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P M Corballis

Publications and source records attributed to P M Corballis.

15 recordsLinked to original sources
PubMed2001

Within grasp but out of reach: evidence for a double dissociation between imagined hand and arm movements in the left cerebral hemisphere.

What roles are played by the cerebral hemispheres in planning object-oriented reaching and grasping movements? In an attempt to address this question, we compared the abilities of the left and right hemispheres of commissurotomy patient J.W. to imagine hand manipulation (i.e., grasp) or arm transportation (i.e., reach) movements. A graphically rendered manipulandum (dowel) was briefly presented to the left (LVF) or right (RVF) visual fields in a variety of different orientations. In the grasp selection task (experiment 1), J.W. was required to determine which side of a dowel his thumb would be on if he were to engage the stimulus in a power grip using either his dominant (right) or non-dominant hand. In the reach selection task (experiment 3), J.W. judged which end his elbow would be on if he treated the dowel as an armrest for his dominant or non-dominant forearm. No actual movements were allowed in either task. Movements selected in the imagery tasks were compared with those chosen during actual motor control under comparable circumstances. These comparisons revealed a left hemisphere advantage for representing grasping movements involving the right hand, and reaching movements involving the left arm. The right hemisphere, by contrast, displayed moderate accuracy when representing grasping movements with the left hand, but appeared incapable of imagining reaching movements with either arm. The double dissociation between imagery for hand and arm movements in the left cerebral hemispere is consistent with the hypothesis that grasping and reaching components of prehension involve dissociable planning mechanisms.

Arm↗
PubMed2000

Toward noninvasive 3-D imaging of the time course of cortical activity: investigation of the depth of the event-related optical signal.

The event-related optical signal (EROS) has been recently proposed as a method for studying noninvasively the time course of activity in localized cortical areas (G. Gratton and M. Fabiani, 1998, Psychonomic Bull. Rev. 5: 535-563). Previous data have shown that EROS has very good temporal resolution and can provide detailed surface activity maps. In the present study we investigated whether the depth of the active area can also be estimated. Nine subjects were run in a study in which the eccentricity of the visual stimuli was varied, and EROS was recorded from medial occipital areas using multiple source-detector distances. Seven of the same subjects were also run through a functional magnetic resonance imaging (fMRI) study using the same protocol. The fMRI data indicated that the depth from the head surface to the cortical area activated increased systematically with the eccentricity of the visual stimuli. The EROS recording indicated a response with a latency of 60-80 ms from stimulation. This response varied systematically with eccentricity, so that the greater the eccentricity of the stimuli, the longer the source-detector distance (and thus the depth) at which the EROS effect was observed. The depth of the brain area generating the EROS effect was estimated using a simple algorithm derived from phantom studies on homogeneous media. The average depth estimates for each eccentricity condition obtained with EROS corresponded with those obtained with fMRI, with discrepancies of less than 1 mm. These data demonstrate that multiple source-detector distances can be used to estimate the depth of the cortical areas responsible for the EROS effects.

Adult↗
PubMed2000

Effect of luminance on successiveness discrimination in the absence of the corpus callosum.

Three split-brained subjects, one with full forebrain commissurotomy and two with callosotomy, were impaired at judging whether pairs of lights in opposite visual fields were successive or simultaneous. This impairment did not vary with luminance when the lights were grey against a dark background, but was more pronounced when the lights were equiluminant with a yellow background. All three subjects were also better able to discriminate succession from simultaneity when the lights were both in the left visual field than when they were both in the right. A fourth subject with callosal agenesis was only slightly impaired relative to normal subjects, who were virtually errorless.

Adult↗
PubMed2000

Insights into the functional specificity of the human corpus callosum.

Patient VP underwent complete callosotomy for the control of intractable epilepsy at the age of 27 years. Subsequent MRI, however, revealed spared callosal fibres in the rostral and splenial ends of the corpus callosum. We report a series of experiments designed to determine whether these fibres support functional transfer of information between the two cerebral hemispheres. Although we found no evidence for transfer of colour, shape or size information, there is good evidence for transfer of word information. This suggests that the spared splenial fibres in VP's corpus callosum are material-specific. The results of these experiments illustrate the remarkable degree of functional specificity within the corpus callosum

Brain↗
PubMed1999

A dissociation between spatial and identity matching in callosotomy patients.

Although they are structurally similar, the two hemispheres of the human brain have many functional asymmetries. Some of these, such as language and motor control, have been well characterized. Others, such as visuospatial asymmetries, are less well understood. Many researchers have noted that the right hemisphere appears to be specialized for visuospatial processing. We investigated the abilities of the divided cerebral hemispheres of two callosotomy patients to perform discriminations based on spatial or identity information. The data revealed a robust right-hemisphere superiority for spatial judgments. In contrast, the left hemisphere was somewhat better than the right at making identity judgments. These results suggest that the right hemisphere is specialized for spatial processing, and the left is specialized for pattern recognition.

