Background
We have recently shown that visual deprivation from birth exacerbates responses to painful thermal stimuli. However, the mechanisms underlying pain hypersensitivity in congenital blindness are unclear.

Methods
To study the contribution of Aδ- and C-fibres in pain perception, we measured thresholds and response times to selective C- and Aδ-fibre activation in congenitally blind, late blind and normally sighted participants. Ultrafast constant-temperature heat pulses were delivered to the hand with a CO2 laser using an interleaved adaptive double staircase procedure. Participants were instructed to respond as quickly as possible when detecting a laser-induced sensation. We used a 650 ms cut-off criterion to distinguish fast Aδ- from slow C-fibre–mediated sensations.

Results
Congenitally blind participants showed significantly faster reaction times to C- but not to Aδ-fibre–mediated sensations. In contrast, thresholds for Aδ- and C-fibre stimulation did not differ between groups. Late blind individuals did not differ from sighted controls in any aspect. A follow-up experiment using only suprathreshold stimuli for Aδ- and C-fibre activation confirmed these findings and further showed that congenitally blind individuals detected significantly more C-fibre–mediated stimuli than sighted controls. A decomposition analysis of the reaction times indicated that the faster response times in the congenitally blind are due to more efficient central processing of C-fibre–mediated sensations.

Conclusion
The increased sensitivity to painful thermal stimulation in congenital blindness may be due to more efficient central processing of C-fibre–mediated input, which may help to avoid impending dangerous encounters with stimuli that threaten the bodily integrity.