So, on Day 3 after activating my other ear, here’s what I know so far:
1. The implant works. Good strong neural response to all electrodes.
2. It can understand speech when aided by reading cues, and it gets bits and pieces of uncued speech.
3. It helps a lot in noise.
So today I’ve been wondering why speech sounds so eerily real but not yet understandable, for the most part. I’m not impatient; it took weeks for me to get to this point with the other ear, back in 2001. Progress has been terrific. But it’s a neurological mystery. Today I’ve done some detective work.
To start with, I decided to listen to Winnie-the-Pooh and Some Bees to figure out what, exactly, my right ear is hearing.
My overriding impression is that speech sounds abrupt and shortened – as if words that actually take 100 milliseconds to say had been shoehorned into 70 milliseconds. Another way to put it is that it feels like pieces of the words are missing.
It’s like watching a squirrel. Instead of smooth glidings from one position to the next there’s discontinuous jumps, as if random frames had been edited out of reality.
But the “editing” isn’t so random. I listened closely to one passage and wrote down what it sounded like to me. Here’s the original:
It is, as far as he knows, the only way of coming downstairs, but sometimes he feels that there really is another way, if only he could stop bumping for a moment and think of it. And then he feels that perhaps there isn’t. Anyhow, here he is at the bottom, and ready to be introduced to you. Winnie-the-Pooh.
And here’s what I hear with my right ear:
It is, as fur as he knuzz, th’ only wy of coming downstrrrs, but sumtimes he fils that there rilly is another wy, if only hih could stop bumping for a moment and think of it. And then he fls that purhaps there iznt. Unyhw, here hih is at the bottom, and ready to be intriduced to yu. Winnie-the-Pooh.
Instead of hearing that long orotound o – as in knooows – I hear an abrupt foreshortened version of it: knuzz. It’s not that the word is actually shorter, although it sounds that way. It’s that the percept of the o is missing or incomplete. The speech sounds brittle and chopped up.
It throws me off enough that I can’t follow the sense of the soundstream. But, as you can see, there is plenty remaining, which makes it very easy to follow if I’m reading along with the text. And peculiarly, there are isolated instants of lucidity: the long o in Pooh comes through loud and clear.
Now, here’s the crux of it. I listened to the same passage just now with my left ear, which has been online and working since 2001. All the vowels were in their proper places sounding like their normal selves. Nice long o’s, good proper even a’s. I can understand the tape perfectly well. So the vowel-decoding software is functioning just fine in my brain.
What’s going on? Let’s consider the possibilities.
Possibility 1: The right ear’s software is not giving me vowel information. I doubt this. It’s the same ‘ware as is running in my left ear – 16-channel Hi-Res P – and the map is approximately the same. If I could put my right processor on my left ear (which I can’t, as each processor is keyed to one specific implant), my left ear would hear pretty much the same as it’s hearing now. No, the information has to be there. Which brings us to –
Possibility 2: My auditory nerves aren’t picking up certain kinds of information. They’ve been unused for many years. But neural atrophy seems to be only a partial explanation at best, because I’m hearing consonants and environmental sounds. Not only that, neural response telemetry showed that the nerves are responding strongly across the board.
Possibility 3: My brain is getting the information but doesn’t know how to use it. It seems to me that this has to be the case. But why, if my brain already knows how to interpret neural input coming from 16 electrodes refreshing themselves 5,156 times per second?
It’s well-known that the right ear sends most of its signal to the left hemisphere for high-level processing, and that the hemispheres have only partial access to what each other knows. But there is significant crossover as the signal ascends neural pathways to the auditory cortex. A quick glance at a diagram from Yost’s Fundamentals of Hearing makes that clear. (It’s the 4th edition, 2000, p. 228.)
The cochlear nucleus – the auditory’s nerve’s terminus in the brain – sends signals to the right superior olivary complex as well as the left one. The inferior colliculus on both sides are connected to each other, too. Basically, the right ear’s input makes its way to the auditory cortex on both sides, although the path on the right side is more convoluted.
I’m guessing that the echo I’ve been hearing comes from crosstalk between the right and left halves of my auditory system. As I noted in an earlier entry, my left ear rings whenever sound goes into the right ear. Right now, as I’m typing, each keystroke is followed by a bright ringing sound in the left ear that takes 3 to 5 seconds to fade.
This is a mystery for two reasons. First, why is it happening at all? And why do I hear the ringing in the other ear this time, whereas before, in 2001, I heard it in the implanted ear?
One wild idea I’m having is that some of the missing vowels are in that ring, somehow. (It’s reduced by about 50% when the left ear is active, by the way. Then it sounds like a brief bright chime.) The ringing did go away last time – so I’m paying careful attention this time to see if it happens again, and when.
To answer Jeff’s question in my last post, pitches do sound different in both ears. Music sounds different, in that certain pitches I hear with my left ear are less “pure” in my right ear. I’m not sure which pitches are different, because I tend to be poor at identifying pitch, especially on Hi-Res 16. (I’ll explain in a later post why I’m using Hi-Res 16 in both ears at the moment instead of Fidelity 120; it’s a strategic decision to scale down the complexity of mapping two ears.)
I spent about an hour studying Fundamentals of Hearing this afternoon. Now I’m going to try a different tack: I’m going to walk about town and hang out at Dubliner’s on 24th Street for a while, and see what happens. Perhaps a pint of Fat Tire will bring illumination.