Introduction: Spoken languages speakers with Phonological Output Buffer (POB) impairments make phonological errors in production, repetition, and reading aloud of morphologically simple words and nonwords, whereas they make whole-unit errors (i.e., substitutions, omissions, and insertions of whole-units from the same category) in morphological-affixes, function-words, and number-words, a phenomenon called STEPS – Stimulus Type Effect on Phonological and Semantic errors (Dotan & Friedmann, 2015). We aimed to identify for the first time deaf signers with low/impaired POB and examine their pattern of errors, by testing their production, repetition and comprehension of morphologically complex structures in sign language.

Methods: We tested 26 deaf native-signers of Israeli Sign Language (ISL) using 5 sequence-recall tests we developed to identify LOOPS – participants with LOw Output Phonlogical Spans suspected to have impairment to the POB. We compared the performance of the LOOPS and the controls in 3 tests: production of classifiers and morphological facial-expressions, repetition of sentences with morphologically complex structures, and comprehension and production of agreement-verbs.

Results:  The LOOPS made more phonological errors than the controls in morphologically simple signs and in bases of morphologically complex signs, but whole-unit errors in number-signs, function signs, and morphological-affixes. Unlike the controls, the LOOPS did not show a recency effect in serial recall, mirroring the pattern reported by Vallar & Papagno (1986) for a hearing pWM-impaired patient.

Conclusions:  POB-impaired signers showed a similar error pattern to the one reported for spoken-language users, suggesting that similar pWM mechanisms are responsible for both sign-language and spoken-language processing.