Research reportEnhanced heat discrimination in congenital blindness
Introduction
There is growing evidence that congenitally blind individuals outperform age-and sex-matched normally sighted individuals in various sensory tasks [1]. Indeed, congenitally blind individuals have supra-normal discrimination skills in tactile [2], [3], [4], auditory [5], [6], [7] and olfactory [8], [9], [10] modalities. In a previous study, we measured warmth and cold detection thresholds as well as heat and cold pain thresholds and responses to supra-thresholds heat stimulation in congenital blind subjects [18]. Although blind individuals showed increased responses to pain stimulation, thresholds for innocuous warmth and cold were not different from normal sighted controls. These results do not imply, however, that blind individuals would not perform any better than sighted controls in more complex temperature discrimination tasks. To date, nothing is known about the blind's ability to discriminate thermal stimuli. Based on anecdotal accounts from blind individuals about their use of thermal cues in daily-life activities, e.g. the difference in temperature gradient caused by sunlight hitting the forehead for purposes of spatial navigation, we hypothesized that they would have better heat discrimination skills.
It has been shown that people can discriminate between a broad range of materials by relying solely on thermal diffusivity properties [11], [12], [13]. Because of their lack of vision, blind individuals might rely more strongly on these thermal cues for object recognition, possibly leading to an enhanced sensitivity to detect subtle differences in thermal properties. Furthermore, thermoception also plays a role in avoiding thermal injury [14]. Indeed, nociceptive heat is encoded by the combined activity of thermoceptors and nociceptors, suggesting that warm fibers contribute to the experience of pain [14], [15], [16], [17]. Therefore, a rapid increase in temperature, even within the innocuous range, can be encoded as dangerous. Since congenitally blind individuals have lower heat pain thresholds compared to the sighted [18], [19], they may be more attentive to temperature shifts that may be indicative for an impeding painful stimulus.
Thermal perception is not only dependent on stimulus intensity but also on spatial summation [12], [20], [21]. Indeed, changing the size of a thermal stimulus drastically affects the perceived intensity. This property is especially important in warmth perception in which intensity and spatial extent of the stimulus have equal influence on the perceived intensity [12], [22]. Unpublished preliminary data from our lab suggested that the spatial extent of thermal stimulation more strongly affects perceptual decision making in blind compared to sighted participants. Therefore, we investigated here in a more systematic manner whether congenitally blind differ from normal controls with respect to spatial summation of heat.
Section snippets
Methods
Participants were recruited from our database of congenitally blind subjects or by advertisement. Our study population consisted of 12 congenitally blind (5F; mean age: 39.0 ± 12.2 years; range: 24–61) and 12 normally sighted (5F; mean age: 38.8 ± 14.7 years; range: 21–66) participants. One blind participant and her matched control were excluded from the data analysis due to non-completion of the experiment. All blind participants suffered from blindness due to peripheral origin. Blindness due to
Results
Levene's tests indicated equality of variance for all data distributions, as illustrated in Table 2. The first ANOVA showed that congenitally blind (CB) participants had a higher accuracy in detecting temperature changes than the normally sighted (NS) (d′: CB = 2.4 ± 1.0, NS = 1.8 ± 0.7; F = 5.903, df = 1, p = 0.025).
Further comparisons indicated that all participants –regardless of the group– performed better when using the large 9 cm2 thermode (d′: 2.56 cm2 = 1.8 ± 0.7, 9 cm2 = 2.5 ± 0.7; F = 7.636, df = 3, p < 0.001).
Discussion
The aims of this study were to test whether congenitally blind individuals (1) are better in discriminating small increases in innocuous warmth and (2) are more prone to spatial summation effects of heat. In accordance with our hypothesis, results showed that congenitally blind participants are better than the normally sighted at discriminating temperature changes, regardless of the amplitude of the temperature increase or the spatial extent of the stimulated area. Our findings further
Conclusions
Altogether, our findings indicate that congenitally blind individuals’ hypersensitivity to nociceptive thermal stimuli extends to innocuous warmth and add to a growing literature on cross-modal compensatory plasticity in congenitally blind individuals [1]. An improved capacity for thermal information processing may help blind individuals in object recognition based upon thermal diffusivity characteristics of materials [11], [12]. Our data therefore suggest that when sight is absent since birth
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
HS, MP and RK conceived and designed the experiments. RK and MP contributed with experimental equipment and analysis tools. HS performed the experiments and the data analysis. HS, MP and RK wrote and edited the manuscript. All authors read and approved the final manuscript.
Acknowledgments
We thank Ricarda Gmeinder for help with the data collection and Prof. Leon Plaghki for in-depth discussions. The Lundbeckfonden and the Harland Sanders Foundation have supported this research. Thanks to the Vision Research Network, the Faculty of Graduate Studies of the University of Montreal, and the Ministry of Education of Quebec for their support. We are grateful to all the participants to this study.
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