Effects of Output Order on Hemispheric Asymmetry for Processing Letter Trigrams

Observers identified consonant–vowel–consonant trigrams with the letters arranged vertically by pronouncing the stimulus (treating the bottom letter as the first letter) and spelling it from bottom to top. On each trial, the trigram was presented to the left visual field/right hemisphere (LVF/RH), to the right visual field/left hemisphere (RVF/LH), or to both visual fields simultaneously (BILATERAL trials). Quantitative and qualitative visual field differences were identical to those found when observers used a more natural response output order, treating the top letter of the trigram as the first letter. The results suggest that, regardless of output order, attention is distributed across the three letters in a relatively slow, top-to-bottom fashion on LVF/RH and BILATERAL trials, whereas attention is distributed more rapidly and evenly across the three letters on RVF/LH trials.     

Can the spotlight of attention be shaped like a doughnut? Evidence from steady-state visual evoked potentials

Visual attention enables observers to extract and process high-priority information in the visual field. Controversy remains as to whether or not observers can ignore information that falls within the spatial beam of attention. We used an objective physiological measure, the steady-state visual evoked potential (SSVEP), to investigate this question. A stream of flickering small, uppercase letters was embedded in the center of a stream of large, uppercase letters. A unitary beam would result in no difference of the SSVEP amplitude elicited by the small letter stream when it was attended versus ignored (i.e., when subjects attended the large letter stream). Contrary to this prediction, SSVEP amplitude increased by almost 100% when the small letter stream was attended compared with when it was ignored. The results support the notion that the attentional spotlight can be formed like a doughnut, processing central information differentially depending on whether it is attended or ignored.     

Interference effect caused by repetition in visual recognition of letters: Examination of the orthographic difference between two same letters1

Abstract: The present study examined the effect of phonological identity between two letters on the visual recognition of the letters. Participants were required to identify the two same or different letters that were successively presented for a short duration. In order to manipulate the phonological identity of the two letters, the orthography of the Kana letters was varied. In half of the trials, the first and second letters were presented in Hiragana (the same‐orthography condition). In the other half, the first letter was presented in Katakana, and the second letter in Hiragana (the different‐orthography condition). The results revealed that the identification performance for the second letter was reduced in the same‐orthography condition when the two letters were the same, compared with when they were different. In contrast to this, in the different‐orthography condition, the identification performance of the second repeated letter was marginally superior to that of the nonrepeated letter. Considering the present findings together with those of the author's previous study (Kuwana, 2004), it is suggested that the interference effect caused by repetition could result from the reduction in the availability of visual pattern information stored in long‐term memory.     

Effects of perceptual quality and visual field of probe stimulus presentation on memory search for letters

Observers indicated whether a single probe letter presented to the left visual field/right hemisphere (LVF-RH) or to the right visual field/left hemisphere (RVF-LH) was contained in a memory set of 2, 3, 4, or 5 letters. For positive trials, the increase in reaction time caused by perceptually degrading the probe letter became progressively larger as memory set size became larger when the probe was presented to the LVF-RH but not when the probe was presented to the RVF-LH. These results were obtained regardless of whether the case of the probe letter varied randomly (Experiment 1) or only capital letters were used (Experiment 2). The results on LVF-RH trials suggest a relatively visuospatial memory comparison process, whereas the results on RVF-LH trials suggest a more abstract memory comparison process. In addition to these effects, the intercept of the memory set size function was lower on LVF-RH trials than on RVF-LH trials when the probe letter was perceptually degraded, consistent with the hypothesis that the right hemisphere is more efficient than the left at early visuospatial processes. Perhaps it is this efficiency at early visuospatial processes that produces the bias toward visuospatial memory comparison on LVF-RH trials.     

Retinal locus and the letter-span error function

The effect of retinal locus on the letter-span error function was investigated by requiring Ss to fixate at the left, middle, or right of lines of letters presented for 100 msec. Stimulus presentations consisted of lines of eight letters presented across the visual field, and lines of four letters presented alternately in the left and right visual hemifields. Locus of fixation and relative letter position were found to be significantly related to whether or not a letter was correctly localized. A significant interaction between locus of fixation and letter position was observed.     

Effect of level of confusability on reporting letters from briefly presented visual displays

In a report paradigm, two letters are presented on a trial which are either confusable(e.g., P and R) or nonconfusable (e.g., P and M) in terms of visual features. Across trials, interletter distance, retinal location, duration, and visual field are varied. Identification accuracy on confusable trials was generally lower than on nonconfusable trials, and this effect of level of confusability increased with distance from the fixation point, decreased with duration, and was smaller on the central letter than on the more peripheral letter. A quantitative model, incorporating aspects of the interactive channels model (Estes, 1972) and feature perturbation model (Wolford, 1975), is developed and tested. One parameter of the model measures the effective similarity between two letters after lateral inhibition has occurred, and other parameters measure the probability of feature perturbation in foveal and peripheral directions.     

