Blake W. Johnson

Comparison of the N300 and N400 ERPs to picture stimuli in congruent and incongruent contexts.

OBJECTIVES The aim of this study was to examine the N300 and N400 effect to pictures that were semantically incongruous to a prior object name. Based upon theories of object identification, the semantic incongruity was manipulated to occur early or late in the object processing stream. METHODS High-density visual event-related potentials were measured in response to passively viewed black and white line drawings of common objects. Pictures were preceded with an object name at either the basic (categorical) or subordinate (specific) level. The object either matched or mismatched with the name. With subordinate level names, mismatches could be within- or between-category. RESULTS The N400 effect was found for both basic and subordinate level mismatches. The N400 was found for both the subordinate-within and subordinate-between. Comparison of the scalp distributions between these N400 effects suggested a common effect was found for all conditions. The N300 effect, however, was only found for between-category mismatches, and only when semantic expectations were high in the match baseline (subordinate matches). CONCLUSIONS The findings are consistent with theories of object identification that suggest that objects are initially categorized prior to being identified at more specific levels. The N300 appears to reflect the categorisation while the N400 effect appears to be responsive to all semantic mismatches. Comparison of scalp topographies, functional differences, and different estimated cortical source locations suggest that the N300 and N400 are two distinct semantic effects that reflect aspects of object identification.

Long‐term potentiation of human visual evoked responses

Long‐term potentiation (LTP) is a candidate synaptic mechanism underlying learning and memory that has been studied extensively at the cellular and molecular level in laboratory animals. To date, LTP has only been directly demonstrated in humans in isolated cortical tissue obtained from patients undergoing surgery, where it displays properties identical to those seen in non‐human preparations. Inquiry into the functional significance of LTP has been hindered by the absence of a human model. Here we give the first demonstration that the rapid repetitive presentation of a visual checkerboard (a photic ‘tetanus’) leads to a persistent enhancement of one of the early components of the visual evoked potential in normal humans. The potentiated response is largest in the hemisphere contralateral to the tetanized visual hemifield and is limited to one component of the visual evoked response (the N1b). The selective potentiation of only the N1b component makes overall brain excitability changes unlikely and suggests that the effect is due instead to an LTP process. While LTP is known to exist in the human brain, the ability to elicit LTP from non‐surgical patients will provide a human model system allowing the detailed examination of synaptic plasticity in normal subjects and may have future clinical applications in the assessment of cognitive disorders.

Cerebral asymmetry for mental rotation: effects of response hand, handedness and gender.

We assessed lateralization of brain function during mental rotation, measuring the scalp distribution of a 400–600 ms latency event-related potential (ERP) with 128 recording electrodes. Twenty-four subjects, consisting of equal numbers of dextral and sinistral males and females, performed a mental rotation task under two response conditions (dominant vs non-dominant hand). For males, ERPs showed a right parietal bias regardless of response hand. For females, the parietal ERPs were slightly left-lateralized when making dominant hand responses, but strongly right-lateralized when making non-dominant hand responses. These results support the notion that visuo-spatial processing is more bilaterally organized in females. However, left hemisphere resources may be allocated to response preparation when using the non-dominant hand, forcing visuo-spatial processing to the right hemisphere.

Turn that frown upside down: ERP effects of thatcherization of misorientated faces

When inverted, thatcherized faces appear normal. This may be due to a decrease in configural and an increase in featural processing. It is not known whether this processing is continuous or reflects two distinct processing systems. Using event-related potentials (ERPs), we investigated the Thatcher effect on thatcherized and normal faces at varying orientations. The ERPs paralleled the perceptual illusion. The effect of thatcherization on upright faces was visible in P1 and N170 ERP components, possibly reflecting attentional engagement due to unpleasantness of thatcherized faces. Effects were also found over two later components, the P250 component, which has been related to configural recognition, and a late parietal component possibly reflecting featural processing. The effect of thatcherization on the two later components decreased gradually (for the P250 component) and abruptly (for the late parietal component) as the faces were rotated away from the upright.

High-density mapping in an N400 paradigm: evidence for bilateral temporal lobe generators

OBJECTIVE The aim of this study was to obtain a more detailed description of N400 scalp topography than has previously been reported. METHODS High-density (128 channel) visual event-related potentials were measured in an N400 paradigm using semantically incongruous sentence endings. RESULTS The stimuli elicited an N400 with a centroparietal scalp distribution. In addition, P400s with similar timing and functional characteristics were observed at non-standard recording locations inferior to the temporal lobes. CONCLUSIONS The data are consistent with intracranial evidence for bilateral activation of anterior medial temporal lobe structures. These structures are oriented such that the positive regions of their scalp fields lie largely outside of the area sampled by standard electrode montages. P400s at other non-standard scalp locations, including infraorbital and infraoccipital sites, may reflect volume conduction from the same generators, or activation of non-temporal lobe generators.

Non-identical neural mechanisms for two types of mental transformation: event-related potentials during mental rotation and mental paper folding

Reaction times, accuracy and 128-channel event-related potentials (ERPs) were measured from 14 normal, right-handed subjects while they performed two different parity-judgment tasks that require transformations of mental images: a relatively simple task requiring a single transformation (mental letter rotation), and a more complex task involving a coordinated sequence of transformations (mental paper folding). Reaction times increased monotonically with larger angular displacements from the upright (for mental rotation) and with number of squares carried (for mental paper folding). Both the tasks resulted in amplitude modulation of an approximately 420-700 ms latency ERP component at parietal electrodes. Scalp topographies indicated that right parietal cortex was activated during mental rotation, but bilateral parietal regions were activated during mental paper folding. Our results support the notion of a right hemispheric superiority for tasks involving simple, single mental rotations, but indicate greater involvement of the left hemisphere when a more complex sequence of transformations are required. This task-dependent lability of hemispheric function may account for some of the inconsistent results reported by previous neuroimaging and electrophysiological studies.

