Early Identification

Auditory Responsiveness of Premature Infants Utilizing Visual Reinforcement Audiometry (VRA) John Mick Moore, PhD; Gary Thompson, PhD; Richard C. Folsom, PhD Director, Special Services, Puget Sound Educational Service District, Seattle, Washington (J.M.M.) and Department of Speech & Hearing Sciences, University of Washington, Seattle, Washington (G.T., R.C.F.)

ABSTRACT The purpose of this investigation was to study the relationship between visual reinforcement audiometry (VRA) performance and age (i.e., corrected and mental) with 60 premature infants. VRA performance was classified as unacceptable, marginal, or acceptable based on conditionability and number of responses obtained before habituation to the task. The results indicated that mental age and corrected age were significantly related to VRA performance. It was found that a corrected age of 8 mo and/ or a mental age of 6 mo is typically required for acceptable performance using VRA. A lack of responsiveness to the VRA procedure at these ages would most likely be due to hearing loss as opposed to a general developmental delay. The data from this study would not support a large scale behavioral hearing screening program for premature infants below 8 mo corrected age or 6 mo mental age. (Ear Hear 13 3:187-194)

TECHNOLOGICAL AND MEDICAL advances in the delivery and care of premature infants have resulted in a dramatic reduction in neonatal mortality. Bennett (1 985) reported that data from medical centers in the United States, Canada, and Europe indicate that the chances of survival for the low birthweight preterm infant have doubled during the past 20 yr. Consequently, smaller and less viable infants are living longer and surviving the traumas of premature birth. Increased survival of preterm infants has contributed to a rise in the number of children with major handicaps (Hagberg, 1979; Stanley, 1979). Current information indicates that approximately 20% of the preterm population is diagnosed as having the major handicap of cerebral palsy, mental retardation, hydrocephalus, visEar and Hearing, Vol. 13, No. 3, 1992

ual impairment, or hearing loss (Bennett, 1984; Parmelee, 1981). Estimates of hearing loss in the preterm population have been reported as high as 12.4 to 17.5% (Drillien, 1964; Schulte & Stennert, 1978). Most researchers have suggested a range of hearing loss between 1.5 and 3.4% for preterm infants (Davies & Tizard, 1975; SchulmanGalambos & Galambos, 1979; Stewart & Reynolds, 1974). Differences in sampling procedures and in operational definitions of both prematurity and hearing loss have accounted for this variability (Roberts, Davis, Phon, Reichert, Sturtevant, & Marshall, 1982). The elevated incidence of hearing loss in premature infants causes this population to be categorized as high risk for hearing loss with a potential need for early intervention (Allen & Capute, 1986; Cunningham, 1971;Gerber & Mencher, 1979; Thompson & Folsom, 1981). The High Risk Registry for Deafness, developed by a task force of the American Speech-LanguageHearing Association (ASHA), has listed very low birthweight (directly associated with premature birth) as one of the major indicators for suspicion of hearing loss (Gerkin, 1984). More recently, the ASHA Committee on Infant Hearing published “Audiologic Screening of Newborn Infants Who Are At Risk for Hearing Impairment” (ASHA, 1989). The committee recommended that at-risk infants should receive ABR screening before hospital discharge with appropriate audiological follow-up for those who fail the ABR screen. The most commonly used audiological procedure for either screening or assessing infants in the 6-mo through 2-yr age category is visual reinforcement audiometry (VRA).

Visual Reinforcement Audiometry Shepherd (197 1) reported that localization behavior is perhaps the most frequently evoked response in the auditory assessment of infants. Newborn infants demonstrate reflexive eye movements toward very brief sounds (Mendelson & Haith, 1976; Turkewitz, Birch, & Cooper, 1972). Many newborns will turn to sound at birth and during the 1st mo, perform poorly during the 2nd and 3rd mo, and localize well during the 4th mo (Field, Muir, Pilon, Sinclair, and Dodwell, 1980; Muir, Abraham, Forbes, and Hams, 1979; Muir & Field, 1979). Muir and his colleagues felt that the 0196/0202/92/1303-0187$03.00/0EARAND HEARING Copyright Q 1992 by Williams & Wilkins Printed in the U.S.A.

