A behavioral evaluation of brain-scan estimates of lesion size

Time-on-task effect in pseudoneglect

Neurologically normal subjects systematically misbisect space during visual line-bisection or similar tasks, generally erring to the left of the veridical center when bisecting horizontal lines, a phenomenon referred to as pseudoneglect. This phenomenon is usually interpreted as enhanced attention toward the left hemispace resulting in an overestimation of the leftward extent of a line. While most studies have examined the role of attention in spatial bias using spatial cueing methods in bisection tasks, Manly et al. (Neuropsychologia 43(12):1721-1728, 2005) proposed an original paradigm in which the participants' alertness was diminished by sleep deprivation or prolonged execution of a line-bisection task. The authors reported a significant rightward shift in attention related to declining alertness, but they did not control eye movements and, consequently, modifications of scanning and fixation strategies with fatigue cannot be ruled out in their study. Here we examine whether a diminution in alertness induced by a 60-min-long Landmark task would diminish (or even reverse) this attentional bias, when eye movements are absent. Participants performed a forced-choice judgment about the location of a transaction mark in relation to the veridical center of a horizontal line. The results confirmed a significant decrease in the leftward bias over the course of the session but, in contrast to the findings of Manly et al. (2005), we did not observe a reverse bias from the left to the right hemispace. The results are discussed within the context of the hemisphere-activation model.

Factor analysis and the cerebral hemispheres: "unlocalized" functions

Previous factor analyses have employed lateralized task tapping processes (functions) whose localizations are thought to be known at the level of the cerebral lobe or finer. Here nine tasks are introduced to factor analysis whose underlying processes have unknown localization. A new function identified as auditory emotional is found to be independent of asymmetry in a visual emotion judgment task, suggesting that lateralized emotion judgments depend on modality specific processes. A second new function identified as visual temporal is found to be independent of asymmetries in visual language recognition, posing constraints on theories that link lateralized language processes to processes dealing with temporal sequencing. Another important finding is that the recognition of dichotic syllables relies on a previously identified process, indicating that the process is better labelled auditory linguistic than auditory lexical and suggesting that lexical status may be irrelevant to auditory language asymmetries. Also, a word naming task is found to produce visual field asymmetry because of a stimulus encoding (visual lexical) process, with no clear evidence found of influence from asymmetric response processes. Taken together, the full set of lateralized processes appear to show high degrees of modality specificity and mutual independence (modularity).

Re-evaluating the effect of unilateral brain damage on simple reaction time to auditory stimulation

Results of a previous experiments designed to test if there is a right hemisphere superiority over the left hemisphere in the capacity to react quickly to a stimulus have not lead to an unambiguous conclusion, although the majority of studies indicate that right-brain-damaged patients are generally slower than left-brain-damaged patients. The present experiment re-examines the speed of reaction time to auditory stimulation in a population of brain-damaged patients. Each case was presented with 120 auditory events and variable inter-trial intervals (from 1 s to 15 s). To ensure homogeneity of the population tested, only patients with a cerebro-vascular accident were chosen. The results show that the right-brain-damaged patients were not generally slower than left-brain-damaged patients. However, a marked deterioration in performance over time was observed in the right-hemisphere group while a small amount of improvement was found in the normal and left-brain-damaged patients. The outcome is discussed in terms of a sustained attention deficit associated with right-hemisphere damage.

Further evidence for focal effect of right hemisphere damage on simple reaction time

Changes of simple visual reaction time were analyzed in two groups of unilateral brain damaged patients in order to evaluate to what extent intrahemispheric localization of lesions affects performance. Possible interactions with size were avoided by selecting a localization criterion, i.e. the median section of lesion, uncorrelated with size of damage. By preliminarly establishing that the distribution of lesions in our hemispheric groups did not differ, we were enabled to confirm that intrahemispheric localization of damage has a different bearing on performance depending on side of lesion. In the right hemisphere group slower performances were generally associated with frontal damage whereas in the left hemisphere group localization did not influence the performance. The use of independent indexes for size and locus of lesion proved the existence of an interaction between these parameters in the right hemisphere group.

Sex differences in visual information processing following right cerebrovascular accidents

This study examined whether males and females who had sustained right CVA's but did not show gross asymmetries in processing left- and right-sided visual information would demonstrate visual hemiimperception when tested with more demanding evaluation procedures. Testing involved tasks of reaction time, oral reading and searching for embedded words and figures. Males and females differed in their response patterns for tasks involving searching for embedded target stimuli; specifically, males required greater lengths of time to locate stimuli in left hemispace, while females showed no such pattern of results. This suggests that closer attention must be paid to screening procedures used to detect hemispatial visual imperceptions, particularly among males.

