when portions of speech are masked by background sounds and thus not available to the auditory system , the brain may restore the missing segments using linguistic knowledge ; syntactic , semantic , and lexical constraints ; expectations ; and context ( warren 1970 ; warren and sherman 1974 ; bashford and warren 1979 ; verschuure and brocaar 1983 ; kashino 2006 ; shahin et al . 2009 ) , enhancing speech intelligibility ( warren 1983 ; grossberg 1999 ; sivonen et al . one paradigm that is commonly used to quantify the effect of such top - down filling is phonemic restoration ( pr ) , where better intelligibility of interrupted speech with periodic silent intervals is observed after these silent intervals are filled with loud noise bursts ( warren 1970 ; warren and obusek 1971 ; warren and sherman 1974 ; bashford and warren 1979 ; verschuure and brocaar 1983 ; bregman 1990 ; bashford et al . huggins ( 1964 ) explained this finding from the point of bottom - up cues only ; applying interruptions to speech introduces spurious cues , such as sudden offsets and onsets , which could potentially disrupt speech intelligibility . adding a loud filler noise in the gaps makes these distortions inaudible . 1994 ) suggested that the excitation caused from the noise could be reallocated to restore speech . other studies demonstrated that listener expectation and linguistic and acoustic context can play important roles in pr ( warren and sherman 1974 ; warren 1983 ; bashford et al . , pr seems to be a result of a complex interplay between bottom - up signal cues and top - down cognitive mechanisms . another observation that hints at the interactive nature of bottom - up cues and top - down mechanisms for pr is the connection between the continuity illusion and the pr benefit observed in increased intelligibility ( thurlow 1957 ; repp 1992 ; warren et al . using simpler stimuli such as tones or noise , interrupted sounds combined with a filler noise can be illusorily perceived as continuous if there is no perceptual evidence against continuity . for example , noise bursts that have sufficient intensity and appropriate spectral content to potentially mask the sound segments induce a strong illusion ( warren and obusek 1971 ; bregman 1990 ; kashino and warren 1996 ) . additionally , it helps if there is no evidence of the interruptions when the interrupted speech is combined with the filler noise ( bregman 1990 ) . when bregman and dannenbring ( 1977 ) introduced amplitude ramps on the tone signals at the signal  noise boundaries , the continuity illusion became weaker . similar principles also apply to perceived continuity of speech ( powers and wilcox 1977 ; bashford and warren 1979 ; warren et al . 1994 ; kashino 2006 ) . moreover , this illusory continuity percept of speech may help the cognitive auditory system to group the speech segments into an object , which in return may enhance speech intelligibility ( heinrich et al . in fact , when amplitude ramps similar to those used by bregman and dannenbring ( 1977 ) were introduced at the speech  noise boundaries , bakent et al . ( 2009 ) observed a reduction in perceived continuity of speech as well as a reduction in the pr benefit . these results suggest that when the bottom - up cues that are important for perceived continuity of speech are degraded , the pr benefit may be reduced . the focus of the present study was on the degradation of bottom - up cues that may be caused by hearing impairment and/or front - end processing of hearing devices and how such degradation may affect phonemic restoration . we hypothesized that they may nevertheless reduce the pr benefit . in partial support of this idea , 2010 ) have previously shown that elderly hearing - impaired listeners with moderate levels of sensorineural hearing loss may not benefit from pr . this population is also known to suffer disproportionately from background noise for understanding speech ( plomp and mimpen 1979 ; dubno et al . 1984 ; schneider et al . 2005 ) , and the reduced or non - existent benefit from pr could be a contributing factor . as pr strongly relies on top - down speech perception mechanisms , cognitive factors may play an important role in it . therefore , it is logical to conclude that cognitive resources and skills changed by aging ( van rooij and plomp 1990 ; salthouse 1996 ; wingfield 1996 ; baltes and lindenberger 1997 ; gazzaley et al . 2005 ; gordon - salant 2005 ) might have caused the reduction of pr observed in this population . the degradations in the signal quality due to peripheral hearing impairment might be the main reason for the reduced restoration benefit . this would be interesting because it would imply that the quality of the bottom - up speech signal , which may have nothing to do with the continuity illusion of interrupted speech , may still be important for the high - level speech perception mechanisms necessary for pr . was not designed to show these effects separately for age and bottom - up degradations due to hearing impairment , and additionally , it is difficult to find young listeners with typical forms of hearing impairment or elderly listeners with no hearing impairment ( echt et al . 2010 ; hoffman et al . 