Marc Chaderjian

Asymmetrical visual-spatial attentional performance in ADHD: Evidence for a right hemispheric deficit

This study was designed to confirm the presence of a lateralizing deficit in visual-spatial attention in children with ADHD, to further characterize the nature of this deficit and to specify the likely hemispheric locus of dysfunction. Two versions of the covert orienting of attention procedure which evaluated separately endogenous and exogenous cuing effects were administered to 20 unmedicated children aged 9-12 with ADHD and 20 matched controls. Both groups also underwent thorough psychiatric assessment and testing using the TOVA and the Wisconsin Card Sorting Task (WCST). Children with ADHD showed an asymmetrical performance deficit characterized by a loss of costs on controlled (endogenous) attentional orienting to invalidly cured left visual field targets. The degree of cost asymmetry correlated negatively with the number of categories sorted on the WCST. It was concluded that unmedicated children with ADHD show an asymmetrical performance deficit on the covert orienting procedure characterized by a disruption of right hemispheric attentional mechanisms. This deficit may be related to diminished right hemispheric frontal-striatal catecholamine activity.

Abnormal processing of irrelevant information in attention deficit hyperactivity disorder

The presence of a selective attention deficit in children with attention deficit hyperactivity disorder (ADHD) was investigated by administering a trial-by-trial version of the Stroop Color-Naming Task to children, aged 9-12, with ADHD (n = 19) and age-matched normal control children (n = 19). Performance was evaluated on both interference and facilitation components of the task. On the standard version of the task, with equal numbers of color words and neutral words, children with ADHD showed increased Stroop interference (prolongation of color-naming times by color-incongruent stimuli) but normal amounts of facilitation (speeding of color naming by color-congruent stimuli). This finding suggests that children with ADHD show increased disruption of color-naming performance by task-irrelevant information, probably secondary to decreased attentional control over the interference process. In contrast to findings of studies in adults, both groups of children failed to use an attentional strategy to reduce interference when they were administered blocks of trials that varied their expectancy for color word trials. This precluded a direct test of the diminished control hypothesis. There were no significant correlations between abnormal Stroop performance and impairment on the Continuous Performance Test or the Wisconsin Card Sorting Test or measures of IQ or reading performance. The implications of these findings for our understanding of information-processing deficits in children with ADHD and of the neurobiological underpinnings of these deficits are discussed.

Attentional Asymmetry in Schizophrenia: Controlled and Automatic Processes

Two versions of Posners covert orienting task were administered to 14 drug-free schizophrenic patients and 12 normal controls. In the schizophrenic subjects, automatic orienting to exogenous cues in the right visual field was impaired. However, this lateralizing general deficit was not present when the schizophrenics were able to direct attention effortfully in the second version of the task using endogenous cues. These findings support the hypothesis that there is a deficit in left hemispheric mechanisms mediating visual spatial attention in schizophrenia. However, when schizophrenics are given the opportunity to use an attentional strategy they are able to partially overcome this lateralized processing deficit.

Abnormal processing of irrelevant information in schizophrenia: The role of illness subtype

In a study using a trial by trial version of the Stroop color naming task, we previously found that unmedicated patients with schizophrenia show a pattern of abnormal performance characterized by increased facilitation (speeding) of color-naming, color-congruent words but normal amounts of interference (slowing) of color-naming, color-incongruent words (Carter et al., 1992). Since a similar finding had recently been reported in patients with Parkinsons disease, we suggested that this finding was consistent with hypotheses about the neurobiological substrates of cognitive impairment that draw upon parallel patterns of cognitive performance in the two illnesses. We now report results from an enlarged group of unmedicated patients with schizophrenia that extend our original finding by allowing us to evaluate the role of illness subtype in abnormal performance on the Stroop task. We found that patients with the undifferentiated subtype of the disorder account for the increased Stroop facilitation effect. Patients with the paranoid subtype show their own pattern of abnormal performance, with normal amounts of facilitation and increased interference. These findings are consistent with the results of other studies which suggest that illness subtype is an important source of variability in studies of cognitive functioning in schizophrenia.

