Lateralized word recognition: assessing the role of hemispheric specialization, modes of lexical access, and perceptual asymmetry.

The processing advantage for words in the right visual field (RVF) has often been assigned to parallel orthographic analysis by the left hemisphere and sequential by the right. The authors investigated this notion using the Reicher-Wheeler task to suppress influences of guesswork and an eye-tracker to ensure central fixation. RVF advantages obtained for all serial positions and identical U-shaped serial-position curves obtained for both visual fields (Experiments 1-4). These findings were not influenced by lexical constraint (Experiment 2) and were obtained with masked and nonmasked displays (Experiment 3). Moreover, words and nonwords produced similar serial-position effects in each field, but only RVF stimuli produced a word-nonword effect (Experiment 4). These findings support the notion that left-hemisphere function underlies the RVF advantage but not the notion that each hemisphere uses a different mode of orthographic analysis.

Central fixations are inadequately controlled by instructions alone: implications for studying cerebral asymmetry.

A fundamental concern when using visual presentations to study cerebral asymmetry is to ensure that stimuli are presented with the same degree of retinal eccentricity from a central fixation point in either visual field. However, a widely used procedure intended to control fixation location merely instructs participants to fixate appropriately without any other means of ensuring that central fixations actually occur. We assessed the validity of assuming that instructions alone ensure central fixation by using the traditional RVF advantage for words and either (a) only instruction to fixate centrally, or (b) eye-tracking device that ensured central fixation on every trial. Experiments 1 and 2 found that when only instructions were given, the vast majority of fixations were not central, and more occurred to the right of centre than to the left. Moreover, the prevalence of non-central fixations was otherwise disguised by the finding that both fixation procedures produced similar RVF advantages in overt performance. The impact of typical non-central fixations on performance was revealed by systematically manipulating fixation location in Experiment 3, where deviations in fixation of only 0.25 degrees from centre had a reliable impact on visual field effects. Implications of these findings for studies of cerebral asymmetry are discussed.

Event-related potentials and the recollection of associative information.

Event-related potentials (ERPs) were recorded from 13 scalp sites during the performance of an associative recall task. At study, subjects were presented with a series of word pairs and were required to incorporate the two members of each pair into a sentence. At test, the first members of each pair were presented intermixed with an equal number of unstudied items. Subjects were required to discriminate between new and studied (old) words and, for each word judged old, to attempt to recall the word with which it had been associated at study. Compared to the ERPs elicited by new words, the ERPs elicited by words correctly judged to be old and for which the associate was correctly recalled showed a sustained, positive-going shift (the "parietal old/new effect"). This effect was strongly lateralised to the left hemisphere and was maximal at temporo-parietal electrodes. The effect was absent in ERPs elicited by words that were correctly judged to be old, but for which the studied associate could not be recalled. The findings are taken as support for the idea that the parietal old/new effect reflects neural activity associated with the recollection of specific past episodes, and hence that the effect may index retrieval operations supported by the medial temporal lobe memory system.