2010 ) . therefore , their results only showed a reduction in pr with typical forms of hearing impairment of moderate to severe levels without exploring either hypothesis explicitly . in the present study , we specifically aimed to test the alternative hypothesis mentioned above , namely , that the degradations in the bottom - up speech signal ( in this study , due to simulated speech perception with cochlear implants ( cis ) ) may prevent the benefit from high - level top - down speech perception mechanisms ( shown using pr ) . two versions of noise - band vocoder processing were used to spectrally degrade speech : one that simulates speech perception with cis and one that simulates speech perception with electric  acoustic stimulation ( eas ) , where acoustic low - frequency input from preserved residual hearing accompanies high - frequency electric input from the ci . by employing a young normal - hearing population , we have eliminated the potential effects of age and age - related cognitive changes . by using the vocoder processing , we have systematically varied the degree of degradations in speech stimuli in a manner that can occur in hearing - impaired listeners using cis ( friesen et al . moreover , because the vocoder processing makes the speech signals noisier , thereby making it easier for the listener to perceptually blend the speech segments with the noise bursts , we do not expect these degradations to counteract the continuity illusion of speech ( bhargava and bakent 2011 ) . interrupted speech should theoretically be perceived as a continuous stream when combined with the filler noise under all degradation conditions , producing no direct bottom - up cue that would hint to interruptions in speech . the results , thus , will show only the effect of degradations in the bottom - up speech signal on the top - down repair mechanism of pr . sixteen young normal - hearing listeners ( 20  db hl or better hearing thresholds at the audiometric frequencies of 2508000  hz ) , six men and ten women , with no history of hearing problems and between the ages of 17 and 33  years ( average  =  21.8    5.5  years ) , participated in the study . all listeners were fully informed about the study procedure , and written informed consent was collected before participation . the pr benefit was quantified with a method that uses highly contextual sentences interrupted with periodic silent intervals ( powers and wilcox 1977 ; verschuure and brocaar 1983 ; bakent et al . speech recognition is measured once with the silent interruptions and once with these interruptions filled with loud noise bursts . the latter condition produces an illusory perceived continuity of interrupted speech , and additionally , an increase in intelligibility is observed , even though the noise bursts do not add speech information . the increase in intelligibility with the addition of noise is accepted as the objective measure for the pr benefit . speech stimuli of the present study were meaningful conversational dutch sentences with high context ( example : buiten is het donker en koud [ outside is it dark and cold ] ) , digitized at a 44,100-hz sampling rate and recorded at the vu university amsterdam ( versfeld et al . 2000 ) . originally acquired the sentences in text from large databases , such as old dutch newspapers . these were then pruned to only retain sentences with reasonable and comparable length , which represent daily - life conversational speech , are grammatically and syntactically correct , and semantically neutral . the resulting database has 78 balanced lists of 13 sentences , with 39 lists spoken by a male speaker and the other 39 by a female speaker . each sentence is four to nine words long , containing words with up to three syllables . two randomly selected lists , one spoken by the male talker and the other by the female talker , were used for each test condition . sentences were first interrupted with silent intervals ( with or without the filler noise ) . a speech - shaped steady noise , produced from the long - term speech spectrum for each speaker and provided with the recordings , was used as the filler . the interrupted sentences were then further manipulated using the noise - band vocoder processing ( with reduced or full spectral resolution ) . the filler noise were amplitude - modulated with a periodic square wave ( interruption rate  =  1.5  hz , duty cycle  =  50  % , 5-ms raised cosine ramp applied at onsets and offsets ) . a slow interruption rate was purposefully selected to induce the best pr effect based on previous studies ( powers and wilcox 1977 ; bakent et al . the modulating square wave started with the on phase ( amplitude  =  1 ) for sentences and the off phase ( amplitude  =  0 ) for noise . the last noise burst was always played for the full duration , instead of adjusting to the sentence duration , to prevent the listeners from using potential timing cues that shorter duration noise bursts could provide . the rms presentation levels were 60 and 70  db spl for sentences and noise , respectively , again selected based on previous studies ( powers and wilcox 1977 ; bakent et al . spectral degradations were applied to interrupted sentences ( with or without the filler noise ) with noise - band vocoder processing , a technique based on ci signal processing and commonly used to simulate speech perception with a ci ( dudley 1939 ; shannon et al . first , the entire bandwidth of the processing was limited to 1507,000  hz . then , spectral degradation conditions of 4 , 8 , 16 , and 32 channels were implemented using a bank of bandpass filters ( butterworth filter , filter order 3 ) . the cutoff frequencies of the filters were determined based on greenwood s mapping