Cerebral Cortex

The cognitive operations discussed in this chapter take place among a large network of cortical cells and connections, the neural switchboard that gives rise to conscious thinking. The cortical mantle of the human brain is very large compared with animal brains, containing more than 14 billion neurons. The information stored in the human cerebral cortex rivals that found in large libraries. Within the cortical mantle, the areas that have expanded the most from animal to human are the association cortices, cortical zones that do not carry out primary motor or sensory functions but rather interrelate the functions of the primary motor and sensory areas. According to Nauta and Feirtag’s 1986 text, 70% of neurons in the human central nervous system reside in the cerebral cortex, and 75% of those are in the association cortex. Higher cortical functions, with few exceptions, take place in the association cortex.

The neuroanatomy of the cerebral cortex has been known in considerable detail since the 1800s. Primary cortical sensory areas include the visual cortex in the occipital lobe, the auditory cortex in the temporal lobe, the somatosensory cortex in the parietal lobe, and probably gustatory and olfactory cortices in the frontal and temporal lobes. Each of these primary cortices receives signals in only one modality (vision, hearing, or sensation) and has cortical-cortical connections only to adjacent portions of the association cortex also dedicated to this modality, called unimodal association cortex. Sensory information is sequentially processed in an increasingly complex fashion, leading from raw sensory data to a unified percept. Within each cortical area are columns of cells with similar function, called modules.

The organization of the primary sensory cortex and unimodal association cortex has been especially well worked out in the visual system through the Nobel Prize–winning research of Hubel and Wiesel and others. Retinal ganglion cells are activated by light within a bright center, with inhibition in the surround. These cells project through the optic nerve to the lateral geniculate body of the thalamus, then via the optic radiations to the primary visual cortex in the occipital lobes. In the primary visual cortex, a vertical band of neurons may be dedicated to the detection of a specific bright area, but in the cortex this is usually a bar or edge of light rather than a spot. These “simple” cells of the visual cortex respond to bright central bars with dark surrounds. Several such cells project to complex cells, which may detect an edge or line with a specific orientation or a specific direction of movement but with less specificity about the exact location within the visual field. Visual shapes are perceived by the operation of these cells. Complex cells in turn project to cells in the visual unimodal association cortex (the Brodmann areas 18 and 19), where cells may detect movement or patterns. Complex cells also respond to movement anywhere in the visual field, an important characteristic because of the organism’s need to maintain visual attention for possible hazards in the environment. In the visual association cortex, columns may respond to specific shapes, colors, or qualities such as novelty. In this fashion, the functions of cell columns or modules become more sophisticated from the primary cortex to the association cortex. In Fodor’s model, the modules of primary visual perception project to central systems. Cognitive science has made tremendous strides in the understanding of the neurobiology of specific functions such as vision, but it has yet to fathom the higher perceptual functions such as the concept of beauty in a starry sky or in a painting, or the cross-modality processes that underlie, for example, the adaptation of a ballet to a specific musical accompaniment.

Unimodal association cortices communicate with each other via still more complex connections to the heteromodal association cortex, of which there are two principal sites. The posterior heteromodal association cortex involves the posterior inferior parietal lobe, especially the angular gyrus. The posterior heteromodal cortex makes it possible to perceive an analogy between an association in one modality (e.g., a picture of a boat or the printed word boat in the visual modality) with a percept in a different modality (e.g., the sound of the spoken word boat). These intermodality associations are difficult for animals, even chimpanzees, but easy for human beings. Cross-sensory associations involve the functioning of cortical networks of multitudes of neurons; the analogy drawn by neuroscientists is to the vast arrays of circuits active in computer networks. The product of such associations is a concept.

The second heteromodal association cortex involves the lateral prefrontal region (Goldman-Rakic, 1996). This region is thought to be involved with attention or “working memory” and with sequential processes such as storage of temporally ordered stimuli and the planning of motor activities. This temporal sequencing of information and motor planning is referred to by neuropsychologists as the executive function of the brain—the decisions we make every instant regarding which of the myriad of sensory stimuli reaching the sensory cortices merit attention, which require a motor response, and in what sequence and timing these motor responses will occur.

Another frontal cortical area, the orbitofrontal portion of the prefrontal cortex, is thought to be involved in emotional states, appetites, and drives, or in the integration of internal bodily states with sensations from the external world. The orbitofrontal cortex is known as the supramodal cortex (Benson, 1996) because it relates the functions of the heteromodal cortex regarding attention and sequencing of responses with interoceptive inputs from the internal milieu of the body. The orbitofrontal area has close connections with the limbic system and autonomic, visceral, and emotional processes. In studying brain evolution from primitive reptiles to humans, the neurobiologist Paul MacLean hypothesized that the internal and emotional parts of the brain, the limbic system, must be tied into the newer neocortical areas responsible for intellectual function, and that the linking of these two systems must underlie the phenomenon of consciousness. In a review of neuronal mechanisms of consciousness, Ortinski and Meador (2004) defined conscious awareness as “the state in which external and internal stimuli are perceived and can be intentionally acted on” (p. 1017). Benson and Ardila (1996), in reviewing clinical data from individuals with frontal lobe damage, state that the supramodal cortex is the brain system that “anticipates, conjectures, ruminates, plans for the future, and fantasizes.” In other words, this part of the brain brings specific cognitive processes to conscious awareness and may be responsible for the phenomena of consciousness and self-awareness themselves.

