We used the DTT to identify a population of individuals with tune deafness. Several factors could have been responsible for low DTT scores in this group. One likely source of a low DTT scores is hearing impairment, and indeed, seven subjects (8%) were disqualified in the Phase I screening portion of the study because of hearing deficits. Another source of poor performance could have been deficits in attention. Our results suggest that ADD/ADHD was common among the tune deaf participants and was also present in the control group. However, the low correlation of scores on the TOVA with those on the DTT suggests that attention deficits are not the primary source of low DTT scores. A third possible source of low DTT scores could have been memory deficits. Such deficits may have caused difficulties in judging whether or not melodies are played correctly. Because we wished to examine memory as broadly as possible, we used well-characterized tests outside the musical domain, as musical memory tests have been employed previously in congenital amusia (Ayotte, Peretz, and Hyde, 2002
). As evaluated by rapid naming tests, long-term memory was found to be normal in our tune deaf group. Short term auditory memory was also normal in our subjects. Our results, combined with previous results obtained in a smaller number of individuals with congenital amusia (Ayotte, Peretz, and Hyde, 2002
), indicate a broad array of memory functions remain intact in tune deafness. Our data support and extend previous findings that have concluded that abnormalities in pitch perception underlie deficits in the perception of music (Ayotte, Peretz, and Hyde, 2002
; Hyde and Peretz, 2004
; Koelsch and Siebel, 2005
). In addition, we found that deficits exist in a range of auditory pitch discrimination and frequency discrimination abilities in tune deaf individuals. Our tune deaf group had significantly larger DLFs than the normal control group. This measure is distinct from a number of other pitch recognition tests employed previously. Previous tests have measured gliding pitch detection, pitch sequence difference detection and a number of other dynamic stimuli (Foxton et al, 2004
). The DLF, which uses one variant pure tone and a reference pure tone, measures a fundamental aspect of auditory processing that may involve other, subcortical areas of auditory processing. In addition, our tune deafness group performed significantly poorer on pitch pattern recognition, gap detection, and duration pattern recognition tasks.
Individuals in our study were identified as tune deaf because they reproducibly scored 18 or below on the DTT. However, there was wide variation in DLF and PPT scores in these tune deaf individuals. In particular, a number of participants with tune deafness performed within the normal range on both of the DLF and PPT. This suggests that the tune deaf population is heterogeneous in its ability to process both frequency and pitch. It may be that there are two types of tune deaf individuals: those who have deficits in pure tone frequency discrimination and those who have normal frequency discrimination. Further analysis of the tune deaf individuals who have normal pure tone frequency discrimination could lead to the identification of new aspects of sound perception and sound processing in humans.
The heterogeneity in DLF scores among our participants with tune deafness raises the question of whether frequency discrimination was a major determinant of performance on the other auditory processing tests we administered. If so, we would expect DLF scores would be correlated with auditory processing test scores. However, this was not the case, and the participants with tune deafness with normal DLF scores did not perform significantly different than those with abnormal DLF scores on any of the auditory processing tests.
Our frequency discrimination test differed somewhat from tests used in previous studies of pitch recognition, which have used other procedures and stimuli (Peretz, Ayotte, Zatorre, Mehler, Ahad, Penhune, et al., 2002
). To focus on frequency perception rather than musical pitch perception, our DLF test employed non-musical frequencies, using half-log frequency differences rather than semitone musical pitch differences. The frequencies of the tones used in the DLF did not correspond to any standard notes on the western musical scale. At the 1000 Hz frequency of our reference tone, the mean DLF of our tune deaf subjects was 35 Hz, which corresponds to approximately one half semitone (35 Hz vs. 59 Hz for the difference between the notes “B” and “C”) in this frequency range. This is in close agreement with the degree of musical pitch perception impairment reported in previous studies of amusic individuals (Hyde and Peretz, 2004
). Our adaptive paradigm and stopping criteria incorporated subject training within the test to account for learning effects. Trials and trial runs were repeated until subjects displayed no further improvement and stable performance.
Our DLF test employed pure tones rather than the complex (piano) tones employed in other studies (Peretz et al., 2002
). This helped to insure that our study was specifically measuring frequency perception, and not pitch perception influenced by overtones, timbre, or other acoustical information present in complex tones.
