Rain G. Bosworth

The Effects of Spatial Attention on Motion Processing in Deaf Signers, Hearing Signers, and Hearing Nonsigners

Visual abilities in deaf individuals may be altered as a result of auditory deprivation and/or because the deaf rely heavily on a sign language (American Sign Language, or ASL). In this study, we asked whether attentional abilities of deaf subjects are altered. Using a direction of motion discrimination task in the periphery, we investigated three aspects of spatial attention: orienting of attention, divided attention, and selective attention. To separate influences of auditory deprivation and sign language experience, we compared three subject groups: deaf and hearing native signers of ASL and hearing nonsigners. To investigate the ability to orient attention, we compared motion thresholds obtained with and without a valid spatial precue, with the notion that subjects orient to the stimulus prior to its appearance when a precue is presented. Results suggest a slight advantage for deaf subjects in the ability to orient spatial attention. To investigate divided attention, we compared motion thresholds obtained when a single motion target was presented to thresholds obtained when the motion target was presented among confusable distractors. The effect of adding distractors was found to be identical across subject groups, suggesting that attentional capacity is not altered in deaf subjects. Finally, to investigate selective attention, we compared performance for a single, cued motion target with that of a cued motion target presented among distractors. Here, deaf, but not hearing, subjects performed better when the motion target was presented among distractors than when it was presented alone, suggesting that deaf subjects are more affected by the presence of distractors. In sum, our results suggest that attentional orienting and selective attention are altered in the deaf and that these effects are most likely due to auditory deprivation as opposed to sign language experience.

Visual Field Asymmetries for Motion Processing in Deaf and Hearing Signers

Recently, we reported a strong right visual field/left hemisphere advantage for motion processing in deaf signers and a slight reverse asymmetry in hearing nonsigners (Bosworth & Dobkins, 1999). This visual field asymmetry in deaf signers may be due to auditory deprivation or to experience with a visual-manual language, American Sign Language (ASL). In order to separate these two possible sources, in this study we added a third group, hearing native signers, who have normal hearing and have learned ASL from their deaf parents. As in our previous study, subjects performed a direction-of-motion discrimination task at different locations across the visual field. In addition to investigating differences in left vs right visual field asymmetries across subject groups, we also asked whether performance differences exist for superior vs inferior visual fields and peripheral vs central visual fields. Replicating our previous study, a robust right visual field advantage was observed in deaf signers, but not in hearing nonsigners. Like deaf signers, hearing signers also exhibited a strong right visual field advantage, suggesting that this effect is related to experience with sign language. These results suggest that perceptual processes required for the acquisition and comprehension of language (motion processing in the case of ASL) are recruited by the left, language-dominant, hemisphere. Deaf subjects also exhibited an inferior visual field advantage that was significantly larger than that observed in either hearing group. In addition, there was a trend for deaf subjects to perform relatively better on peripheral than on central stimuli, while both hearing groups showed the reverse pattern. Because deaf signers differed from hearing signers and nonsigners along these domains, the inferior and peripheral visual field advantages observed in deaf subjects is presumably related to auditory deprivation. Finally, these visual field asymmetries were not modulated by attention for any subject group, suggesting they are a result of sensory, and not attentional, factors.

Left-Hemisphere Dominance for Motion Processing in Deaf Signers

Evidence from neurophysiological studies in animals as well as humans has demonstrated robust changes in neural organization and function following early-onset sensory deprivation. Unfortunately, the perceptual consequences of these changes remain largely unexplored. The study of deaf individuals who have been auditorily deprived since birth and who rely on a visual language (i.e., American Sign Language, ASL) for communication affords a unique opportunity to investigate the degree to which perception in the remaining, intact senses (e.g., vision) is modified as a result of altered sensory and language experience. We studied visual motion perception in deaf individuals and compared their performance with that of hearing subjects. Thresholds and reaction times were obtained for a motion discrimination task, in both central and peripheral vision. Although deaf and hearing subjects had comparable absolute scores on this task, a robust and intriguing difference was found regarding relative performance for left-visual-field (LVF) versus right-visual-field (RVF) stimuli: Whereas hearing subjects exhibited a slight LVF advantage, the deaf exhibited a strong RVF advantage. Thus, for deaf subjects, the left hemisphere may be specialized for motion processing. These results suggest that perceptual processes required for the acquisition and comprehension of language (motion processing, in the case of ASL) are recruited (or “captured”) by the left, language-dominant hemisphere.

Effects of iconicity and semantic relatedness on lexical access in american sign language.

Iconicity is a property that pervades the lexicon of many sign languages, including American Sign Language (ASL). Iconic signs exhibit a motivated, nonarbitrary mapping between the form of the sign and its meaning. We investigated whether iconicity enhances semantic priming effects for ASL and whether iconic signs are recognized more quickly than noniconic signs are (controlling for strength of iconicity, semantic relatedness, familiarity, and imageability). Twenty deaf signers made lexical decisions to the 2nd item of a prime-target pair. Iconic target signs were preceded by prime signs that were (a) iconic and semantically related, (b) noniconic and semantically related, or (c) semantically unrelated. In addition, a set of noniconic target signs was preceded by semantically unrelated primes. Significant facilitation was observed for target signs when they were preceded by semantically related primes. However, iconicity did not increase the priming effect (e.g., the target sign PIANO was primed equally by the iconic sign GUITAR and the noniconic sign MUSIC). In addition, iconic signs were not recognized faster or more accurately than were noniconic signs. These results confirm the existence of semantic priming for sign language and suggest that iconicity does not play a robust role in online lexical processing.

Effects of gestational length, gender, postnatal age, and birth order on visual contrast sensitivity in infants.

