in a grating orientation task (GOT) was the consequence of visual deprivation alone or whether tactile experience-dependent mechanisms were the key determinant. The authors followed up on a previous study showing that tactile acuity is enhanced in blindness (Goldreich and Kanics, 2003), and they compared the passive tactile spatial acuity on the index, middle, and ring fingers of blind participants with varying levels of Braille expertise to that of sighted participants.…”