We expected our subjects could show learning effects during the course of the DLF test, and that their DLF thresholds would decrease over repeated trial runs (Foxton et al., 2004
). We did not observe this, which suggests that in agreement with previous studies, learning did not significantly affect frequency discrimination using our test paradigm (Foxton et al., 2004
). Thus, our DLF test appears to measure an innate ability in adults, consistent with that suggested by other researchers (Peretz, Ayotte, Zatorre, et al., 2002
The other notable aspect of our DLF test results was the inter-trial variation in performance by tune deaf participants. The minimum detectable frequency difference varied widely across successive trial runs in this group. This outcome was unlikely to have come from stochastic variation for two reasons. First, this variation did not occur in normal controls. Normal subjects displayed very stable frequency discrimination across successive trial runs (a mean of 8.7 Hz), with a low standard deviation. Second, each trial consisted of a minimum of 42 test stimuli, with a minimum of 14 reversals of stimulus presentation. This constituted at least seven successive approaches to the respondent’s minimum detectable difference during each trial run. The extent of variation during the successive trial runs suggests that frequency perception may be unstable in tune deaf individuals within the time frame of this test. While a fraction of the tune deaf individuals performed in the normal range on the DLF test, the converse was not true, in that none of the normal control individuals scored in the abnormal range. This suggests the DLF test is more robust for confirming normal pitch perception status than it is for demonstrating musical pitch perception deficits.
Temporal processing is of particular interest in tune deafness. Temporal measures such as rhythmicity have been previously examined in a limited group of amusic subjects (Hyde and Peretz, 2004
) and found to be largely normal. The DPT and the PPT both reveal deficits in temporal order processing in our tune deaf subjects. Poor pitch pattern recognition was expected in the tune deaf group, but poor duration pattern recognition was not. However our results may not represent a contradiction with the results of Hyde and Peretz, as temporal order processing measured by the DPT and the PPT is quite different than the comprehension of rhythm in music, which is a substantially different stimulus compared to those in the DPT and the PPT. Because musical rhythm is characterized by a continuous and repetitive auditory stimulus, it could well employ functions different from those used in the discrete and separated stimuli in the temporal order tests we used.
The GIN also measures a form of auditory temporal resolution. It is important to note that all participants in both the tune deaf group and the normal control group performed within the normal range. However the scores were significantly poorer in the tune group compared to the control group based on both the percentage of correct answers and on gap detection thresholds. While it is unlikely that such small differences are significant in a clinical audiology setting, they reinforce the view that temporal processing is affected in tune deafness.
Auditory temporal processing thus appears to be affected in several different ways in tune deafness. The DPT measures ordering of auditory stimuli over the course of trials about one second in length. The GIN measures the detection of millisecond-length gaps in noise, and thus the DPT and the GIN test perception on very different time scales. Thus the functional extent of the deficits and the sparing of auditory temporal processing in tune deaf/amusic individuals is now a question of some interest..
Previous studies have provided evidence for differences between brain hemispheres in the processing of musical sounds versus non-musical sounds (Peretz, 2001
, Koelsch et al 2002
), so it was of interest to determine whether asymmetries in auditory processing would be significant in individuals with tune deafness. In the two auditory processing tests in which each ear was tested separately (the PPT and the DPT), the performances were not different between ears, and thus we saw no evidence for lateral asymmetry in tune deaf subjects.
Because scores on tests of memory were normal in the tune deaf group, we expected that scores on memory tests would show only weak correlations with their scores on any of the auditory tests. This was generally the case, however scores on the Rapid Dignit Naming and Rapid Object Naming tests often showed moderate negative correlation with the pitch and auditory processing tests in tune deaf individuals (). Taken together, our results raise the possibility that not only is memory unimpaired in tune deaf individuals, it may be modestly enhanced.
Correlations between other tests provide some additional intriguing findings. Scores on gap detection, pitch pattern detection, and duration pattern detection showed low correlation with each other in tune deaf subjects. This suggests that although these functions are impaired in tune deaf individuals, these deficits are not obligatorily related. In addition, scores on the DTT did not show high correlation with DLF scores in tune deaf individuals. This is consistent with our observation that many tune deaf individuals score normally on the DLF. It also suggests that tune deafness, which is the inability to perceive the correct relation between successive pitches, is not the same as a deficit in pure tone frequency discrimination. Finally, scores on both the DTT and the DLF showed substantial correlation with scores on the duration pattern, pitch pattern, and gap detection tests. This reinforces the view that perception of auditory temporal information is often co-impaired with pitch and frequency perception.
Deficits in performance on a number of different auditory tests have been observed previously in amusic subjects, at it has been suggested that such deficits may form a cascade originating with a congenital pitch processing defect (Peretz et al, 2002
; Foxton, Dean, Gee, et al, 2004
). At the current time, it is not clear which of the many auditory deficits we have identified are primary in this disorder and which are secondary. They could be related to each other in a cascade fashion, or they could be associated in other ways.
We hypothesized that tune deafness is a disorder that extends to other auditory functions beyond musical pitch recognition. On all measures of auditory processing that we examined, the tune deaf group scored significantly poorer than the normal controls. Our results support the view that both frequency (tonotopic) information and timing (temporal) information play an important role in the processing of both music and non-musical sounds. It is clear, however, that tune deaf individuals vary widely in their auditory processing functions. Our results suggest that future stratification of the tune deaf population may provide additional insights into these aspects of auditory processing.