function ( greenwood 1990 ) by using equal cochlear distance , and the same cutoff frequencies were applied to both analysis and synthesis filters . using half - wave rectification and low - pass filtering ( butterworth filter , filter order 3 ; cutoff frequency , 160  hz ) , the noise carriers in each channel were modulated with the corresponding extracted envelope . adding the modulated noise bands from all vocoder channels produced the final speech stimuli . in a slightly different configuration , eas was simulated by replacing the low - frequency channels of the vocoder with speech low - pass - filtered ( lpf ) at 500  hz ( butterworth filter , filter order 3 ) in a manner similar to previous studies ( qin and oxenham 2006 ; bakent and chatterjee 2010 ) . for the reduced spectral resolution conditions of 4 , 8 , 16 , and 32 channels , hence , the lowest one , two , four , and eight channels , respectively , were replaced with the unprocessed lpf acoustic speech . the interrupted and spectrally degraded sentences were diotically presented using a matlab graphical user interface through the s / pdif output of an external echo audiofire 4 soundcard , a lavry da10 d / a converter , and sennheiser hd 600 headphones . the listeners were instructed to verbally repeat what they heard after each sentence was presented . moreover , they were also told that they would hear only meaningful words in contextual and conversational sentences . guessing and repeating only parts of the sentences were encouraged . with these instructions , even if some words were perceived ambiguously , the listeners had the chance to report what they thought they heard . a student assistant who did not know the experimental conditions scored the recorded responses off - line . all words of the sentences were included in the scoring and no points were taken for wrong guesses . the percent correct scores were calculated by counting the number of the correctly identified words and taking its ratio to the total number of the words presented . these rules were followed in deciding correct identification : the propositions ( de , het [ the ] ) were only counted in scoring when the accompanied noun was also correctly reported . confusions in personal pronouns ( hij / zij [ he / she ] ) , demonstrative determiners ( dit / dat [ this / that ] ) , past or present tenses of verbs ( heeft / had [ has / had ] ) , singular or plural forms of verbs ( heeft / hebben [ has / have ] ) , and diminutives ( hond / hondje [ dog / doggy ] ) were ignored , and these were counted as correct when reported in either form . if personal pronouns and demonstrative determiners were the only words reported from a sentence and if they were not entirely correct , then they were not accepted as correct . for compound words ( vensterbank [ windowsill ] ) , reporting the partial words ( venster [ window ] and bank [ sill ] ) was not counted as correct . the same rule was applied to the verbs that are formed by adding prefixes to other verbs ( vertellen [ to tell ] and tellen [ to count ] ) as the meaning usually differed between such verbs . the listeners of the present study were naive about the purpose of the experiment , and they had no prior experience with the sentence materials or the manipulations used in the experiment . a short training session was provided before actual data collection began using one list of uninterrupted sentences that were processed with a four - channel vocoder . the practice run was repeated with each listener until an intelligibility threshold of 30  % was reached ( selected based on previous studies with similar materials and processing ) . the order of the blocks was counterbalanced using a latin square design , and the order of the trials within each block was randomized . for each trial , one list of 13 sentences was used , and no sentence was played more than once to the same listener . the listeners were able to finish each block in 2025  min and all blocks in one session . including the explanation of the study and instructions , filling of the written informed consent , the audiometric test , and occasional breaks , one session lasted 3  h or less.table  1summary of the data collection blocksdata collection blockspectral resolution ( no . of vocoder channels)speakerinterruptionvocoder simulation methodno . of trialsno . of sentences14 , 8 , 16 , 32 , fullmalesilent or with filler noiseci5    213024 , 8 , 16 , 32malesilent or with filler noiseeas4    210434 , 8 , 16 , 32 , fullfemalesilent or with filler noiseci5    213044 , 8 , 16 , 32femalesilent or with filler noiseeas4 2104ci refers to the vocoder simulation of cochlear implant processing ; eas refers to the second version of the vocoder simulation that was combined with unprocessed acoustic lpf speech , simulating combined electric  acoustic stimulation summary of the data collection blocks ci refers to the vocoder simulation of cochlear implant processing ; eas refers to the second version of the vocoder simulation that was combined with unprocessed acoustic lpf speech , simulating combined electric  acoustic stimulation sixteen young normal - hearing listeners ( 20  db hl or better hearing thresholds at the audiometric frequencies of 2508000  hz ) , six men and ten women , with no history of hearing problems and between the ages of 17 and 33  years ( average  =  21.8    5.5  years ) , participated in the study . all listeners were fully informed about the study procedure , and