Two successive sequences of five tones with the same frequency, intensity, duration and
attack, and decay time were presented on each trial (one regular and one irregular). The
subject had to decide if the irregular sequence was in the first or second position (see Figure
14.1). Five conditions with an IOI of 80, 300, 500, 800, or 1000 ms were prepared in order
to test the effect of tempo on anisochrony discrimination. The anisochrony was introduced
by delaying either the second or the fourth sound of the isochronous sequence. To obtain
reliable measures of anisochrony discrimination, a psychophysical procedure developed by
Levitt^38 was used to determine the minimum temporal shift necessary for each individual
to discriminate the regular from the irregular sequence. The size of the first shift corre-
sponded to 10 per cent of the base interval (i.e. 50 ms for an IOI of 500 ms). The threshold
of the shift detection was determined by an adaptive procedure in which the subject’s
response on one trial determined the size of the shift on the next trial. The perceptual
threshold was then expressed as a percentage of the base IOI to allow comparison with
other differential thresholds reported in the literature. Further details about the methods
are provided in another paper.^39
Eighteen patients with medically intractable epilepsy, candidates for surgical treatment
at La Salpêtrière hospital (Paris) as well as a group of normal control subjects (n6), par-
ticipated in this study. They all presented medial temporal lobe epilepsy associated with lat-
eralized hippocampal sclerosis as identified by magnetic resonance imaging (MRI). None
of them suffered from language disturbances and language function was lateralized in the
left-hemisphere for all subjects. These patients were divided into two groups: those with
right hippocampal atrophy (n8) and those with left hippocampal atrophy (n10).
According to the literature, we hypothesized that the thresholds of patients with LTL
lesions would be significantly higher than the thresholds of patients with RTL lesions or
normal control subjects for tempos with IOIs at or below 300 ms. In contrast, no specific
deficit was predicted for slower tempos with IOIs greater than 300 ms. Finally, we predicted
that for all subjects the thresholds obtained for the fastest tempo (80 ms) would be higher
than those obtained for slower tempos (300 ms) as indicated by previous results in
normal listeners.
In keeping with our predictions, the results displayed in Figure 14.2 clearly showed that
anisochrony discrimination thresholds for the rapid tempo (80 ms IOI) were significantly
higher for the patients with LTL dysfunction (mean threshold: 27.5 per cent) than for the
patients with RTL dysfunction (17.7 per cent) and for the normal control subjects (16.4 per
cent). However, there were no differences between the groups for the slower tempos (from
300 to 1000 ms IOI). This finding is compatible with results of aphasic patients with left-
hemisphere lesion1,2,4and provides strong evidence suggesting that fast auditory sequential
information processing depends specifically on the integrity of LTL structures.208 Figure 14.1Example of the two sequences presented at one trial in the anisochrony discrimination task.
Sequence 1 Sequence 2