It Is All Quiet in the Minor Hemisphere: An Enquiry into the Laterality of Consciousness, Attention and Vision in Human Brain.

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Biomedicine
Biomedicine International (2010) 1: 3-15 International
INTRODUCTION
Next to language, handedness is the most remarkable of
all human characteristics. Not even the congenitally
blind escape the compelling grip of the neural circuitry
governing handedness; they display the same ratio of
right to left handedness as those who are not blind.1,2
However, the structural foundations of speech and
handedness have been a source of controversy since the
emergence of neurology as a scientific/medical disci-
pline. This is because lesions causing aphasia (or ne-
glect of one side of the body) do not always correspond
to the avowed (behavioral) handedness of subjects.
These irregularities include crossed aphasia,3,4 crossed
non-aphasia5 and crossed right hemisphere syndromes,6
which are exceptions to the law of “constant conjunc-
tion” of speech and handedness.
The recent discovery of handedness anatomy7-13 ex-
*Address correspondence to Iraj Derakhshan, MD, 415 Morris St,
Suite 401, Charleston, WV 25301; Tel: 304 343 4098; Fax: 304 343
4598; E-mail: idneuro@hotmail.com
Submitted April 19, 2009; accepted in revised form July 15, 2009.
Advance Access Publication 15 December 2010 (see
www.bmijournal.org)
plains these ambiguities surrounding the subject area by
recognizing that speech is a function of the executive
(major) hemisphere, from which all signals originate -
including those intended for the nondominant side of
the body; which in turn travel to the minor hemisphere
via the corpus callosum before becoming implemented
by the latter hemisphere. In this article, the laterality of
the command center is referred to as both neural hand-
edness and brainedness. This command center is always
on the opposite side of the body with the shortest reac-
tion times, including those effectors for moving the
eyes sideways. Thus, there is no motor communication
from the minor to the major hemisphere.8,11
There is a statistical relationship between behavioral
handedness and neural handedness (see above), which
is valid approximately 80 % of the time. This relation-
ship concerning the laterality of motor control may also
extend to other areas in which hemispheric asymmetries
play a role, including “attention,” neglect and epilepsy.
13-15
This report is a meta-analysis of the literature con-
cerning instances in which neural and behavioral hand-
edness of subjects did not match. Moreover, it becomes
evident that Newton’s concept of the visual sense of
space (i.e. contralateral representation of hemispace in
REVIEW
It Is All Quiet in the Minor Hemisphere: An Investigation into
the Laterality of Consciousness, Attention and Vision in the
Human Brain
Iraj Derakhshan1*
1Case Western Reserve and Cincinnati Universities, Ohio, USA
ABSTRACT
This is a meta-analysis of the literature on laterality of motor control in relation to vision, neglect and attention. It
documents the role of directionality of signal transfer between the two hemispheres. By indexing cases in which macular
vision was spared or damaged in lesions affecting the minor or major hemispheres it is shown that the minor hemi-
sphere’s role is to dilate (extend) the space in which vision occurs rather than providing for the elementary aspects of vi-
sion. The statistical relationship between the laterality of seeing and speaking and that of behavioral handedness of sub-
jects in studies employing inspection time and lexical decision paradigms are reviewed; the distinction between the two
modes of handedness (i.e. behavioral and neural) is defined. Evidence is provided indicating that brainedness (i.e. the lat-
erality of the executive hemisphere) encompasses the destination of all signals relating to consciousness, and that the mi-
nor hemisphere relates to three-dimensional (3-D) spatial awareness. However, this 3-D spatial awareness relies on sig-
nals traveling from the non-dominant side of the body/space to the major hemisphere via the posterior aspect of the callo-
sum; which may explain why neglect occurs exclusively in lesions affecting the minor hemisphere, as defined in this arti-
cle. This is a comprehensive review of clinical, neuropathological and physiological data, and indicates that macular vi-
sion occurs in the major hemisphere alone and that handedness is a statistical reflection of the laterality of conscious
awareness. Biomed. Int. 2010; 1: 3-15. ©2010 Biomedicine International, Inc.
Key words: attention, cerebral laterality, consciousness, vision

4 Derakhshan / Biomedicine International (2010) 1: 3-15
the occipital lobes) is no longer tenable and should be
replaced with the new scheme advanced here.9,11
LATERALITY OF VISION AND EYE
MOVEMENTS
Because of symmetry and simplicity, it has been ac-
cepted that each hemisphere controls the movements of
the opposite side of the body16, and that the sensations
arising from each side achieve consciousness in the op-
posite hemisphere; i.e. there are two “endpoints of the
visuomotor stream,” one for each side of the body.17 In
addition, it is widely believed that each hemisphere
specializes in certain functions, the major hemisphere in
speech and the minor hemisphere in “visuo-spatial”
abilities. These beliefs are based on the observations
that an appropriately placed lesion of the major hemi-
sphere interferes with speech (e.g. aphasia) and a lesion
of the minor hemisphere results in neglect of the oppo-
site side of the body. There is tacit acceptance that be-
havioral (self-declared) handedness lies opposite the
major hemisphere (hemisphere of speech), at least in
right-handed subjects.5 Such canonical views date back
to 1704, when Sir Isaac Newton suggested that the fi-
bers arising in the nasal retinae crossed at the chiasma
before their final destination in the calcarine cortex
(Newton’s Opticks, 1704).