Brain Mapping↗
PubMed1999

A deficit in perceptual matching in the left hemisphere of a callosotomy patient.

Decades of research have demonstrated dramatic differences between the hemispheres of the brain. While the most obvious asymmetries are in the areas of language and motor control, the visuospatial abilities of the left hemisphere are also known to differ from those of the right hemisphere. This hemispheric difference has been demonstrated empirically but its basis is thus far unclear. In this paper, we investigate the hypothesis that the left hemisphere is capable of sophisticated visual processing, but represents spatial information relatively crudely compared to the right hemisphere. The implication of this hypothesis is that pattern recognition is a function of both hemispheres but the right hemisphere is further specialized for processing spatial information. In a series of seven experiments we examined perceptual matching of mirror-reversed stimuli by the divided cerebral hemispheres of a callosotomy patient. In each experiment the left hemisphere's performance was impaired relative to the right hemisphere. This finding was independent of stimulus type, response bias and stimulus duration. These results are consistent with the idea that visual processing in the left hemisphere is directed towards pattern recognition at the expense of spatial information.

Adult↗
PubMed1999

Illusory contour perception and amodal boundary completion: evidence of a dissociation following callosotomy.

A fundamental problem in form perception is how the visual system can link together spatially separated contour fragments to form the percept of a unitary shape. Illusory contours and amodal completion are two phenomena that demonstrate this linking process. In the present study we investigate these phenomena in the divided hemispheres of two callosotomy ("split-brain") patients. The data suggest that dissociable neural mechanisms are responsible for the generation of illusory contours and amodal completion. Although both cerebral hemispheres appear to be equally capable of perceiving illusory contours, amodal completion is more readily utilized by the right hemisphere. These results suggest that illusory contours may be attributable to low-level visual processes common to both hemispheres, whereas amodal completion reflects a higher-level, lateralized process.

Adult↗
PubMed1997

Fast and localized event-related optical signals (EROS) in the human occipital cortex: comparisons with the visual evoked potential and fMRI.

Localized evoked activity of the human cortex produces fast changes in optical properties that can be detected noninvasively (event-related optical signal, or EROS). In the present study a fast EROS response (latency approximately 100 ms) elicited in the occipital cortex by visual stimuli showed spatial congruence with fMRI signals and temporal correspondence with VEPs, thus combining subcentimeter spatial localization with subsecond temporal resolution. fMRI signals were recorded from striate and extrastriate cortex. Both areas showed EROS peaks, but at different latencies after stimulation (100 and 200-300 ms, respectively). These results suggest that EROS manifests localized neuronal activity associated with information processing. The temporal resolution and spatial localization of this signal make it a promising tool for studying the time course of activity in localized brain areas and for bridging the gap between electrical and hemodynamic imaging methods.

Adult↗
PubMed1995

Shades of gray matter: noninvasive optical images of human brain responses during visual stimulation.

Recent theories about human brain function emphasize the need for imaging methods that allow the study of dynamic interactions among different structures. In this paper, we report on a new technique, based on the measurement of parameters of migration of near-infrared photons, that yields functional images of the human occipital cortex, combining a spatial resolution of 0.5 cm and a temporal resolution of 50 ms. This technique appears to be suitable for studying the dynamics of cortical activation.

Adult↗
PubMed1995

Removing the heart from the brain: compensation for the pulse artifact in the photon migration signal.

Various factors, including variations in the concentration of hemoglobin, determine changes in the transparency of living tissue to near-infrared light. Hence, optical measures have been proposed as a noninvasive method for investigating regional changes in brain activity. However, the amount of near-infrared light traversing a region of the head is also influenced by the periodic changes in blood pressure that occur during the cardiac cycle (pulse). These large changes may obscure smaller, localized events associated with brain activity. We developed a least-squares regression algorithm for compensating for the artifact introduced by the pulse. This procedure takes into account beat-to-beat variability in heart rate and differences in the shape of the pulse among subjects and among recording conditions.

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↗
PubMedDate not supplied

An evolutionary perspective on hemispheric asymmetries.

The evolution of hemispheric specialization of function has obvious benefits in terms of overall processing capacity, but it may also have associated costs to each hemisphere in isolation. These costs are not apparent in an intact brain since information can be readily transferred between the hemispheres via the cortical commissures. We suggest that the evolution of language in the left hemisphere may have occurred at the expense of some visuospatial functions. Because the right hemisphere was still capable of performing those functions, the relative left-hemisphere impairment for visuospatial processing would be invisible. If the cortical commissures are severed, as in callosotomy patients, the costs of specialization may become more apparent. We report data supporting the idea that the left hemisphere may have lost visuospatial abilities that it once possessed, and suggest that this process may also result in right hemisphere deficits in abilities related to linguistic processing.

Animals↗