12.31.06

Dear Editor,

The conundrums, depicted in Dr. Pavao Martins’ well documented report, 1 have recently been resolved. The author described five patients who became aphasic during repeated episodes of complicated migraine affecting the right cranium, followed by complete recovery. Based on the fact that they were all (behaviorally, see below) right handed, the author engaged in speculative assumptions related to possible transfer of “speech center” from left to right hemisphere; ignoring recent documentations that speech is merely a convenient marker of the hemisphere of action where all commands are initiated regardless of the laterality of the effector addressed by that command (i.e. the command center or the major hemisphere). Thus, it has been shown that commands destined for those effectors ipsilateral to the command center arrive at their destination late, by an amount equal to transcallosal interhemispheric transfer time (IHTT).2-5 In vast majority of behavioral right handers the command center lies on the left, whereas it lies on the right side in about 50 % of those who avow left handedness. 6 The twin fact that the ratio of right to left handedness in the congenitally blind mirrors that of the same in general public and that of the existence of an eight fold increase in the incidence of left handedness among the families of a sighted left hander point to the role of volition in adopting a favorite hand different from that designated by nature (i. e. neural handedness).

In the past, this disparity of the two modes of laterality had caused chaos in the classical literature which was satisfied by cataloguing these instances as “crossed” aphasias or “crossed” nonaphasias (as the case may be).

Thus, it is the proximity to the command center that underpins the laterality of the dominant side of the body; with other side being further away from the command by a callosum-width. This differential is manifested by an increased reaction time of the nondominant side of the body by an amount equal to the IHTT.

The following observation provides an example, while corroborating Pavao-Martin’s findings:

DW is a 50 years old right handed woman with history of migraine since childhood. These headaches are usually right sided although on occasion they may start as frontal headaches affecting both sides. She describes three “strokes” associated with migraines, all affecting the left side of the body, with the last one involving her left leg (lasting two hours, without a sequel). She has difficulty with orientation to place and time with some of her headaches. In other occasions she cannot express herself and has difficulty understanding what she hears. These speech episodes typically lasted minutes to an hour or more. The latest attack was associated with “tremor” of the left arm. This headache which was associated with nausea, vomiting and blurred vision, did not respond to Imitrex (6 mg, subcutaneously). There was no history of epilepsy or head trauma. Her MRI of brain was unremarkable. In the absence of headaches, neurological examination in the areas of visual fields, cranial nerves, motor and sensory examination and equilibrium has been unremarkable.

Method and Results: DW underwent a reaction time study, using proprietary software prepared for such purpose. The latter allowed determination of time (in milliseconds) taken from the appearance of a GO signal (after a GET READY warning) in the center of the computer screen to the pushing of a designated button (in this case the buttons for letters “a” and ‘l,’ for the blocks using the left and right hand (respectively). Each block consisted of 39 trials. Results of two consecutive trials are depicted below:

Average response delays (reaction time) (a) left hand 240 ms, (l) right hand 263 ms.

a 281, 1 329

a 266 , l 250

a 281 , l 187

a 250 , l 219

a 281 , l 250

a 266 , l 265

a 265 , l 250

a 235 , l 235

a 218 , l 234

a 235 , l 203

a 172 , l 235

a 203 , l 218

a 250 , l 250

a 234 , l 266

a 250 , l 266

a 219 , l 250

a 234 , l 218

a 188 , l 219

a 187 , l 406

a 250 , l 438

a 219 ,l 297

a 250 , l 250

a 344 , l 250

a 250 , l 219

a 218 , l 234

a 203 , l 359

a 266 , l 219

a 250 , l 313

a 313 , l 281

a 235 , l 219

a 281 , l 265

a 234 , l 235

a 313 , l 296

a 265 , l 250

a 235 , l 235

a 218 , l 265

a 219 , l 250

a 234 , l 250

a 219 , l 266

Conclusion: As demonstrated above, there is a delay of 23 ms for performing a simple reaction time with the ostensibly dominant hand in DW, making her a neurally left handed subject. 2-5,9 The relationship of avowed handedness to that underpinned by the abovementioned circuitry is statistical (not biological).

Iraj Derakhshan

References:

1. Pavao Martins I. Crossed aphasia during migraine aura. Transcallosal spreading depression? J Neurol Neurosurg Psychiatry. 2006; 1-3.

2. Derakhshan I. Laterality of seizure onset and the simple reaction time: revamping the Poffenberger's paradigm for seizure surgery. Neurol Res. 2006; 28:777-784.

3. Derakhshan I. Nonconvulsive status epilepticus with an unusual EEG: a fresh look at lateralities of motor control and awareness. Epilepsy Behav. 2006; 9:204-210.

4. Derakhshan I. Crossed-uncrossed difference (CUD) in a new light: anatomy of the negative CUD in Poffenberger's paradigm. Acta Neurol Scand. 2006; 113:203-208.

5. Derakhshan I. How do the eyes move together? New understandings help explain eye deviations in patients with stroke. CMAJ. 2005; 172:171-173.

6. Goodglass H, Quadfasel FA. Language laterality in left-handed aphasics. Brain. 1954; 77:521-548.

7. Kutas M, McCarthy G, Donchin E. Differences between sinistrals' and dextrals' ability to infer a whole from its parts: a failure to replicate. Neuropsychologia. 1975; 13:455-464.

8. Oishi M, Suzuki K, Sasaki O, Nakazato S, Kitazawa K, Takao T, Koike T. Crossed aphasia elicited by direct cortical stimulation. Neurology. 2006; 67:1306-1307.

9. Derakhshan I. Crossed aphasia elicited by direct cortical stimulation. Correspondence, accepted for publication in Neurology (copy attached and available online).