written informed consent was collected before participation . the pr benefit was quantified with a method that uses highly contextual sentences interrupted with periodic silent intervals ( powers and wilcox 1977 ; verschuure and brocaar 1983 ; bakent et al . , speech recognition is measured once with the silent interruptions and once with these interruptions filled with loud noise bursts . the latter condition produces an illusory perceived continuity of interrupted speech , and additionally , an increase in intelligibility is observed , even though the noise bursts do not add speech information . the increase in intelligibility with the addition of noise is accepted as the objective measure for the pr benefit . speech stimuli of the present study were meaningful conversational dutch sentences with high context ( example : buiten is het donker en koud [ outside is it dark and cold ] ) , digitized at a 44,100-hz sampling rate and recorded at the vu university amsterdam ( versfeld et al . 2000 ) . originally acquired the sentences in text from large databases , such as old dutch newspapers . these were then pruned to only retain sentences with reasonable and comparable length , which represent daily - life conversational speech , are grammatically and syntactically correct , and semantically neutral . the resulting database has 78 balanced lists of 13 sentences , with 39 lists spoken by a male speaker and the other 39 by a female speaker . each sentence is four to nine words long , containing words with up to three syllables . two randomly selected lists , one spoken by the male talker and the other by the female talker , were used for each test condition . sentences were first interrupted with silent intervals ( with or without the filler noise ) . a speech - shaped steady noise , produced from the long - term speech spectrum for each speaker and provided with the recordings , was used as the filler . the interrupted sentences were then further manipulated using the noise - band vocoder processing ( with reduced or full spectral resolution ) . the sentences and the filler noise were amplitude - modulated with a periodic square wave ( interruption rate  =  1.5  hz , duty cycle  =  50  % , 5-ms raised cosine ramp applied at onsets and offsets ) . a slow interruption rate was purposefully selected to induce the best pr effect based on previous studies ( powers and wilcox 1977 ; bakent et al . the modulating square wave started with the on phase ( amplitude  =  1 ) for sentences and the off phase ( amplitude  =  0 ) for noise . the last noise burst was always played for the full duration , instead of adjusting to the sentence duration , to prevent the listeners from using potential timing cues that shorter duration noise bursts could provide . the rms presentation levels were 60 and 70  db spl for sentences and noise , respectively , again selected based on previous studies ( powers and wilcox 1977 ; bakent et al . spectral degradations were applied to interrupted sentences ( with or without the filler noise ) with noise - band vocoder processing , a technique based on ci signal processing and commonly used to simulate speech perception with a ci ( dudley 1939 ; shannon et al . first , the entire bandwidth of the processing was limited to 1507,000  hz . then , spectral degradation conditions of 4 , 8 , 16 , and 32 channels were implemented using a bank of bandpass filters ( butterworth filter , filter order 3 ) . the cutoff frequencies of the filters were determined based on greenwood s mapping function ( greenwood 1990 ) by using equal cochlear distance , and the same cutoff frequencies were applied to both analysis and synthesis filters . using half - wave rectification and low - pass filtering ( butterworth filter , filter order 3 ; cutoff frequency , 160  hz ) , the envelopes were extracted from the analysis filters . the noise carriers in each channel were modulated with the corresponding extracted envelope . adding the modulated noise bands from all vocoder channels produced the final speech stimuli . in a slightly different configuration , eas was simulated by replacing the low - frequency channels of the vocoder with speech low - pass - filtered ( lpf ) at 500  hz ( butterworth filter , filter order 3 ) in a manner similar to previous studies ( qin and oxenham 2006 ; bakent and chatterjee 2010 ) . for the reduced spectral resolution conditions of 4 , 8 , 16 , and 32 channels , hence , the lowest one , two , four , and eight channels , respectively , were replaced with the unprocessed lpf acoustic speech . the interrupted and spectrally degraded sentences were diotically presented using a matlab graphical user interface through the s / pdif output of an external echo audiofire 4 soundcard , a lavry da10 d / a converter , and sennheiser hd 600 headphones . the listeners were instructed to verbally repeat what they heard after each sentence was presented . moreover , they were also told that they would hear only meaningful words in contextual and conversational sentences . guessing and repeating only parts of the sentences were encouraged . with these instructions , even if some words were perceived ambiguously , the listeners had the chance to report what they thought they heard . a student assistant who did not know the experimental conditions scored the recorded responses off - line . all words of the sentences were included in the scoring and no points were taken for wrong guesses . the percent correct scores