Evidence and theory behind the symmetrical connections joining the brain's hemispheres

Abstract

This dissertation considers how left-right symmetry of body and brain influences how the two brain hemispheres represent visual/spatial topology. A “mirror-posed” model (for the mirror superposition of both visual fields) is proposed as a general framework to examine effects of bilateral symmetry on perception, action, and memory. The model explicitly considers whether mirror reversals in neural representation are necessarily accompanied by motor or perceptual reversals. This thesis begins with the hypothesis that the transfer of a visual representation from one hemisphere to the other is accompanied by left-right mirror reversal. This can result in the superposition of veridical and mirrored representations within the same hemisphere. It is proposed that this occurs primarily in the right cerebral hemisphere. Chapter 2 examines whether this mirroring can be detected in the way people draw squares with opposite hands. There was little evidence that right-handers drew squares in opposite directions with the two hands. Left-handers were more likely to do so, but an analysis in Chapter 3 showed that they were also more likely to use an inverted drawing posture when drawing with the left hand, ruling out the possibility that the reversed drawing with the left hand was due simply to a reversal of manual motor representations (praxicons) in the right hemisphere. Chapter 4 examines whether the left-right sense of a shape might be coded with reference to the dominant eye rather than with respect to hand movements. Participants copied asymmetrical shapes with one or other hand, and when viewing with one or other eye. They later viewed the shapes with either the same or opposite eye, and were asked to indicate whether the viewed shape was in the same or opposite left-right orientation. There was no evidence that switching eyes during initial copying and subsequent testing impaired discrimination, and indeed some indication that discrimination was better with the eye that was dominant for reading following copying with the nondominant eye. More generally, there was no evidence for systematic reversal under any of the conditions. Chapter 5 investigates the processing of picture in two phases, a priming phase in which single pictures were presented in one or other visual field and viewed with one or other eye, and a testing phase in which they were presented in the same field, the opposite field, or in both fields. In both phases viewing was restricted to either nasal or temporal visual fields. In the testing phase RT to decide whether the pictures represented living or nonliving objects was faster when they were mirrored between fields, but only when the initial viewing was by the eye that was nondominant for reading. This suggested a modification of the original model, such that mirroring was more likely in the hemisphere nondominant for reading, rather than in the right hemisphere as such. Chapter 6 investigates the reading of normal and left-right reversed (mirrored) three-letter words. The words were presented either singly in one or other visual field, or bilaterally. Although mirrored words were read more slowly than normal words, the difference was less marked for those who preferred the left eye for reading, suggesting greater mirror-image representation in the right hemisphere in these participants. Left-handers with normal (rather than inverted) hand posture showed evidence of right-hemispheric dominance for the representation of word forms, and were better able than right-handers to read mirrored words in the right visual field. Chapter 7 replicated this study, with some minor modification and better control of attention, and showed that the redundancy gain with bilateral presentation found in the previous experiment, with faster reading of right-visual-field words when either a normal or a mirrored copy was present in the left visual field, was probably due to focused attention to the right visual field. The effect was no longer present when attention was divided between fields. During reading of symmetrical word pairs, addition of the forwards direction word copy to the LVF facilitated reading while inhibition occurred when the RVF contained the forwards-direction word copy. The mirror-posed model explains this as homotopic resonance that preferentially aligns images within the right hemisphere. Chapter 8 replicates this experiment as a case study whereby a natural left-hander forced to write with his right hand, also being a stutterer, depends heavily on the right hemisphere for reading aloud. It is proposed that reliance on the (less capable) right hemisphere is the common ground between stuttering and hand-switching. Chapter 9 presents a model to explain how the patient of Nyman et al. (1982) mysteriously showed three different percepts when viewing a pair of letters, depending on the length of the presentation. 1) Only the quickest presentations allowed accurate perception. 2) Slightly slower presentation created an illusory superimposition of the two letters. 3) Longer presentations resulted in a perceptual left-right reversal of the two letters. The Ipsifov model proposes a single neural structure that can explain all three of these percepts, as well as invoking the known principles of symmetrical topology that apply during axonal migration for achiasmatic and albino people. This serves as an illustration of how the left-right symmetry between the two brain hemispheres may create inadvertent perception of spatial exchange following developmental deviations. Chapter 10 summarizes and integrates the findings of this dissertation.