Further studies concerning ocular rivalry have indi-
cated that only one eye is at work at any one time, and
that there is no choice as to the laterality of the viewing
eye.18-20 In addition, incongruous visual field defects in
unilateral lesions of central visual pathways,21 and a 3:1
ratio of presentation of one eye with a full thickness
macular hole22 (compared to the other eye affected with
the same condition), have been ineffective in challeng-
ing the common belief in Newton’s formulation.23 Two
other asymmetries, limitation of apraxia to the limbs
ipsilateral to lesions of the major hemisphere, and that
of neglect to limbs contralateral to lesions in the minor
hemisphere, have both remained without verifiable ex-
planations.
Using the one-way callosal traffic theory,7-13 the
above asymmetries can be correlated with chronometric
data from event-related cortical potentials and reaction
time, expanding our knowledge of the mechanism(s)
underlying “hemispheric specialization”, and explaining
the exceptions mentioned earlier. Briefly, evidence in-
dicates that in right handers, sensations from the left
side of the body (the nondominant side) are delayed in
arriving at consciousness until the related somato-
sensory signals reaching the right parietal lobe have
traveled to their final destination in the left hemisphere
(via the splenium of the corpus callosum).9,24,25 Fur-
thermore, evidence from a study of penetrating wounds
to the skull has demonstrated the vulnerability of the
left-hand side to lesions affecting the parietal lobe of
the left (ipsilateral) hemisphere. This indicated contigu-
ous or overlapping representation of both hands in the
same hemisphere (among the right handers).26 The op-
posite result was documented in epileptic subjects who
had undergone surgical removal of the post-
central/parietal cortex of the right (minor) hemisphere;
complete corticectomy resulted in sensory defects lim-
ited to the left side of the body.27 However, some left-
handed individuals present with an ‘opposite direction-
ality of signal transfer’.7
According to the one-way callosal traffic circuitry,
handedness is a code for the directionality of callosal
communication from one hemisphere (major) to the
other (minor), and the trans-callosal direction of com-
munication in the motor domain is opposite to that in
the somato-sensory domain. Therefore, the unexpected
“anomalies” mentioned above result from a disparity
between a person’s neural wiring and behavioral hand-
edness. This can be traced to the fact that as children we
choose the laterality of the hand that we eventually
adopt as our preferred hand; an option sometimes exer-
cised in opposition to the unchangeable mandate of na-
ture (neural wiring).8,11 However, we have no choice as
to which hemisphere controls our speech.9 Irrespective
of the existence of a choice as to one’s behavioral
handedness, the ‘wiring’ connecting the executive
hemisphere to the minor hemisphere is not modifiable.
The minor hemisphere is dedicated to the nondominant
side of the body/space while both sides of the body are
represented in the major hemisphere.9, 24-27 Directional-
ity of callosal traffic can be discerned by measuring
reaction time, which reveals the remoteness of the non-
dominant side of the body from the command center by
a value equal to the interhemispheric transfer time
(IHTT).8,11 The intimate relationship between handed-
ness and brainedness can be demonstrated by the fact
that excessive verbal demand on the major hemisphere
is associated with bilateral disruption of hand func-
tion.29 This conclusion was also reached in studies in-
volving readiness potentials when self-paced tasks were
employed.30 Therefore, speech disturbance is simply a
convenient marker of injuries affecting the executive
hemisphere. The distinction between the two modes of
handedness, i. e. neural (speech, laterality of motor con-
trol) and behavioral (self-declared) is, at the societal
level, evidenced by robust chronometric differentials of
the two sides of the body (regardless of the effectors
tested) and by functional emission and transcranial
magnetic stimulation studies.7,8
For example, a recent study by Shen and Franz, in-
vestigating simple reaction times in two groups of right-
and left-handed subjects (40 individuals per group),
documented an almost equal ratio of left to right hand
lead among the left-handed subjects studied (48/45 %,
respectively). Among the right handers this ratio was
26/69 %, a highly significant difference (p = 0.008).31
The same differentials are observed in other symmetri-
cally opposed effectors of the body.1,7,11 Iacoboni and
Zaidel32 demonstrated such laterality by indexing reac-
tion time differences in thousands of trials and con-
cluded that these “reflect hard-wired mechanisms of
callosal transmission”. Failure to recognize the dichot-
omy of motor control reflected in the above data has
impeded understanding of the role of laterality in sei-

Biomedicine International (2010) 1: 3-15 / Cerebral Laterality 5
zure onset and in employing effective therapeutic
measures for those affected with neglect syndromes.9,12
The evidence that macular vision from both eyes is
transmitted directly to the major hemisphere, and that
the role of minor hemisphere in “vision” is to expand
(dilate) the visual field via a callosally-mediated excita-
tory synapse from the minor to the major hemisphere,
will be reviewed here.33,34 The unexplored role of sac-
cades in mediating the right visual field advantage (in
right handers) in inspection and lexical decision timing
paradigms among neural right handers, and in the oppo-
site direction among neural left handers, will also be
discussed. Clinical and time-resolved data demonstrat-
ing the role of the corpus callosum in channeling soma-
tosensory data arising from the nondominant side of the
body to the major hemisphere for conscious apprehen-
sion of events occurring ipsilaterally will be provided.
