Deficits in temporal-order judgments in dyslexia: evidence from diotic stimuli differing spectrally and from dichotic stimuli differing only by perceived location

Abstract

The main debate concerning dyslexia focuses on the question of whether dyslexia is a language-specific disorder or a general nervous system dysfunction manifested in deficits of temporal processing. According to the temporal-order deficit hypothesis, dyslexia manifests difficulty in discriminating the temporal order of stimuli. Evidence has usually involved testing the ability to discriminate series of phonemes or pure tones whose components are separated by very short intervals. One of the difficulties in interpreting the data is the confound of changes in the spectrum with changes in temporal order. Two experiments are reported. In the first experiment, we verified the difficulty by adult dyslexics in judging the temporal order of two tones differing in frequency and presented diotically. The second experiment was designed to isolate temporal-order judgment (TOJ) from holistic frequency-based pattern discrimination processes. We tested temporal-order judgments with 15 ms duration tones of equal frequency presented dichotically (left–right, right–left) with ISI intervals ranging from 8 to 400 ms. Dichotic temporal threshold was significantly lower for adult normal readers than for the adult dyslexics. The results support the claim that adult dyslexics have difficulty in discriminating temporal order even when no spectral changes are involved.

Introduction

The question of whether dyslexia is a language-specific disorder or a more general nervous system disorder, with language being the major, but not the only symptom has been the focus of dyslexia research for the past 20 years. According to the phonological model, dyslexia reflects a deficit on the level of the phonological module that impairs the ability to segment the written word into its underlying phonological components. In particular, it has been suggested that children with dyslexia have poor phonological awareness, that is, poor knowledge of the internal phonological structure of words, resulting in poor grapheme to phoneme recoding (Heath, Hogben, & Clark, 1999; Mody, Studdert-Kennedy, & Brady, 1997; Rayner, Pollatsek, & Bilsky, 1995).

The competing, temporal-order model holds that underlying dyslexia is a more fundamental perceptual deficit in processing rapid temporal, auditory, and visual sequences, degrading the ability to perceive critical elements in the speech stream accurately, thus disrupting the development of a veridical phonological code (Farmer & Klein, 1995; Heim, Freeman, Eulitz, & Elbert, 2001; Keen & Lovegrove, 2000Kujala et al., 2000, Reed, 1989, Tallal, 1984). The temporal-order model does not preclude the phonemic/phonological deficit, but regards it as a symptom of a more general deficit in processing rapid temporal sequences of linguistic and non-linguistic cues. Supporting the temporal-order model are findings that reading-impaired individuals show not only auditory temporal-order deficits but also significant problems in processing rapid sequences of visual stimuli (Keen & Lovegrove, 2000; Kinsbourne, Rufo, Gamzu, Palmer, & Berliner, 1991; May, Williams, & Dunlap, 1988), and that temporal-order judgment (TOJ) thresholds between modalities do not differ markedly from those within a modality (Hirsh & Sherrick, 1961).

Temporal processing is a very broad concept referring to processing procedures involving two or more stimuli presented non-simultaneously. A variety of tasks have been used to investigate temporal processing, such as: detection of gaps between tones (Trainor & Adams, 2000), discrimination of short auditory intervals (Rousseau, Hebert, & Cuddy, 2001), temporal processing of visual stimuli (Keen & Lovegrove, 2000), rapid naming of words, objects, and letters (Semrud, Guy, Griffin, & Hynd, 2000), auditory saltation illusion (Phillips & Hal, 2001), and the identification and discrimination of temporal order (De-Martino, Espesser, Rey, & Habib, 2001; Heim et al., 2001, Kinsbourne et al., 1991; McGivern, Berka, Languis, & Chapman, 1991; Reed, 1989, Tallal, 1980).

When subjects are presented with a sequence of very brief stimuli (usually two), several temporal thresholds can be assessed. The “simultaneity threshold,” or “fusion threshold” refers to the threshold for detecting the presence of two, instead of one stimulus; that is, fusion threshold separates the impression of simultaneity and non-simultaneity. It is found to be between 660 μs to about 2 ms and considered to reflect sensory limitations (Lotze, Wittmann, von Steinbüechel, Pöeppel, & Roenneberg, 1999; Moore, 1993). A different temporal threshold, the “order threshold” refers to the ability to detect which of the stimuli was presented first and is believed to reflect perceptual limitation (see Pastore & Farrington, 1996, for review). Investigations of order thresholds typically focus on two types of such values. One is stimulus-duration threshold, usually measured with sequences of stimuli presented without gaps, and assumed to be around 200 ms/item (Warren, 1993). The second, inter-stimulus interval (ISI) threshold refers to the minimal stimulus onset asynchrony for detecting the order of two stimuli with 75% accuracy (Hirsh, 1959, Lotze et al., 1999; Pastore & Farrington, 1996). Unlike fusion threshold, ISI order threshold appears to be independent of sensory modality (Hirsh & Sherrick, 1961), relative amplitude, and frequency (Pastore, Layer, Morris, & Logan, 1988), suggesting that central mechanisms underlie identification of order processes.

