Science of Aphasia 5 Cross Linguistic Aspects of Aphasia ...

DISCRIMINATION OF SIMILAR ENVIRONMENTAL SOUNDS FOLLOWING STROKE:
LEFT HEMISPHERE PATIENTS FAIL TO USE SEMANTIC FACILITATION
Alessandra LORENZI, Luigi A. Vignolo
University of Brescia
Auditory nonverbal agnosia (auditory agnosia sensu strictiori) consists in the selective inability to recognize
nonverbal sounds. Carl Kleist (1928) maintained that the inability to perceive isolated sounds or noises
(perceptive Geräuschtaubheit) should be distinguished from the inability to understand the meaning of noises
(Geräuschsinntaubheit). Quantitative behavioural studies carried out since the 1960s (see Vignolo, 1982;
Schnider et al., 1994) confirmed this dichotomy and investigated its anatomical bases. They led to define the
existence of two types of auditory agnosia: an acoustic-discriminative one, preferentially associated with lesions
of the right hemisphere and a semantic-associative one, specifically associated with lesions of the left
hemisphere and aphasia. The former consists of imperception for the acoustic structure of sounds. The latter
consists of defective recognition of the meaning of sounds and may be considered to be a cognitive rather than a
perceptual disorder, involving one or several sensory modalities and consisting in the inability to put together
different aspects of the same concept.
The problem with all these studies was that acoustic discrimination and semantic recognition were investigated
by tests involving different sensory modalities. Discrimination was tested by a purely auditory same/different
test, while identification was tested by an auditory-visual multiple choice test. This difference made it difficult to
compare results.
The present study was undertaken in order to verify the right-left difference employing same/different
discrimination tests strictly confined to the auditory modality. We wished to investigate the ability to
discriminate pairs of acoustically similar environmental sounds, with or without semantic help. We assumed that
the discrimination between two acoustically similar sounds should be facilitated when they originate from two
different semantic sources ( e.g. miaowing cat– crying baby ) as opposed to when they come from two sources
belonging to the same semantic group ( e.g. cat 1 – cat 2 ). In the first case the semantic difference adds to the
acoustic difference in order to permit the discrimination, while in the second case no such facilitation is present.
If this is true, patients with right hemisphere damage (who can identify the meaning of sounds) should be able to
take advantage of the semantic help, like normal controls, while patients with left hemisphere damage (who have
problems in identifying the meaning) should not be able to make good use of this help.
A new test, the Environmental Sounds Discrimination Test, was administered to 20 controls and 12 unilateral
hemispheric stroke patients (5 RBD [right brain damaged], 7 LBD [left brain damaged]) harbouring a small or
medium sized lesion (diameter less than or equal to 3 cm) on the CT scan. Exclusion criteria were: a history of
hearing difficulties before stroke, multiple lesions, trauma, neurodegenerative diseases. The test consists of 40
items, each made up of a pair of meaningful environmental sounds. 14 items are pairs of acoustically similar
sounds, originating from different semantic sources (e.g. applause-tape-writer): they constitute Task 1. 14 items
are pairs of acoustically similar sounds, originating from sources belonging to the same semantic category (e.g.
thunder 1 – thunder 2; wind 1 – wind 2): they constitute Task 2. The remaining 12 items are pairs of identical
sounds (e.g. hen-hen). The patient was told that he was going to hear two familiar noises one after the other. He
was asked to listen to them carefully and to say whether they where the same or different.
We expected that a. controls would perform both tasks well, but Task 1 (with semantic help) better than Task 2 (
without semantic help ); b. RBD patients would perform both tasks much worse than the other groups but, like
controls, Task 1 better than Task 2 (because both controls and RBD use the semantic help); c. LBD patients
would perform both tasks better than RBD, without significant differences between Task 1 and Task 2 (being
unable to use the semantic help).
Results confirmed our expectations. The total number of errors made by RBD patients were double with respect
to the LBD and more than triple with respect to controls. Both controls and RBD patients could easily
discriminate among similar sounds originating from different semantic sources (Task 1). Virtually all errors
made by these two groups concerned the discrimination of sounds produced by the same semantic source (Task
2). This indicates that in both controls and right brain damaged, the semantic help makes acoustic
discrimination significantly easier. By contrast, LBD patients were the only group in which a remarkable
number of errors in Task 1 was found. This indicates, in our view, that semantic help is less efficient in LBD than
in RBD and controls.
In conclusion, this study, carried out with a test specifically confined to the acoustic sphere, provided further
evidence of the semantic impairment typical of the left damaged subjects.