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decline in performance during the 2nd and 3rd mo of life may reflect a disruption in an innate response mechanism and/or that orientation to sound may decline until the infant reaches a cognitive organizational level where there are reinforcing consequences for turning toward sounds. Cowan (1 978) theorized that reflexive responses operate independently from birth to 6 weeks of age, coordination between sensory motor schemes begins to form between 2 and 4 mo and intentional responding directed toward external events emerges between 4 and 6 mo of age. There appears to be a reduction in localization behavior as the sensory motor schemes develop between 2 and 4 mo of age. Chun, Pawsat, and Forster (1960) and Lyons-Ruth ( 1975)suggested that infants do not show reliable visual orienting to sounds until after 4 or 5 mo of age. Northern and Downs (1978) reported that the ability to locate sounds in the vertical and oblique planes emerges later than for the horizontal plane during the 4 to 6 mo time frame. Suzuki and Ogiba (1960, 1961) developed a procedure called conditioned orientation reflex (COR) audiometry utilizing the localization response. Liden and Kankkunen (1 969) used the term visual reinforcement audiometry (VRA) to describe a modified COR procedure. Moore, Thompson, and Thompson (1975) investigated auditory localization behavior as a function of reinforcement using four conditions: ( 1) no reinforcement; (2) social reinforcement (a smile, verbal praise and/or pat on the shoulder); (3) simple visual reinforcement (a blinking light); and (4)complex visual reinforcement (an illuminated toy animal which moved in place when activated). Only a clear head turn in the direction of a loudspeaker was counted as a response. Infants who received reinforcement responded significantly more often than infants in the control group (no reinforcement). In addition, the complex visual reinforcer group showed significantly more responses than the other reinforcement groups. The results of this study implied that auditory localization behavior of 12 to 18 mo old infants is strongly influenced by reinforcement and that the type of reinforcement used has a differential effect on response behavior.

VRA and AGE In the preceding section, reference was made to several studies which discussed auditory localization and cognitive development (Cowan, 1978; Field et al, 1980; Muir et al, 1979; Muir & Field, 1979). Based on these articles, it might be predicted that the lower age boundary for effective use of VRA on normally developing full-term infants would be approximately 6 mo. Indeed, it has been demonstrated that normal full-term infants condition to the VRA task as young as 5 to 6 mo of age, but 4 mo olds do not (Moore, Wilson, & Thompson, 1977). Mental Development Premature infants have been shown to display significantly poorer performance on standardized measures 18.9

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of mental ability (Bayley Scales) when compared to fullterm infants of the same postpartum age (Goldstein, Caputo, & Taub, 1976; Rubin, Rosenblith, & Balow, 1973). Although the differences tend to lessen when the groups are matched on conceptional age, they are nonetheless evident during the first year of life (Bakeman & Brown, 1980; Hunt & Rhodes, 1977; Parmelee & Schulte, 1970). Kopp ( 1974) concluded that the quality and quantity of cognitive exploration, of which preterm infants do less when compared to full-term infants, may account for reduced motor development. Rubin, Rosenblith, and'Balow (1973) and Goldstein, Caputo, and Taub (1976) reported that preterm infants have mental delays that result in less motorical exploration when compared to full-term infants. There are currently no research studies available which have systemically looked at the relationship between VRA performance and age in preterm infants. Is it possible to predict VRA performance on the basis of corrected age (subtracting the estimated weeks of prematurity in gestational age from each infant's chronological age), or must other factors such as mental development be considered in order to increase predictability? The present study was designed to investigate VRA performance of premature infants by asking the following questions: (1) What is the relationship between corrected age of premature infants and VRA performance? (2) What is the relationship between mental age of premature infants and VRA performance? METHODS

Subjects The subjects were 60 premature infants (28 female and 32 male) selected from the Neonatal Intensive Care Unit (NICU) at the University of Washington Hospital. Prematurity was based on a gestational age of 36 weeks or less as determined by the Ballard Scale (Ballard, Kazmaier, & Driver, 1977). All infants weighed 2500 g or less at birth and were considered to have normal weight for gestational age. The consideration of appropriate birthweight for gestational age assisted in the exclusion of sick babies, who are often small for gestational age. All infants in this study were tested at a corrected age of 4 to 9 mo. Corrected age was determined by subtracting the estimated weeks of prematurity in gestational age from each infant's chronological age. The intent of this study was to focus on the relationship between prematurity and auditory responsiveness while ruling out severe neonatal factors and handicapping conditions, which individually might contribute to hearing loss or major developmental delays. Therefore, premature infants with documented conditions of visual impairment, cerebral palsy, hydrocephalus, grade four intracranial hemorrhage, chronic lung disease requiring home oxygen, genetic syndromes, and central nervous system infection were not included in the subject pool. Criteria for inclusion in this study were (1) preterm history without major handicapping conditions, (2) no unusual history of middle ear involvement or insertion of pressure equalization (PE) tubes, (3) no parental concern about hearing status, and (4) normal tympanometry on the day of testing. Ear and Hearing, Vol. 13, NO. 3,1992

