The interpretation of the mental status examination depends on an understanding of behavioral neuroanatomy. As in the rest of neurology, the examination leads to localization, followed by a differential diagnosis of disease processes. This chapter gives the essentials necessary for localization of mental status abnormalities in the brain. It includes the different cortical lobes and the limbic structures that line the medial aspect of the cerebral hemispheres and contribute to memory and emotions. It covers the essentials only, as it would require the entire book for a complete and exhaustive review of behavioral neuroanatomy.
This chapter has three parts. The first part discusses general brain-behavior principles. They involve hemispheric specialization and cerebral dominance, disconnection phenomena, and the “anomalous dominance pattern.” The second part discusses the regional localization of specific behaviors in the brain. This section assumes a “localizationist” view that testable neurocognitive domains are located in different brain regions or centers. The last part of this chapter challenges this localizationist view and introduces neural networks and circuits. It emphasizes that, although we use a modular centers approach for localization of neurocognitive disorders, the reality is that they arise from changes in complex and distributed, interconnected circuits.
General Brain-Behavior Principles
The brain is composed of two cerebral hemispheres (telencephalon) along with subcortical white matter and basal ganglia, corresponding thalamic and hypothalamic nuclei (diencephalon), the brain stem, and the cerebellum. The six-layered neocortex is a convoluted structure, organized into gyri and sulci, which spans four major lobes: frontal, temporal, parietal, and occipital. The brain contains approximately 86 billion neurons, with 100 trillion synapses, resulting in numerous and complex networks. Histological areas of the brain are composed of “Brodmann areas” (BA) ( Fig. 4.1 ), which aid in localizing brain-behavior functions.
A basic principle of brain organization of importance to the mental status examination is the presence of hemispheric specialization and the related concept of cerebral dominance. The strongest evidence of differences between the two brains comes from the occurrence of distinct disorders from focal lesions in the left and right hemispheres ( Table 4.1 ). Further evidence for hemispheric specialization comes from studies on “split-brain” patients. To prevent seizure generalization, some patients have undergone a surgical transection of the corpus callosum, the main interhemispheric fiber bundle, with resultant inability to transfer information between the two hemispheres. The split hemispheres may manifest different emotional states along with interhemispheric cognitive differences evident on specialized testing of each side separately ( Fig. 4.2 ). With the left hand they may not be able to name (tactile anomia), perform learned motor movements (ideomotor apraxia), or write without linguistic errors (agraphia), whereas with their right hand they may not be able to copy or perform visuospatial tasks (dyscopia, left hemineglect).
| Left Hemisphere Abnormalities |
| Aphasias |
| Alexia and agraphia |
| Ideomotor apraxia |
| Acalculia and Gerstmann syndrome |
| Loss of detail in drawings/constructions |
| Somatotopagnosia or autotopagnosia |
| Associative visual agnosia |
| Color anomia or color agnosia |
| Right Hemisphere Abnormalities |
| Visuospatial difficulties |
| Abnormal visuospatial constructions with decreased overall pattern |
| Environmental disorientation or topographagnosia |
| Dressing apraxia |
| Left hemispatial neglect |
| Anosognosia |
| Aprosodia and amusia |
| Face discrimination difficulties |
| The following lists are not exhaustive, and localization is approximate for many items. B = bilateral; L = left; R = right. |
| Frontal Lobes |
| Abstraction impairment on idioms and proverbs (B, L>R) |
| Amusia and aprosodia, expressive (R) |
| Apathy, abulia, and akinetic mutism (B) |
| Aphasia, non-fluent or Broca (L) |
| Aphemia or isolated verbal language expression difficulty (L) |
| Attentional disengagement (R) |
| Complex problem-solving difficulty (L>R) |
| Compulsive or repetitive behaviors (B?) |
| Delusions or impaired reality testing (especially with temporal limbic involvement) (R) |
| Depression (L) |
| Design fluency impairment (L) |
| Disconnection ideomotor apraxia (L) |
| Disinhibition (B) |
| Dysexecutive syndrome (B) |
| Emotional lability (R>L) |
