Jeffrey D. Holtzman

A computational analysis of mental image generation: Evidence from functional dissociations in split-brain patients

Recent efforts to build computer simulation models of mental imagery have suggested that imagery is not a unitary phenomenon. Rather, such efforts have led to a modular analysis of the image-generation process, with separate modules that can activate visual memories, inspect parts of imaged patterns, and arrange separate parts into a composite image. This idea was supported by the finding of functional dissociations between the kinds of imagery tasks that could be performed in the left and right cerebral hemispheres of two patients who had previously undergone surgical transection of their corpus callosa. The left hemisphere in both subjects could inspect imaged patterns and could generate single and multipart images. In contrast, although the right hemisphere could inspect imaged patterns and could generate images of overall shape, it had difficulty in generating multipart images. The results suggest a deficit in the module that arranges parts into a composite. The observed pattern of deficits and abilities implied that this module is not used in language, visual perception, or drawing. Furthermore, the results suggest that the basis for this deficit is not a difficulty in simply remembering visual details or engaging in sequential processing.

A left hemisphere basis for visual mental imagery

The lateralization of visual mental imagery was investigated by presenting each hemisphere of a commissurotomy patient with a letter classification task known to require imagery and with a pair of control tasks designed to require all of the same processes as the imagery task except for the imagery processing itself. Whereas both hemispheres performed well on the control tasks, only the left hemisphere performed the imagery task.

MRI assessment of human callosal surgery with neuropsychological correlates

MRI imaging using recovery and spin-echo techniques was carried out on three patients after surgical section of the corpus callosum to control intractable epilepsy. The scans revealed that the total callosotomy had been obtained in two patients, while partial sparing of splenial and rostra1 fibers was seen in the third.

Cortical mechanisms involved in praxis Observations following partial and complete section of the corpus callosum in man

In patients who have undergone complete section of the corpus callosum for intractable epilepsy, lateralized presentation of visual nonverbal stimulation showed that the coordination of motor acts by either hand is controlled exclusively by the contralateral hemisphere. When two patients had serial operations consisting of an initial division of the splenium and posterior 3 cm, followed by complete callosal division, an opportunity arose to test the explicit cortical pathways involved in ipsilateral control. Between operations, these patients could not coordinate movements of the hand ipsilateral to the hemisphere receiving the command. This suggested that for visual nonverbal stimulation, the posterior 3 cm of corpus callosum is necessary for control of the ipsilateral hand; the rostra1 callosum cannot transfer sensorimotor commands. Also, contrary to current views, each hemisphere can carry out sequentially dependent motor activity.

Further characterization of patients with amnesia after cardiac arrest: preserved recognition memory

We observed that patients with amnesia after cardiac arrest had preserved recognition memory despite profound loss of recall memory. In the present study, rate of forgetting was measured in six amnesic subjects for both recall and recognition memory of verbal material. The data show that recall decayed significantly faster for the amnesic subjects than for controls, whereas the rate of forgetting for recognition memory was comparable in both groups. Dissociation between recall and recognition performance is a feature of the amnesic syndrome after cardiac arrest.

Enhanced dual task performance following corpus commissurotomy in humans

A commissurotomy patient and two neurologically intact control observers were required to encode spatial patterns presented concurrently to the two hemifields. Under conditions of maximal perceptual load, in which different patterns appeared in the two fields, the commissurotomy patient encoded more information than the control observers. Based on these findings, it is concluded that competition for common internal processing mechanisms interferes with overall processing efficiency.

Speech without conscious awareness.

Following commissurotomy, it is usually the case that information presented to the left hemisphere can be named and described, while information presented to the mute, right hemisphere cannot be spoken about. In the present study, it was discovered that under special test conditions, an MRI-verified, callosally sectioned adult could name or write about information presented to the right hemisphere. Studies revealed this was not an instance of right hemisphere expression. Rather, the right hemisphere was somehow able to program the left hemisphere for a spoken or written response. Further, the studies also showed that the left hemisphere was not consciously aware that it possessed the information transmitted to it by the right half brain.

The Integration of Motor Control and Visual Perception

A basic strategy in attempting to reach some understanding of the brain and its relation to behavior has been to simplify this forbiddingly complicated task by subdividing brain processes into functional subsystems that can then be studied more or less in isolation. The fruitfulness of this strategy obviously depends in part on an appropriate selection of meaningful subsystems to study, and over the years, various adjustments in this partitioning have been made. One of the most enduring divisions has been that between sensory/perceptual and motorIresponse systems.

The Integrity of Attentional Control Following Commissural Section

One of the most dramatic and reliable behavioral consequences of the surgical transection of the corpus callosum in humans is the perceptual disconnection of the visual world. In general, following callosal surgery each hemisphere only perceives stimuli that are presented to the contralateral visual field: the perception of left field stimuli is isolated to the right hemisphere, and the perception of right field stimuli is isolated to the left hemisphere (Gazzaniga, 1970; Gazzaniga and LeDoux, 1978). As a consequence, each hemisphere can identify stimuli that appear within its sensory field, but the patient is unable to perform perceptual tasks that require the integration of visual information from both hemifields. Typically such tasks involve a forced choice decision such as whether two stimuli are the same or whether they are members of the same class. When both items appear in the same hemifield performance is generally quite accurate. When they are presented to different fields performance usually falls to chance levels.