Normal tympanometry was defined as observable compliance peaks less negative than -200 mm daPa. Fifty-seven out of 60 subjects demonstrated bilateral normal tympanometric results. Three subjects demonstrated unilateral normal compliance peaks and could not be tested in the other ear due to uncooperative behavior. None of the three subjects had a cold on the test day and none had ever had an ear infection according to parental report. Auditory Stimulus Band-pass complex noise was used as the auditory stimulus. This stimulus has its major concentration of energy between 500 and 2000 Hz,with decreasing energy in the higher and lower frequencies. Band-pass complex noise was used as the auditory stimulus because it could easily be calibrated in the sound field and because a similar stimulus was used to determine normative auditory responsiveness with full-term infants using visual reinforcement audiometry (Moore et al, 1977; Wilson, Moore, & Thompson, 1976). Use of a similar auditory stimulus and comparable experimental designs for the previous studies and the current study allowed for a comparison of VRA performance between full-term and preterm infants. All auditory signals were presented at 60 dB SPL as measured at the position of the infant’s head in the test room. Electroacoustic calibration was checked before each test session with a Bruel and Kjaer SPL meter (Model 2203). Instrumentation and Test Room Arrangement The test room arrangement is shown in Figure 1. The auditory stimulus was presented by a standard audiometer (Madsen Electronics Model OB77) located in one room of a two-room IAC sound-treated audiometric test suite. The infant was observed through a one-way mirror/window in the adjacent sound-treated room. All auditory signals were delivered through an Electro Voice SP-12 loudspeaker located at ear level, approximately 1.3 m from the infant and at a 45“ angle to the right of the infant’s midline of vision. Each subject was placed on a parent’s lap in the center of the test room. The parent was not masked during testing. The parent was instructed not to respond in any way during testing and to remain quiet. The examiner in the test room attempted to maintain each subject’s attention focused to the midline with soft colorful toys. Auditory stimuli were presented only when the subjects were in a ready state. The ready state was accepted when the infant was quietly facing forward and not vocalizing or demonstrating fretful behaviors. A visual reinforcer was located at eye level next to the loudspeaker on the infant’s right side. The visual reinforcer consisted of a colorful 12-in high animated toy animal which moved various body parts when electrically activated. The toy animal was contained in a smoked Plexiglas box and could only be observed when illuminated by two 60 watt light bulbs located inside the Plexiglas container. The visual reinforcer was controlled by a logic circuit and AND-gate. The examiner behind the audiometer and the examiner seated to the child’s left side held push-button switches which were depressed when they voted that a head turn response had occurred. The instrumentation logic determined the response window of 4 sec after the onset of the auditory stimulus. The visual reinforcer could only be activated if both examiners voted that a response occurred and the response window was still open. The duration of the reinforcement was set at 2 sec and was controlled by the instrument logic circuit. A manual ovemde push-button switch was controlled by the examiner during training trials. Ear and Hearing, Vol. 13, No. 3, 1992

Figure 1. Room arrangement. El, experimenter 1; E2. experimenter 2, P, parent; I, infant; VR, visual reinforcer.

