Two-part stimulus integration and specific reading disability.

29 dyslexics and 29 control children were presented with two halves of a black cross, one-half being presented at varying interstimulus intervals after the other half. Both dichoptic and binocular presentations were used and separation thresholds were at greater interstimulus intervals for the dichoptic condition. Dyslexics had thresholds at greater intervals than controls. These differences were significant and there was no significant interaction of group by dichoptic condition. The relative magnitudes of difference between dyslexics and controls were as previously reported by Stanley and Hall (1973) and support the notion that dyslexics have longer visual persistence than controls.     

Iconic memory and visible persistence

There are three senses in which a visual stimulus may be said to persist psychologically for some time after its physical offset. First, neural activity in the visual system evoked by the stimulus may continue after stimulus offset (“neural persistence”). Second, the stimulus may continue to be visible for some time after its offset (“visible persistence”). Finally, information about visual properties of the stimulus may continue to be available to an observer for some time after stimulus offset (“informational persistence”). These three forms of visual persistence are widely assumed to reflect a single underlying process: a decaying visual trace that (1) consists of afteractivity in the visual system, (2) is visible, and (3) is the source of visual information in experiments on decaying visual memory. It is argued here that this assumption is incorrect. Studies of visible persistence are reviewed; seven different techniques that have been used for investigating visible persistence are identified, and it is pointed out that numerous studies using a variety of techniques have demonstrated two fundamental properties of visible persistence: theinverse duration effect (the longer a stimulus lasts, the shorter is its persistence after stimulus offset) and theinverse intensity effect (the more intense the stimulus, the briefer its persistence). Only when stimuli are so intense as to produce afterimages do these two effects fail to occur. Work on neural persistences is briefly reviewed; such persistences exist at the photoreceptor level and at various stages in the visual pathways. It is proposed that visible persistence depends upon both of these types of neural persistence; furthermore, there must be an additional neural locus, since a purely stereoscopic (and hence cortical) form of visible persistence exists. It is argued that informational persistence is defined by the use of the partial report methods introduced by Averbach and Coriell (1961) and Sperling (1960), and the term “iconic memory” is used to describe this form of persistence. Several studies of the effects of stimulus duration and stimulus intensity upon the duration of iconic memory have been carried out. Their results demonstrate that the duration of iconic memory is not inversely related to stimulus duration or stimulus intensity. It follows that informational persistence or iconic memory cannot be identified with visible persistence, since they have fundamentally different properties. One implication of this claim that one cannot investigate iconic memory by tasks that require the subject to make phenomenological judgments about the duration of a visual display. In other words, the so-called “direct methods” for studying iconic memory do not provide information about iconic memory. Another implication is that iconic memory is not intimately tied to processes going on in the visual system (as visible persistence is); provided a stimulus is adequately legible, its physical parameters have little influence upon its iconic memory. The paper concludes by pointing out that there exists an alternative to the usual view of iconic memory as a precategorical sensory buffer. According to this alternative, iconic memory is post-categorical, occurring subsequent to stimulus identification. Here, stimulus identification is considered to be a rapid automatic process which does not require buffer storage, but which provides no information about episodic properties of a visual stimulus. Information about these physical stimulus properties must, in some way, be temporarily attached to a representation of the stimulus in semantic memory; and it is this temporarily attached physical information which constitutes iconic memory.     

Suppression of visible persistence

Five experiments were conducted to examine duration of visible persistence in sequences of stimuli. The basic display consisted of a point that stepped around a circular path on the face of an oscilloscope. Observers estimated the number of points seen simultaneously. Results were compared with a control condition in which the points were plotted in random order rather than sequentially. It was found that visible persistence of a point is suppressed if other points are shown nearby and after an appropriate delay. The degree of suppression depended on the spatial proximity of successive points. It was also found that both duration of visible persistence and degree of suppression increase with eccentricity in the visual field. The results are discussed in terms of two independent processes, persistence and suppression, that operate in a hierarchically antithetical relation.     

Continuous measurement of visible persistence

In the synchrony judgment paradigm, observers judge whether a click precedes or follows the onset of a light flash and, on other trials, whether or not a click precedes light termination. The interclick interval defines the duration of visible persistence. An elaboration of this method consists of two phases: In Phase 1, the luminance of a reference stimulus is psychophysically matched to the peak brightness of the test flash. Five luminance values between .1 and 1.0 of the reference stimulus are used subsequently. In Phase 2, a random one of the five reference stimuli, a test flash, and a click are presented; the observer judges whether the click occurred before or after the brightness of test flash reached the reference value (on onset trials) or decayed below it (on termination trials). This method was validated on 3 subjects with test stimuli whose luminance rises and decays slowly in time, and then was used to trace out the precise subjective rise and decay (temporal brightness response function) of brief flashes.     

Abnormalities of visual evoked potentials by checkerboards in children with specific reading disability

Visual evoked potentials by checkerboards of varying check sizes were recorded in the two hemispheres of 16 specific reading disabled and 8 normal children. In most of the disabled subjects a gross hemisphere asymmetry was assessed, while in the control group the usual evoked potential symmetry was observed. In some disabled subjects the evoked potentials had a larger amplitude in the right hemisphere, while in others the amplitude was larger in the left hemisphere. In a small subgroup the evoked potentials were symmetrical, but they had a smaller amplitude than in the control subjects. The results, giving evidence of a dysfunction in basic visual processing, are discussed in the context of current literature on clinical subgroups and the interhemispheric relationship in the dyslexic syndrome.     