| Empathy impairment (R>L) |
| Environmental dependency, echolalia, echopraxia, imitation/utilization behavior (B) |
| Feedback responsiveness impairment (B) |
| Frontal lobe personality change (R) |
| Insight impaired and loss of personal and social awareness (R>L) |
| Jocular affect, euphoria (B, R>L) |
| Judgment impairment (B) |
| Manic-like affect (R) |
| Motor programming difficulty (B, L>R?) |
| Perseveration, intrusions, stimulus-bound (B, R>L?) |
| Phonagnosia, or inability to identify voices (R) |
| Psychomotor slowed, poor initiation (B) |
| Response inhibition impairment (B) |
| Set changing difficulty (L>R) |
| Social dysregulation (R>L) |
| Social judgment, tact, and social responsibility impairments (R>L) |
| Theory of mind impairment (R>L) |
| Verbal fluency or word-list generation impairment (L) |
| Working memory impairment (B) |
| Temporal Lobes (Lateral Neocortical and Mesial Limbic) |
| Amnesia, particularly declarative, episodic memory loss (B) |
| Amusia, receptive difficulty in music appreciation (B, R>L) |
| Aphasia, fluent, Wernicke (L) |
| Aprosodia, receptive difficulty in the melody and intonation of language (R) |
| Auditory agnosia (B, R>L) |
| Auditory perception disturbances (B, R>L) |
| Biological motion, difficulty in detection (B, R>L) |
| Conduction aphasia (L) |
| Contralateral superior quadrantanopia (L or R) |
| Cortical deafness (B) |
| Emotional empathy impairment (R) |
| Emotional learning or conditioning impairment (B) |
| Emotional reaction impairment (B) |
| Expert visual system disturbance (B, R>L) |
| Kluver-Bucy (placid, hypermetamorphosis, hyperoral, hypersexual, visual agnosia) (B) |
| Landmark agnosia (R) |
| Prosopagnosia, or impaired recognition of familiar faces (B, R>L) |
| Pure word deafness (B or L) |
| Rhythm agnosia (L) |
| Semantic anomia (L) |
| Semantic knowledge difficulty (B) |
| Sense of perceptual familiarity altered (R) |
| Social concepts impaired (R) |
| Surface agraphia (L) |
| Temporal lobe personality changes (B, R or L) |
| Theory of mind impairment (R>L) |
| Topographagnosia, or environmental disorientation from landmark agnosia (B, R>L) |
| Visual hallucinations, formed (R) |
| Visual word form alexia (L) |
| Word-selection anomia (L) |
| Parietal Lobes |
| Acalculia, or abnormal calculation (L>R) |
| Agency attribution impairment (R) |
| Agraphia, or writing disturbance (L) |
| Akinetopsia, or abnormal movement detection (R) |
| Alexia, or reading disturbance (L) |
| Alexithymia, or inability to recognize one’s feelings (R) |
| Anarithmetia, or loss of basic mathematical processes (L) |
| Anosodiaphoria, or loss of concern for illness (R) |
| Anosognosia, or loss of awareness or denial of illness (R) |
| Aphasia, transcortical sensory (L) |
| Asomatognosia, or the loss of awareness of parts of one’s body (R) |
| Attentional disengagement and disorientation (B, R>L) |
| Balint syndrome, or simultanagnosia, optic ataxia, oculomotor apraxia (B) |
| Body identity disorders or distortions (R) |
| Constructions with impaired basic form (R) |
| Constructions with impaired detail (L) |
| Contralateral inferior quadrantanopia (L and R) |
| Depersonalization or derealization (R) |
| Depth perception impairment (global stereopsis, monocular depth cues) (R) |
| Digit agnosia or difficulty knowing fingers or toes, and right-left confusion (L) |
| Dressing apraxia (R) |
| Egocentric spatial disorientation (R) |
| Environmental disorientation from topographic memory impairment (R) |
| Facial discrimination impairment (R) |
| Gerstmann syndrome (acalculia, agraphia, right-left confusion, digit agnosia) (L) |
| Gestalt grouping perceptual impairment (R) |
| Ideomotor apraxia (L) |
| Idiokinetic apraxia (L>R)) |
| Neglect, hemispatial, contralateral sensory or body, or conceptual (R) |
| Oculomotor apraxia (B, R>L) |
| Optic ataxia (B, R>LR) |
| Planotopokinesia, or impaired map reading (R) |
| Spatial acalculia (R) |
| Spatial agnosia including visual search difficulty (R) |
| Visual line orientation impairment (R) |
| Occipital Lobes |
| Alexia without agraphia (L) |
| Apperceptive visual agnosia (B) |
| Associative visual agnosia (L) |
| Body form and body action agnosia (B) |
| Color agnosia, color anomia, and color aphasia (L) |
| Cortical blindness (B) |
| Figure-ground disturbance (B, R>L) |
| Hemiachromatopsia (L or R) |
| Homonymous hemianopsia (L or R) |
| Stereopsis disturbances (B) |
| Visual hallucinations, unformed (B) |
| Visual illusions (R>L) |
| Visual synthesis disturbance (B, R>L) |
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