Age-related alterations in human neocortical plasticity.

Age-related changes in neuroplasticity may be central to the cognitive decline associated with healthy ageing. Modulated Long-Term Potentiation (LTP) and Long-Term Depression (LTD) have been repeatedly demonstrated in aged rodents, however the translation to human research has been limited by a scarcity of non-invasive methods for doing so. We have previously demonstrated that, following a block of high frequency presentations of a visual stimulus (referred to as a "visual tetanus"), there is a LTP-like enhancement of the N1b component of the visually evoked potential (VEP) to subsequent low frequency presentations of the same stimulus. The aims of the current study were, firstly, to use this electroencephalography (EEG) paradigm to assess age group differences in neocortical plasticity in humans, and secondly, to expand on the visual LTP paradigm by examining plasticity in another component of the VEP; the P2a. While a young participant group (N=29, age range=19-35) demonstrated the expected LTP-like enhancement of the N1b immediately following the visual tetanus, an older participant group (N=19, age range=68-91) did not. However, both age groups demonstrated a positive shift of the P2a component after repeated presentations of low frequency baseline blocks, which is hypothesized to be an LTD-like shift in the VEP. These results support the rodent literature indicating an age-related shift in threshold for LTP, but a relative preservation of the threshold for LTD. This study not only provides valuable insight into healthy age-related alterations in neocortical plasticity, but is also the first to identify an LTD-like modulation of the VEP in humans.

Sex hormonal modulation of interhemispheric transfer time.

It is still a matter of debate whether functional cerebral asymmetries (FCA) of many cognitive processes are more pronounced in men than in women. Some evidence suggests that the apparent reduction in women's FCA is a result of the fluctuating levels of gonadal steroid hormones over the course of the menstrual cycle, making their FCA less static than for men. The degree of lateralization has been suggested to depend on interhemispheric communication that may be modulated by gonadal steroid hormones. Here, we employed visual-evoked EEG potentials to obtain a direct measure of interhemispheric communication during different phases of the menstrual cycle. The interhemispheric transfer time (IHTT) was estimated from the interhemispheric latency difference of the N170 component of the visual-evoked potential from either left or right visual field presentation. Nineteen right-handed women with regular menstrual cycles were tested twice, once during the menstrual phase, when progesterone and estradiol levels are low, and once during the luteal phase when progesterone and estradiol levels are high. Plasma steroid levels were determined by blood-based immunoassay at each session. It was found that IHTT, in particular from right-to-left, was generally longer during the luteal phase relative to the menstrual phase. This effect occurred as a consequence of a slowed absolute N170 latency of the indirect pathway (i.e. left hemispheric response after LVF stimulation) and, in particular, a shortened latency of the direct pathway (i.e. right hemispheric response after LVF stimulation) during the luteal phase. These results show that cycle-related effects are not restricted to modulation of processes between hemispheres but also apply to cortical interactions, especially within the right hemisphere. The findings support the view that plastic changes in the female brain occur during relatively short-term periods across the menstrual cycle.

The perception of real and illusory motion in schizophrenia.

An illusion of rapid movement is normally perceived when an attentional cue (such as a peripheral flash) preceeds the onset of a line. The movement is perceived as receding away from the cue. This study investigated how this illusion was perceived by people with schizophrenia. Nineteen participants with schizophrenia and 26 healthy matched controls were presented with a series of real, illusory, no motion or combined real and illusory motion stimuli at various target speeds. Detection thresholds were measured to determine the reliability of motion perception. The participants with schizophrenia were not distinguished from the control group in the perception of real motion. However, the motion detection curves for the schizophrenia group revealed a reduction in the perceptual effect of illusory motion in comparison to controls. The findings revealed that people with schizophrenia may be less easily deceived by illusory motion in comparison to healthy participants.

The effects of illusory line motion on incongruent saccades: implications for saccadic eye movements and visual attention.

