Jillian Grose-Fifer

Repetition and semantic priming of nonwords: implications for theories of N400 and word recognition.

ERPs were elicited by two types of orthographically legal, pronounceable nonwords. One nonword set was derived from and resembled real words, whereas the other set did not. Nonwords derived from related root words elicited N400 semantic priming effects similar to those obtained for words, indicating semantic activation of the root words. N400 repetition priming effects from nonderived nonwords were similar to those obtained for words. The elicitation of N400 by only derived nonwords would have suggested it was generated by the activation of word meanings, per se. However, both types of nonwords produced an N400, and an N400 priming effect. Because nonderived nonwords are not associated with meaning, the N400 cannot be generated by semantic activation per se. Rather, the N400 appears to be generated by orthographic/phonological analysis and is attenuated by the top-down feed of semantic information to the orthographic/phonological level.

Newborn infants learn during sleep

Newborn infants must rapidly adjust their physiology and behavior to the specific demands of the novel postnatal environment. This adaptation depends, at least in part, on the infants ability to learn from experiences. We report here that infants exhibit learning even while asleep. Bioelectrical activity from face and scalp electrodes was recorded from neonates during an eye movement conditioning procedure in which a tone was followed by a puff of air to the eye. Sleeping newborns rapidly learned the predictive relationship between the tone and the puff. Additionally, in the latter part of training, these infants exhibited a frontally maximum positive EEG slow wave possibly reflecting memory updating. As newborns spend most of their time sleeping, the ability to learn about external stimuli in the postnatal environment during nonawake states may be crucial for rapid adaptation and infant survival. Furthermore, because eyelid conditioning reflects functional cerebellar circuitry, this method potentially offers a unique approach for early identification of infants at risk for a range of developmental disorders including autism and dyslexia.

Development of accommodation and convergence in infancy

Paraxial photorefraction was used to assess the development of accommodation and convergence in a large sample of infants under 1 year of age. The infants viewed small dolls placed at various distances (200-25 cm). The majority of infants at all ages demonstrated appropriate convergence for target distance, regardless of manifest refractive error. However, accommodation lagged behind convergence in development. Infants under 2 months tended to demonstrate either flat accommodation responses with a fixed plane of focus at around 30 cm, or accommodated appropriately for near targets, but failed to relax their accommodation sufficiently for the more distant targets. Thus, the focussing error increased with increasing target distance. Since the manifest refractive error was estimated by extrapolating the accommodation function to 0 diopters demand, these infants demonstrated spuriously myopic behavior. After 2 months, the majority of infants showing emmetropic behavior had accommodation responses that changed appropriately with target distance. However, infants with myopic or hyperopic manifest refractive errors displayed a variety of accommodative styles.

Evidence for a new conceptualization of semantic representation in the left and right cerebral hemispheres

Two experiments are reported that examined qualitative differences in how semantic information is represented in the two hemispheres. In the first experiment, items that were associatively related but did not share semantic features or membership in semantic categories produced priming when delivered to the LH (RVF) but not to the RH (LVF). In the second experiment items that shared semantic features but were neither associates nor in the same category produced priming in the RH (LVF), but not in the LH (RVF). Together, the two experiments support the theory that, in the right hemisphere, semantic memories are represented within a distributed system, on the basis of semantic features, whereas, in the left hemisphere representations are, as in local models, relatively more holistic, and are connected via associative links.

Priming by natural category membership in the left and right cerebral hemispheres

The cerebral representation of category information was examined in a single word priming paradigm, during which the N400 component of the event-related potential (ERP) was recorded. The visual half-field paradigm was employed in order to selectively stimulate the two hemispheres. To investigate which aspects of category membership are relevant in producing priming, two types of related stimuli were presented. In one condition pairs of exemplars had a higher amount of feature overlap (e.g., MOSQUITO-FLEA) than in the other (e.g., SOFA-VASE). Significant priming was obtained only for stimuli in the high feature overlap condition and then only when these were presented to the left visual field (LVF)/right hemisphere (RH). This finding was interpreted within our recent model of semantic memory wherein the right hemisphere represents items on the basis of distributed individual features, whereas the left hemisphere (LH) represents semantic information locally, within a spreading activation system, where priming occurs exclusively through associative links. It was concluded that knowledge regarding category membership is maintained in the RH, on the basis of feature coding.

The feedback-related negativity (FRN) in adolescents.

