Speech and language development is a complex process influenced by biological and social factors. Proper language acquisition requires accurate sound reception, dependent on a correctly functioning auditory system, from peripheral (conductive and perceptive hearing apparatus) to cortical levels. Auditory processing disorders (APD) occur when the brain, despite receiving correct auditory information, fails to process sounds accurately, causing difficulties in understanding and memorizing sounds, especially verbal information. This can hinder language development, leading to difficulties in reading and writing. APD is frequently diagnosed in early school-age children with normal peripheral hearing who struggle with auditory problems. Auditory discrimination, a higher auditory function involving the perception of differences in sound volume, duration, and frequency, plays a crucial role in speech development. Assessment methods include electrophysiological techniques (like Mismatch Negativity) and behavioral methods (like the Frequency Pattern Test, FPT), often using nonverbal stimuli. Difficulties in sound frequency differentiation can lead to deficits in phoneme discrimination, impacting speech comprehension. While phoneme discrimination develops intensively from the end of the first year of life, the exact age at which this development concludes remains unclear. This study aimed to examine auditory discrimination in children with and without APD, using the FPT and a phoneme discrimination test (PDT) to determine the necessity of including PDT in APD diagnostic standards. The hypothesis was that children with APD would perform poorly on phoneme discrimination tasks and that phoneme discrimination continues to develop during school age.

Existing research on phoneme discrimination across different language systems doesn't definitively establish the age at which this ability fully matures. Studies have shown a relationship between higher auditory functions and language development, with phoneme discrimination crucial for understanding, acquiring, and producing speech. Clinical observations suggest that categorical perception of phonemes may remain immature even at school age, contradicting some reports suggesting that school-age children no longer experience difficulties. Previous studies on phoneme discrimination have varied in sample size, age groups studied, the number of phonological oppositions tested, and the languages used. Some studies have used word-based tests, potentially leading to a ceiling effect where even children with subtle deficits achieve high scores. Other studies have highlighted the relationship between phoneme discrimination skills and language proficiency, and the need to include phoneme discrimination tests in the diagnostic process for speech and language disorders, particularly APD. However, these studies often had small sample sizes or lacked age-specific analysis. The present study aimed to address these gaps by using a large sample size, employing age-specific analysis, and utilizing nonsense words to minimize the influence of language proficiency.

This study included 366 Polish-speaking children (aged 6-9 years) with normal hearing. Participants were divided into two groups: 220 with APD (based on at least two psychoacoustic tests showing deficits) and 146 TD children (with a maximum of one sub-normative result). Exclusion criteria included intellectual disability, developmental disorders (such as autism), neurological problems (such as epilepsy), and congenital genetic disorders. All participants underwent pure-tone audiometry, speech audiometry, tympanometry, and otoacoustic emissions testing to confirm normal hearing. Higher auditory functions were assessed using a battery of psychoacoustic tests from the ATS Neuroflow platform, including the Adaptive Speech in Noise Test (ASPN), the Dichotic Digits Test (DDT), and the Frequency Pattern Test (FPT). Phoneme discrimination was evaluated using a standardized test with 25 pairs of nonsense words, with 18 pairs differing by one phoneme and 7 pairs identical. A certified Neuroflow specialist conducted the psychoacoustic tests in an acoustically treated room, and a speech-language pathologist administered the phoneme discrimination test. Statistical analysis using MS Excel and Statistica for Windows involved non-parametric tests (Kruskal-Wallis ANOVA and post hoc tests) due to non-normal distribution of some data. Spearman's rank correlation and Pearson's correlation tests assessed correlations between FPT and PDT results.

The median FPT results for APD participants were more than twice as low as those of TD participants (20% vs. 50%, p < 0.05). This difference was consistent across all age groups. Nine-year-old APD participants performed significantly worse on the FPT than 6-year-old TD children (30% vs. 40%, p < 0.05). The median PDT results were also significantly higher for TD participants than for APD participants (24 vs. 21, p < 0.05). The PDT results showed age-related differences within each group, with older children scoring better. However, 9-year-old APD children performed worse than 6-year-old TD children on the PDT (median 22 vs. 23). Spearman's and Pearson's correlation tests revealed statistically significant, fair correlations between FPT and PDT results in both groups, slightly stronger in the TD group. The standard deviation (SD) of PDT results decreased with age in the TD group, indicating an equalization of phoneme discrimination skills, while the SD in the APD group remained higher, demonstrating imbalanced development.

This study's findings demonstrate a significant correlation between deficits in FPT and the presence of APD, with APD children consistently exhibiting more than twice the deficit in FPT scores compared to their TD peers. The consistent age-related improvement in FPT and PDT results across both groups reflects the physiological maturation of the central nervous system and auditory pathway. However, the persistent difference in performance between 9-year-old APD children and 6-year-old TD children in both tests underscores the continued development of auditory discrimination skills beyond the acquisition of phonemes. The use of nonsense words in the PDT made the results independent of language proficiency, improving the test's sensitivity in identifying children with even subtle auditory perception difficulties. This strengthens the argument for including the PDT in APD diagnostic procedures, enabling the creation of tailored therapeutic programs.

This study confirmed the hypothesis that children with APD have significantly more difficulty with phoneme discrimination than their TD peers. It highlights the ongoing development of phoneme discrimination during school age and the significant correspondence between FPT deficits and APD. The results strongly suggest the inclusion of the phoneme discrimination test (PDT), using nonsense words, in the standard APD diagnostic battery to enable more precise and individualized therapeutic interventions.

This study did not assess working memory capacity, which could influence FPT results. Future research should investigate the impact of cognitive factors on both FPT and PDT scores. A comparison with a group of children with Specific Language Impairment (SLI) would further enhance the understanding of auditory discrimination deficits in different populations. More sophisticated statistical models could be used in future studies to analyze the data more thoroughly.