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

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 β phase-locking factor at frontal, parietal, and occipital electrode sites in HC but not SZ. In SZ, however, there was a correlation between the β attentional load effect and overall d′, indicating that high-performing SZ had relatively normal β attentional load effects. These results show that variations in attentional load are associated with β oscillations and provide a link between attentional dysfunction and β-generating neural circuitry in schizophrenia

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 interhemispheric 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—30 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 occipital-temporal sites in current source densitytransformed 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 ultimately underpinned by abnormalities in neural timing

Attentional spatial selection and identity extraction are separable cognitive processes

While searching for objects in a cluttered environment, observers confront two tasks: selecting where to search, and identifying the targets. Chapter 1 reviews major theories of visual search, and highlights their approaches to these two functions. While anatomical, neurological, and behavioural evidence suggests dissociation between spatial selection and identity extraction, there is a vast controversy about this issue among visual-search accounts. This review demonstrates that none of these theories has adopted the right tool to independently manipulate the two functions. A new methodology is suggested in which the two functions are manipulated independently using spatial cueing to manipulate localization, and the attentional blink – AB - to manipulate identification (AB: impaired identification of the second of two briefly-displayed sequential targets). In examining the separability of spatial selection and identity extraction, additive-factors logic is adopted: if two factors (here: spatial cueing and AB) influence independent stages of processing, they will have additive effects on the dependent measure. Conversely, whenever additivity occurs, the underlying mechanisms can be assumed to be independent. Experiments in Chapter 2 show that cueing and the AB have additive effects, confirming the hypothesis that the two functions are separable. The results are accounted for by relating them to two major parallel pathways in the visual system: the dorsal and ventral pathways. Based on the characteristics of each pathway, it is plausible to assume that spatial cues (indexing spatial selection) are processed along the dorsal pathway while identification is processed along the ventral pathway. The two functions are therefore separable because they are mediated by mechanisms that are anatomically and functionally distinct. The experiment in Chapter 3 was designed to address contrary evidence regarding the separability of location and identity processing. It shows that those results were due to a procedural, artefactual ceiling. In Chapter 4, a prediction is tested based on the interpretation of results in the first study: if cueing involves both the dorsal and ventral pathways it should interfere with the AB; the results support this prediction. Chapter 5 discusses how these results collectively support the separability of spatial and identity processing, and also discusses future directions.Arts, Faculty ofPsychology, Department ofGraduat

Different attentional mechanisms subserve the attentional blink and visual search

Joseph et al. (1997) found that a search task that was performed efficiently as a single task was impaired under dual-task conditions. This led to the claim that visual search is not performed preattentively. But that claim is questionable because different criteria (efficiency of visual search and mean level of performance) were used to assess attentional requirements under single- and dual-task conditions. In four experiments, I found that the two measures are affected in different ways by attentional manipulations and, therefore, represent different attentional processes. This finding questions the validity of the evidence used by Joseph et al. to substantiate the claim that all forms of visual search require attention.Arts, Faculty ofPsychology, Department ofGraduat