The examinerswere experienced pediatric audiologistswho were certified by ASHA. The examiner in the test room wore earphones in order to hear comments from the examiner located behind the audiometer. In addition, a broadband noise was presented via earphones to the examiner in the test room to mask the presence or absence of the auditory stimulus during test and control trials and to signal the onset of a 4 sec hit window. The examiner in the test room voted if a head turn response was observed and because of the masking noise, did not know if an auditory stimulus had been presented or whether a control trial had been initiated. The examiner behind the audiometer did have knowledge of the stimulus events and noted whether or not the assistant voted. This information was used to gather data on responses to auditory stimuli, falsepositive responses, and interjudge reliability. Schedule of Trials Phase 1: Training Trials Phase 1 consisted of conditioning subjects to the VRA task. The initial three presentations were training trials. During these training trials, the auditory stimulus was presented for 2 sec and the visual reinforcer was activated and paired with the auditory stimulus. The auditory stimulus preceded the visual stimulus by 1 sec before simultaneous pairing of the two stimuli. If the infant “naturally” demonstrated a head turn response after the initiation of the sound source, the visual reinforcer was immediately activated. If an infant did not demonstrate a head turn response after the addition of the visual reinforcer, the examiner in the test room shaped a head turn response by assisting the infant in locating the source of the auditory/visual reinforcer. Each infant was given an opportunity during the initial three trials (i.e., training trials) to receive assistance, if needed, in shaping the conditioned head turn. The visual reinforcer was controlled by the ovemde push button during the training phase. Phase 2: Experimental Trials After the three training trials, a series of stimulus and control trials began (phase 2). The stimulus trials consisted of a 2 sec presentation of bandpass complex noise with the initiation of visual reinforcement only if the infant demonstrated a head turn response to the auditory signal within a 4 sec hit window. The visual reinforcer was only activated if both examiners voted within 4 sec Auditory Responsivenessof Premature Infants

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Table 1. Number of subjects demonstrating unacceptable, marginal, or acceptable VRA performance in relation to corrected age (N = 60). Corrected Age

Category I (unacceptable performance) Category II (marginal performance) Category 111 (acceptable performance)

Group A 4-5 mo (N = 25)

Group B 6-7 mo (N = 18)

Group C 8-9 mo (N = 17)

18 2 5

4 4 10

0

after the initiation of the auditory stimulus. If the infant did not respond, the visual reinforcer was not activated. The infant was considered to be conditioned if three responses were observed out of four consecutive stimulus trails. If the infant did not meet the conditioning criterion, additional training trials were introduced to give the infant a second training opportunity. Subjects were considered not conditionable if they failed to meet the conditioning criterion after the second series of three training trials. Testing continued for those infants who demonstrated conditionability on a 100% reinforcement schedule until the subject reached 60 stimulus trials or habituated to the task. Habituation was defined as four no-response behaviors among five consecutive stimulus trials. In addition to the stimulus trials, each infant received one control trial randomly included in each set of three stimulus trials to record false-positive behavior. Each control trial lasted 4 sec, during which time the two examiners voted whether or not the infant turned his/her head toward the sound source in the absence of an auditory stimulus (falsepositive response). The examiner in the test room was cued to vote by a tone heard through earphones and was blind as to whether the judging period was a stimulus presentation or control trial. RESULTS

The purpose of this study was to investigate VRA performance of preterm infants. Sixty premature infants met the selection criteria and were utilized as subjects. The results will be presented in accordance with the two research questions with which this study is concerned: (1) What is the relationship between corrected age of premature infants and VRA performance? Table 1 presents an overview of subject data with respect to VRA performance and corrected age. Subjects were placed in one of three categories on the basis of VRA performance. Category I (unacceptable performance) represented infants who were not conditionable. This group revealed no head turn responses or a few inconsistent responses without meeting the conditioning criterion of three responses during four stimulus trials. Category I1 (marginal performance) represented infants who were conditionable, but responded fewer * A typical VRA screening protocol conducted in the sound field requires a minimum of 15 stimuli. For example, if three or four stimulus presentations are required for establishing a response at a level of 20 dB HL for a particular auditory stimulus, and if three or four different auditory stimuli are used in the screening protocol (e.g., 1000, 2000, and 4000 Hz narrow bands or warble tones and a response to speech), the total number of responses required to complete the screen would be approximately 15.