were calculated by counting the number of the correctly identified words and taking its ratio to the total number of the words presented . these rules were followed in deciding correct identification : the propositions ( de , het [ the ] ) were only counted in scoring when the accompanied noun was also correctly reported . confusions in personal pronouns ( hij / zij [ he / she ] ) , demonstrative determiners ( dit / dat [ this / that ] ) , past or present tenses of verbs ( heeft / had [ has / had ] ) , singular or plural forms of verbs ( heeft / hebben [ has / have ] ) , and diminutives ( hond / hondje [ dog / doggy ] ) were ignored , and these were counted as correct when reported in either form . if personal pronouns and demonstrative determiners were the only words reported from a sentence and if they were not entirely correct , then they were not accepted as correct . for compound words ( vensterbank [ windowsill ] ) , reporting the partial words ( venster [ window ] and bank [ sill ] ) was not counted as correct . the same rule was applied to the verbs that are formed by adding prefixes to other verbs ( vertellen [ to tell ] and tellen [ to count ] ) as the meaning usually differed between such verbs . the listeners of the present study were naive about the purpose of the experiment , and they had no prior experience with the sentence materials or the manipulations used in the experiment . a short training session was provided before actual data collection began using one list of uninterrupted sentences that were processed with a four - channel vocoder . the practice run was repeated with each listener until an intelligibility threshold of 30  % was reached ( selected based on previous studies with similar materials and processing ) . the order of the blocks was counterbalanced using a latin square design , and the order of the trials within each block was randomized . for each trial , one list of 13 sentences was used , and no sentence was played more than once to the same listener . no feedback was provided during training or data collection . the listeners were able to finish each block in 2025  min and all blocks in one session . including the explanation of the study and instructions , filling of the written informed consent , the audiometric test , and occasional breaks , one session lasted 3  h or less.table  1summary of the data collection blocksdata collection blockspectral resolution ( no . of vocoder channels)speakerinterruptionvocoder simulation methodno . of trialsno . of sentences14 , 8 , 16 , 32 , fullmalesilent or with filler noiseci5    213024 , 8 , 16 , 32malesilent or with filler noiseeas4    210434 , 8 , 16 , 32 , fullfemalesilent or with filler noiseci5    213044 , 8 , 16 , 32femalesilent or with filler noiseeas4 2104ci refers to the vocoder simulation of cochlear implant processing ; eas refers to the second version of the vocoder simulation that was combined with unprocessed acoustic lpf speech , simulating combined electric  acoustic stimulation summary of the data collection blocks ci refers to the vocoder simulation of cochlear implant processing ; eas refers to the second version of the vocoder simulation that was combined with unprocessed acoustic lpf speech , simulating combined electric  acoustic stimulation figure  1 shows the percent correct scores , averaged across all participants and combined for stimuli spoken by the male and female speakers , as a function of spectral resolution . the left and right panels show the results with the two vocoder configurations , namely , ci and eas simulations , respectively . the black circles and the red diamonds in each panel show the scores for stimuli with silent intervals or when the gaps were filled with the noise , respectively . the difference between the two scores for each spectral resolution condition shows the pr benefit , i.e. , the improvement in intelligibility of interrupted speech after the noise bursts were added into the silent intervals . right panel results with vocoder processing combined with lpf acoustic speech ( eas simulation).fig . 2.improvement in speech performance with the addition of noise bursts , calculated from the difference in scores in figure  1 and shown as a function of spectral resolution . right panel results with vocoder processing combined with lpf acoustic speech ( eas simulation ) . improvement in speech performance with the addition of noise bursts , calculated from the difference in scores in figure  1 and shown as a function of spectral resolution . figure  1 shows that reducing spectral resolution reduced speech intelligibility as all scores decrease with decreasing numbers of vocoder channels , for both configurations . adding lpf speech to the vocoder ( simulating an additional hearing aid ) improved intelligibility as the scores in the right panel are better in general than those in the left panel . both of these observations have been previously reported , but only with intelligibility of speech interrupted with silent intervals ( bakent and chatterjee 2010 ) . the main interest of the present paper was the effect of spectral degradation on the high - level perceptual mechanism of pr ; therefore , we will focus our analysis on the restoration benefit , i.e. , the improvement in scores after noise was added ( the difference between the scores shown with diamonds and circles in fig .  1 for each spectral resolution condition and the scores of fig .  