Biomedical subjects

M C Corballis

Publications and source records attributed to M C Corballis.

At least 37 records · Page 2Linked to original sources

PubMed1997

Mental rotation and the right hemisphere.

Mental rotation may be considered a prototypical example of a higher-order transformational process that is nonsymbolic and analog as opposed to propositional. It is therefore a paradigm case for testing the view that these properties are fundamentally right-hemispheric. Evidence from brain-imaging, unilateral brain lesions, commissurotomy, and visual-hemifield differences in normals is reviewed. Although there is some support for a right-hemispheric bias, at least for the holistic rotation of relatively simple shapes, it is unlikely that this bias approaches the degree of left-hemispheric dominance for language-related skills. An evolutionary scenario is sketched in which the characteristically symbolic mode of the left hemisphere evolved relatively late and achieved the quality of recursive generativity only in the late stages of hominid evolution. This forced an increasingly right-hemispheric bias onto analog processes like mental rotation. Such processes nevertheless remain important and are integral even to those processes we think of as highly symbolic, such as language and mathematics.

Animals↗

PubMed1997

The genetics and evolution of handedness.

At some point in hominid evolution, a mutation may have produced a "dextral" (D) allele, strongly biasing handedness in favor of the right hand and control of speech toward the left cerebral hemisphere. An alternative (chance [C]) allele is presumed directionally neutral, although there are probably other genes that influence asymmetries and that may create a weak bias toward right-handedness (and other asymmetries). Simulations show that the D allele could have spread quite quickly through a population, given even a minuscule advantage of CD heterozygotes over CC and DD homozygotes in terms of reproductive fitness. This heterozygotic advantage would also explain the apparent stability in the relative proportions of left-handers and right-handers. This putative, uniquely human allele may have emerged with the evolution of Homo sapiens in Africa some 150,000 to 200,000 years ago.

Adaptation, Biological↗

PubMed1997

Mental time travel and the evolution of the human mind.

This article contains the argument that the human ability to travel mentally in time constitutes a discontinuity between ourselves and other animals. Mental time travel comprises the mental reconstruction of personal events from the past (episodic memory) and the mental construction of possible events in the future. It is not an isolated module, but depends on the sophistication of other cognitive capacities, including self-awareness, meta-representation, mental attribution, understanding the perception-knowledge relationship, and the ability to dissociate imagined mental states from one's present mental state. These capacities are also important aspects of so-called theory of mind, and they appear to mature in children at around age 4. Furthermore, mental time travel is generative, involving the combination and recombination of familiar elements, and in this respect may have been a precursor to language. Current evidence, although indirect or based on anecdote rather than on systematic study, suggests that nonhuman animals, including the great apes, are confined to a "present" that is limited by their current drive states. In contrast, mental time travel by humans is relatively unconstrained and allows a more rapid and flexible adaptation to complex, changing environments than is afforded by instincts or conventional learning. Past and future events loom large in much of human thinking, giving rise to cultural, religious, and scientific concepts about origins, destiny, and time itself.

Animals↗

PubMed1996

Location of the handedness gene on the X and Y chromosomes.

Accumulated data from five handedness surveys show that concordance for sex is slightly but reliably higher among siblings of the same handedness than among those of opposite handedness. This is consistent with Crow's theory that the genetic locus for handedness is in an X-Y homologous region of the sex chromosomes. The small size of the effect is predicted from genetic models in which there is a substantial random component underlying phenotypic left handedness. The findings are relevant to the putative role of cerebral asymmetry in the aetiology of psychosis.

Chromosome Mapping↗

PubMed1996

Perception of apparent motion across the retinal midline following commissurotomy.

One subject (L.B.) with complete forebrain commissurotomy, another (D.K.) with posterior callosotomy, and 12 normal controls, were shown either single lights, simultaneous pairs, or successive pairs, presented either within the left or right visual fields or bilaterally. Regardless of location, all subjects scored at or near ceiling in discriminating: (1) simultaneous pairs from single lights, (2) successive pairs from single lights, (3) simultaneous pairs from successive pairs, and (4) leftward succession from rightward succession. However, with bilateral presentation, L.B. was often slow to respond to successive lights, and his accuracy in detecting bilateral succession deteriorated when successive presentations were intermixed with simultaneous pairs and single lights. These and other results suggest that three mechanisms may contribute to the discrimination of apparent motion: the detection of simultaneous events, a subcortically mediated switch in attention from first to second location, and cortical tracking between locations. Cortical tracking across the midline is incapacitated following complete forebrain commissurotomy.

Adult↗

PubMed1996

A dissociation in naming digits and colors following commissurotomy.

Pairs of red and green digits were flashed in the opposite visual fields of L.B., a man with complete forebrain commissurotomy. He was highly accurate in naming both digits, the green ones alone, or the red ones alone, regardless of field. In contrast, he was relatively poor at naming the colors of the digits in the left visual field, and in comparing colors between fields. This dissociation suggests either (a) that information about shape was tagged by color and transferred from right to left hemisphere, but information about color itself was not transferred, or, more likely (b) that L.B.'s right hemisphere has acquired the ability to overtly name highly familiar stimuli, such as digits, but not colors.

