In addition, an effect of tempo was systematically obtained for all the subjects. The
thresholds for very rapid sequences (80 ms IOI21.43 per cent) were always higher than
the thresholds obtained with slower tempos ranging from 300 to 1000 ms IOI (7.6 per cent
on average), for which the different thresholds remained proportional to IOI. This finding
confirmed results of a previous study carried out in healthy subjects^34 and is in agreement
with the assumption that brief auditory information is processed differently from longer
information.^35 –^37
In a previous paper,^39 we proposed that the decreased sensitivity observed for the fastest
tempo could be related to an auditory memory saturation. According to echoic memory
studies (Ref. 40; see Ref. 41 for a review), acoustic properties of sounds necessitate a
200 – 300 ms delay following the presentation of a tone to be adequately retained. This time
delay was obviously shorter for the fastest tempo (80 ms IOI). Indeed, the interval between
the two sequences to be discriminated was twice the base interval, in order to maintain
isochrony across the trial. Thus, the inter-sequence interval for the fastest tempo (80 ms
IOI) was 160 ms whereas a minimum of 600 ms separated the sequences for the other tem-
pos. Therefore, a saturation of echoic memory could have been responsible for the
observed difference between the thresholds of the fasted tempo and the slower tempos,
because the inter-sequence interval would be too short to allow consolidation of the first
sequence. If such an explanation is validated in the future, the deficit documented in
patients with LTL dysfunction in fast sequential auditory perception could be interpreted
as an echoic memory dysfunction. Although an eventual relation between a temporal pro-
cessing dysfunction and an auditory memory deficit in dysphasic children has already been
suggested by Tallal and collaborators,^42 the possible link between an echoic memory dys-
function and an anisochrony discrimination deficit of fast auditory information in patients
with LTL lesion remains to be clarified.
209Figure 14.2 Relative thresholds of the three groups of subjects (LHAleft hippocampal atrophy, RHAright
hippocampal atrophy and NCNormal controls) for Inter Onset Intervals (IOI) of 80, 300, 500, 800, and
1000 msec. The bars represent the standard error of the mean.
Relative threshold (% IOI)NC
RHA
LHATempo (IOI in ms)80 300 500 800 10000510152025303540