Cerebral Hemispheres and Consciousness

Lockean views of personal identity maintain that we are essentially persons who persist diachronically by virtue of being psychologically continuous with our former selves. In this article, I present a novel objection to this variant of psychological accounts, which is based on neurophysiological characteristics of the brain. While the mental states that constitute said psychological continuity reside in the cerebral hemispheres, so that for the former to persist only the upper brain must remain intact, being conscious additionally requires that a (...) structure originating in the brainstem—the ascending reticular activating system—be functional. Hence, there can be situations in which even small brainstem lesions render individuals irreversibly comatose and thus forever preclude access to their mental states, while the neural correlates of the states themselves are retained. In these situations, Lockeans are forced to regard as fulfilled their criterion of diachronic persistence since psychological continuity, as they construe it, is not disrupted. Deeming an entity that is never again going to have any mental experiences to be a person, however, is an untenable position for a psychological account to adopt. In their current form, Lockean views of personal identity are therefore incompatible with human neurophysiology. (shrink)

If we were to connect two individuals’ brains together, how would this affect the individuals’ conscious experiences? In particular, it is possible for two people to share any of their conscious experiences; to simultaneously enjoy some token experiences while remaining distinct subjects? The case of the Hogan twins—craniopagus conjoined twins whose brains are connected at the thalamus—seems to show that this can happen. I argue that while practical empirical methods cannot tell us directly whether or not the twins share conscious (...) experiences, considerations about the locality of content processing in the brain entails that they most likely do so. (shrink)

Research on electroencephalogram (EEG) characteristics associated with major depressive disorder (MDD) has accumulated diverse neurophysiologic findings related to the content, topography, neurochemistry, and functions of EEG oscillations. Significant progress has been made since the first landmark EEG study on affective disorders by Davidson 35 years ago. A systematic account of these data is important and necessary for building a consistent neuropsychophysiologic model of MDD and other affective disorders. Given the extensive data on frequency-dependent functional significance of EEG oscillations, a frequency (...) domain approach may reveal the types of brain functions involved and disturbed in MDD. In this review, we systematize and integrate diverse and often unconnected observations on the content, topography, neurochemistry, and functions of EEG oscillations involved in MDD within the general concept of an EEG oscillatory pattern. (shrink)

This paper reviews the neuroanatomical bases of language processing in the brain. It argues that the highly detailed anatomical structures that process different aspects of language render any extracorporeal mind superfluous. Though conceivable, the reality of a mind that can exist independently of the brain would make redundant the neural architecture and complex processing mechanisms necessary for the production and understanding of language. Since these structures and mechanisms are manifestly not redundant, how could normal language function be preserved after their (...) destruction by death, allowing the dead to understand or produce language? -/- 1. Introduction -- 2. Language and the Left Hemisphere: Language is Not Just One Thing - A. Broca’s (Expressive) Aphasia - B. Wernicke’s (Receptive) Aphasia - C. Conduction Aphasia -- 3. Language and the Right Hemisphere -- 4. Other Language Functions -- 5. Anatomical, Neurochemical, and Comparative Findings -- 6. Dualism and the Brain. (shrink)

After the first callosotomy surgeries were performed, the general consensus among the medical community was that severing the corpus callosum had relatively little, if any, effect on an individual's behavior. Nearly twenty years later, researchers discovered that, under experimental conditions, the two hemispheres could simultaneously maintain very different interpretations of the same stimulus. These findings immediately called into question the unity of subjective experience, a fundamental characteristic of human consciousness. How could the split‐brain patient not experience any disruption in his (...) experience as a unified self when the two hemispheres are physically and functionally disconnected? In this chapter, we review the research that has led to a conceptualization of the split brain as two minds within one body and the implications of this research for the scientific study of consciousness. We argue that consciousness is a neural function that emerges from the integration of information across available functional modules. (shrink)