Cases described in sufficient detail to allow determina-
tion of neural laterality of the subject versus their be-
havioral handedness will be identified for this purpose.8,
35
Among the paradigms confirming the role of direc-
tionality of callosal traffic in hemispheric asymmetries
is the lateralized word recognition task (also called the
inspection time test). The procedure consists of flashing
stimuli to each side of the midline for a limited time
(usually 90-100 milliseconds), demonstrating the supe-
riority of the right visual field (RVF) over the left visual
field (LVF) in retaining information contained in a
stimulus. Despite the consistency of the results, the
critical role of proximity of the dominant side of the
body to the command center (i.e. faster turning of the
eyes to the right side and thus RVF advantage) has not
been considered as a reason for this advantage, despite
recognition that increasing the exposure time eliminated
the RVF advantage in this and similar paradigms36 and
the knowledge of a latency differential in moving the
eyes to the sides. In some instances, this time differen-
tial has been explained in terms of the time taken for
interhemispheric transfer of visual information from the
occipital cortex of the right (via the splenium) to the
left. However, other observations indicate the preserva-
tion of macular vision upon injury of the occipital lobe
of the minor hemisphere and occurrence of cortical
blindness in lesions affecting the occipital cortex of the
major hemisphere; negating Newton’s formulation of
contralateral representation. Data from tachistoscopic
procedures in callosotomy patients, wherein preserva-
tion of macular vision is also documented, leads to a
similar conclusion (see below).37-39
FURTHER EVIDENCE AGAINST NEWTON’S
FORMULA
It could be claimed that the preservation of macular
vision in those who lack a chiasma owing to a congeni-
tal anomaly (achiasmata) is the prima facie evidence
against Newton’s formula.40 Nevertheless, exploring the
relevant issues is useful in indicating where the concep-
tual weaknesses of the Newtonian formulation lie, thus
removing the remaining doubts concerning the untena-
bility of “visual field” representation as proposed by
Newton.
Schulte-Altedorneburg et al.41 described a previously
normal (presumably right handed) patient who became
cortically blind after sustaining a parenchymal contrast
enhancing lesion of the right occipital lobe and a large
infarct in the distribution of the left middle cerebral
artery (following coronary angiography). During the
procedure the patient became unresponsive, disoriented
and confused. In the intensive care unit he showed cor-
tical blindness, dysarthria, acalculia, perseveration, im-
pairment of short term memory and anosognosia. The
next day the patient’s sight returned, but he remained
dysarthric, perseverating and had difficulty distinguish-
ing right from left. A week later, he displayed dyslexia
and dysgraphia. CT scan performed within hours of the
onset of these symptoms showed intense gyral en-
hancement of the right occipital lobe involving both
cortical and subcortical areas. The occipital lobe on the
left was intact. A further CT scan carried out the fol-
lowing day revealed that the abnormal contrast en-
hancement had disappeared, and had been replaced by a
sharply demarcated hypodense area in the right occipi-
tal lobe, as well as the presence of a left-sided, large,
middle cerebral infarction with slight hemorrhagic
transformation. On the basis of current understanding of
anatomical-functional correlations, the authors were
surprised to find “cortical blindness rather than a he-
mianopia that would be expected with unilateral occipi-
tal enhancement”. Therefore, the fast and favorable
resolution of the cortical blindness was attributed to the
right occipital lobe lesion and the persistent dyslexia
and dysgraphia was attributed to the findings that were
confirmed after a third CT scan carried out one week
after the angiography. The left parieto-temporal infarct
was considered embolic in origin, but the occipital lobe
lesion was ascribed to neurotoxic effects of the contrast
medium. The anosognosia and right-left disorientation
were attributed to the lesion of the left hemisphere, as-
sumed to be the major hemisphere in this patient. Ac-
cording to the new understanding, however, this case is
an example of “crossed right hemisphere syndrome”
reviewed by Marchetti et al.6 with aphasia and transient
cortical blindness due to the lesion in the right hemi-
sphere and anosognosia and left-right disorientation
caused by the lesion in the minor left hemisphere. Simi-
lar cases of cortical blindness associated with unilateral
occipital lobe lesions from angiographic contrast injec-
tion have been described.42,43
Transient as well as permanent visual disturbances in
patients with epilepsy and foci in the occipital lobe of
the major hemisphere constitute another category of
cases in which cortical blindness has been described.44-
47 Cases of cortical blindness in the context of reversi-
ble posterior leukoencephalopathy with epilepsy have
been described in individuals suffering from eclampsia
and after chemotherapy treatment, with lesions limited
to one occipital lobe.48-53 The occurrence of cortical
blindness in a patient with a remaining right eye follow-

6 Derakhshan / Biomedicine International (2010) 1: 3-15
ing infarction of the right occipital lobe can be inter-
preted the same way. In this case, six days after an in-
farction, central vision was restored and visual acuity
returned to normal, emphasizing the role of the right
hemisphere in sustaining macular vision of the remain-
ing right eye.54 The findings in this case cannot be ex-
plained by contralateral representation of vision in hu-
mans as envisioned by Newton.
Similarly, a stable right visual field superiority in in-
spection time and lexical decision task across visual
fields (not attributable to chance, sampling error, eye
dominance or monocular versus binocular vision) is in
opposition to Newtonian concept too.36,39,55-59 High-
lighting this point, the elaborate study of Sadler et al.57
is remarkable. Employing a wide range of exposure
times (35-110 msec), a left hemispheric superiority
(RVF) was documented for ten right-handed subjects.
However, as figure 10 of the report demonstrates, at
least two individuals (subjects 6, 8) had LVF superior-
ity, i.e. scores in the inspection time test were higher
when the stimulus arrived from the left. Such events
demonstrate an inherent flaw in group studies, namely
the nullification of a behavioral trait in quantitative data
derived from a biologically nonhomogenous group.
