together in pseudorandom order. Each subject also performed two control tasks, the WAIS-
III Digit Memory Span task, assessing auditory working memory capacity, and a simple
auditory detection reaction time task, assessing simple motor reaction time for detecting
audible sounds.
As shown in Figure 17.13, patients’ pitch discrimination thresholds were on average 5.5
times that for controls (p0.0001). The degree of impairment was correlated (r0.70,
p0.003) with the severity of patients’ pancerebellar ataxia. Furthermore, the sizes of
patients’ pitch discrimination thresholds were uncorrelated with their loudness thresholds
for the standard tone (r0.02,p0.79) and for the wider range from 125 to 8000 Hz
(r0.06,p0.22). Patient and control groups did not differ on digit span memory or
auditory detection reaction time.
As a group, then, patients with pancerebellar degeneration, but no other neurological
conditions, show strongly impaired pitch discrimination. However, the degree of impair-
ment for individuals is proportional to the severity of their pancerebellar ataxia. The sever-
ity of the pancerebellar ataxia (e.g. with symptoms in gait, speech, limb, and eye behaviour)
is assumed to be indicative of the degree of pancerebellar atrophy. Although these data do
not indicate the location of auditory processing regions in the cerebellum, it is assumed
that atrophy in auditory regions of the cerebellum is proportional to the pancerebellar
ataxia. Such atrophied auditory regions are assumed to be responsible for the pitch dis-
crimination impairment. These cerebellar patients’ auditory abilities are intact in a funda-
mental way, as indicated by reasonably normal auditory loudness thresholds. This
observation is consistent with the idea that primary sensory processing is subserved in
appropriate regions of cerebral cortex, which are spared in these patients. However,
patients’ loudness thresholds (ranging from normal hearing to mild hearing loss) are
uncorrelated with the degree of their pitch discrimination impairment. Central to the idea
of cerebellar involvement in sensory data acquisition is the suggestion that cerebellar effects
would be most clearly evident in tasks that require highly optimized sensory data. In the
current case, it seems reasonable to assume that the simple detection of the presence of a
tone is unlikely to require the same quality sensory data as the fine discrimination of pitch.
Thus, these results suggest that the cerebellum has a fundamental supporting role in the
information processing necessary for fine auditory discrimination. This conclusion is con-
sistent with cerebellar activations reported in recent neuroimaging studies involving pitch
261Figure 17.13 Average lower and upper difference discrimination thresholds for auditory pitch in pancerebellar
degeneration patients and healthy controls.^75
PatientsControls460 470 480 490 500
Differnce threshold (Hz)Standard = 500 Hz510 520 530 540