Attentional Load Effects on Beta Oscillations in Healthy and Schizophrenic Individuals.

Attentional deficits are prominent among the cognitive disturbances found in schizophrenia. Given that schizophrenia is also characterized by abnormalities in high-frequency oscillations, we investigated whether attentional function in schizophrenia is related to abnormalities in high-frequency oscillations in a visual discrimination task in which attentional load was manipulated. Sixteen healthy control subjects (HC) and 23 chronic schizophrenia patients (SZ) discriminated between target discs (p = 0.2) and standard discs (p = 0.8). Attentional load was manipulated by varying the size difference between the target and standard discs across blocks: large (Easy condition), medium (Medium), and small (Difficult). The electroencephalogram was recorded and the oscillations evoked by the standard stimuli were analyzed using the Morlet wavelet transform. Subjects' performance decreased as attentional load increased, but HC and SZ did not differ. Attentional load increased ß phase-locking factor at frontal, parietal, and occipital electrode sites in HC but not SZ. In SZ, however, there was a correlation between the ß attentional load effect and overall d', indicating that high-performing SZ had relatively normal ß attentional load effects. These results show that variations in attentional load are associated with ß oscillations and provide a link between attentional dysfunction and ß-generating neural circuitry in schizophrenia.

Oscillatory dynamics of Gestalt perception in schizophrenia revisited.

BACKGROUND: Abnormalities in γ oscillations (30-100 Hz) in the scalp-recorded electroencephalogram (EEG) have been proposed to reflect neural circuitry abnormalities in schizophrenia. Oscillations in the γ band are thought to play an important role in visual perception, mediating the binding of visual features into coherent objects. However, there is relatively little evidence to date of deficits in γ-mediated processes associated with Gestalt perception in schizophrenia. METHODS: Fourteen healthy control subjects (HC) and 17 chronic schizophrenia patients (SZ) discriminated between illusory Kanisza Squares and No-Square control stimuli, indicating their judgment with a manual button press. Time-frequency decomposition of the EEG was computed with the Morlet wavelet transform. Time-frequency maps of phase locking factor (PLF) values were calculated for stimulus- and response-locked oscillations. RESULTS: HC and SZ did not differ in reaction time, error rate, an early ERP effect associated with Gestalt processing, nor an early visual-evoked γ oscillation. Two response-locked high γ effects had greater PLF for Square than No-Square stimuli in HC, and the reverse pattern in SZ. One of these effects was correlated with thought disorder symptom ratings in SZ. CONCLUSIONS: SZ demonstrated abnormalities in γ oscillations associated with the perception of Gestalt objects, while their early visual-evoked γ activity was mostly normal, contrary to previous results. This study supports the hypothesis that high-frequency oscillations are sensitive to aspects of psychosis.

Predicting inter-hemispheric transfer time from the diffusion properties of the corpus callosum in healthy individuals and schizophrenia patients: a combined ERP and DTI study.

BACKGROUND: Several theories of schizophrenia have emphasized the role of aberrant neural timing in the etiology of the disease, possibly as a consequence of conduction delays caused by structural damage to the white-matter fasciculi. Consistent with this theory, increased inter-hemispheric transmission times (IHTTs) to unilaterally-presented visual stimuli have been reported in patients with schizophrenia. The present study investigated whether or not these IHTT abnormalities could be underpinned by structural damage to the visual fibers of the corpus callosum. METHODS: Thirty three schizophrenia patients and 22 matched controls underwent Event Related Potential (ERP) recording, and a subset of 19 patients and 16 controls also underwent 3T Diffusion-Tensor Imaging (DTI). Unilateral visual stimuli (squares, 2×2 degrees) were presented 6 degrees lateral to either side of a central fixation point. IHTTs (ipsilateral minus contralateral latencies) were calculated for the P1 and N1 components at parietal-occipital sites in current source density-transformed ERPs. The visual fibers of the corpus callosum were extracted with streamline tractography and the diffusion metrics of Fractional Anisotropy (FA) and Mode calculated. RESULTS: While both subject groups exhibited highly significant IHTTs across a range of posterior electrode pairs, and significantly shorter IHTTs from left-to-right hemisphere than vice versa, no significant groupwise differences in IHTT were observed. However, participants' IHTTs were linearly related to their FA and Mode, with longer IHTTs being associated with lower FA and more prolate diffusion ellipsoids. CONCLUSIONS: These results suggest that IHTTs are estimable from DTI measures of white matter integrity. In light of the range of diffusion abnormalities that have been reported in patients with schizophrenia, particularly in frontal fasciculi, these results support the conjecture that schizophrenia is associated with abnormalities in neural timing.

