Impaired pitch identification as a potential marker for depression

Cognitive deficits in depression arise among others in the auditory domain [1]. Thus, some authors considered impaired auditory processing as a potential marker for depression [2, 3]. There is a controversy, however, whether cognitive dysfunctions in depression are specific [4] or general [5, 6]. Variable findings of correlations between depression and pitch perception may indicate the involvement of specific impairments: depressive patients responded less [1], similarly [7], or even more accurately [8] than healthy controls in pitch perception tasks. The variable outcomes suggest that heterogeneous processes may be differentially affected during a depressive episode. The present study aimed to confirm that a specific pitch perception skill - pitch identification - and not pitch perception in general is impaired in depressive patients.

Three different skills contribute to pitch perception: pitch identification, pitch contour perception, and pitch discrimination [9, 10]. There are reasons to hypothesize that depression impairs specifically pitch identification: Pitch identification was assessed using a choice reaction task [11], and visual choice reaction was poor in depression [6, 12, 13]. Moreover, pitch identification activated the intraparietal sulcus which was associated with numerosity encoding [11], and non-verbal encoding strategies were impaired in depressive patients [14]. In contrast, little data support that depression affects pitch contour perception or pitch discrimination.

The test design can bias outcomes in a clinical study. For instance, too easily distinguishable pitches could level out differences due to a ceiling effect. A ceiling effect may explain why Knott et al. found no impairment of auditory perception in depression [7]. Indeed, depressive patients performed poorly only during effortful tasks [15]. A second reason for variable outcomes may be a speed-accuracy tradeoff in depressive patients as described by Kalb et al. [8]. Thirdly, reduced performance may reflect low motivation to respond, i.e. poor ‘responsiveness’ [1, 16]. To minimize misinterpretations due to such confounding factors, auditory tests in depression should employ difficult tasks and monitor speed-accuracy tradeoff and responsiveness.

The present investigation applied pitch identification, pitch contour perception, and pitch discrimination tasks which reflected different auditory skills and neurophysiologic processes [10, 11, 17]. The tests were adjusted to high task difficulty by selection of small pitch variations such that the tests were sensitive to impairments [10, 15]. To detect a potential speed-accuracy tradeoff, reaction times were recorded. Moreover, we considered the sum of correct and incorrect responses as measure of responsiveness.

Based on the quoted literature the hypothesis was tested, that auditory accuracy during pitch identification but not during pitch contour perception and pitch discrimination is impaired in depressive patients. This finding was expected to emerge independent of potential covariates, i.e. speed-accuracy tradeoff, responsiveness, motivation as well as sex, age, musical experience, education, and smoking.

Most data approximated a normal distribution (S-W > 0.93, p > 0.110) but the sum of correct and false responses in pitch identification (S-W = 0.57, p = 0.001) and self-rated motivation scores (S-W = 0.90, p = 0.025) yielded a ceiling effect. A task-specific impairment emerged (interaction depression x tasks: F(2,44) = 3.3, G-G = 0.98, η² = 0.13, p = 0.047). Post-hoc tests revealed that depressive patients identified pitch less accurately than controls without impairment in pitch contour perception and pitch discrimination (Table 2); during pitch identification, depressive patients yielded 24% fewer correct responses than non-depressive participants. Depression did not slow-down response times (interaction depression x tasks: F(2,44) = 0.4, G-G = 0.96, p = 0.620). No significant speed-accuracy tradeoff emerged during pitch identification (r(24) = −0.19, p = 0.368) and pitch contour perception (r(24) = −0.22, p = 0.297) but during pitch discrimination (r(24) = −0.47, p = 0.020). Responsiveness did not differ between depressive and non-depressive participants (interaction groups x tasks F(2,22) = 0.5, G-G = 0.96, p = 0.575; main difference between groups F(1,11) = 0.1, p = 0.793). Thus, responsiveness cannot explain differential accuracy scores. Self-rated motivation was identical in depressive and non-depressive participants (Z = −0.1, p = 0.910).