This was noted by Sadler et al., who commented that “a
few subjects did show some hemispheric dominance in
their performance of the task…Even among right-
handed subjects there appeared to be different hemi-
spheric advantages in inspection time” (pages 290,
293).57 Geffen et al. reported similar findings in a reac-
tion time (RT) study of right-handed participants where
seven of the 36 subjects “showed reversed RTs (i.e.
faster processing of digits in LVF, or faster RTs for
faces in RVF).”60 Constantinidis et al.61 reported the
most recent instance of laterality indexed nullification
from inappropriate “grouping.” The study involved
measuring left and right latencies of saccades in 676
healthy adults. The authors deny the validity of earlier
works on latency differentials (many of which are cited
in this article) on the basis of selective manipulation of
the data they obtained. Some of these concerns have
been addressed in an article dealing with the issue of
“idiosyncrasy” in reaction time studies on the sac-
cades.9
Six specific examples of quantitative data indicating
the presence of impurities in handedness groups are
provided below, followed by an explanation of the find-
ings according to the new insight:
1. In a tachistoscopic study involving verbal reaction
time in a group of seven right-handed subjects, Amadeo
et al. (1977) recorded a faster response time to RVF
stimuli in four individuals (416.3 msec). The remaining
subjects had a quicker response to LVF presentations
(416.7 msec). After averaging the results, the authors
concluded that verbal reaction times were of the same
magnitude regardless of the laterality of the stimulus.
They could not explain how they had arrived at a result
contrary to all other investigations on the subject.62
2. In a study involving 26 right-handed subjects that
investigated tachistosopic recognition of single and
multiple letters, Bryden (1966) reported a 4:1 ratio in
favor of right visual field exposures (exposure times 20
and 30 msec).63
3. The second experiment described by Fisk and
Goodale (1985) concerning eye and limb movements
during unrestricted reach to targets involved four right
handed subjects. Three of these individuals had signifi-
cantly shorter response latencies for the right hand and
RVF compared to the first experiment. The addition of
a fourth right-handed subject (but neurally left handed,
see below) to the group for the second experiment
changed the findings from a significantly faster re-
sponse to right-sided stimuli (seen in experiment 1) to a
failure to show superiority of the RVF .64
4. In an elegant study by Viviani et al. involving con-
tinuous monitoring of bimanual asynchrony in right-
and left-handed subjects, the average lead of the right
hand over the left in a group of right-handers engaged
in simultaneous bilateral movements was 36 msec.
However, the lead of the left hand over the right among
the left-handed participants was –13 msec (left-leading
was designated negatively). Reviewing the performance
of individual subjects explained the reason for the di-
vergent expression of the leading hand between the two
groups. According to the new insight, one of the left
handers (subject 7) was a neural right hander as indi-
cated by his right hand lead of 24 ± 13 msec. Removing
this outlying subject from analysis of the performance
of left handers changed the lead of the left hand to - 26
± 38 msec (a value much closer to the lead obtained for
the right handers).65
5. In a dot detection paradigm involving sixteen right
handed subjects using verbal responses to lateralized
binocular exposures of 100 msec, McKeever et al.58
found a faster response to LVF stimuli in four of the
subjects (table 1, subjects 3,4,8,15). In another experi-
ment, the same authors reported the results of a similar
procedure carried out on 20 right- and 20 left-handed
subjects (using 50 msec exposures). While the verbal
responses from the RVF were faster than those from the
left in every right-handed subject (by an average of 36.9
msec), the same measure in favor of the RVF exposures
in the left handed group was valued at 13.6 msec. Only
five of the 20 left handers were neurally left-handed,
with an average negative crossed uncrossed differential
(CUD) of 32 msec (subjects # 2, 3, 9, 17, 18). Subtract-
ing this figure from the group average changed the RVF
advantage to 28.75 msec, which was close to the CUDs
measured in right handers. In agreement with Hardyck
et al.,14 the authors of this study denied any role for “di-
rectional scanning” in relation to the RVF advantage in
verbal reaction times of the subjects (p 372).58
6. Three cases of right-sided neglect in right-handed
patients with left brain damage were reported by Bar-
tolomeo et al. (table 1 and pages 1016, 1017) and by
Bartolomeo (Fig. 2).66,67 These individuals were neural
left-handers who were nevertheless behaviorally right,
evidenced by slower reaction times, more so of the right
hand, and LVF advantage in tachistoscopic studies,
rather than the RVF advantage expected in right-handed

Biomedicine International (2010) 1: 3-15 / Cerebral Laterality 7
subjects.33,68
In all the examples outlined above, if the authors had
had knowledge of the negative crossed-uncrossed dif-
ference (CUD) they obtained, they could have recog-
nized the non-homogeneity of the population they were
studying.11
In the past, callosum-meditated nondominant delay in
motor tasks has been attributed to a longer processing
time needed for events involving the right hemisphere
such as moving non-dominant limbs69 and the eyes.36
However, the shorter reaction time of the neutrally-
dominant side, regardless of the involvement of vision
or the effector tested, attests (one-) sidedness of the ex-
ecutive hemisphere and the callosum-width proximity
of that side to the hemisphere of action. This is consis-
tent with the observation that increasing the time of
tachistoscopic exposure or increasing the difficulty of
decision making (words versus non-words) eliminates
the RVF advantage.11,14,15,33 As highlighted by Hardyck
and colleagues, the results of these experiments show
that “attentional bias can be effectively eliminated as a
source of variation in visual field effects at least for
lexical design tasks”.14 Meanwhile, the “unimportance”
of the splenium in macular vision is demonstrated by
the bilateral redundancy gain in simple reaction time
during tachistoscopic testing of callosotomy cases and
in control subjects.9,15,38 A similar conclusion was
reached by Berlucchi et al., who carried out a simple
reaction time study on a subject that indicated stability
of the CUD regardless of the eccentricity of the stimu-
lus. The authors of the study concluded that the “inter-
hemispheric integration of simple visuomotor tasks
does not depend crucially upon the callosal connections
of the visual cortex.”70
Hutton and Palet71 demonstrated similar ratios of
“impurity” among right- and left-handed individuals in
an investigation involving the latency of right and left
going saccades. In groups of 18 right- and left-handed
subjects, 15 of the right-handers gazed to the right
faster than to the left by an average of 23 msec. The
remaining three subjects performed faster when looking
to the left hand side. Twelve of the 18 left handers had a
shorter reaction time looking to the right side and the
remaining six moved faster when looking to the left.
