Although patients with LTL lesion presented a clear deficit in perceiving temporal
microvariations, they seem to be able to generate temporal expectations to the same extent
as healthy subjects.
To summarize, the results of this study illustrate a dissociation between a selective deficit
in fine grained temporal sensitivity whereas temporal expectations seem to be relatively
spared in the presence of LTL lesions, suggesting that top-down (or descending) process-
ing may function independently from bottom-up (or ascending) processing. In addition,
these data indicate that top-down processing involved in the temporal perception of
musical information can still be preserved despite disrupted bottom-up processing.
Conclusion
The experiments reported in this paper were designed to test the hypothesized role of LTL
structures to musical temporal processing. We focused our interest on the perception of
brief sequential information ranging from few tens to hundreds of milliseconds by com-
paring results obtained with very simple auditory sequences and real musical excerpts. In
keeping with our predictions, the data reported in this paper suggest that LTL structures
are predominantly involved in perceiving inter-onset interval increments in familiar tunes
as well as in isochronous sequences, extending to the musical domain findings previously
reported in speech.12,43
The patients included in both studies had lesions located within the medial temporal
lobe structures and more specifically in the hippocampus. It remains difficult to know if
the deficit could be attributed to the hippocampal lesion or to a more global dysfunction
of the temporal neocortical structures. Evidence from a functional cerebral imaging study
(Positron Emission Tomography with 18 Fluorodeoxyglucose, FDG-PET) carried out in a
similar population of epileptic subjects indicates that the temporal lobe dysfunction asso-
ciated with unilateral hippocampal sclerosis is not restricted to the hippocampus but
extends to include the anterior part of the temporal neocortex corresponding to the tem-
poral pole.^51 Although the structural lesion is apparently limited to the hippocampal forma-
tion, it does not mean that the connections between the medial temporal lobe structures
and the neocortex or the surrounding and external temporal cortex per seremain func-
tional. The present data does not allow to differentiate the role of the hippocampus from
the contribution of the adjacent cortical areas in auditory sequential information process-
ing. However, it seems relevant to suggest that abnormal metabolism reported in these sur-
rounding cortical structures reflects a temporal lobe dysfunction or an interruption of the
hippocampo-neocortical loop that may be responsible for the deficits obtained in our
patients with LTL dysfunction.
In the present studies, the time-related deficit following LTL lesion was reproduced in
two different tasks involving purely temporal stimuli as well as multidimensional musical
sequences in the context of an adaptive or a fixed level paradigm, respectively. Despite
numerous methodological differences, the results of these two studies provide strong sup-
port for the selective sensitivity of LTL structures in processing rapid time-related events.
Indeed, the two methodologies require the detection of subtle temporal variations which
was made possible by the use of regular sequences or very familiar tunes producing strong
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