Adult↗

PubMed1996

The roles of stimulus-response compatibility and mental rotation in mirror-image and left-right decisions.

People were shown rotated letters and timed as they decided (a) whether the letters were normal or backward, or (b) whether a dot was to the left or right of each letter with respect to its upright orientation. In the viewer-centered version of (b), the judgement was to be independent of whether the letter was normal or backward, so that stimulus-response (S-R) compatibility was confounded with angular orientation. In the letter-centered version, the judgement was relative to the letter's own coordinates, so that the confounding between S-R compatibility and orientation was reversed for backward letters. The functions relating RT to angular orientation were parallel across the three tasks, suggesting that S-R compatibility played no role, and that the participants mentally rotated the letters to the upright in each case. A marked increase in RT to backward letters in the letter-centered task suggested a second rotation in depth to restore the letters to normal.

Adult↗

PubMed1996

Predicting reading ability from handedness measures.

Word reading, hand skill on a peg-moving task and hand preference were measured in 203 children aged from 11-13 years. Differences in skill between the hands were largely attributable to variations in left-hand skill, consistent with the view that manual and cerebral asymmetry are due to the loss (pruning) of cells in the right hemisphere during development-a mechanism that may be dependent on the "right shift' (rs+) gene postulated by Annett (1985). There was only weak evidence for the curvilinear relation between differential hand skill and reading predicted by Annett's (1985) theory of balanced polymorphism, which postulates a heterozygotic advantage. Reading was better predicted by absolute measures of hand skill than by the difference between the hands, especially among boys. For the boys, word reading was predicted significantly from right-hand skill, while for girls left-hand skill predicted slightly more strongly than right-hand skill. This may be related to differential growth gradients in the two hemispheres, and different environmental influences for boys and girls during a period of right hemispheric growth.

Adolescent↗

PubMed1995

Visual integration in the split brain.

A review of evidence suggests that subjects with forebrain commissurotomy can integrate information about location, orientation, and movement between visual hemifields, albeit with some loss of spatial and temporal resolution, even when it is flashed briefly in parafoveal vision. The classic disconnection syndrome for visual stimuli is largely explained by the lack of subcortical transfer of information about form. Voluntary location-based ('spotlight') attention is integrated interhemispherically in these subjects but object-based attention and controlled visual search can function concurrently and independently in the two hemispheres. The data support the distinction between two visual systems, each with its own perceptual and attentional subsystems: (1) a relatively precise cortical system dedicated primarily to the analysis of form and the fine-grained exploration of visual scenes; and (2) a 'second' subcortical system dedicated to a relatively low-resolution processing of movement and location, including 'spotlight' attention.

Animals↗

PubMed1994

Split decisions: problems in the interpretation of results from commissurotomized subjects.

It has been widely accepted that there is little if any interhemispheric transfer of perceptual information in subjects following section of the forebrain commissures, although there may be diffuse transfer of emotional and connotative information, and an ability to direct spatial attention between hemispheres. However Sergent has recently shown that split-brained subjects can make rapid and accurate perceptual judgments based on visual stimuli presented simultaneously to the two visual fields. These include judgments of alignment, judgments of relative quantity, and higher-order judgments such as lexical decisions about letter strings straddling the visual fields. However it is premature to conclude that these tasks are accomplished by subcortical transfer of perceptual information; at least some of them can be explained in terms of strategies based on information available to just one hemisphere, or on the transfer of rudimentary information, perhaps accomplished through cross-cueing.

Attention↗

PubMed1994

Can commissurotomized subjects compare digits between the visual fields?

Sergent (Brain 113, 537-568, 1990) flashed pairs of digits to the opposite visual fields of three commissurotomized subjects (L.B., N.G. and A.A.), and found that they were highly accurate in deciding which digit was the larger, but at little or no better than chance in deciding whether the digits were the same or different. Experiment 1 confirmed that these subjects were better at relative than at same-different judgements. However their performance on relative judgements was considerably lower than in Sergent's study and could be explained largely in terms of the subjects' use of information available to a single hemisphere. A fourth subject, D.K., with section of the posterior corpus callosum only, had little difficulty with either task despite previous evidence of visual disconnection, and appeared able to transfer the information verbally. In Experiment 2, L.B. was better able to make both relative and same-different judgements when the digits were presented within either the left or right visual fields than when they were presented in opposite fields. These results suggest little, if any, interhemispheric transfer of either shape or numerical information following complete forebrain commissurotomy.

Adult↗

PubMed1993

Effects of concurrent memory load on visual-field differences in mental rotation.