Attentional control and word inhibition in schizophrenia

Previous studies have suggested that schizophrenia patients do not utilize contextual information efficiently to modulate attentional performance. The goal of the current study was to compare the utilization of context in modulating responses to irrelevant information on the Stroop task between a group of schizophrenia outpatients and matched controls. A single-trial version of the Stroop task was used to investigate performance on the Stroop task under three expectancy conditions. Eleven schizophrenia outpatients (on and off antipsychotic medication) and sixteen matched controls were tested. The schizophrenia patients showed: (1) augmented facilitation; (2) interference comparable to normals; and (3) normal ability to reduce interference under certain experimental circumstances. Schizophrenia patients were able to utilize contextual information under certain conditions and could modulate the magnitude of irrelevant word interference, although they were not able to overcome the prepotent tendency to read the word during the Stroop task as effectively as normals, which was reflected in greater Stroop facilitation. This suggests that the integrity or impairment of cognitive control functions in schizophrenia is related to the complexity of the context representation required to support that function.

Attentional asymmetry in schizophrenia: The role of illness subtype and symptomatology

1. Patients with undifferentiated and paranoid schizophrenia, and normal controls were compared using 2 versions of the covert orienting of attention procedure which evaluate exogenous (automatic) and endogenous (controlled) cuing mechanisms. 2. For both tasks, attentional performance varied with illness subtype, but in different ways. 3. On measures of automatic orienting undifferentiated patients showed evidence consistent with a mild right visual field deficit, while paranoid showed a reduction of inhibition-of-return, a mechanism which biases against returning to previously attended locations. 4. On measures of controlled orienting only the undifferentiated group showed the asymmetry of costs which has been the emphasis of most previous studies. The pattern of cost asymmetry was similar to that previously associated with prominent negative symptoms. Additionally, the magnitude of cost asymmetry correlated positively with negative symptoms in the overall patient group. 5. These findings show that systematically considering cue type and symptomatology are critical in interpreting varying patterns of performance by different groups of patients with schizophrenia on the covert orienting procedure. The implications of these findings for understanding the psychopathology of attention in schizophrenia and its neurobiological substrates are discussed.

The sum of induced and real motion is not a straight path.

The apparent motion path of a stimulus undergoing simultaneous real motion and orthogonal induced motion (1M) was measured by having subjects draw the perceived path. The average path was not a straight line, but was curved away from the direction of inducer motion. This result is consistent with a previous finding that 1M magnitude increases with stimulus duration, and with a theory that attributes 1Mto the suppression of reflexive eye movements. Implications are discussed for research in which the apparent slope of the motion path of a stimulus undergoing simultaneous 1M and real motion is used as a measure of 1M.

A reevaluation of the effect of velocity on induced motion

Induced motion (IM) was measured as a function of the temporal frequency of inducer oscillation. IM magnitude decreased as frequency increased above 5 Hz. Increasing the amplitude of inducer motion, and thereby its velocity, did not influence the temporal frequency dependence of IM. This suggests that it is the duration of inducer motion, rather than its velocity, that is the critical stimulus feature in studies that report decreased IM with higher frequencies of inducer oscillation. In a separate experiment, the optokinetic nystagmus elicited by the inducing stimulus in the absence of a fixation target displayed frequency-response characteristics similar to those of IM. This finding supports the hypothesis that IM magnitude is proportional to the voluntary effort required to suppress reflexive eye movements while maintaining stable fixation.

Perceived path of oblique motion: horizontal-vertical and stimulus-orientation effects

Subjects adjusted the path of moving stimuli to produce apparent slopes of 45° with respect to horizontal. The stimulus was either a single moving dot or a vertical or horizontal bar. In separate experiments either the stimuli were tracked or fixation was maintained on a stationary fixation target positioned 8 deg to the right of the center of stimulus motion. In both experiments the selected path slopes were in general more horizontal than 45°. This pattern indicates that subjects overestimate the vertical component of motion along an oblique path, and is interpreted as a manifestation of the spatial anisometropy generally termed the ‘horizontal-vertical illusion’. Additionally, paths selected for horizontal bars were more vertical than those for vertical bars. This finding is interpreted in the context of a previous report of the influence of stimulus orientation on perceived velocity.

Effects of lorazepam on the distribution of spatial attention.

Reaction time (RT) to stimulus events was assessed for 2 tasks with different spatial attention demands before and after receiving either a placebo or lorazepam (1 mg). In 1 task (onset), 12 participants responded to the onset of 1 of 5 potential dot targets contained within either a small or large area. In the other task (offset), all 5 targets were illuminated and 12 participants responded to the offset of 1 of them. In the onset task, lorazepam slowed RT equally for both the large and small display areas. In the offset task, substantial impairment was found with the large but not the small display area. Results are consistent with the hypothesis that lorazepam interferes with the processes involved in the movement of spatial attention. The possibility that lorazepam selectively impairs the disengage component of attentional movement is discussed.