Consciousness

All human beings have a subjective understanding of what it means to be conscious and to have a concept of self, yet the neural basis for conscious awareness and the sense of self remain poorly understood. Until recently, many neuroscientists left the study of consciousness to the realm of religion and philosophy. Even Hippocrates knew that consciousness emanated from the brain, but “to consciousness the brain is messenger.” Francis Crick devoted the last part of his career to the understanding of consciousness. For Crick, the best model for the study of consciousness is visual awareness because the anatomy and physiology of the visual system are well understood. Crick argued that neurons in the primary visual cortex likely do not have access to conscious awareness. Stated another way, we do not pay attention to much of what our eyes see and our visual cortex analyzes. A perceived object, however, excites neurons in several areas of the visual association cortex, each with associations that enter consciousness or are stored in short-term memory.

Crick and Koch (1995) hypothesized that activation of the frontal cortex is necessary for visual percepts to enter consciousness, although subconscious awareness in the form of blindsight may exist at the level of the occipital cortex. Conscious visual perception involves interactions between the visual parts of the brain and the prefrontal systems for attention and working memory (Ungerleider, Courtney, and Haxby, 1998). The orbitofrontal cortex contains neurons that integrate interoceptive stimuli related to changes in the internal milieu with exteroceptive sensory inputs such as vision. Ortinski and Meader (2004) also point out the varying latencies of perception of specific sensory stimuli, such as color versus identification of a visual object. A synchronization of inputs through the thalamus to the cortex may be necessary before the perception becomes conscious. As stated earlier, the interaction between attention to external stimuli and internal stimuli underlies conscious awareness.

In the visual system, Goodale and Milner (1992; Milner and Goodale, 2008; see also McIntosh and Schenk, 2009) have divided the visual system, after processing in the occipital cortex, into a ventral and a dorsal stream. The ventral stream, involved in perception of objects, is usually subject to conscious awareness and involves an occipital-temporal pathway, whereas the dorsal stream, involved in spatial localization of perceived objects to plan action, is usually less conscious.

There are many clinical examples of “unconscious” mental processing, and a number of these involve vision. Patients with cortical blindness sometimes show knowledge of items they cannot see, a phenomenon called blindsight. Patients with right hemisphere lesions who extinguish objects in the left visual field when presented with bilateral stimuli nonetheless show activation of the right visual cortex by functional magnetic resonance imaging (MRI), indicating that the objects are perceived, although not with conscious awareness (Rees et al., 2000). Libet (1999) demonstrated experimentally that visual and other sensory stimuli have to persist at least 500 milliseconds to reach conscious awareness, yet stimuli of shorter duration can elicit reactions. An experimental example of unconscious visual processing comes from Gur and Snodderly (1997), who tested color vision in monkeys. When two colors were projected at a frequency of greater than 10 Hz, the monkey perceived a fused color, yet cellular recordings clearly demonstrated coding of information about the two separate colors in the monkey’s visual cortex. Motor responses to sensory stimuli can occur before conscious awareness, as in the ability to pull one’s hand away from a hot stove before feeling the heat. Racers begin running before they are aware of having heard the starting gun (Crick and Koch, 1998). A familiar example of unconscious visual processing is the drive home from work; most individuals can remember very little they see on the trip, yet they avoid oncoming vehicles and obstacles, stop for red lights, and drive without accidents. Crick and Koch (1998) refer to the unconscious visual processing as an “online” visual system. We shall discuss unconscious or “implicit” memories later in this chapter. In language syndromes, patients can match spoken to written words without knowledge of their meaning, suggesting that there are unconscious rules of language. Brust (2000) has called all of these unconscious mental processes the “non-Freudian unconscious.”

Recent research has linked the right frontal cortex to the sense of self. Keenan and colleagues (2001) studied patients undergoing the Wada test, in which a barbiturate is injected into the carotid artery to determine cortical language dominance. They presented subjects with a self-photograph and a photograph of a famous person, followed by a “morphed” photograph of a famous person and the patient. When the left hemisphere was anesthetized, the subjects said that the morphed photograph represented the subject himself, whereas with right hemisphere anesthesia, the subject selected the famous face. Patients with frontotemporal dementia also indicate a relationship between the right frontal lobe and self-concept. In the series by Miller and colleagues (2001), six of the seven patients who developed a major change in self-concept during their illness had predominant atrophy in the nondominant frontal lobe. A last example of the sense of self is the so-called Theory of Mind, which alludes to the understanding of another person as a conscious human being. Keenan and colleagues (2005) cite evidence that the right hemisphere frontotemporal cortex is dominant for both the sense of self and the recognition of other people.