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3 14

thdn 15 times before habituation to the task.* Category I11 (acceptable performance) consisted of subjects who were conditionable and responded 15 times or more before habituation to the task. In order to meet requirements for inclusion in either category I1 or category 111, subjects had to demonstrate false-positive response rates below 25%. The infants were placed into one of three corrected age groups [group A (4-5 mo), group B (6-7 mo), group C (8-9 mo)]. Corrected age was determined by subtracting the estimated weeks of prematurity from each infant’s chronological age. A total of 60 subjects were tested in the three corrected age groupings ( N = 25 in group A; N = 18 in group B; N = 17 in group C). The results displayed in Table 1 were tested for significance with the Pearson product-moment correlation (Y) procedure. The statistical results were significant ( Y = 0.763; df = 59; p < 0.05), allowing the inference that VRA performance is related to corrected age. Inspection of Table 1 reveals that all subjects (100%)in group C conditioned to VRA and provided either marginal or acceptable performance before response habituation. In contrast, 18 subjects (72%) in group A failed to meet the conditioning criterion (unacceptable performance), two (8%) demonstrated marginal performance, and five (20%) demonstrated acceptable performance. Some group A subjects demonstrated reflexive-like head turn behavior, which will be discussed later in this article. Subjects in group B demonstrated an emerging pattern of acceptable performance [four (22%) provided unacceptable performance, four (22%) provided marginal performance, and 10 (56 %) provided acceptable performance]. These results imply that premature infants with corrected ages of 4 or 5 mo are not likely to perform acceptably using VRA, that premature infants with corrected ages of 6 or 7 mo may perform acceptably using VRA, and that premature infants with corrected ages of 8 or 9 mo are likely to perform acceptably using VRA. The mean number of responses for subjects in category I (unacceptable performance) was 0.82, with a SD of 1.7. These infants either did not respond to the VRA procedure or demonstrated only a few inconsistent responses without meeting the conditioning criterion. The mean number of responses for subjects in category I1 (marginal performance) was 5.9, with a SD of 2.1. The mean number of responses for subjects in category I11 (acceptable performance) was 28.1, with a SD of 11.4. Table 2 presents descriptive information on the reEar and Hearing, Vol. 13, No. 3,1992

lationship between VRA performance and corrected age for those children who demonstrated acceptable performance (category 111). Question 2: What is the relationship between mental age and VRA performance in premature infants? Mental development of the premature infants was determined using the Bayley Scales of Infant Development (Bayley, 1969). The Bayley Scales were administered by experienced registered physical or occupational therapists. The Bayley mental age raw scores were converted into mental ages in order to classify the subjects into one of three mental age groups (i.e., group A, 5 mo and below; group B, 6-7 mo; and group C, 8-9 mo). In order to make a direct comparison between VRA performance and mental age (in months), it was necessary to test VRA performance on the same day that mental age was tested by using the Bayley Scales of Infant Development. Forty-three infants (out of the total N of 60) were administered the Bayley and VRA procedures on the same day. Table 3 displays the relationship between mental age and VRA performance. The results were tested for significance using the Pearson product-moment correlation ( r ) procedure. There was a significant correlation between VRA performance and mental age ( Y = 0.604; df = 42; p < 0.05). Table 3 reveals that all 15 subjects (100%) in groups B and C were conditionable, with 12 subjects (80%) demonstrating acceptable performance. In contrast, 20 subjects (7 1%) in group A did not meet conditioning criteria (unacceptable performance), three (1 1%) demonstrated marginal performance, and five (18%)demonstrated acceptable performance. Further analysis revealed that subjects who demonstrated unacceptable performance ( N = 20) had a mean mental age of 4.4 mo. Premature infants who were conditionable and demonstrated marginal ( N = 6) or acceptable ( N = 17) performance had a mean mental age of 6.1 and 6.7 mo, respectively. These results suggest that premature infants must have a mental age of approximately 6 mo in order to perform VRA.

False-Positive Responses and Interjudge Reliability The majority of subjects (83%) demonstrated zero or one false-positive response during control trials. The percentage of false-positive responses observed during control trials was 8.5% for subjects who were 4 or 5 mo corrected age, 7.3% for infants with a corrected age of 6 or 7 mo, and 9.1% for subjects who were 8 or 9 mo corrected age. It was concluded that random head turn responses were not a major influence on responses recorded during stimulus trials. The examiner in the test room and the examiner behind the audiometer independently recorded the same response, 99.876, after the stimulus trials and 99.7% after the control trials. Eliminated Infants Seventy-one premature infants were tested. Sixty infants met the selection criteria and were utilized as subjects. Eleven premature infants did not qualify as subjects. Three infants were rejected due to visual problems. One infant was found to have severe developmental delays (most likely due to mental retardation). One infant was rejected due to age (older than 9 mo corrected age at the time of test) and six infants failed the screening criteria due to flat tympanograms or negative pressure peaks beyond -200 mm daPa. No infants were rejected due to a false-positive rate in excess of 25% during control intervals. DISCUSSION

VRA Performance and Corrected Age The first research question addressed in this study investigated the relationship between VRA performance and corrected age of premature infants. After meeting a conditioning criterion, acceptable performance was defined as a minimum of 15 responses before habituation to the VRA task. Results suggest that premature infants with a corrected age of 8 or 9 mo are

Table 2. Responses of subjects demonstratingacceptable performance in relationship to corrected age (N = 29).