Evidence for an addition to the reading scan hypothesis

Letters were tachistoscopically presented in pairs to the left or right of fixation for 40 msec. The pair consisted of a target letter (B, D, P, or T) and a noise letter (all other letters of the alphabet except I), with the noise letter appearing either to the inside or the outside of the stimulus letter. S’s task was to report verbally the target letter as fast as possible. Reaction times and number of errors indicated that there was more interference when the noise letter occurred further from the fixation point than the target letter for both the left and right visual field. Since a reading scan cannot account for the results in the right visual field, an outside-toward-fixation scan is proposed. An analysis of the different kinds of errors indicated that the outside to fixation scan is more primitive than the reading scan in that it produces less detailed information about the letters than does the reading scan.     

A unilateral field advantage for detecting repeated elements

Previous studies have shown that letter repetitions are detected more rapidly when presented bilaterally (one letter in each visual field) than when presented unilaterally (both in the same field) when subjects have to report matches independently of case or font (e.g., Aa). This pattern of results is referred to as a bilateral field advantage. Here, we present evidence of an opposite pattern of results for detecting repeated items when they are physically identical. In our repetition detection paradigm, subjects indicated whether there was a repetition of any two of four presented items, one in each quadrant of the visual field. Stimulus classes tested included letters, color, size, orientation, and motion paths. The subjects were significantly faster at detecting unilateral versus bilateral repetitions for four out of the five stimuli classes tested, with a trend in the same direction for the fifth. This unilateral field advantage suggests that low-level processes group physically identical items more efficiently within hemifields than across.     

Effect of list length on recall after dichotomous visual presentation

A visual analogue of an auditory dichotic listening experiment was carried out using what may be termed dichotomous visual presentation of lists of either three or four stimuli. Each stimulus consisted of several identical letters which were arranged so as to form the outline of a second, large letter. Subjects attended to either the small or large letters, and recalled either the attended or unattended letters first. It was found that the results for the three-stimulus and four-stimulus lists differed substantially; they were interpreted as consistent with the model of short-term memory proposed by Broadbent (1975).     

The integrative action of the cerebral hemispheres

In four experiments, right-handed subjects were asked to identify uppercase letters presented tachistoscopically in the right (R) or left (L) visual field or in both visual fields simultaneously (RL). When R, L, and RL trials were randomized together and RL trials consisted of the same letter in each visual field (Experiments 1 and 2), 11 of 21 male subjects (Experiment 1) and 9 of 18 female subjects (Experiment 2) showed a strong left-visual-field advantage, and the remainder in each experiment showed an equivalently strong right-visual-field advantage. When the RL trials consisted of a different letter in the two visual fields (Experiment 3), a consistent right-visual-field advantage was observed. It is argued that these results reflect predominantly analytical (left-hemisphere) processing of “different” pairs and relatively holistic processing of “same” pairs, which induces a shift toward a right-hemisphere advantage in some subjects. The main purpose of three of the four experiments was to test five probability models of hemispheric integration: (1) statistical summation, which assumes that the hemispheres operate independently; (2) redundancy, which assumes that RL decisions reflect only processing by the more specialized hemisphere; (3) complete dependency, according to which RL decisions are the mean of Rand L decisions; (4) integration, which assumes that R and L decisions can be represented as vectors in a joint RL space; (5) correlation, according to which the Rand L decisions are assumed to be statistically dependent. Whether R, L, and RL trials were randomized together or RL trials were presented in separate blocks, models 1 and 4 could be clearly rejected. In general, the best predictor of RL performance was model 5. It is argued that the hemispheres function as an integrated system in letter identification.     

Effects of concurrent spatial and verbal memory loads on serial position functions of laterally presented letter strings

In this study the influence of visuospatial and verbal memory loads on the identification of laterally presented letter strings was investigated. In the asymmetry task without concurrent loads, a clear right visual field advantage for letter identification was obtained. Hemisphere-specific effects due to concurrent loads were particularly observed when the difficulty of the load tasks was increased. The effects of visuospatial loads were found to be sex related, suggesting that under heavy load conditions mental rotation selectively overloads the processing capacity of the right hemisphere in males, while in females capacity limitations were observed in both hemispheres. Concurrent visuospatial loads produced more facilitation (or less interference) for letters in the outermost positions of each visual field than for letters in the innermost positions of each visual field. The results of the verbal memory load tasks revealed that an easy verbal load task facilitated performance which was particularly manifest for the right-most letter of both the left visual field and the right visual field. A difficult verbal memory load task interfered with recognition accuracy of letters which was most marked for the center letter in the right visual field. Letter position effects obtained in this study were interpreted in terms of various processing mechanisms influencing the serial position curves.     