Spatial selection and target identification are separable processes in visual search.

Visual search involves deciding both where to look (spatial selection) and whether any given object is a target or a non-target (identification). The aim of the present study was to determine whether these two functions are separable in performance. Spatial selection was manipulated by an exogenous cue and identification was manipulated by whether a second target appeared after a short or long delay following a first target (the attentional blink, AB). Experiment 1 indicated an additive relation between non-informative spatial cueing and the AB, pointing to independent spatial and identification processes. Experiment 2 tested an informative spatial cue with similar results. Experiment 3 also showed an additive relationship, using a response measure that avoided possible floor effects. We interpret the separability of spatial selection and identification as reflecting the independent operation of dorsal and ventral visual pathways, respectively, at least at the early stages of processing.

Are spatial selection and identity extraction separable when attention is controlled endogenously?

Visual search for a target involves two processes: spatial selection and identity extraction. Ghorashi, Enns, and Di Lollo (2008) found these processes to be independent and surmised that they were carried out along distinct visual pathways: dorsal and ventral, respectively. The two experiments that are described in the present article evaluated this hypothesis. Attentional-blink methodology was combined with voluntary spatial cuing in a visual search task: Intertarget lag was used to manipulate identity extraction; predictive cues were used to signal target locations. Central digit cues in Experiment 1 required participants to identify digits before voluntarily directing attention to a corresponding location, whereas flashed dots in Experiment 2 (indicating an opposite location) required attentional redeployment without prior cue identification. Consistent with the dual-pathway hypothesis, cuing was impaired only when the first target and the number cue competed for ventral-pathway mechanisms. Collectively, the results support the dual-pathway account of the separability of spatial selection and identity extraction.

Spatial cuing does not affect the magnitude of the attentional blink.

Identification of the second of two targets is impaired when the second target is presented less than about 500 msec after the first. Nieuwenstein, Chun, van der Lubbe, and Hooge (2005, Experiment 4) reported that the magnitude of this attentional blink (AB) is reduced when the location of the second target is precued. Here we show how that finding resulted from an artifact brought about by a ceiling imposed by data limitation. Instead of using an accuracy measure, the present work used a dynamic threshold-tracking procedure that was not constrained by a performance ceiling. The results show that, when the ceiling is removed, spatial cuing does not affect and is not affected by the AB. These results are consistent with the hypothesis that cue localization and target identification may take place along separate (dorsal and ventral) visual pathways.

Ignorance is bliss: the role of observer expectation in dynamic spatial tuning of the attentional focus.

When two sequential targets (T1, T2) are inserted in an RSVP stream of distractors, perception of T2 is impaired at intertarget lags shorter than 700 msec. Paradoxically, this deficit disappears when T2 is presented directly after T1 (lag-1 sparing). Visser, Bischof, and Di Lollo (1999) found that lag-1 sparing occurs only when T1 and T2 are presented in the same stream. In contrast, Shih (2000) obtained lag-1 sparing with targets insepa-rate streams. Four experiments addressed this inconsistency and revealed lag-1 sparing with targets in different streams, but only when observers had no foreknowledge of T1's location. We hypothesized that when T1 location is known, attention is focused narrowly on that stream; if T2 then appears in the other stream it is missed, and lag-1 sparing does not occur. When T1 location is not known, attention is focused broadly, encompassing both streams, and lag-1 sparing ensues.