Corrected Age Group A 4-5 mo (N = 5) Group B 6-7 mo (N = 10) Group C 8-9 mo (N = 14)

% of Subjects Demonstrating Acceptable VRA Performance

Mean

SD

Range

False-Positive Rate

20% 56% 82%

31.8 31.6 24.3

14.5 13.9

21-48 18-55 16-44

13% 6% 9%

Number of Responses

7.3

Table 3. Number of subjects demonstratingunacceptable, marginal or acceptable VRA performance in relation to mental age (N = 43).

Mental Age

Category I (unacceptable performance) Category II (marginal performance) Category 111 (acceptable performance)

Ear and Hearing, VOl. 13, NO.3, 1992

Group A 5 mo and Below (N = 28)

Group B 6-7 mo (N = 9)

Group C 8-9 mo (N = 6)

20 3 5

0 1 8

0 2 4

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likely to perform acceptably to VRA (100% conditioned to task and 82% performed acceptably), that premature infants with a corrected age of 6 or 7 mo may perform acceptably to VRA (78% conditioned to task and 56% performed acceptably), and that premature infants with a corrected age of 4 or 5 mo are not likely to respond to the VRA procedure (28% conditioned to task and 20% performed acceptably). A comparison of these data to results of previous studies on full-term infants discloses that although full-term infants are likely to perform acceptably to VRA by a chronological age of 6 mo (Moore et al, 1977; Wilson et al, 1976), premature infants are not likely to respond acceptably to VRA until approximately 8 mo corrected age. It should be noted that subjects were classified according to corrected age rather than chronological age in this investigation. Corrected age was determined by subtracting the estimated weeks of prematurity in gestational age from each infant’s chronological age. Although there is some controversy about whether to classify premature infants according to corrected or chronological age, the overwhelming majority of NICU in the United States prefer to use corrected age because corrected age more nearly approximates cognitive-developmental functioning levels of young premature infants (Bennett, 1992). The infants in this study averaged 9 weeks of prematurity. The infants who performed the VRA task in an acceptable manner were approximately 8 mo corrected age and approximately 10 mo chronological age. The mean number of responses for subjects who were conditionable and who provided either marginal or acceptable VRA performance was 22.8. This number can be compared to VRA studies on full-term infants. Moore et a1 (1977) investigated VRA responsiveness with a suprathreshold protocol that did not exceed 30 stimulus presentations. They reported that full-term infants between 5 and 11 mo of age averaged approximately 26 responses to the 30 stimulus presentations. Primus and Thompson (1985) reported a mean number of responses at 3 1.3 for full-term 12 mo old infants who were tested with VRA using a test protocol, including stimulus and intensity level, similar to the present study. Comments are in order with respect to the number and quality of responses obtained from some premature infants. Most infants in the youngest (4-5 mo) corrected age category failed to condition, but infants who did condition were likely to respond to a very high rate. For example, two 5 mo old subjects did not reach the habituation criterion and were continuing to respond when testing was terminated at 60 stimulus trials. One infant was presented 25 additional response trials and did not reach the habituation criterion after 85 stimulus presentations. These infants also demonstrated a qualitatively different type of response. They responded with a “reflexive” head turn, which appeared to be a startle-like head jerk as opposed to a more gradual orientation response toward the loudspeaker and visual 192