2 ) . the rightmost data points in each panel in the figures show the baseline scores with interrupted speech before the vocoder processing was applied ( full spectrum resolution ) . hence , there was a baseline pr effect of 9  % before the interrupted sentences were spectrally degraded . data show that the greater the spectral degradation was , the smaller was the pr benefit ( fig .  2 ) . to analyze the significance of these effects , a two - factor repeated - measures anova was applied with the within - subjects factors of number of vocoder channels and addition of noise for each vocoder configuration . the anovas showed that there was a significant main effect of number of vocoder channels ( f(4,60 )  =  533.96 , p  <  0.001 , power  =  1.00 ; f(4,60 )  =  370.40 , p  <  0.001 , power  =  1.00 , for ci and eas simulations , respectively ) , confirming that the performance decreased significantly as the spectral resolution decreased . there also was a significant main effect of adding the noise ( f(1,15 )  =  8.10 , p  < 0.05 , power  =  0.71 ; f(1,15 )  =  28.20 , p  <  0.001 , power  =  1.00 , for ci and eas , respectively ) , confirming the pr benefit . the interaction was significant for the vocoder data shown in the left panel ( f(4,159 )  =  5.52 , p  <  0.001 , power  =  0.93 ) and almost significant for the right panel ( f(4,159 )  =  2.39 , p  =  0.061 , power  =  0.40 ) , indicating that the pr benefit varied for different spectral resolution conditions . to further identify at what spectral resolution conditions a pr benefit was observed , a post hoc pairwise multiple comparison tukey s test was applied . this test showed that the baseline pr effect with full spectrum resolution ( no vocoder processing ) was significant ( p  <  0.001 ) . when the spectral resolution was reduced the pr effect was significant only at the highest spectral resolution condition of 32 channels ( p  <  0.01 ) with the ci simulation , and at the spectral resolution conditions of 16 and 32 channels ( p  <  0.05 and p  <  0.001 , respectively ) with the eas simulation . at lower spectral resolutions , there was no significant benefit from restoration . the main focus of the present study was on the bottom - up cues that could be degraded due to hearing impairment and/or front - end processing of hearing devices , and the effects of these degradations on the top - down speech repair mechanisms , quantified using the pr paradigm . the classical pr studies showed negative effects of bottom - up cue degradations on restoration when these degradations worked against perceived illusory continuity ( e.g. , bakent et al . this is not entirely unexpected as a continuity percept is presumably related to the grouping of speech segments and the eventual object formation of interrupted speech , and such formation may contribute to the increased intelligibility ( heinrich et al . different from these studies , we hypothesized that the hearing impairment and/or hearing device - related degradations in bottom - up cues could also reduce the restoration benefit , even though they were not expected to work against perceived continuity ( bhargava and bakent 2011 ) . the pr benefit observed in the present study was thus assumed to be primarily a measure of top - down repair ( e.g. , sivonen et al . 2006 ) . as a result , the study emphasizes that the top - down repair may not function adequately when substantial degradations are present in the bottom - up speech signals , and this may additionally contribute to speech perception problems that the hearing - impaired listeners have in challenging real - life listening situations . in line with the present study s hypothesis , bakent et al . ( 2010 ) had previously shown that the benefit from pr was reduced or absent in moderate levels of sensorineural hearing loss . however , in this study , the degree of hearing loss and the age of the participants co - varied ; the listeners with more hearing loss were also older in general . as cognitive resources may decline with aging ( e.g. , salthouse 1996 ; wingfield 1996 ; gordon - salant 2005 ) , and due to the importance of cognitive mechanisms for pr , age could potentially have contributed to the reduced pr benefit observed in the study by bakent et al . in the present study , the effect of bottom - up signal degradations on pr benefit was studied for another form of degradation , reduced spectral resolution , which could occur with hearing - impaired people , specifically with users of cis . by simulating implant processing and testing young normal - hearing listeners , the potential effects from age and age - related changes in cognitive resources were entirely eliminated . because noise - band vocoder processing produces stimuli that are more noise - like in quality , we assumed that vocoded speech segments would be easier to be perceptually bound with filler noise bursts , strengthening the perceived continuity . additionally , the spectral degradation could be applied systematically by varying the number of the vocoder processing channels . this systematic approach was expected to produce a gradual reduction in the restoration benefit so that the limit of the top - down repair could be explored . in line with our expectations , the results with the vocoder simulation of cis showed that there was a benefit from pr only in the highest spectral resolution condition of 32 channels . all lower spectral resolution conditions , 16 channels or less , prevented the pr benefit . a second simulation was also implemented in the present study to simulate speech perception with eas . adding acoustic low frequencies to the electric perception through the ci has been shown to help speech intelligibility in adverse listening conditions ( kong and carlyon 2007 ; bchner et al . one of the advantages that eas provides is the addition of strong voicing information ( brown and bacon 2009 ; cullington and zeng 2010 ; zhang et al . 