Similar observations were made by Pirozzol and
Rayner,72 who found that half the subjects showed left-
handedness and half a preference for right-handedness.
Significantly, in this experiment, the right visual field
advantage was present for both letters and symbols.
According to the authors, “normal right handed adults
do exhibit gaze laterality while the sinistrals, as a
group, do not”. The most recent investigation docu-
mented a faster speed for looking to the right in 20 out
of 21 right-handed subjects examined.73 The relative
homogeneity of right-handed groups compared to left-
handed groups provides the basis for the previously-
mentioned findings in left-handed groups. The nonho-
mogeneity of left handed groups, resulting in smaller
right-left differentials than in right handers, seems to be
the sole reason that some authors deny the significance
of laterality indexed differentials of saccades as the ba-
sis of the RVF advantage.74 It is very rare for non-
homogeneity to be equally distributed among a small
group of right handers undergoing manual reaction
time; but of the six right-handed participants in a reac-
tion time study by Diedrichsen et al., three were neu-
rally right- and three neurally left-handed, with the
leading hand preceding the other by an average of 20
msec.75 However, this event does not negate the find-
ings of numerous time-resolved and clinical observa-
tions,7,75,76 and Diedrichsen and colleagues are misled
in this respect.
Hyona and colleagues77 compared the performance of
free viewing versus fixating procedures in a lexical de-
cision task involving 22 right-handers. The authors
found a significantly improved performance in “move”
conditions but only in exposures from the left visual
field. They also reported a longer manual reaction time
in presentations to the left of the midline than those to
the right (table 1), confirming the findings of
Goodale.78 The stability of right RVF advantage with
the eyes fixating or moving is not surprising as a fixat-
ing eye is not stationary79 and the microsaccades im-
posed on the fixating eyes have “the same dynamics as
the larger saccades”. According to Hafed et al., such
microsaccades may be used as an overt measure of cov-
ert “attention shifts” and are subject to the same lateral-
ity indexed influences as the larger saccades (p 2540-
2543).80
However, the remarkable lack of attention in this field
of study to the existence of laterality-indexed differen-
tial of the saccades as a possible factor in visual field
advantages in inspection and lexical decision tasks is
worrying. In two of the largest studies concerning this
subject,14,15 RVF advantage was limited to meaningful
words and absent for non-words at the expense of the
RVF (both of these studies employed 100 msec expo-
sure times). In these studies, the reaction times and the
rate of correct responses produced similar curves. One-
way callosal traffic circuitry and laterality of macular
vision (see above) provide these observations with a
“structural” basis; i.e. the RVF advantage relates to the
laterality of motor control for gaze. They also assign the
same laterality to apprehension of meaningfulness or
lack thereof. 9 The fact that the same differential (60
msec) exists in the nonverbal sympathetic skin response
as shown by Danilov et al. (table 3) also points to the
generality of the design at hand, i.e becoming aware is
an active process that occurs exclusively in the major
hemisphere.81
Anatomical studies related to the laterality of lesions
causing cortical blindness deserve special attention. The
study carried out by Van Buren82 concerning a posterior
cerebral lesion causing “dyslexia” following resection
of the left occipital pole due to an underlying hemor-
rhagic melanoma, and that of De Renzi et al.53 investi-
gating resection of the same lobe, will be summarized
here. Van Buren described the patient as becoming con-
fused after surgery, and although she was blind to light
and movements, she denied having any difficulty with

8 Derakhshan / Biomedicine International (2010) 1: 3-15
vision. Ten days later the visual acuity had improved in
both eyes but a dense right homonymous hemianopia
remained. Later, she could read slowly, word by word,
with occasional paraphasia present. At autopsy the le-
sion was found to be limited to the left occipital lobe
and the corpus callosum was intact. The author ascribed
the patient’s ability to perceive large moving objects in
the far periphery of the right visual field to the preser-
vation of a small remaining fragment in the anterior
portion of the left calcarine cortex, a finding consistent
with electro-diagnostic evaluation of similar cases indi-
cating that sparing the primary visual cortex was re-
quired for any perception to occur.83 De Renzi et al.,
referring to the proposition of Coslett and Saffran84,
appreciated the anatomical-clinical relationship be-
tween apperceptive and associative agnosias in lesions
affecting the major hemisphere. A similar case has been
described by Benke.85
The opposite result is evident in the preservation of
macular vision after injury or removal of the occipital
lobe of the minor hemisphere, referred to as visual ne-
glect (hemianopia), causing restriction of the horizontal
visual field.5,86,87 The syndrome is often multimodal,
affecting the ability to perceive or to hear from the non-
dominant side.87- 89 It does not involve macular vision,
as can be demonstrated with the eyes fixed or when
they are allowed to move as shown in Fig. 1 (adapted
from Joenette et al.90). In such cases, unaffected macu-
lar vision can be confirmed by confrontation testing,
where the stimulus is moved from the left side of the
patient and closer to the midline. Similarly, when the
patient is asked to make a firm fist with his paretic left
hand or to write using this hand, hemianopia disappears
(Derakhshan, unpublished observations),. However, this
is only true when the callosum remains extant at the
genu, allowing signal transfer from the major to the
minor hemisphere.11,91,92 Lesions of the parietal lobe
can result in neglect involving both visual fields, which
is most severe contralaterally.9,68,93 This phenomenon
can also occur when the callosum is injured.94-95 In all
such cases, the sensory deficit may be multimodal.96-98
Fig. 1. Note the absence of neglect of stimuli within central vision
regardless of the laterality of the responding hand in all three cases.