Asymmetric distribution of function between the cerebral hemispheres has been widely investigated in the auditory modality. The current approach borrows heavily from visual local–global research in an attempt to determine whether, as in vision, local–global auditory processing is lateralised. In vision, lateralised local–global processing likely relies on spatial frequency information. Drawing analogies between visual spatial frequency and auditory dimensions, two sets of auditory stimuli were developed. In the high–low stimulus set we manipulate frequency information, and in the fast–slow stimulus set (...) we manipulate temporal information. The fast–slow stimuli additionally mimic visual hierarchical stimulus structure, in which the arrangement of local patterns determines the global pattern. Unlike previous auditory stimuli, the current stimulus sets contain the experimental flexibility of visual local–global hierarchical stimuli allowing independent manipulation of structural levels. Previous findings of frequency and temporal range priming were replicated. Additionally, by presenting stimuli monaurally, we found that priming of frequency ranges (but not temporal ranges) was found to vary by ear, supporting the contention that the hemispheres asymmetrically retain traces of prior frequency processing. These results contribute to the extensive literature revealing cerebral asymmetries for the processing of frequency information, and extend those results to the realm of priming. (shrink)

Chicks reared in the absence of light and rat pups reared without extra stimulation fail to exhibit behavioral laterality, implying that a threshold amount of environmental stimulation is necessary for the brain to follow an asymmetry pathway. Reverse asymmetry has been reported in the chick, but not the rat, though a sex difference resembling reverse asymmetry has been found in the rat.

The target article proposes that behavioral asymmetries evolved in response to social pressures, accounting for the unequal distribution of handedness across the population. In contrast, we provide evidence that human handedness reflects individual adaptations that enhance movement skill, and that the distribution across the population is best explained by a genetic polymorphism, either balanced or tending toward fixation for right-handedness.

To assess the ability of the disconnected cerebral hemispheres to recognize images of the self, a split-brain patient was tested using morphed self-face images presented to one visual hemifield at a time while making “self/other” judgments. The performance of the right and left hemispheres of this patient as assessed by a signal detection method was not significantly different, though a measure of bias did reveal hemispheric differences. The right and left hemispheres of this patient independently and equally possessed the ability (...) to self-recognize, but only the right hemisphere could successfully recognize familiar others. This supports a modular concept of self-recognition and other-recognition, separately present in each cerebral hemisphere. (shrink)

Hemisphere differences in word reading were examined using explicit and implicit processing measures. In an inclusion task, which indexes both conscious and unconscious word reading processes, participants were briefly presented with a word in either the right or the left visual field and were asked to use this word to complete a three-letter word stem. In an exclusion task, which estimates unconscious word reading, participants completed the word stem with any word other than the prime word. Experiment 1 showed that (...) words presented to either visual field were processed in very similar ways in both tasks, with the exception that words in the right visual field were more readily accessible for conscious report. Experiment 2 indicated that unconsciously processed words are shared between the hemispheres, as similar results were obtained when either the same or the opposite visual field received the word stem. Experiment 3 demonstrated that this sharing between hemispheres is cortically mediated by testing a split-brain patient. These results suggest that the left hemisphere advantage for word reading holds only for explicit measures; unconscious word reading is much more balanced between the hemispheres. (shrink)

Previous research found that mixed handers were more likely than strong handers to update their beliefs . It was assumed that this was due to greater degrees of communication between the two cerebral hemispheres in mixed handers. Niebauer and Garvey made connections between this model of updating beliefs and metacognitive processing. The current work proposes that variations in interhemispheric interaction contribute to differences in consciousness, specifically when consciousness is used in rumination versus the metacognitive task of self-reflection. Using the Rumination–Reflection (...) Questionnaire , predictions were supported such that strong handedness was associated with self-rumination; whereas, mixed handedness was associated with increased self-reflection p values < .01, . James’s concept of the “fringe of consciousness” is used to make connections between metacognition, updating beliefs, and self-reflection. Several studies are reviewed suggesting that mixed handers experience fringe consciousness to a greater degree than strong handers. (shrink)

What do the relationships observed in the occurrence of various limb, facial, and speech apraxias following left hemisphere damage mean for Corballis's theory? What does the right hemisphere's role in nonpropositional and automatic speech production tell us about the coevolution of right hand preference and speech; how could the possibility that the right hemisphere may be “dominant” for some aspects of speech be accommodated by his theory?