2010 ) , a cue important for speech perception , especially in complex auditory scene analysis with interfering background sounds ( bregman 1990 ) . this cue is also considered to be important for binding sound segments that are audible through distortions , such as fluctuating noise or temporal interruptions ( grossberg 1999 ; plack and white 2000 ; bakent and chatterjee 2010 ; chatterjee et al . 2010 ) . because a continuity percept seems to be important for the restoration of interrupted speech ( powers and wilcox 1977 ; bashford and warren 1979 ; heinrich et al . 2009 ) , a strong voicing cue could conceivably provide better benefits from pr as well . the results of the present study indeed showed that there was a slight improvement in restoration benefit in the eas simulation compared to the ci - only simulation . while with the ci simulation , a spectral resolution of 32 channels was required for pr to be observed ; once the unprocessed lpf speech was added , a spectral resolution of 16 channels was also sufficient . however , at lower spectral resolutions , even though the overall scores were higher with the eas ( as shown by the comparison of the scores in the right panel to the scores in the left panel of fig .  1 ) , restoration benefit was still not observed . while the present study indicates a deficit in pr as a result of spectral degradations , it does not specify what specific factors have contributed to this finding . inspecting figure  1 , one could conclude that a certain amount of speech intelligibility is required ( above 50  % in this case ) to be able to restore as the pr effect was only observed for these levels of intelligibility . however , earlier data by verschuure and brocaar ( 1983 ) showed that , with normal - hearing listeners and using interrupted speech that was not degraded otherwise , pr benefit was actually larger when speech intelligibility was lower . reanalyzing data from an earlier study ( bakent et al . 2010 ) , bakent ( 2010 ) later showed that even at a high baseline speech intelligibility , no restoration was observed with moderately hearing - impaired listeners , ruling out this potential explanation . hence , a low - level intelligibility of interrupted speech alone does not seem to be the cause of reduced pr benefit . however , the factors that cause low intelligibility could affect the pr differently for degraded and non - degraded speech ( or between hearing impairment and normal hearing ) . some such factors are relevant to speech intelligibility in general while also relevant to restoration . for example , glimpsing of speech through noise segments ( barker and cooke 2007 ) and integration of these speech segments into an auditory object ( shinn - cunningham and wang 2008 ) can be affected by peripheral degradations . the vocoded speech segments are noisy in nature and therefore should be easier to integrate with the noise segments . assuming an overlap between perceived continuity and the top - down repair of degraded speech ( heinrich et al . 2008 ; bakent et al . hence , further research is needed to identify what specific factors play a role in the restoration of interrupted and spectrally degraded speech . the results of the present study fit well within the general research area of how degradations in bottom - up speech signals may have effects on a variety of top - down processes . for example , sarampalis et al . ( 2009 ) showed that speech presented in background noise increased listening effort , indicated by longer reaction times in a dual - task paradigm , even when there was no change in intelligibility . rabbitt ( 1991 ) observed that elderly hearing - impaired listeners had problems remembering words presented to them auditorily , even though they could understand and repeat back these words . ( 2006 ) , and janse and ernestues ( 2011 ) showed that the semantic facilitation from context was reduced with both elderly hearing - impaired listeners and young listeners who were tested with simulations of hearing impairment . shinn - cunningham and best ( 2008 ) argued that peripheral degradations caused by hearing impairment could negatively affect selective attention and object formation , and hafter ( 2010 ) similarly argued for potential negative effects of hearing device processing on cognitive processes . these studies , combined , point to the strong connection between top - down processes and the effects of bottom - up degradations in speech signals caused by hearing impairment , aging , or other distortions on these processes . furthermore , one can conceive that other factors , such as increased listening effort , reduced memory , semantic facilitation from context , or selective attention , may negatively affect the cognitive resources needed for top - down repair . overall , from a scientific point of view , the findings of the present study further emphasize the crucial interaction between the bottom - up speech signals and their top - down interpretation and enhancement for robust speech perception , especially for speech distorted due to noise or hearing impairment ( grossberg 1999 ; alain et al . 