Normal right-handers mentally rotated letters flashed in the left or right visual fields under different conditions of working-memory load. In Experiment 1, the subjects held patterns of eight dots or sequences of eight letters in working memory, and these conditions produced a progressively increasing right visual field (RVF) advantage in rate of mental rotation relative to a control condition in which there was no load. Experiment 2 confirmed the shift toward increasing RVF advantage with a load of eight digits relative to two control conditions, one in which there was no load and another in which the subjects recalled digits before carrying out the mental rotation on each trial. These results are discussed in terms of the priming of the hemispheres by the concurrent loading of working memory.

Adolescent↗

PubMed1993

Effect of spatial attention on mental rotation.

Normal right-handed subjects performed a task requiring mental rotation of the letter R, presented in varying angular orientations in the left or right visual fields. They were told to attend to the side indicated prior to each trial by a centrally located arrow, while maintaining visual fixation on the arrow. On 75% of trials (compatible trials) the arrow pointed to the side of the letter, while on 25% (incompatible trials) it pointed to the other side. Although overall RT was shorter on compatible than on incompatible trials, there was no evidence that spatial attention affected mental-rotation rate. However, estimated rate was higher when attention was to the left side of space, consistent with right hemispheric specialization for mental rotation. This effect was especially marked when presentation of the letter was to the right.

Adult↗

PubMed1993

How apparent motion affects mental rotation: push or pull?

Subjects were timed as they judged whether a small bar perpendicular to one side of a clockhand would point left or right if the hand was pointing upward (i.e., at the 12:00 position). The clockhand was shown in two successive orientations 30 degrees apart, so that it was perceived to jump from one to the other, but the bar was included at only one of the two orientations. Analysis of reaction times as a function of angular orientation showed that the subjects "mentally rotated" the clockhand to the upright position before making their decisions. When the bar appeared on the second presentation, the jump had no significant influence on mental rotation but when it appeared on the first presentation, the estimated orientation from which the clockhand was mentally rotated was "dragged" in the direction of the jump.

Adult↗

PubMed1992

On the evolution of language and generativity.

One of the properties that most conspicuously distinguishes human language from any other form of animal communication is generativity. Language with this property therefore presumably evolved with the Homo line somewhere between H. habilis and H. sapiens sapiens. Some have suggested that it emerged relatively suddenly and completely with H. sapiens sapiens, and this view is consistent with (a) linguistic estimates as to when vocal language emerged, (b) the relatively late "explosion" of manufacture and cultural artifacts such as body ornamentation and cave drawings, and (c) evidence on changes in the vocal apparatus. However, evidence on brain size and developmental patterns of growth suggests an earlier origin and a more continuous evolution. I propose that these scenarios can be reconciled if it is supposed that generative language evolved, perhaps from H. habilis on, as a system of manual gestures, but switched to a predominantly vocal system with H. sapiens sapiens. The subsequent "cultural explosion" can then be attributed to the freeing of the hands from primary involvement in language, so that they could be exploited, along with generativity, for manufacture, art, and other activities.

Brain↗

PubMed1992

Judgements about numerosity by a commissurotomized subject.

A commissurotomized subject, L.B., was shown asterisks flashed at random locations, up to four in each field, and attempted either to compare the numbers in the two fields or to report the total number. The main results were: (a) Report was more accurate with unilateral than with bilateral presentation, suggesting that the difficulty integrating across fields was partly attentional; (b) in integrating across fields, attention was focused on one field, with only crude 'one-or-many' information from the other; (c) in cross-field comparisons, the focus was on the LVF, but in reporting the number it was on the RVF when report was oral or right-handed, and on the LVF when report was left-handed; (d) cross-field comparisons were improved when the locations were mirrored across the midline.

Adult↗

PubMed1990

The effect of apparent movement on mental rotation.

Eleven subjects were timed as they judged whether a small bar perpendicular to one side of a clockhand would point left or right if the hand was pointing upward (i.e., at the "12 o'clock" position). The clockhand was shown in two successive orientations 30 degrees apart, so that it was perceived to jump from one to the other in either a clockwise or a counterclockwise direction. Reaction times were consistent with the interpretation that the subjects "mentally rotated" the clockhand from its perceived orientation back to the upright before making their decisions. The direction of the jump influenced perceived orientation but did not influence either the direction or rate of mental rotation itself.

Adult↗

PubMed1989

Mental rotation in a commissurotomized subject.

The commissurotomized subject L.B. showed a strong right-hemispheric advantage on a task requiring him to judge rotated letters normal or backward, but a left-hemispheric advantage in a task requiring discrimination of the same letters, implying that the right-hemispheric advantage has to do with mental rotation. However, his right hemisphere proved unable to perform a task requiring mental rotation of a simple unfamiliar pattern to a designated "upright", and was inferior to the left on a task requiring two such patterns, at varying orientations relative to one another, to be judged same or different. We suggest that the deficiency of the right hemisphere on these tasks was unrelated to mental rotation per se, but reflected a failure to comprehend the tasks properly.

Adult↗