Reaction time in brain disease: some reflections

Two factors associated with measurements of RT, viz., the role of task complexity and the relationship of RT to locus and size of lesion, are considered. Analysis of the literature suggests that the two factors are interrelated. There appears to be a trend for the complexity gradient to be steeper in patients with widespread brain damage than in those with focal lesions. Although simple RT is generally reported to be slower in patients with right hemisphere lesions, there are indications that complex RT may be slower in patients with left hemisphere lesions. The multiplicity of procedures utilized in different studies is probably responsible for some of the inconsistent findings in the literature. Explicit definition of these procedures and the employment of combinations of them should resolve these inconsistent findings and help to elucidate the relationship between RT and cerebral status.

Continuous reaction times in cerebral dysfunction

The Continuous Reaction Times (CRT) method was used to examine pharmacological, psychiatric, medical and neurological problems in clinical practice where a detailed neuropsychological examination with a test battery was not possible. Either 100 visual or 150 auditory stimuli were given within a period of 6 1/2 or 10 min respectively, and the patients were instructed to react as quickly as possible to every stimulus. The same CRT method was applied to heterogeneous groups of patients with cerebral dysfunction and control subjects. The results demonstrated that it was useful as a screening test for the presence of cerebral dysfunction and was especially sensitive to progressive diseases. Its discriminative power was equivalent to more sophisticated and complex psychological tests. Even if CRT is a valid and reliable method of testing cerebral affection, the neural background for the reaction time process is still unknown and the test did not distinguish between patients with right- or left-hemisphere lesions and was not influenced by the etiology of the disease. CRT is therefore considered a provisional method, suggestive only for the actual diagnostic procedures. Only when the patients have a well-defined diagnosis e.g. a diffuse cerebral affection caused by a drug or a metabolic disorder can CRT be recommended as a measure of the degree of lowered consciousness.

Simple reaction-time changes in patients with unilateral brain damage

Changes of simple visual reaction time were analysed in two groups of unilateral brain-damaged patients in order to evaluate to what extent properties of lesions, clinical parameters and experimental variables might influence speed of motor response. The results confirmed that brain damage, independent of its side, produces a retardation of speed. However, the two hemispheric groups differed in so far as volume of damage had a different bearing depending on side of lesion. In spite of such a difference the presence of a general interaction between size of damage and rate of progression of lesion was noted in both the hemispheres, reminiscent of Jackson's concept of 'lesion momentum'. Aphasia was related to a significant retardation of speed in left-hemisphere-diseased patients, although a specific effect of the disturbance of language could not be demonstrated. Experimental variables such as warned vs unwarned stimulation did not affect significantly the performance of brain-damaged patients.

Reaction time and brain disease: relations to location, etiology and progression of cerebral dysfunction

Using the results from 485 patients with various forms of cerebral dysfunctions and from 60 hospitalized controls, it was shown that measurement of continuous reaction times (CRT) is sensitive to cerebral lesions. Reaction times were more impaired by progressive than by non-progressive brain diseases. The test did not distinguish between patients with right- or left-hemisphere lesions and was not influenced either by etiology, chronicity, age or sex. Our conclusion is that CRT is useful as a "screening" test for the presence of cerebral dysfunction and is especially sensitive to progressive diseases. Its discrimination power is equivalent to more sophisticated and complex psychological tests.

Assessment of cognitive asymmetries in brain-damaged and normal subjects: validation of a test battery

A test battery designed to assess cognitive functions normally related to the left and right cerebral hemispheres was validated on 30 patients with unilateral (16 right, 14 left) lesions. The tests were preselected to reflect typical functioning of the hemispheres according to general agreement in the literature. A Cognitive Laterality Quotient (CLQ) was calculated from the difference in performance between the "right" and "left" test batteries and, therefore, reflected the relative functioning attributed to the right and left hemispheres. Using the CLQ measurement and a control group of 30 non-neurological patients matched for age and education, 28 out of 30 brain-damaged patients (93%) were categorised correctly according to side of lesion; the other two were considered to have either abnormal lateralisation (one was left handed) or asymmetrical premorbid cognitive profiles. Using only one (paired) test whose two subparts were designed to vary only slightly in task requirements to measure either right or left functioning, 29 out of 30 patients were correctly categorised. It is suggested that the concept of relative assessment of basic cognitive functions is more fruitful than general assessment of intellectual functions for use in diagnosis and rehabilitation of neurological patients or normal subjects with developmental or acquired behavioural cognitive abnormalities.

Hemispheric differences in dot detection

Two experiments are reported using normal subjects which show a superior detection for dots in the left visual field. This is shown to be more pronounced for stimuli of reduced contrast and for male subjects. The cerebral activation hypothesis of hemispheric asymmetries is tested and found to contribute towards a visual field advantage to some extent but a structural account is also suggested. It is considered that the right hemisphere is prepotent for dealing with simple perceptual stimuli even when there is no spatial component for the task.

The reliability of reaction time determinations

The reliability of simple and two-choice reaction time (RT) as a function of number of trials and measure of central tendency was examined. It was found that, within the particular conditions of the study, 18 trials were sufficient to obtain a highl reliable determination of simple RT but 30 trials were required for a highly reliable determination of two-choice RT. The selection of measure of central tendency was not related to degree of reliability.