2001 ; grimault and gaudrain 2006 ; pichora - fuller and singh 2006 ; zekveld et al . 2006 , the results are potentially important for hearing - impaired listeners with cis . for speech intelligibility , the vocoder simulation with eight spectral channels has been shown to be functionally closest to a typical ci user ( friesen et al . more surprisingly , however , with eight channels , there was also no pr benefit with the eas simulation . this is partially in contrast with the previous studies that showed an advantage from the added unprocessed low - frequency acoustic speech . for example , qin and oxenham ( 2006 ) and kong and carlyon ( 2007 ) showed an improvement in intelligibility of ( uninterrupted ) speech in noise with similar simulations after unprocessed low - frequency speech was added to the vocoder . with a similar manipulation , bakent and chatterjee ( 2010 ) recently also showed better perception of interrupted speech ( with periodic silent intervals ) with eas over ci simulation . this improvement was largest for the lowest spectral resolution conditions of four and eight channels . the perception of speech interrupted with silence intervals is one of the conditions of the present study as well . the comparison between the filled circles in the left and right panels of figure  1 shows an improvement due to eas , similar to that reported by bakent and chatterjee ( 2010 ) . however , this improvement in baseline scores with interrupted speech was not reflected in an improvement of the pr benefit ( fig .  2 ) . hence , while the additional cues provided by the unprocessed lpf speech were useful to enhance one form of restoration , i.e. , the perceptual restoration of speech interrupted with silence intervals , they were not sufficient for the other form of restoration , i.e. , the top - down filling when the loud noise bursts were added . a recent study by benard and bakent ( under review ) explored perceptual learning with the two forms of interruptions : one with silence and the other with silent intervals filled with noise . as they observed similar learning trends with the two forms , they concluded that similar cognitive mechanisms should play a role in the restoration of speech with silent intervals and speech with noise - filled intervals . however , the present study implies that the restoration of the latter may be more sensitive to the bottom - up signal degradations . all results combined , thus , show that the top - down repair mechanisms have limitations , and as the degradations in bottom - up speech signals increase , these mechanisms may fail to be helpful . thus , the speech intelligibility difficulties that ci users encounter in interfering background noise could partially be caused by the top - down repair mechanisms that do not function properly as a result of the degraded input from the periphery . the current speech intelligibility tests used in the clinical diagnostic and rehabilitation procedures are not yet designed to show the effects from high - level speech perception mechanisms ( or their failure ) ; therefore , such deficiencies may be difficult to identify in the clinical settings . for future research , our results imply that increasing the fidelity of the speech signals processed and sent to the electrodes may also increase the functional use of top - down processing for better speech perception in ci users .
the brain , using expectations , linguistic knowledge , and context , can perceptually restore inaudible portions of speech . such top - down repair is thought to enhance speech intelligibility in noisy environments . hearing - impaired listeners with cochlear implants commonly complain about not understanding speech in noise . 
 we hypothesized that the degradations in the bottom - up speech signals due to the implant signal processing may have a negative effect on the top - down repair mechanisms , which could partially be responsible for this complaint . to test the hypothesis , phonemic restoration of interrupted sentences 
 was measured with young normal - hearing listeners using a noise - band vocoder simulation of implant processing . 
 decreasing the spectral resolution ( by reducing the number of vocoder processing channels from 32 to 4 ) systematically degraded the speech stimuli . supporting the hypothesis , 
 the size of the restoration benefit varied as a function of spectral resolution . 
 a significant benefit was observed only at the highest spectral resolution of 32 channels . with eight channels , which resembles the resolution available to most implant users 
 , there was no significant restoration effect . 
 combined electric 
 acoustic hearing has been previously shown to provide better intelligibility of speech in adverse listening environments . in a second configuration , combined electric  acoustic hearing was simulated by adding low - pass - filtered acoustic speech to the vocoder processing . 
 there was a slight improvement in phonemic restoration compared to the first configuration ; the restoration benefit was observed at spectral resolutions of both 16 and 32 channels . 
 however , the restoration was not observed at lower spectral resolutions ( four or eight channels ) . 
 overall , the findings imply that the degradations in the bottom - up signals alone ( such as occurs in cochlear implants ) may reduce the top - down restoration of speech .