This figure has been reproduced from the article by Joanette et al.,
1986, by permission from the publisher.
Hemianopia can be ameliorated by moving the non-
dominant side of the body or stimulating other modali-
ties of sensation, provided the callosum remains extant
at the genu. Movement of the eyes to the nondominant
side of the body has a widening effect on the visual
field.99 However, patients exhibiting hemianopia due to
injury of the right parietal lobe are unresponsive to acti-
vation of the right visual cortex by an extinguished
stimulus.100 This suggests that the nondominant occipi-
tal lobe has no role in elementary vision; its role con-
cerns expansion of the visual field.9,87,91,92
Evidence suggests that loss of central vision is a fea-
ture of injury to the occipital lobe of the major hemi-
sphere, but it is unknown what anatomical features
within that hemisphere underpin the asymmetry of
function. However, Stensaas and colleagues docu-
mented an asymmetry of the line of Gennai.101 In a
study measuring the total striate area of the visual cor-
tex of 23 brains with paired hemispheres, 15 of the left
hemispheres had more total striate and larger mesial
cortices than those on the right side. This amounted to a
left to right hemispheric advantage of 2:1 in the 46
paired hemispheres examined by the authors of that
study. The authors were in agreement with Brodmann
that there was a strong tendency towards lateralization,
but they did not know the handedness of the subjects
studied. However, examination of the case notes indi-
cates no bias in selecting individuals, so it is likely that
the above ratio reflected laterality of motor control at
the societal level. In contrast, Murphy’s study of the
visual cortex of both hemispheres (which were volu-
metrically equal) was the subject of selective sampling,
as evidenced by the exclusion of 31 patients from the
study. In addition, as Leuba et al. reported, there were
no controls for brain shrinkage. Furthermore, the meas-
urements taken could have been affected by a problem
with the plane of sampling.102,103 A recently docu-
mented MRI scan of a congruous volumetric asymme-
try in the optic tract, lateral geniculate body and cal-
carine cortex supports the view that macular vision is
exclusively controlled by the major hemisphere (An-
drews et al., 1997).104
LATERALITY OF NEGLECT AND
ANOSOGNOSIA
Clinically, the occurrence of alien hand syndrome in the

Biomedicine International (2010) 1: 3-15 / Cerebral Laterality 9
right hand of a right-handed subject is a manifestation
of neuro-behavioral mismatching. The case of a boiler
maker, described by Denny-Brown and Banker, is a
classic example where a right-handed patient with a left
parietal lesion initially denied ownership of his right
hand and used his left hand instead as he still had a full
control of it owing to its direct connection to the major
hemisphere (in this case the right). The absence of
aphasia and presence of anosognosia compelled the
authors to declare the patient as “an unexplained excep-
tion to Babinski’s rule that [only] right hemisphere le-
sions produce anosognosia”.105 McNabb et al. 106 re-
ported a case of bilateral movement-related cortical
potentials upon moving the neurally nondominant right
hand, providing evidence that in such cases the right
hemisphere houses the command center, as moving the
nondominant side required bi-hemispheric activity of
the motor cortices, and movement of the dominant side
of the body is associated with activity of the major
hemisphere only.7-13,91,92 In a recent study of 78 patients
who had had a left hemisphere stroke (83 % of them
right handed), anosognosia was present in five indi-
viduals (6%) and ten cases (13.2 %) had signs of right-
sided neglect detected by a drawing test.107 These num-
bers concur with statistics relating to the rates of impu-
rity among right-handers as determined by other stud-
ies.7,108 This point is illustrated by the drawing of a
clock by a neural and behavioral left hander who
showed complete denial of the right hemispace follow-
ing a large left parietal infarct.109
Such manifestations of neglect regress temporarily if
the patient performs a task with the nondominant hand.
This is because the excitatory commands from the ma-
jor hemisphere stimulate the dormant nondominant
hemisphere.9, 91,92 Moving the dominant hand does not
effect any visual changes as such movements are han-
dled by the major hemisphere alone, whereas those of
the nondominant side are associated with activity in
both hemispheres.7, 92
In contrast to vision, where the role of nondominant
hemisphere is to provide space in which vision can oc-
cur (see above), the post central parietal lobe of the mi-
nor hemisphere plays a role in receiving primary sen-
sory data from the contralateral side of the body and in
transferring the same to its dominant counterpart for
conscious apprehension to occur. Therefore, according
to the new concept, there is no duality of “end-points of
visuomotor streams” as is currently believed.17 Rather,
movements and somatosensory perceptions occur in the
same hemisphere.8,11
According to De Renzi et al., neglect in isolation
never occurs in lesions affecting the left hemisphere.110
However, anosognosia associated with hemianopia and
hemiplegia occurred in six and ten percent of patients
with left brain damage, respectively (Beis et al., Table
1).107 This is in contrast to the incidence of “neglect of
the left limbs” in patients with right-sided brain dam-
age, which occurs seven times more often (Azouvi et
al., Table 4).111 Among left handers, there are accounts
of neglect in the right hemispace in lesions involving
the left hemisphere, without occurrences of apha-
sia.86,112 According to one-way callosal traffic circuitry,
such instances demarcate neural and behavioral left
handers, in contrast to the three cases described by Bar-
tolomeo et al., all of whom were neural left- but behav-
ioral right-handers.66 In general, the laterality of the
executive hemisphere is evenly divided between both
hemispheres in behavioral left handers, as shown in
reaction time studies and data from intracarotid sodium
amytal tests concerning lateralization of the speech
hemisphere (Branch et al., Table 6).77,108 However, the
distribution of the executive hemisphere is skewed for
right handers, as demonstrated by the skewed occur-
rence of anosognosia associated with hemiplegia in
these individuals. In a study of 50 patients presenting
with right or left hemispheric lesion resulting in hemi-
plegia, 90% of patients with anosognosia had an acute
lesion in the right hemisphere. Anosognosia associated
with hemiplegia was not present in 24 patients with left
hemispheric lesions. One patient presented with
anosognosia associated with right-side hemiplegia, but
also presented with a hematoma of the left thalamus
associated with right hemianesthesia. Each of the 10
anosognostic patients in this study showed no response
in either hemisphere upon stimulation of the median
nerve on the hemiplegic side.113 Green et al. concluded
that impaired awareness of the hemiplegic side was
caused by blockage of signals arising from the damaged
(minor) hemisphere to the major hemisphere (via the
callosum). However, the authors could not explain
“why the somatosensory potential was absent over the
involved left hemisphere in only one patient (p. 1144)”.