The analysis of further covariates revealed a sex effect. Female but not male depressive patients performed poor in pitch identification (female vs. male depressive patients: Z = −2.2, p = 0.025; female patients vs. female controls: -14.5 ± 9.5 with 45% fewer correct responses, one-tailed Z = −2.4, p = 0.031; male patients vs. male controls: -0.8 ± 10.6, one-tailed Z = −0.1, p > 0.500). Differential correlations of BDI depression scores with pitch identification reflect this sex effect: A high BDI score predicted inaccurate pitch identification only in women (r(12) = −0.82, p = 0.001; β = −0.73, R² = 0.68) but not in men (r(12) = −0.10, p = 0.745; β = −0.06, R² = 0.01; see Figure 1). Types of drugs did not differ between sexes (χ² = 0.4, p = .649). The difference in body weight failed significance (women: 70.6 ± 15.0 kg, men: 86.0 ± 14.0 kg, Z = 1.4, p = 0.180) thus an interaction between sex and relative dosage appears unlikely. Finally, impaired pitch identification was not significantly associated with age (r(12) = 0.28, p = 0.379), musical experience (Z = −0.6, p = 0.564), education (χ² = 5.4, p = 0.062 with a tendency to a lower effect in high-educated participants), or smoking (Z = 1.1, p = 0.299). Thus, depression and no other factor led to impaired pitch identification in women.

Pitch identification was selectively impaired in depression. Pitch identification differed from other pitch-related skills, and this supports a model of circumscribed cognitive deficits rather than a global impairment. The choice reaction task used for assessing pitch identification activated the intraparietal sulcus [11] and not memory-related mesio-temporal or frontal regions [23]. The intraparietal sulcus was associated with the encoding of numerosity [24]. Conceivably, numerosity contributed to poor performance of depressive patients during effortful tasks in the study of Hamar et al. [15]. These authors increased effort by applying a higher number of stimuli. In view of these findings, we suggest that poor pitch identification reflects a reduced mental representation of numerous alternatives in depression.

We could not confirm a bias from speed-accuracy tradeoff, responsiveness, and motivation. Though several studies reported a slowing in depression (e.g. [6]), choice reaction time remains a controversial issue: neither the present study nor Chase et al. [25] found a deceleration. Possibly, speed becomes less important in difficult tasks [26]. Malone and Hemsley [1] hypothesized that demand characteristics of the task enhance responsiveness, which was not supported in the current study. Depressive patients performed worse during the pitch identification task, which demanded a response in each trial. The total number of responses did not differ between depressive and non-depressive participants. A previous study did not find a generally reduced responsiveness as well because false positive responses and omissions counterbalanced each other [27]. Finally, the data did not support low motivation as origin of cognitive impairment [16] but a distinction of neuropsychological deficits from motivation in depression [28].

Inaccurate pitch identification emerged in women but not in men. This sex effect was unexpected. In view of the low number of participants, this finding is exploratory and needs a replication. Another study reported hearing impairment in male depressive patients [29] – though on a very basic sensory level and not directly comparable to the pitch identification task. The test which presented the lowest pitch differences in the present study – pitch discrimination – did not distinguish depressive and non-depressive participants. Therefore it is unconceivable that sex interacted with pitch identification due to different perception thresholds. Poor pitch identification in depressive women resembled the outcome after serotonergic medication in healthy (male) participants [17]. Serotonergic neurotransmission and specific auditory impairments in depression may be associated; in this regard, differences between women and men have not been thoroughly investigated so far.

Auditory processing has been considered as marker of depression reflecting an underlying serotonergic dysfunction [30]. In the present study, depression in women impaired pitch identification; the effect was task-specific, not biased from reaction time and motivational factors, and persisted despite treatment. Thus, we suggest that inaccurate pitch identification merits attention as a potential marker for depression in women.