The frontal lobes, as the executive center of the brain and the determining agent for attention and motor planning, are the origin of several critical networks for cognition and action. Cummings (1993) described five frontal networks for consciousness and behavior. The frontal cortex projects to the basal ganglia, then to thalamic nuclei, and back to the cortex.

Clinical neurology provides important information about how lesions in the brain impair consciousness. The functioning of the awake mind requires the ascending inputs referred to as the reticular activating system, with its way stations in the brainstem and thalamus, as well as an intact cerebral cortex. Bilateral lesions of the brainstem or thalamus produce coma. Very diffuse lesions of the hemispheres produce an “awake” patient who shows no responsiveness to the environment, a state sometimes called coma vigil or persistent vegetative state, as in the well-known Terri Schiavo case (Bernat, 2006; Perry, Churchill, and Kirshner, 2005). Patients with very slight responses to environmental stimuli are said to be in a minimally conscious state (Wijdicks and Cranford, 2005). Recently, functional brain imaging studies have suggested that at least in a few patients labeled as having persistent vegetative state or minimally conscious state after traumatic brain injury, patients can think of playing tennis or standing in their home and seeing the other rooms, and the brain areas activated are similar to those of normal subjects. These same subjects, a small minority of patients with chronically impaired consciousness secondary to traumatic brain injury, showed evidence of conscious modulation of brain activity to indicate “yes” or “no” responses (Monti et al., 2010). This report has engendered controversy over our ability to determine when a patient truly lacks consciousness. In an accompanying editorial, Ropper noted that activation on brain imaging studies does not equal conscious awareness, and the concept that “I have brain activation, therefore I am…would seriously put Descartes before the horse” (Ropper, 2010).

Still less severe diffuse abnormalities of the association cortex produce encephalopathy, delirium, or dementia. These topics involve very common syndromes of clinical neurology. Stupor and coma are discussed in Chapter 5, and encephalopathy, or delirium, is covered in Chapter 4.

Focal lesions of the cerebral cortex generally produce deficits in specific cognitive systems. A detailed listing of such disorders would include much of the subject matter of behavioral neurology. Examples include Broca aphasia from a left frontal lesion, Wernicke aphasia from a left temporal lesion, Gerstmann syndrome (acalculia, left-right confusion, finger agnosia, and agraphia) from a left parietal lesion, visual agnosia or failure to recognize visual objects (usually from bilateral posterior lesions), apraxia from a left parietal lesion, and constructional impairment from a right parietal lesion. Multiple focal lesions can affect cognitive function in a more global fashion, as in the dementias (Chapter 66). Some authorities separate “cortical” dementias such as Alzheimer disease, in which combinations of cortical deficits are common, from “subcortical” dementias, in which mental slowing is the most prominent feature.

The frontal lobes are heavily involved in integration of the functions provided by other areas of cortex, and lesions there may affect personality and behavior in the absence of easily discernible deficits of specific cognitive, language, or memory function. In severe form, extensive lesions of the orbitofrontal cortex may leave the individual awake but staring, unable to respond to the environment, a state called akinetic mutism. With lesser lesions, patients with frontal lobe lesions may lose their ability to form mature judgments, reacting impulsively to incoming stimuli in a manner reminiscent of animal behavior. Such patients may be inappropriately frank or disinhibited. A familiar example is the famous case of Phineas Gage, a worker who sustained a severe injury to the frontal lobes. Gage became irritable, impulsive, and so changed in personality that coworkers said he was “no longer Gage.” Bedside neurological testing and even standard neuropsychological tests of patients with frontal lobe damage may reveal normal intelligence except for concrete or idiosyncratic interpretation of proverbs and similarities. Experimentally, subjects with frontal lobe lesions can be shown to have difficulty with sequential processes or shifting of cognitive sets, as tested by the Wisconsin Card Sorting Test or the Category Test of the Halstead-Reitan battery. Luria introduced a simple bedside test of sequential shapes (Fig. 6.1). In contrast to the subtlety of these deficits to the examiner, the patient’s family may state that there is a dramatic change in the patient’s personality.

Fig. 6.1 Luria’s test of alternating sequences.

(Adapted from Luria, A.R., 1969. Frontal lobe syndromes, In: Vynken P., Bruyn, G.W. (Eds.), Handbook of Clinical Neurology, vol. 2, Elsevier, New York. Reprinted with permission from Kirshner, H.S., 2002. Behavioral Neurology: Practical Science of Mind and Brain, second ed. Butterworth Heinemann, Boston.)