According to one-way callosal traffic circuitry, this was
an exceptional case in which a neural left hander had
the characteristics of a right-handed person, and in
whom the left hemisphere was the minor.5,6 This re-
sulted in the bilateral loss of somatosensory evoked
potential after stimulating the patient’s nondominant
side (i.e. right median nerve).113
Another characteristic of hemispheric asymmetry is
the rapid (but temporary) improvement of neglect by
stimulating the nondominant side of the body,114 such
as the disappearance of hemianopia when a patient
writes with the left hand,11,91 stimulating the left poste-
rior neck muscles with a vibrator or electrical stimula-
tion of the left hand. Moving the eyes to the left by
cold-water calorics of the left ear, or warm water calo-
rics of the right ear (or by optokinetic nystagmus), abol-
ished both left-sided anosognosia associated with motor
impairment and left-sided hemianopia.115-119 These
events only occur in right handers with right brain dam-
age and left side neglect. The only exceptions to this are
“ambidextrous” patients with left-sided brain damage
and right-sided neglect, who improve with cold-water
calorics of the right side.117 Vallar et al.120 documented
two exceptions among a group of 11 right handers with
left brain damage; cases 30 and 31 showed neglect of
the right side, with improvement after calorics on the
right. A third case concerned a patient who was one of a
group of four right handers with left-sided brain dam-

10 Derakhshan / Biomedicine International (2010) 1: 3-15
age.121 This latter group underwent transcutaneous elec-
trical stimulation for symptom amelioration. Neglect
was only evident in one of the four individuals (case #
11, Fig. 2) and he was also the only patient who showed
significant improvement after electrical stimulation of
the contralesional side of the neck. Although Vallar et
al.120 investigated the mechanism of such improvements
(activation of the dormant/injured right hemisphere)
they did not acknowledge the specific role played by
eye movements in directing these changes, i.e. activa-
tion of the executive hemisphere when moving an ef-
fector (the eyes) to the nondominant side. Instead, the
authors attributed the changes to the sensory stimulation
of the ear canal by “temporarily activating the hypo-
aroused right hemisphere.” However, in another study,
the authors noted that the sparing of somatosensory
processes in the minor hemisphere “did not entail per-
ceptual awareness”. 121 The fact that merely moving the
eyes to the nondominant side, either by volition or by
warm water calorics of the right ear, causes an im-
provement in neglect, implies that the physiological
interpretation suggested by one-way callosal traffic cir-
cuitry is more likely to be accurate than that proposed
by the authors. Interestingly, in a group of 18 patients
with neglect, the individual who failed to improve
showed no eye movements in response to caloric stimu-
lation (case #9); this was presumed to be due to a previ-
ously sustained gentamycin induced vestibular dam-
age.115 Neglect contralateral to left hemisphere damage,
and associated with compulsory “peeking” to the left,
was found in four of 43 (9 %) right handed patients ex-
amined by De Renzi et al. (p. 236).68
DISCUSSION
According to the evidence reviewed here, the underly-
ing mechanism of a human’s inability to move both
hands (or any other pair of symmetrically located effec-
tors) simultaneously, and the laterality of the leading
hand in speeded or self-paced tasks, is the callosum-
width proximity of the dominant side of the major
hemisphere compared to that present on the nondomi-
nant side. It would have been most remarkable if such a
timing differential had gone completely unnoticed (ex-
cept for its functional equivalent, i.e. the dexterity of
the subject) since accurate timing of movement became
feasible. Recognition of the melody lead of the right
hand in piano players a century and a half ago, and the
more recent demonstration of the same trait in violin
players (between the bowing and the fingering
hands),122 as well as the onomatopoeic “flam” of
drummers, all demonstrate the indispensable role of the
Fig. 2. A schematic representation of visuo-motor pathways in a neural and behavioral left hander, whose macular vision and command center
both lie in the right hemisphere. Reproduced from Figure 5 of Azemar et al.,135 based on the one-way callosal traffic model described above.