This study examined age-related differences in the ERP correlates of external feedback processing (i.e., the feedback-related negativity [FRN]) in adolescent and young adult males, using a simple gambling task involving unpredictable monetary losses and gains of low and high magnitude. The FRN was larger after losses than gains, and was modulated by the magnitude of gains, but not the magnitude of losses, for all participants regardless of age. FRN amplitude was larger in adolescents than adults and also discriminated relatively less strongly between gains and losses in adolescents. In addition, the morphology of the waveform after high losses suggests that feedback in this condition may have been processed less efficiently by adolescents. Our results suggest that, although the FRN in adults and adolescents share some common characteristics, the neural processes that generate the FRN are still developing in midadolescence. These findings are discussed in the context of adolescent risk taking.

Attentional capture by emotional faces in adolescence

Poor decision making during adolescence occurs most frequently when situations are emotionally charged. However, relatively few studies have measured the development of cognitive control in response to emotional stimuli in this population. This study used both affective (emotional faces) and non-affective (letter) stimuli in two different flanker tasks to assess the ability to ignore task-irrelevant but distracting information, in 25 adults and 25 adolescents. On the non-emotional (letter) flanker task, the presence of incongruent flanking letters increased the number of errors, and also slowed participants’ ability to identify a central letter. Adolescents committed more errors than adults, but there were no age-related differences for the reaction time interference effect in the letter condition. Post-hoc testing revealed that age-related differences on the task were driven by the younger adolescents (11-14 years); adults and older adolescents (15-17 years) were equally accurate in the letter condition. In contrast, on the emotional face flanker task, not only were adolescents less accurate than adults but they were also more distracted by task-irrelevant fearful faces as evidenced by greater reaction time interference effects. Our findings suggest that the ability to self-regulate in adolescents, as evidenced by the ability to suppress irrelevant information on a flanker task, is more difficult when stimuli are affective in nature. The ability to ignore irrelevant flankers appears to mature earlier for non-affective stimuli than for affective stimuli.

A NOVEL QUANTITATIVE MEASURE OF TRACE-ALTERNANT EEG ACTIVITY AND ITS ASSOCIATION WITH SLEEP STATES OF PRETERM INFANTS

This study describes the application of a novel quantitative method for classifying patterns of EEG activity that are associated with the predominant sleep-states of newborn infants. Periods in which there are bursts of high-voltage slow wave activity in the EEG that alternate with periods of low-voltage activity are termed Tracé-alternant. During active or REM sleep. Tracé-alternant is absent and EEG activity is characterized by a variable mixture of frequencies including intermittent high frequency (10-20 Hz) activity superimposed on slower frequencies. Results show that an analytic method previously developed in fetal baboons for identifying EEG segments with and without Tracé-alternant successfully distinguishes homologous patterns of EEG activity in preterm infants. This method provides an excellent objective approach for monitoring changes in EEG patterns that are coincident with behaviorally defined sleep states.

Contrast-dependent responses in the human visual system: Childhood through adulthood

Visual evoked potentials (VEPs) to temporal modulation of spatial patterns, recorded from humans ranging in age from 4-42 years, demonstrated that contrast-dependent responses exist in early childhood and change dramatically throughout childhood. Bright or dark isolated-check stimuli were used to emphasize contributions from ON or OFF pathways to the VEP. (ON and OFF pathways constitute one major pair of parallel subsystems, which process brightness [positive-contrast] and darkness [negative-contrast] information, respectively.) The developmental effects observed for each pathway were similar in magnitude and time course, suggesting maturation of a common physiological mechanism dependent on spatial contrast. Childrens responses were more variable and larger than those of adults, and exhibited a relative phase lag. In addition, we recorded transient VEPs to a conventional contrast-reversing checkerboard pattern. The latency of the major positive wave (P100) was found to decrease, while the latency of the initial positive wave (P60) was found to increase, with increasing age. We propose a vector-summation model, which posits a relative decrease in cortical excitation with increasing age, to explain our major findings.

Physiological evidence that a masked unrelated intervening item disrupts semantic priming: Implications for theories of semantic representation and retrieval models of semantic priming

Event-related potentials were recorded in a paradigm where an unrelated word was interposed between two related words. In one condition, the intervening item was masked and in another condition it was not. The N400 component indicated that priming of the related word was disrupted by the intervening item whether it was masked or not. The data are interpreted to be inconsistent with retrieval models of priming.