Unlike all other lateral specializations, the necessity for unilateral dominance is clear only for the case of the motor control of the speech organs lying on the midline of the body and innervated from both hemispheres. All functional asymmetries are likely to be a consequence of this asymmetry of executive control.

Corballis seems to have not considered two points: (1) the importance of direct selection pressures for the evolution of handedness; and (2) the evolutionary significance of the polymorphism of handedness. We provide arguments for the need to explain handedness in terms of adaptation and natural selection.

The possibility that two forms of asymmetry underlie handedness is considered. Corballis has proposed that right-handedness developed when gesture encountered lateralized vocalization but may have been superimposed on a preexisting two-thirds dominance. Evidence is reviewed here which suggests that the baseline asymmetry is even more substantial than this, with possible implications for brain anatomy and genetic theories of handedness.

I would like to stress that early development repeats the evolution of the species. Hence, to understand the origins of functional brain asymmetry and the underlying mechanisms involved in handedness, we have to seek information not only from what we know about human evolution, but also from how an early hand preference develops in our own species.

In this commentary we argue that evolution of the human brain to host the language system was accomplished by the selective development of frontal and temporal areas in the left hemisphere. The unilateral development of Broca's and Wernicke's areas could have resulted from one or more transcription factors that have an expression pattern restricted to the left hemisphere.

In this commentaryI evaluate the claim made byKeenan, Nelson, OÕConnor, and Pascual-Leone (2001) that since self-recognition results from right hemispheric activity, self-awareness too is likely to be produced by the activity of the same hemisphere. This reasoning is based on the assumption that self-recognition represents a valid operationalization of self-awareness; I present two views that challenge this rationale. Keenan et al. also support their claim with published evidence relating brain activityand self-awareness; I closelyexamine their analysis of one specific review of (...) literature and conclude that it appears to be biased. Finally, recent research suggests that inner speech (which is associated with left hemispheric activity) is linked to self-awareness—an observation that further casts doubt on the existence of a right hemispheric self-awareness. Ó 2002 Elsevier Science (USA). All rights reserved. (shrink)

Half a Brain is Enough is the extraordinary story of Nico, a three-year-old boy who was given a right hemispherectomy to control his severe intractable epilepsy...

In this commentary I use recent empirical evidence and theoretical analyses concerning the importance of language and the meaning of self-recognition to reevaluate the claim that the right mute hemisphere in commissurotomized patients possesses a full consciousness. Preliminary data indicate that inner speech is deeply linked to self-awareness; also, four hypotheses concerning the crucial role inner speech plays in self-focus are presented. The legitimacy of self-recognition as a strong operationalization of self-awareness in the right hemisphere is also questioned on the (...) basis that it might rather tap a preexisting body awareness having little to do with an access to mental events. I conclude with the formulation of an alternative interpretation of commissurotomy according to self-awareness — a “complete” one in the left hemisphere and a “primitive” one in the right hemisphere. (shrink)

The majority of the chapters in this edition of The Cognitive Neurosciences are new, and those from the first edition have been completely rewritten and updated ...

Grodzinsky's Tree-Pruning Hypothesis can be extended to explain agrammatic comprehension disorders. Although agrammatism is evidence for syntactic modularity, there is no evidence for its anatomical modularity or for its localization in the frontal lobe. Agrammatism results from diffuse left hemisphere damage – allowing the emergence of the limited right hemisphere linguistic competence – rather than from damage to an anatomic module in the left hemisphere.

The volume reviews recent progress on our understanding of memory, consciousness and the brain and identifies a number of acute outstanding problems.

Epilepsy patients who have undergone cerebral commissurotomy exhibit striking behavior patterns, about which many data have been gathered. Philosophers have drawn certain metaphysical and epistemological conclusions from these data, including: That commissurotomy patients possess two streams of consciousness; that they possess two minds; that they are two persons; that all normal human organisms are actually composites of two persons; and that the commonsense concept of the mind must be discarded. I argue that a few such patients possess two stream of (...) consciousness and two minds, yet all are single persons, and that the commonsense concept of mind is so vague that it is pointless to attempt a systematic evaluation of it. (shrink)

The third edition of a work that defines the field of cognitive neuroscience, with extensive new material including new chapters and new contributors.