(Reproduced with permission)

Biomedicine International (2010) 1: 3-15 / Cerebral Laterality 11
laterality of motor control in allowing inter-manual co-
ordination in our daily lives. In each of these examples,
the musicians intend to move both hands at the same
time but fail to do so owing to the callosum-width
closeness of the neurally-dominant hand to the com-
mand center. Therefore, a neurally left-handed person
will never be a successful concert pianist if playing a
piece composed by a right-handed individual who wrote
the music for the piano as a right-handed musical in-
strument. This explains the marketing appeal of a re-
versed panel piano for left handers.123
Clinically, the footprints of handedness and the anat-
omy sustaining it are traceable in essential tremor, focal
dystonia (writer’s or musician’s cramp) and epilepsy,
characterized by lopsided distribution of the affected
side of the body or laterality of seizure onset; as such,
these conditions are classed as diseases of the major
hemisphere.12,13,124-126
The unitary nature of visual fields can be confirmed
by the following additional observations: Clinically, the
terms “right” and “left” used to describe a neglected
stimulus during confrontation testing are better defined
as “right of” or “left of” the respective positions, re-
gardless of the visual field tested;68,93, 27 another fact
that cannot be reconciled with Newton’s concept of
contralateral representation of space. Furthermore,
Newton’s formula fails to predict the following obser-
vations: unaffected central vision in patients with visual
neglect during confrontation tests when the eyes were
either held stationary or allowed to move (Fig. 1);
prompt but temporary disappearance of neglect upon
stimulation of an effector on the nondominant side of
the body;11,91,92 and stability of RVF superiority across
visual fields during inspection time or lexical decision
making, regardless of the side of the retina stimu-
lated.56, 89
On the subject matter of “attention,” it is difficult to
pass by a scene without a quick glance. This is referred
to as the “obligatory coupling between attention and
eye movements” (Hyona et al., p 158).77 Employing the
“cued shifts of attention” technique, Heilman et al.
demonstrated that warning stimuli from the LVF re-
duced the reaction time of the right hand by 14 msec,
whilst similar stimuli from the RVF did not affect the
reaction time.128 The assumption was that subjects had
not moved their eyes toward the stimulus as instructed
and the authors concluded that “the right hemisphere
dominates attention.” Such instructions may appear
futile,129 but evidence also suggests that a visual warn-
ing is only noticed by the person being warned. The
same authors obtained opposing results in a separate
study where “valid attentional information resulted in a
reduction in reaction time, and did so equally to stimuli
from either hemispace”.130 However, the faster reaction
time of the left hand (the subjects were right handed)
suggests that the nonhomogeneity of the group studied
should be the main finding in that report, rather than “a
right hemispheric superiority for mediating intention,”
as claimed. This novel interpretation, while consistent
with the body of evidence reviewed thus far, resolves
the contradiction between the two studies by Heilman
and colleagues.128,130 In addition, these studies highlight
the workings of a twin excitatory loop traversing the
callosum, i.e. an anterior loop starting from the major
hemisphere and ending at the minor hemisphere for
moving the eyes to the left,28 and a posterior loop from
the minor hemisphere to the major hemisphere respon-
sible for the shortening of the reaction time of the right
hand when looking to the left.128,130 Therefore, the LVF
delay in the lexical decision task and the increased ex-
citability of the brain due to (the un-noticed) left-going
saccades caused by the stimuli from the left hemispace
might share the same structural foundations; namely
that the callosum-mediated activation of the minor
hemisphere is caused by signals arising from the major
hemisphere when looking to the left.11, 28, 91, 92 The pos-
terior callosal loop, with directionality from the minor
hemisphere to the major hemisphere, is responsible for
the nondominant delay in sensing stimuli applied simul-
taneously to both hands. This directionality was first
described by Efron131 and subsequently confirmed by
Corwyn et al.,132 Conrad133 and Libet et al.134 As a cli-
nician, Efron was fully aware of the significance of di-
rectionality in callosal traffic, and the fact that it was
the reason for delays in awareness of events arising on
the nondominant side of the body.131 Libet and his neu-
rophysiology colleagues were ignorant as to the origin
of the sensory asynchrony they described. Describing
bilateral stimulation of the hands, Libet et al.134 wrote
“…even young alert normal subjects were not subjec-
tively aware of the order of two somatosensory inputs
when the time interval between them was less than 25-
50 ms”. Finally, a recent review by Azemar et al.135
documents that athletes engaged in sporting duels such
as fencing and boxing are more likely to win if the lat-
erality of their macular vision (i.e. dominant eye) is the
same as that of their command center (Fig. 2), findings
that are in agreement with one-way callosum traffic
circuitry underpinning the lateralities of motor and sen-
sory control.
CONCLUSIONS
A meta-analysis of the literature concerning laterality of
vision, attention, neglect and anosognosia in relation to
hemispheric asymmetries indicates that Newton’s for-
mula relating to visual experience lacks clinical and
experimental support. Numerous case studies have been
cited in which patients experienced blindness as a result
of injury limited to the occipital lobe of one hemisphere
(the major) while macular vision remained intact in
others following hemispherectomy or occipital lobec-
tomy (of the minor hemisphere).
Experimental data indicate that the additional time
required for moving the eyes to the nondominant side of
the space prevents the dominant hemisphere from ex-
tracting sufficient data in those paradigms that limit
exposure of information to a very short time. A com-
prehensive analysis based on one-way callosal traffic
circuitry indicates that the eyes are never stationary

12 Derakhshan / Biomedicine International (2010) 1: 3-15
even when fixating an object. It also indicates that
IHTT-mediated lateral gaze differentials in humans
were overlooked in earlier studies investigating atten-
tion and neglect, or were mistakenly ascribed to the
laziness of the minor hemisphere; in either case, the
role of directionality in callosal traffic was negated. The
reverse situation exists for somatosensory data; signals
arising from the nondominant side of the body will not
reach consciousness until the signals arriving at the mi-
nor hemisphere traverse the posterior callosum and
reach the major hemisphere.
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