Studies involving TOJ in dyslexia typically include repetition of the order of stimuli differing along a given dimension. A now well-known procedure for investigating temporal processing in dyslexia, the repetition test, requires the subject to verbally repeat or reproduce the order of two auditory stimuli, differing in frequency and presented in four possible combinations (low–low, low–high, high–low, high–high). The stimuli are separated by several different inter-stimulus intervals (Reed, 1989, Tallal, 1980, Tallal, 1984). At ISIs shorter than 300 ms, dyslexic children (8–10 years) typically have higher error rates than do normal readers.

Although dyslexia is considered to persist into adulthood, most studies involving TOJ in dyslexia tested children and adolescents and have rarely examined adults (see Table 1). In Experiment 1, we tested whether adult dyslexics also show deficits in temporal-order processing, using the classic repetition test. The similar cognitive and neurobiological profile of children and adults with dyslexia (Schulte-Koerne, Deimel, Bartling, & Remschmidt, 2001; Shapiro, 2000; Shaywitz, Pugh, Fletcher, & Shaywitz, 2000), would allow us to compare our results with previous findings on TOJ with dyslexic children.

However, two stimuli that differ in frequency and temporal order often give rise to spectrally discriminable sequences when the order is reversed. Studies on order threshold, using multi-item sequences of tones differing in duration and presented without gaps, lead researchers to conclude that there are two fundamentally distinct types of human auditory sequence perception: (a) holistic-pattern recognition, relevant when the individual elements in the sequence range in duration from a few milliseconds to few hundred ms, and (b) direct identification of the elements and their order, relevant when the duration of the individual elements range from a few hundred ms and longer (Thomas & Fitzgibbons, 1971; Warren, 1974, 1976). The distinction between holistic-pattern recognition and identification of order processes may have important implications also to the study of temporal-order processes in dyslexia. It is possible that similar holistic-pattern discrimination also occur when subjects are required to judge the temporal order of two brief duration tones of different frequencies separated by short temporal intervals. For example, a 1-K tone followed by a 2-K tone may “sound different” from a 2-K tone followed by a 1-K tone even if the temporal interval separating them is too short for an accurate temporal-order judgment. Therefore, with very short two-stimuli onset asynchronies and multiple trials, as employed in TOJ experiments in dyslexia measuring ISI thresholds, correct responses may be based on judgments of the “quality” of combined pairs rather than on the direct identification of components and their order. Of course, with very short stimulus durations, as employed in temporal-order tasks, the fast Fourier transform of the two 2-tone sequences should not be dependent upon the order of the tones. Therefore, one can argue that if the quality of the two orders is different it must be due to some sort of temporal processing. Nevertheless, no studies have yet addressed this issue directly with regard to tasks involving temporal-order judgments. Thus, it is impossible to determine which of the two processes, spectral or temporal, was employed in the discrimination of sequences of tones differing in frequency. Therefore, the purpose of the second experiment was to test the hypothesis that TOJ deficits in dyslexics, as manifested in the repetition test, reflect deficits in identification of order and cannot be accounted for only by a holistic-pattern recognition deficit.

Moreover, although studies indicate slower performance of dyslexics over normal readers in tasks involving temporal processes, performance in TOJ studies in dyslexia is exclusively measured by accuracy and the ISI threshold for a given accuracy level (Table 1). To the best of our knowledge, no previous studies measured the speed of TOJ responses in dyslexia, albeit the important implications that temporal processing speed might have on reading processes.

The following two studies, therefore, have two aims. The aim of Experiment 1 was to test auditory temporal-order deficits in adult dyslexics using speed, in addition to accuracy, as a dependent variable. The aim of Experiment 2 was to determine whether the temporal-order deficits found in adult dyslexics can be accounted for by differences in the holistic frequency-based discrimination of tone sequences or rather are rooted in the time dimension and reflect difficulty in the identification of temporal order.