Co-Planar Stereotaxic Atlas of the Human Brain—3-Dimensional Proportional System: An Approach to Cerebral Imaging, J. Talairach, P. Tournoux. Georg Thieme Verlag, New York (1988), 122 pp., 130 figs. DM 268

Bartlett viewed thinking as a high level skill exhibiting ballistic properties that he called its “point of no return”. This paper explores one aspect of cognition through the use of a simple model...

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Orienting of attention

The two basic phenomena that define the problem of visual attention can be illustrated in a simple example. Consider the arrays shown in each panel of Figure 1. In a typical experiment, before the arrays were presented, subjects would be asked to report letters appearing in one color (targets, here black letters), and to disregard letters in the other color (nontargets, here white letters). The array would then be briefly flashed, and the subjects, without any opportunity for eye movements, would give their report. The display mimics our. usual cluttered visual environment: It contains one or more objects that are relevant to current behavior, along with others that are irrelevant. The first basic phenomenon is limited capacity for processing information. At any given time, only a small amount of the information available on the retina can be processed and used in the control of behavior. Subjectively, giving attention to any one target leaves less available for others. In Figure 1, the probability of reporting the target letter N is much lower with two accompa­ nying targets (Figure la) than with none (Figure Ib). The second basic phenomenon is selectivity-the ability to filter out un­ wanted information. Subjectively, one is aware of attended stimuli and largely unaware of unattended ones. Correspondingly, accuracy in identifying an attended stimulus may be independent of the number of nontargets in a display (Figure la vs Ie) (see Bundesen 1990, Duncan 1980).

Neural Mechanisms of Selective Visual Attention

: The concept of attention as central to human performance extends back to the start of experimental psychology, yet even a few years ago, it would not have been possible to outline in even a preliminary form a functional anatomy of the human attentional system. New developments in neuroscience have opened the study of higher cognition to physiological analysis, and have revealed a system of anatomical areas that appear to be basic to the selection of information for focal (conscious) processing. The importance of attention is its unique role in connecting the mental level of description of processes used in cognitive science with the anatomical level common in neuroscience. Sperry describes the central role that mental concepts play in understanding brain function. As is the case for sensory and motor systems of the brain, our knowledge of the anatomy of attention is incomplete. Nevertheless, we can now begin to identify some principles of organization that allow attention to function as a unified system for the control of mental processing. Although many of our points are still speculative and controversial, we believe they constitute a basis for more detailed studies of attention from a cognitive-neuroscience viewpoint. Perhaps even more important for furthering future studies, multiple methods of mental chronometry, brain lesions, electrophysiology, and several types of neuro-imaging have converged on common findings.

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The Attention System of the Human Brain

Attention deficit hyperactivity disorder (ADHD) comprises a deficit in behavioral inhibition. A theoretical model is constructed that links inhibition to 4 executive neuropsychological functions that appear to depend on it for their effective execution: (a) working memory, (b) self-regulation of affect-motivation-arousal, (c) internalization of speech, and (d) reconstitution (behavioral analysis and synthesis). Extended to ADHD, the model predicts that ADHD should be associated with secondary impairments in these 4 executive abilities and the motor control they afford. The author reviews evidence for each of these domains of functioning and finds it to be strongest for deficits in behavioral inhibition, working memory, regulation of motivation, and motor control in those with ADHD. Although the model is promising as a potential theory of self-control and ADHD, far more research is required to evaluate its merits and the many predictions it makes about ADHD.

/pdf/behavioral-inhibition-sustained-attention-and-executive-3s4ta68l66.pdf

Behavioral inhibition, sustained attention, and executive functions: Constructing a unifying theory of ADHD.

Visual grasp in corticobasal degeneration

Holmes noted that with hemispheric injuries proprioceptive disturbances were more marked in the distal than proximal limb segments and proposed that this difference was related to the size of cortical sensory representations. An alternative hypothesis is that sensation from distal segments projects to the contralateral hemisphere and sensation from proximal segments projects to both hemispheres. Selective hemispheric anesthesia was used to test these alternative hypotheses and revealed a decrement in distal but not proximal proprioception with hemispheric anesthesia, thereby supporting the bilateral projection hypothesis.

