It's been a while since I did one of these, hasn't it? But sometimes they just hop onto the keyboard and start meowing in my face.
In this paper, Ryugo et al. are mostly concerned with structures at the synapses in certain nerve fibers, rejoicing in the name of "the endbulbs of Held", which they describe in fairly flowery language: "Endbulbs have a calyxlike appearance that is formed from the main axon as several gnarled branches that arborize repeatedly to enclose the postsynaptic cell in a nest of en passant swellings and terminal boutons." These are abnormal in congenitally deaf animals: they don't branch so much, they're enlarged, they've got a flat rather than an undulating profile, and they've got fewer of the vesicles containing neurotransmitters that make synapses work. Not surprisingly, these endbulbs don't seem to transmit signals very well. This is a problem, especially since the nerve pathways where they tend to be found are the ones which encode precise timing information about sounds, important alike for for predators fond of twilight and leaping in ambush, and for chattering East African Plains Apes ("The critical nature of temporal resolution in facilitating speech recognition is underscored by studies that show speech recognition based on temporal cues while spectral content is systematically degraded").
What they did was to take congenitally deaf cats and, as kittens, give them cochlear implants which restored their capacity to hear. The physical capacity was verified by recording the propagation of neural signals; also by the fact that "we could routinely 'call' implanted cats for a food reward." After several months, they examined the development of the end-bulbs of Held in these cats, compared to matched normal animals, and to congenitally deaf cats which received no implant. (Don't ask how.) The results, photographically, are pretty convincing: the endbulbs look a lot more like those of normal cats than deaf, non-implanted cats, and quantitative comparisons of e.g. size are also fairly persuasive.
It would be interesting to know how old cats can be before simply providing the cochlear implant isn't enough for these synapses to develop properly. They speculate (but don't really show) that the same effect takes place in people, and that this is why congenitally deaf children benefit more from implants the earlier they get them. If that's so, it would just reinforce the importance of making sure all the children who need them get such implants swiftly. It would also make it nice to know what (if anything) could be done in conjunction with such implants, to help gnarl-up children's endbulbs.
Notice, by the way, that, as I've had occasion to remark before, that the whole nature-nurture division is not actually useful to understanding what's going on in processes like the development of hearing in these cats. (Go on, calculate a heritability here and tell me it means something, I dare you.) But this is generally the case with cognition.
Posted at December 30, 2005 23:08 | permanent link