Attentional factors in iconic memory and visible persistence

Arrays of 12 fragmentary letters were followed, at various intervals, by one randomly selected complementary letter fragment. On half the trials a partial-report cue (a vertical bar-marker) coincided with the second completion fragment. In Experiment 1 subjects fixated the centre of the to-be-identified letter location, whereas in Experiment 2 subjects fixated the centre of the array. In Experiment 1, the degree of integration and time between successive fragments were inversely related. Integration of fragmentary letters in Experiment 2, however, was uniformly low in the experimental and guessing control conditions. The results were discussed in terms of recent non-traditional accounts of iconic storage that emphasize post-categorical processing.     

Hemispheric symmetry in duration of visible persistence

Nine right-handed subjects performed a visual task requiring perceptual integration of a pattern whose parts were displayed sequentially in time. Correct performance on the task depended critically on the simultaneous visibility of all parts of the pattern; duration of visible persistence could therefore be gauged by varying the duration of the temporal interval between successive portions of the display. The pattern was displayed either foveally or parafoveally in either visual field. Analysis of overall performance and of distribution of errors at each temporal interval revealed more accurate performance for foveal displays but no hemispheric asymmetries in duration of visible persistence. These and other results reported in the literature are interpreted in terms of Moscovitch’s (1979) information-processing model of hemispheric functioning.     

Functional neuroimaging studies of reading and reading disability (developmental dyslexia)

Converging evidence from a number of neuroimaging studies, including our own, suggest that fluent word identification in reading is related to the functional integrity of two consolidated left hemisphere (LH) posterior systems: a dorsal (temporo-parietal) circuit and a ventral (occipito-temporal) circuit. This posterior system is functionally disrupted in developmental dyslexia. Reading disabled readers, relative to nonimpaired readers, demonstrate heightened reliance on both inferior frontal and right hemisphere posterior regions, presumably in compensation for the LH posterior difficulties. We propose a neurobiological account suggesting that for normally developing readers the dorsal circuit predominates at first, and is associated with analytic processing necessary for learning to integrate orthographic features with phonological and lexical-semantic features of printed words. The ventral circuit constitutes a fast, late-developing, word identification system which underlies fluent word recognition in skilled readers.     

Phonemic analysis and severe reading disability in children

Forty-five first-grade children of average intellectual ability were studied, consisting of one group of average readers, one group with mild reading difficulty, and one group with severe reading disability. A striking deficit in phonemic analysis was observed in children with severe reading disability. These children were unable to segment spoken syllables into individual speech sounds, while children with only mild reading difficulty or none were quite proficient at this skill. In fact, using phonemic analysis scores, it was possible to distinguish the severe reading disability group from the others with perfect accuracy.This research was supported in part by U.S. Public Health Service, Maternal and Child Health Project 916, and by Grants HD-03110 and ES-01104 from the National Institutes of Health.     

Immediate memory functions in reading disability subtypes

This study examined immediate memory processes in specific reading disability subtypes. Three subgroups (n = 15 in each subgroup) of reading disabled children were examined: (a) perceptual-motor disorder, (b) verbal disorder-general, and (c) verbal disorder-specific (memory). The three groups were matched for age and full scale IQ. All children received a memory-for-colors task (Color Span Test) designed to evaluate intra- and intermodal serial memory functioning. Comparison of memory profiles for the three reading disability subtypes revealed that patterns varied depending on mode of stimulus presentation or response. Although all three groups performed considerably below normative levels on each of the four subtests of the Color Span Test, all subjects performed significantly better on verbally presented items than on visually presented items. The findings were interpreted to suggest that these reading disabled children may not consistently utilize verbal strategies for coding or retrieval of information when stimuli are visually presented.     

Specific reading disability: a multiplanar view

In the past three decades a revolution has altered the way society approaches people with disabilities. Social changes resulted in a significant increase in fundamental and applied research that seeks to improve the lives of people with disabilities by facilitating better understanding of the mechanisms, manifestations, prevention, and treatment of functional impairment. Specific Reading Disability (SRD) has benefited from this revolution. This review focuses on the evolution of SRD, new information in its neurobiology and management, and the challenges that remain. Evidence from a wide spectrum of research provides strong support for the role of phonology in Specific Reading Disability. Despite the mounting evidence, the case is far from completely established. Adults with compensated SRD read but still demonstrate disordered phonology (Felton et al. [1990] Brain Language 39:485-497). Whether poor phonology is causal or a covariant remains to be demonstrated. Of children with poor phonology, it is not known how many are poor readers. While phonology is associated with SRD, other studies have questioned the uniqueness of SRD. Challenges have been made to the method of classification, the uniqueness of phonological dysfunction as a mechanism in SRD and the response to treatment. In the final analysis all poor readers may have a common core of dysphonology, independent of whether their reading is discrepant from their IQ.