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reinforcer. These reflexive-like head jerk responses and failure to reach the habituation criterion after 60 stimulus trials were also recorded with two additional subjects, who were 7 and 8 mo corrected age. All of these subjects tended to be less mature than their corrected ages would indicate. They all had low birthweights, retention of primitive reflexes according to the movement assessment inventory (MAI) results, and low Bayley motor ages. The test results (startle-like head jerk responses and lack of normal habituation to repeated auditory presentations) from this subgroup of very imtnature premature infants are of interest in relation to theories of habituation. Sokolov ( 1 963) proposed a theoretical model to explain habituation with respect to the orienting response. His premise is that external stimuli travel along two central nervous system pathways. One pathway serves as a memory trace which allows for a comparison of incoming signals. When a difference exists between the memory trace and incoming signal, an orienting or alerting response occurs. As the difference between incoming signals and the comparator diminishes, inhibition occurs. A recovery of the orienting response occurs with a change in stimulus. Milstein, Stevens, and Sachdev (1 969) reported that habituation to redundant stimuli is a requisite for effcient functioning of the central nervous system and is also characteristic of a mature system. The inability to habituate to auditory stimuli by some premature infants may be a reflection of immature neurological development. The high number (and reflexive quality) of responses from some premature infants draws attention to previous observations (Bench & Parker, I97 1; Copriviza & Lima, 1983; Gerber, 1985; Gerber, Lima, & Copriviza, 1982; Hurlock, 1956) that have suggested that premature infants are “hyper-responsive’’ and/or more responsive than full-term infants. It is not possible to make a direct comparison between results from these previous studies and the current study because the previous studies looked for reflexive (unconditioned) responses and the present study used conditioning (visual reinforcement). If, however, the extended number of responses observed for a few subjects in the current study were truly reflexive in nature (as opposed to being conditioned), the data may support the previously expressed position that at least a segment of the premature population is “hyper-responsive” to auditory stimulation due to an immature neurological system. This speculation is restricted to suprathreshold level response behavior because the auditory stimulus in the present study was presented at 60 dB SPL. If the responses from these few subjects were truly reflexive rather than conditioned, it is unlikely, from a clinical perspective, that the subjects would have responded to screening (20 dB HL) or threshold level stimulation. VRA Performance and Mental Age The second research question investigated the relationship between VRA performance and mental age Ear and Hearing, Vol. 13, NO. 3, 1992

using the Bayley Scales of Infant Development (Bayley, 1969). The data revealed a significant relationship between VRA performance and mental age. All subjects ( 100%)who had a Bayley mental age of 6 mo or greater were conditionable and displayed marginal or acceptable VRA performance. The marginal group averaged a mental age of 6.1 mo, whereas the subjects demonstrating acceptable performance had a mean mental age of 6.7 mo. Only 28.6% of the subjects who had a mental age of 5 mo or less were conditionable. These data indicate that premature infants need to have a mental age of approximately 6 mo to ensure a high success rate when using VRA. These results are similar to the expected mental ages of normal developing infants who can perform the VRA task. Moore et a1 (1977) and Wilson et a1 (1976) reported that the VRA procedure was successful with normal developing full-term infants at approximately 6 mo of age.

Summary and Clinical Implications Precise behavioral estimates of hearing sensitivity are not possible until infants can respond appropriately to an operant conditioning test procedure such as VRA. There is considerable evidence that when conditions can be established, VRA performance is unrelated to the nature or intensity of the auditory stimulus (Primus & Thompson, 1985; Thompson & Folsom, 1985), method of conditioning (Thompson & Folsom, 1984) or schedule of reinforcement (Primus & Thompson, 1985). This robust quality of VRA implies that infants who learn (that is, condition to the procedure) are likely to perform well in clinical situations to a variety of auditory signals at threshold or near threshold (screening) levels as well as at suprathreshold levels. With respect to preterm infants, the present study showed that two factors, mental age and corrected age, are highly predictive of VRA performance. At a mental age of 5 mo or less, only 8 of 28 subjects (28.6%) conditioned to VRA, whereas at a mental age of 6 mo or more, 15 of 15 subjects (100%) conditioned to VRA and 12 of 15 (80%) demonstrated acceptable performance. At a corrected age of 4 to 5 mo, only 7 of 25 subjects (28%) conditioned to VRA. At a corrected age of 6 to 7 mo, 14 of 18 subjects (77.8%) conditioned to VRA and 10 of 18 (55.6%)demonstrated acceptable performance. At a corrected age of 8 to 9 mo, 17 of 17 subjects (100%) conditioned to VRA and 14 of 17 (82.4%) demonstrated acceptable performance. Of these two predictors of VRA performance (mental age and corrected age), corrected age may be the more practical one to use in most cases because it can be obtained from hospital records and does not require tests which would be needed to determine mental age. Most large scale NICU incorporate behavioral hearing screening (Le., VRA) as a part of their follow-up program. The results from this study suggest that routine follow-up VRA hearing screens on preterm infants be conducted at either a mental age of 6 to 7 mo or a Ear and Hearing, Vol. 13, No. 3, 1992

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