To investigate effects of visual experience versus preprogrammed mechanisms on visual development, we used multiple regression analysis to determine the extent to which a variety of variables (that differ in the extent to which they are tied to visual experience) predict luminance and chromatic (red/green) contrast sensitivity (CS), which are mediated by the magnocellular (M) and parvocellular (P) subcortical pathways, respectively. Our variables included gestational length (GL), birth weight (BW), gender, postnatal age (PNA), and birth order (BO). Two-month-olds (n = 60) and 6-month-olds (n = 122) were tested. Results revealed that (1) at 2 months, infants with longer GL have higher luminance CS; (2) at both ages, CS significantly increases over a approximately 21-day range of PNA, but this effect is stronger in 2- than 6-month-olds and stronger for chromatic than luminance CS; (3) at 2 months, boys have higher luminance CS than girls; and (4) at 2 months, firstborn infants have higher CS, while at 6 months, non-firstborn infants have higher CS. The results for PNA/GL are consistent with the possibility that P pathway development is more influenced by variables tied to visual experience (PNA), while M pathway development is more influenced by variables unrelated to visual experience (GL). Other variables, including prenatal environment, are also discussed.

Motion detection in normal infants and young patients with infantile esotropia.

The purpose of this study was to investigate asymmetries in detection of horizontal motion in normal infants and children and in patients with infantile esotropia. Motion detection thresholds (% motion signal) were measured in 75 normal infants and in 36 eyes of 27 infants with infantile esotropia (ET), using a forced-choice preferential looking paradigm with random-dot patterns. Absolute motion detection sensitivity and asymmetries in sensitivity for nasalward (N) vs. temporalward (T) directions of motion were compared in normal and patient populations, ranging in age from 1 month to 5 years. In normal infants, N and T thresholds were equivalent under 2.5 months of age, whereas a superiority for monocular detection of N motion was observed between 3.5 and 6.5 months of age. The nasalward advantage gradually diminished to symmetrical T:N performance by 8 months of age, matching that of adults. No asymmetry was observed in 15 normal infants who performed the task binocularly, hence, the asymmetry was not a leftward/rightward bias. In the youngest infantile ET patients tested, at 5 months of age, a nasalward superiority in motion detection was observed and was equivalent to that of same-age normal infants. However, unlike normals, this asymmetry persists in older patients. This greater asymmetry in infantile ET represents worse detection of T than N motion. This is the first report of an asymmetry in motion detection in normal infants across a wide age range. Initially, motion detection is normal in infants with infantile esotropia. Cumulative abnormal binocular experience in these patients may disrupt motion mechanisms.

Effects of prematurity on the development of contrast sensitivity: Testing the visual experience hypothesis

In order to investigate the effects of visual experience on early visual development, the current study compared contrast sensitivity across infants born with different degrees of moderate-to-late prematurity. Here the logic is that at any given postterm age, the most premature infants will have the oldest postnatal age. Given that postnatal age is a proxy for visual experience, the visual experience hypothesis predicts that infants who are more premature, yet healthy, should have higher sensitivity. Luminance (light/dark) and chromatic (red/green) contrast sensitivities (CS) were measured in 236 healthy infants (born -10 to +2 weeks relative to due date) between 5 and 32 weeks postterm age from due date and 8-38 weeks postnatal from birth date. For chromatic CS, we found clear evidence that infants who were most premature within our sample had the highest sensitivity. Specifically, 4-10 additional weeks of visual experience, by virtue of being born early, enhanced chromatic CS. For luminance CS, similar but weaker results were seen. Here, only infants with an additional 6-10 weeks of visual experience, and only at later age points in development, showed enhanced sensitivity. However, CS in preterm infants was still below that of fullterm infants with equivalent postnatal age. In sum, these results suggest that chromatic CS is influenced more by prematurity (and possibly visual experience) than luminance CS, which has implications for differential development of parvocellular and magnocellular pathways.

Effects of spatial attention on motion discrimination are greater in the left than right visual field

In order to investigate differences in the effects of spatial attention between the left visual field (LVF) and the right visual field (RVF), we employed a full/poor attention paradigm using stimuli presented in the LVF vs. RVF. In addition, to investigate differences in the effects of spatial attention between the dorsal and ventral processing streams, we obtained motion thresholds (motion coherence thresholds and fine direction discrimination thresholds) and orientation thresholds, respectively. The results of this study showed negligible effects of attention on the orientation task, in either the LVF or RVF. In contrast, for both motion tasks, there was a significant effect of attention in the LVF, but not in the RVF. These data provide psychophysical evidence for greater effects of spatial attention in the LVF/right hemisphere, specifically, for motion processing in the dorsal stream.

Visual Sonority Modulates Infants' Attraction to Sign Language.

ABSTRACT The infant brain may be predisposed to identify perceptually salient cues that are common to both signed and spoken languages. Recent theory based on spoken languages has advanced sonority as one of these potential language acquisition cues. Using a preferential looking paradigm with an infrared eye tracker, we explored visual attention of hearing 6- and 12-month-olds with no sign language experience as they watched fingerspelling stimuli that either conformed to high sonority (well-formed) or low sonority (ill-formed) values, which are relevant to syllabic structure in signed language. Younger babies showed highly significant looking preferences for well-formed, high sonority fingerspelling, while older babies showed no preference for either fingerspelling variant, despite showing a strong preference in a control condition. The present findings suggest babies possess a sensitivity to specific sonority-based contrastive cues at the core of human language structure that is subject to perceptual narrowing, irrespective of language modality (visual or auditory), shedding new light on universals of early language learning.