Multimodal functional imaging of motor imagery using a novel paradigm

Neuroimaging studies have shown that the neural mechanisms of motor imagery (MI) overlap substantially with the mechanisms of motor execution (ME). Surprisingly, however, the role of several regions of the motor circuitry in MI remains controversial, a variability that may be due to differences in neuroimaging techniques, MI training, instruction types, or tasks used to evoke MI. The objectives of this study were twofold: (i) to design a novel task that reliably invokes MI, provides a reliable behavioral measure of MI performance, and is transferable across imaging modalities; and (ii) to measure the common and differential activations for MI and ME with functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG). We present a task in which it is difficult to give accurate responses without the use of either motor execution or motor imagery. The behavioral results demonstrate that participants performed similarly on the task when they imagined vs. executed movements and this performance did not change over time. The fMRI results show a spatial overlap of MI and ME in a number of motor and premotor areas, sensory cortices, cerebellum, inferior frontal gyrus, and ventrolateral thalamus. MI uniquely engaged bilateral occipital areas, left parahippocampus, and other temporal and frontal areas, whereas ME yielded unique activity in motor and sensory areas, cerebellum, precuneus, and putamen. The MEG results show a robust event-related beta band desynchronization in the proximity of primary motor and premotor cortices during both ME and MI. Together, these results further elucidate the neural circuitry of MI and show that our task robustly and reliably invokes motor imagery, and thus may prove useful for interrogating the functional status of the motor circuitry in patients with motor disorders.

Object-related brain potentials associated with the perceptual segregation of a dichotically embedded pitch.

The cortical mechanisms of perceptual segregation of concurrent sound sources were examined, based on binaural detection of interaural timing differences. Auditory event-related potentials were measured from 11 healthy subjects. Binaural stimuli were created by introducing a dichotic delay of 500-ms duration to a narrow frequency region within a broadband noise, and resulted in a perception of a centrally located noise and a right-lateralized pitch (dichotic pitch). In separate listening conditions, subjects actively discriminated and responded to randomly interleaved binaural and control stimuli, or ignored random stimuli while watching silent cartoons. In a third listening condition subjects ignored stimuli presented in homogenous blocks. For all listening conditions, the dichotic pitch stimulus elicited an object-related negativity (ORN) at a latency of about 150-250 ms after stimulus onset. When subjects were required to actively respond to stimuli, the ORN was followed by a P400 wave with a latency of about 320-420 ms. These results support and extend a two-stage model of auditory scene analysis in which acoustic streams are automatically parsed into component sound sources based on source-relevant cues, followed by a controlled process involving identification and generation of a behavioral response.

Behavioral and multimodal neuroimaging evidence for a deficit in brain timing networks in stuttering: a hypothesis and theory

The fluent production of speech requires accurately timed movements. In this article, we propose that a deficit in brain timing networks is one of the core neurophysiological deficits in stuttering. We first discuss the experimental evidence supporting the involvement of the basal ganglia and supplementary motor area (SMA) in stuttering and the involvement of the cerebellum as a possible mechanism for compensating for the neural deficits that underlie stuttering. Next, we outline the involvement of the right inferior frontal gyrus (IFG) as another putative compensatory locus in stuttering and suggest a role for this structure in an expanded core timing-network. Subsequently, we review behavioral studies of timing in people who stutter and examine their behavioral performance as compared to people who do not stutter. Finally, we highlight challenges to existing research and provide avenues for future research with specific hypotheses.

Neural activity associated with binaural processes for the perceptual segregation of pitch

OBJECTIVE We measured late cortical potentials in a psychophysical procedure for binaural unmasking of a dichotically-embedded pitch. METHODS Late-latency auditory evoked potentials were measured from 128 recording channels in 13 healthy subjects. Control stimuli consisted of 500 ms segments of broadband acoustic noise presented identically to both ears via earphones, evoking a perception of noise localized in the centre of the head. Dichotic pitch stimuli were created by introducing a dichotic delay to a narrow frequency region of the same noise segments, and resulted in a perception of both the centrally-located noise and a right-lateralized pitch. RESULTS Both stimuli evoked late auditory event-related potentials (ERPs) characterized by a P1-N1-P2 complex of waves between 60 and 180 ms after stimulus onset. ERPs associated with the control and dichotic pitch stimuli showed no amplitude differences for the P1 and N1 waves. ERPs to dichotic pitch stimuli became significantly more negative beginning at a latency around 150 ms, an effect that was maximal between 210 and 280 ms. Topographic mapping showed that this late negativity was lateralized to the left hemisphere. CONCLUSIONS The late negative wave elicited by the dichotic pitch stimulus reflects neural processing that is dependent upon binaural fusion within the auditory system. SIGNIFICANCE The dichotic pitch paradigm may provide a useful tool for the electrophysiological assessment and study of the temporal processing capabilities of the auditory system. This paradigm may also be useful for the study of binaural mechanisms for the perceptual segregation of concurrent sound sources.