Flank to the left,flank to the right: Testing the modified receptive field hypothesis of letter-specific crowding

The present study tested for effects of number of flankers positioned to the left and to the right of target characters as a function of visual field and stimulus type (letters or shapes). On the basis of the modified receptive field hypothesis (Chanceaux & Grainger, 2012), we predicted that the greatest effects of flanker interference would occur for leftward flankers with letter targets in the left visual field. Target letters and simple shape stimuli were briefly presented and accompanied by either 1, 2, or 3 flankers of the same category either to the left or to the right of the target, and in all conditions with a single flanker on the opposite side. Targets were presented in the left or right visual field at a fixed eccentricity, such that targets and flankers always fell into the same visual field. Results showed greatest interference for leftward flankers associated with letter targets in the left visual field, as predicted by the modified receptive field hypothesis.     

Hemispheric differences in processing handwritten cursive

Hemispheric asymmetry was examined for native English speakers identifying consonant-vowel-consonant (CVC) non-words presented in standard printed form, in standard handwritten cursive form or in handwritten cursive with the letters separated by small gaps. For all three conditions, fewer errors occurred when stimuli were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH) and qualitative error patterns indicated that the last letter was missed more often than the first letter on LVF/RH trials but not on RVF/LH trials. Despite this overall similarity, the RVF/LH advantage was smaller for both types of cursive stimuli than for printed stimuli. In addition, the difference between first-letter and last-letter errors was smaller for handwritten cursive than for printed text, especially on LVF/RH trials. These results suggest a greater contribution of the right hemisphere to the identification of handwritten cursive, which is likely related visual complexity and to qualitative differences in the processing of cursive versus print.     

Memory for attended and unattended visual stimuli

Subjects were presented with lists of 'compound letters,' letters whose overall shapes were described by repeated use of replicates of other, smaller letters. In Experiment 1 subjects were asked to attend to either the overall letter or the smaller, constituent letter. At the end of list presentation, recall of all letters was required, but a postlist cue determined whether the attended or unattended letters were to be reported first. The results for four-item lists accorded with those of Martin (1978, 1980): order of report had a larger effect upon retention of attended letters than upon retention of unattended letters. The findings for three-item lists did not agree with Martin, however: first, the interaction of attention and order was weak; second, sharp recency for unattended letters was not found.

Experiment 2 required that subjects recall either in temporal pairs or by letter size. The results strongly suggest that the present paradigm does not constitute an analogue to dichotic listening. In particular, there is little evidence for a role for sensory retention of compound letters at time of recall.     

Interhemispheric cooperation and non-cooperation during word recognition: evidence for callosal transfer dysfunction in dyslexic adults

Participants report briefly-presented words more accurately when two copies are presented, one in the left visual field (LVF) and another in the right visual field (RVF), than when only a single copy is presented. This effect is known as the 'redundant bilateral advantage' and has been interpreted as evidence for interhemispheric cooperation. We investigated the redundant bilateral advantage in dyslexic adults and matched controls as a means of assessing communication between the hemispheres in dyslexia. Consistent with previous research, normal adult readers in Experiment 1 showed significantly higher accuracy on a word report task when identical word stimuli were presented bilaterally, compared to unilateral RVF or LVF presentation. Dyslexics, however, did not show the bilateral advantage. In Experiment 2, words were presented above fixation, below fixation or in both positions. In this experiment both dyslexics and controls benefited from the redundant presentation. Experiment 3 combined whole words in one visual field with word fragments in the other visual field (the initial and final letters separated by spaces). Controls showed a bilateral advantage but dyslexics did not. In Experiments 1 and 3, the dyslexics showed significantly lower accuracy for LVF trials than controls, but the groups did not differ for RVF trials. The findings suggest that dyslexics have a problem of interhemispheric integration and not a general problem of processing two lexical inputs simultaneously.     

Effects of informative peripheral cues on eye movements: Revisiting William James’ “derived attention”

Two experiments examined effects of peripheral information on the latency of saccadic eye movements. In Experiment 1, simple target stimuli were presented to the left or right visual field. Prior to each target, a pair of cue letters was presented for 40msec bilaterally. The relative location of the letters (W-S or S-W) was related to target location, but participants were not informed of this contingency. After a brief practice period, saccadic latencies were faster for targets at the likely location, as indicated by the letter pair. This derived peripheral cueing effect was related to participants' awareness of the relation between cue type and target location. Experiments 2A and 2B employed monocular viewing in order to compare performance across the nasal and temporal visual fields. The effect observed in Experiment 1 was confined to the nasal visual field. In a reflexive orienting condition, the effect of a unilateral letter cue was larger in the temporal visual field. It is concluded that the neurocognitive processes responsible for derived peripheral cueing are distinct from those involved in either reflexive or voluntary orienting.     

