Hearing loss has a negative effect on the daily life of 10-15% of the world’s population. One of the most common ways to treat a hearing loss is to fit hearing aids which increases audibility by providing amplification. Hearing aids thus improve speech reception in quiet, but listening in noise is nevertheless often difficult and stressful. Individual differences in cognitive capacity have been shown to be linked to differences in speech recognition performance in noise. An individual’s cognitive capacity is limited and is gradually consumed by increasing demands when listening in noise. Thus, fewer cognitive resources are left to interpret and process the information conveyed by the speech. Listening effort can therefore be explained by the amount of cognitive resources occupied with speech recognition. A well fitted hearing aid improves speech reception and leads to less listening effort, therefore an objective measure of listening effort would be a useful tool in the hearing aid fitting process. In this thesis the Auditory Inference Span Test (AIST) was developed to assess listening effort by measuring an individual’s cognitive spare capacity, the remaining cognitive resources available to interpret and encode linguistic content of incoming speech input while speech understanding takes place. The AIST is a dual-task hearing-innoise test, combining auditory and memory processing, and requires executive processing of speech at different memory load levels. The AIST was administered to young adults with normal hearing and older adults with hearing impairment. The aims were 1) to develop the AIST; 2) to investigate how different signal-to-noise ratios (SNRs) affect memory performance for perceived speech; 3) to explore if this performance would interact with cognitive capacity; 4) to test if different background noise types would interact differently with memory performance for young adults with normal hearing; and 5) to examine if these relationships would generalize to older adults with hearing impairment. The AIST is a new test of cognitive spare capacity which uses existing speech material that is available in several countries, and manipulates simultaneously cognitive load and SNR. Thus, the design of AIST pinpoints potential interactions between auditory and cognitive factors. The main finding of this thesis was the interaction between noise type and SNR showing that decreased SNR reduced cognitive spare capacity more in speech-like noise compared to speech-shaped noise, even though speech intelligibility levels were similar between noise types. This finding applied to young adults with normal hearing but there was a similar effect for older adults with hearing impairment with the addition of background noise compared to no background noise. Task demands, MLLs, interacted with cognitive capacity, thus, individuals with less cognitive capacity were more sensitive to increased cognitive load. However, MLLs did not interact with noise type or with SNR, which shows that different memory load levels were not affected differently in different noise types or in different SNRs. This suggests that different cognitive mechanisms come into play for storage and processing of speech information in AIST and for listening to speech in noise. Thus, the results suggested that a test of cognitive spare capacity seems to be a useful way to assess listening effort, even though the AIST, in the design used in this thesis, might be too cognitively demanding to provide reliable results for all individuals.

Cognitive resources, specifically working memory capacity are used for listening to speech, especially in noise. Cognitive resources are limited, and if listeners allocate a greater share of these resources to recovering the input signal in noise, fewer resources are available for interpreting and encoding its linguistic content. Although the importance of CSC for individual success in communicative situations has been acknowledged, this concept has not hitherto been explored experimentally. In this thesis, a CSC test (CSCT) was developed and administered to young adults with normal hearing and older adults with age-related hearing loss. CSCT required executive processing of speech at different memory loads with and without visual cues in different noise conditions. A free recall task using the same material was administered for comparison purposes and a battery of cognitive tests was administered to understand the relation between CSC and established cognitive concepts. The aims of the thesis were to investigate how CSC is influenced by 1) different executive demands and memory loads; 2) background noise; 3) visual cues; 4) aging and concomitant hearing loss. The results showed that 1) CSC was sensitive to memory load, and updating demands reduced CSC more than inhibition demands; 2) CSC was reduced in background noise compared to quiet; 3) visual cues enhanced CSC especially in noise; 4) CSC was reduced with ageing and concomitant hearing loss especially when visual cues were absent, memory demands were  increased and background noise was speech-like. The main finding of this thesis was that visual cues enhanced CSC for older individuals with hearing loss, specifically in adverse listening conditions. This demonstrates the importance of audiovisual testing in audiological assessment. Further, specific cognitive resources depleted during listening in noise were at least partially compensated by other cognitive functions. This thesis is the first step towards a theoretical understanding of CSC and in future, tests of CSC may play a crucial role in planning rehabilitation of persons with hearing loss. / Kognitiva resurser, speciellt arbetsminneskapacitet, förbrukas när vi lyssnar på tal, framförallt i bakgrundsbrus. De kognitiva resurserna är begränsade och ju större del som används till att rekonstruera en inkommande signal, desto färre finns fortsatt tillgängliga för att koda in och tolka dess språkliga innehåll. Trots att betydelsen av kognitiv reservkapacitet för framgångrik kommunikation är erkänd har kognitiv reservkapacitet hittills inte blivit undersökt experimentellt. I detta avhandlingsarbete utvecklades CSCT som ett test av kognitiv reservkapacitet. CSCT administrerades till unga vuxna med normal hörsel och äldre individer med åldersrelaterad hörselnedsättning. CSCT kräver exekutiv bearbetning av talat språk under olika minnesbelastningar, med och utan tillgång till visuell information och med och utan bakgrundsbrus. En fri återgivningsuppgift, baserad på samma material som i CSCT, administrerades för att kunna göra jämförelser. Slutligen administrerades ett kognitivt testbatteri för att förstå relationen mellan kognitiv reservkapacitet och andra kognitiva funktioner. Avhandlingens syfte var att undersöka hur kognitiv reservkapacitet påverkas av 1) olika krav på de exekutiva förmågorna och olika grad av minnesbelastning; 2) bakgrundsbrus; 3) tillgång till visuell information; 4) åldrande och åldersrelaterad hörselnedsättning. Resultaten visade att kognitiv reservkapacitet 1) var känsligt för minnesbelastning och reducerades mer av uppdaterings- än inhibitionskrav; 2) reducerades av bakgrundsbrus; 3) ökade med tillgång till visuell information, framförallt i bakgrundsbrus; 4) var reducerad hos äldre med åldersrelaterad hörselnedsättning, speciellt när visuell information saknades, minnesbelastningen ökades och bakgrundsbruset bestod av talspråk. Huvudfyndet var att visuell information frigjorde kognitiv reservkapacitet hos äldre individer med nedsatt hörsel, speciellt när lyssningssituationen var ogynnsam. Detta visar på betydelsen av audiovisuell testning vid audiologisk bedömning. Resultaten visade vidare att när specifika kognitiva resurser förbrukades under ogynnsamma lyssningsförhållanden kunde andra kognitiva funktioner kompensera för detta. Denna avhandling utgör det första steget mot en teoretisk förståelse av kognitiv reservkapacitet. På sikt kan utvärdering av kognitiv reservkapacitet spela en avgörande roll för planering av rehabilitering i samband med hörselnedsättning.