A complex neural problem must be solved before a voluntary eye movement is triggered away from a stimulus (antisaccade). The location code activated by a stimulus must be internally translated into an appropriate signal to direct the eyes into the opposite visual field, while the reflexive tendency to look directly at the stimulus must be suppressed. No doubt these extra processes contribute to the ubiquitous slowing of antisaccades. However, there is no consensus on the cognitive mechanisms that contribute to the antisaccade programme. Visual attention is closely associated with the generation of saccadic eye movements and it has been shown that attention will track an illusion of line motion. A series of experiments combined this illusion with a saccadic eye movement that was congruent (i.e. directed towards), or incongruent with (i.e. direct away from), a peripheral target. Experiment 1 showed that congruent saccades had faster reaction times than incongruent saccades. In contrast, Experiments 2 and 3 demonstrated that, with illusory line motion, incongruent saccades now had faster reaction times than congruent saccades. These findings demonstrate that an illusory phenomenon can accelerate the processing of an incongruent relative to a congruent saccade.

Induction of LTP in the human auditory cortex by sensory stimulation.

High-frequency, repetitive, auditory stimulation was used to determine whether induction of a long-lasting increase of the human auditory evoked potential (AEP) was possible. Recording non-invasively with electroencephalogram scalp electrodes, stable increases in amplitude were observed in the N1 component of the AEP, which is thought to reflect activity within auditory cortex (N1). The increase was maintained over an hour and was shown to be independent of alterations in the state of arousal. This is the first demonstration of the induction of long-lasting plastic changes in AEPs, and suggest that this represents the first direct demonstration of long-term potentiation in the auditory cortex of normal, intact humans.

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.

Perceptual and motor mechanisms for mental rotation of human hands.

We measured brain potentials from human subjects performing a mental rotation task requiring right-left judgments of misoriented hands, and a control task requiring palm-back judgments of the same stimuli. High-density, 128-channel event-related potentials (ERPs) were recorded from 16 normal, right-handed subjects. There was a main effect of task at five different latencies: 148 ms (occipital), 180 ms (parietal), 388 ms (vertex), 556 ms (central-parietal), and 900 ms (vertex). Source estimations derived from topographic data indicate that frontal brain regions were strongly activated after 300 ms in the control task, but not until about 900 ms in the rotation task. We conclude that the neural computations underlying mental hand rotation may be recruited from relatively early stages of visuo-perceptual analysis; these early computations influence subsequent processing within a parietal-prefrontal system for the integration of perception with action.

A paradox in the laterality of melody processing.

Melody lateralisation has been examined extensively, using well known behavioural techniques--dichotic/monaural listening and dual tasks. The literature using these techniques has produced paradoxical results for the lateralisation of melody processing particularly with non-musicians. Research using the dichotic/monaural listening paradigm suggests right hemisphere processing for non-musicians while the dual task paradigm has suggested a left hemisphere dominance. The current study utilises both monaural listening and dual task techniques within trials. The results replicated the paradox in the literature. The dichotic/monaural listening melody measure (d') suggest right hemisphere processing of melodies for both musicians and non-musicians. In contrast, the dual task measure suggests left hemisphere processing for the non-musicians and bilateral processing for the musicians. Both measures were collected from the same stimulus presentations and are therefore paradoxical particularly for the non-musicians. Irrespective of which task participants are primarily attending, both laterality measures should produce internally consistent patterns if they are measuring the same aspects of melody processing. Although the paradox for the musicians may be resolved by postulating an attentional bias towards the tapping task, no such explanation is possible for the non-musicians. In light of the paradoxical findings produced by these two measures, their utility for providing unambiguous information about lateralisation of mental processing is questionable.

Object-oriented millisecond timers for the PC.

Object-oriented programming provides a useful structure for designing reusable code. Accurate millisecond timing is essential for many areas of research. With this in mind, this paper provides a Turbo Pascal unit containing an object-oriented millisecond timer. This approach allows for multiple timers to be running independently. The timers may also be set at different levels of temporal precision, such as 10(-3) (milliseconds) or 10(-5) sec. The object also is able to store the time of a flagged event for later examination without interrupting the ongoing timing operation.

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.

Effects of orientation on the identification of rotated objects depend on the level of identity.

Matching names and rotated line drawings of objects showed effects of object orientation that depended on name level. Large effects, in the same range as object naming, were found for rotations between 0 degrees and 120 degrees from upright with subordinate names (e.g., collie), whereas nonsignificant effects were found with superordinate (e.g., animal) and basic names (e.g., dog). These results support image normalization, after contact with orientation-invariant representations, that provide basic-level identity. They consequently fail to support theories of object recognition in which rotated object images are normalized to the upright position before contact with long-term object representations.