Attentional distribution in the visual field duringsame-different judgments as assessed by response competition

Judgments ofsame anddifferent on a comparison task have been found to be subject to response competition if an irrelevant stimulus is presented in the display along with the target stimuli. For example, the reaction time for judging two letters the same is markedly increased if a different but irrelevant letter is also present in the display (C. W. Eriksen, O’Hara, & B. [A.] Eriksen, 1982). We have made use of this competition effect to map the visual attentional field in two dimensions. In two experiments, we varied the size of the attended area by varying the separation of the comparison stimuli. The boundaries of the attended area were mapped by varying the location of a response-competitive irrelevant noise letter. On this task, the attended area was found to be elliptical in shape, with the location of the target stimuli defining the major axis. The minor axis of the ellipse increased in direct proportion to increases in the major axis. Rather than interpret these field effects in terms of areas of enhanced processing, we propose that instead they represent the limits or failures in inhibition of competing stimulation.     

Hemispheric asymmetry,early visual processes,and serial memory comparison

Observers indicated whether a single probe letter presented to the left visual field/right hemisphere (LVF-RH) or to the right visual field/left hemisphere (RVF-LH) matched one of two, three, four, or five set letters in both name and case. For positive trials during the initial experimental session, the slope of the linear memory set size reaction time function was increased by perceptually degrading the probe letter on LVF-RH trials, but not on RVF-LH trials. In addition, perceptual degradation of the probe letter increased the intercept of the memory set size function more on RVF-LH trials than on LVF-RH trials. During subsequent experimental sessions, the same pattern of intercept results was obtained but perceptual degradation of the probe no longer changed the slope for either LVF-RH or RVF-LH trials. However, the slopes were uniformly lower on RVF-LH trials than on LVF-RH trials. The major results are consistent with hypothesized right-hemisphere efficiency for early processing stages and left-hemisphere efficiency for serial processing of alphanumeric information. The results further illustrate the importance of separating stages of processing in studies of cerebral laterality and indicate that the relative difficulty of the various stages can be a determinant of laterality results.     

Repeating the target neither speeds nor slows its detection: Evidence for independent channels in letter processing

A target letter at a predesignated location typically is identified less readily when extraneous letters are added to the display. This disruption has been attributed to lateral interference via interactive or inhibitory channels or to attempts to encode the string as a unit. In the present study, subjects saw a single letter (e.g., B), a repeated-letter string (e.g., BBBB), or an extraneous-letters string (e.g., BCLD) and had to decide whether the leftmost letter in the string matched a target letter. Since trials were blocked by string type, letter position did not have to be discriminated on repeated-letter trials, nor was response competition present on those trials. With normal letter spacing, RT was virtually the same on repeated-letter trials as on single-letter trials. (Increasing the letter spacing in Experiment 3 did produce a slight, but nonsignificant, 22-msec increment on the repeated-letter trials.) The results indicate that individual letters are perceived as such just as well when presented in a group as when presented individually and thus provide support for the parallel, independent-channels model.     

Sequential processing in hemispheric word recognition: the impact of initial letter discriminability on the OUP naming effect

The cerebral hemispheres have been proposed to engage different word recognition strategies: the left hemisphere implementing a parallel, and the right hemisphere, a sequential, analysis. To investigate this notion, we asked participants to name words with an early or late orthographic uniqueness point (OUP), presented horizontally to their left (LVF), right (RVF), or both fields of vision (BVF). Consistent with past foveal research, Experiment 1 produced a robust facilitatory effect of early OUP for RVF/BVF presentations, indicating the presence of sequential processes in lexical retrieval. The effect was absent for LVF trials, which we argue results from the disadvantaged position of initial letters of words presented in the LVF. To test this proposition, Experiment 2 assessed the discriminability of various letter positions in the visual fields using a bar-probe task. The obtained error functions highlighted the poor discriminability of initial letters in the LVF and latter letters in the RVF. To confirm that this asymmetry in initial letter acuity was responsible for the absent OUP effect for LVF presentations, Experiment 3 replicated Experiment 1 using vertical stimulus presentations. Results indicated a marked facilitatory effect of early OUP across visual fields, supporting our contention that the lack of OUP effect for LVF presentations in Experiment 1 resulted from poor discriminability of the initial letters. These findings confirm the presence of sequential processes in both left and right hemisphere word recognition, casting doubt on parallel models of word processing.