Interhemispheric transfer and the processing of foveally presented stimuli

Abstract

Two arguments are commonly given in favor of a nasotemporal overlap along the vertical meridian of the visual field: anatomical findings and the existence of macular sparing in hemianopia. A review of the literature, however, points to the weakness of the evidence. The anatomical indications are exclusively based on horseradish peroxidase studies, which can not give an unequivocal answer to the amount of overlap in central vision, and which were not supported by a recent study that made use of the more direct ["C]2deoxy-D-glucose technique. The argument of macular sparing in hemianopia appears to be derived evidence that depends on the validity of the anatomical findings. In addition, behavioral studies consistently failed to find functional confirmation of the overlap. To further test the possibility of bilateral representation in central vision, a new paradigm is proposed. It is argued that if interhemispheric transfer is needed for the processing of foveally presented stimuli, the word-beginning superiority effect should be larger for subjects with left hemisphere dominance than for subjects with right hemisphere dominance. Results are in line with the hypothesis and point to the fact that interhemispheric transfer of visual information may be involved in more processing than usually accepted. It is also noted that transfer time seems to depend on the amount of information that must be transferred, and is significantly shorter than the estimates obtained in visual half field studies.Key words: Corpus callosum; Interhemispheric transfer; Macular sparing; Foveal vision; Optimal viewing position; Visual half field studies vision in the foveal region, a phenomenon called macular or foveal sparing. I will discuss the two arguments successively. Anatomical evidence of a hemiretinal overlapThe anatomical evidence for an overlap between the nasal and the temporal hemiretina comes from studies of unilateral sectioning of the optic tract or unilateral electrophoretic injection of horseradish peroxidase (HRP) into the dorsal lateral geniculate nucleus (dLGN) [ 19,20,35,36,52,53,55]. This technique allows tracing the ganglion cells of the retina which project to the dLGN and from there to the visual cortex. The general finding is that in a strip of about 0.2 mm along the vertical midline, ipsilaterally and contralaterally projecting ganglion cells intermingle. This is true both for cats and monkeys. According to Leventhal et al. [35,36] the overlap is primarily due to the presence of ipsilaterally projecting cells in the nasal hemiretina, rather than to contralaterally projecting cells in the temporal hemiretina. Interhemispheric transfer and the processing of foveally presented stimuliDue to the partial decussation of the optic nerves at the optic chiasm, the human visual field is split in two halves: the left and the right visual field (LVF and RVF). Both hemifields project to the contralateral cerebral hemisphere. That is, stimuli presented in the LVF are transmitted to the right cerebral hemisphere, and stimu...

The Split Fovea Theory and the Leicester critique: What do the data say?

“…Because precise assessment of language dominance requires fMRI testing of left-handed participants (Hunter & Brysbaert, 2008b), we limited our studies to right-handed individuals who in addition were right eye dominant (needed because we tracked the right eye). The default expectation for these participants is that they will be left hemisphere dominant and, indeed, none of our participants showed the flat curve typically observed in right dominant participants (Brysbaert, 1994a;Hunter et al, 2007;Hunter & Brysbaert, 2008a).…”

“…The present study is an attempt to assess the impact of inadequate fixation control on the OVP effect, in particular on the left-right asymmetry between fixations on the word beginning and the word end, which has been interpreted as evidence for interhemispheric transfer in foveal word recognition (Brysbaert, 1994a;Hunter et al, 2007).…”

“…The OVP effect is the outcome of four factors (Brysbaert & Nazir, 2005): (1) the first letters contain the most information about the identity of the word; (2) visual acuity decreases when the distance between the fixation location and the letter increases; (3) the eyes tend to land on the first half of the word, so that readers have more practice processing words from this location; and (4) fixating the left side of a word makes most letters fall in RVF and hence most information is sent directly to the LH, the language dominant hemisphere for the majority of readers. 1 Evidence for the hypothesis that brain laterality is involved in the OVP-effect was obtained by comparing the OVP effect for participants with left and right hemisphere dominance (Brysbaert, 1994a;Hunter et al, 2007). For left dominant participants, factors 1, 3 and 4 result in a word beginning advantage and, hence, we can expect to find a hefty processing cost when participants are forced to fixate on the last letters.…”

“…Even adding a fixation control task, such as asking participants to name briefly presented digits that appear on some of the trials (e.g., Brysbaert, 1994a;Hunter et al, 2007), does not fulfill the requirement. The only way to properly control fixation positions is by using an eye-tracking device.…”

“…In the experiments reported below we repeated the OVP studies reported by Brysbaert (1994a) and Hunter et al (2007) and introduced various degrees of fixation control. In addition, we made the stimuli small enough to fit within the foveal area and used monocular viewing.…”