Proprioception more impaired distally than proximally in subjects with hemispheric dysfunction.

According to Liepmann, patients with limb-kinetic apraxia (LKA) have a loss of upper limb deftness-dexterity. Prior studies have revealed in right-handed patients that, whereas injury of the left hemisphere induces an ipsilesional LKA, injury to the right hemisphere does not induce an ipsilesional LKA. There are at least two possible means by which the left hemisphere may influence the deftness of the left hand, either by callosal connections or by ipsilesional corticospinal projections. The purpose of this study was to learn whether a patient with a focal lesion of the corpus callosum had a callosal disconnection LKA. This 57-year-old right-handed man had a memory impairment, and upon brain imaging, was found to have a septum pellucidum cyst, which was causing mild ventricular obstruction to the occipital and temporal horns. He underwent an endoscopic-assisted fenestration of the septum pellucidum. Postoperative imaging revealed a lesion of the mesial portion of his corpus callosum and an assessment of praxis revealed that he had both a limb-kinetic and ideomotor apraxia of his left but not his right hand. The observation that this man had a callosal disconnection LKA of his left hand suggests that in some people it is the left hemisphere's premotor or motor cortex that enables the right hemisphere's motor system to program deft movements of the left hand.

Callosal disconnection and limb-kinetic apraxia

Objective—Patients who misperceive that they are moving their paralysed arm (phantom movements) may not recognise its weakness. Therefore, the relation between phantom limb movements and anosognosia for hemiplegia during selective right hemispheric anaesthesia (the Wada test) was examined. Methods—Nine patients with intractable epilepsy underwent the Wada test. During the right hemispheric injection, after the onset of hemiparesis, anosognosia was assessed by asking patients if they were weak. The patient’s vision was limited such that they could not see the position of their limbs.Phantom movements were tested for by asking patients to attempt to lift their left upper limb, and to demonstrate their left limb’s position by placing their right limb in the same position as their left. Proprioception was tested by lifting the patient’s paretic upper limb and having patients demonstrate this position by lifting their right limb to the same position. Results—Three patients experienced left phantom limb movements, and five were anosognosic for their hemiplegia.However, phantom movement occurred in only one patient with anosognosia. The other two patients with phantom movement were without anosognosia. The patient with phantom movement and anosognosia had impaired proprioception. The two patients with phantom movement but without anosognosia had intact proprioception. Conclusions—Phantom movement in the presence of a proprioceptive deficit could contribute to anosognosia. However, anosognosia and phantom movement are dissociable; therefore phantom movement cannot alone account for anosognosia. Because phantom movement occurred with and without proprioceptive deficits, proprioceptive loss is not a prerequisite for phantom movement. (J Neurol Neurosurg Psychiatry 2000;69:820‐823)

https://jnnp.bmj.com/content/jnnp/69/6/820.full.pdf

Dissociation of anosognosia and phantom movement during the Wada test

In hemiplegics, anosognosia (unawareness of deficit) rests on a mismatch between expected and actual movement: a feedback hypothesis emphasizes sensory deficits or neglect, a feedforward hypothesis postulates impaired intention to move. Anosognosia for other problems is less studied. The authors report a man without sensory deficits who was unaware of choreiform movements, except on videotape delay. The authors believe that a feed-forward mechanism underlies his "on-line" unawareness.

Kenneth M. Heilman

Papers

Visual grasp in corticobasal degeneration

Proprioception more impaired distally than proximally in subjects with hemispheric dysfunction.

Callosal disconnection and limb-kinetic apraxia

Dissociation of anosognosia and phantom movement during the Wada test

On-line anosognosia: Unawareness for chorea in real time but not on videotape delay