Recall of letters from the left and right visual half-fields under unilater and bilateral presentation.

Bilateral presentation in the visual half-field greatly increases superiority of the right visual half-field in tachistoscopic recognition of words when fixation is controlled using a center digit. Two experiments explored left-right asymmetry with bilateral presentation on a visual half-field short-term memory task, with fixation controlled by a sequence of letters at fixation. A total of 40 subjects served in the two experiments, which compared recall under unilateral versus bilateral presentation to the visual half-field. Bilateral presentation increased over-all recall from the last serial position but did not alter asymmetry of the visual half-field. As in previous experiments, the superiority of the right visual half-field was greatest from the initial serial positions. It was concluded that asymmetry of the visual half-field on this recall task with controlled fixation depends primarily on masking and short-term memory but is independent of unilateral-bilateral presentation.     

Perceptual interference and hemispheric specialization

Two experiments evaluated the effect of stimuli presented at fixation on the recognition of faces or random shapes presented to the left or right visual half-field (VF). Increasing the processing demands of the center stimulus produced a large, linear decrease in recognition from both VFs for both faces and shapes. Recognition of random shapes was decreased more in the right visual field by center digits and in the left VF by center faces and shapes. In addition, interference was found between the VF faces and the center digits to the left of fixation. It was concluded that differences in the processing capacity of the two hemispheres are a function of the verbal-nonverbal nature of the stimuli at a later stage in processing but that the two hemispheres may also differ along other perceptual dimensions at an earlier stage of visual recognition.     

Simon-type effects: chronometric evidence for keypress schemata in typewriting

In 4 experiments, chronometric evidence for keypress schemata in typing was sought by presenting stimuli to be typed in positions that were displaced from a central fixation point. Reaction times were shorter when stimulus positions corresponded to keyboard locations of the letters to be typed, suggesting that position was an important part of the internal representation of the response. Experiment 1 presented single letters left and right of fixation. Experiment 2 presented single letters above and below fixation. Experiment 3 presented words left and right of fixation and found evidence of parallel activation of keypress schemata. Experiment 4 found no effect of the eccentricity of the keyboard locations and responding fingers, suggesting that response-location codes are categorical, not metric. The results are consistent with D. E. Rumelhart and D. A. Norman's (1982) theory of typewriting.     

Part-whole relationships in the processing of small visual patterns

Subjects seem to react to a word faster than they react to a letter within a word. One interpretation is that words are processed holistically; another is that all visual stimuli are processed in terms of components, but that more stimulus information is available for use when the targets are words than when they are letters within words. The results of three experiments indicate that the word or pattern-level advantage occurs even when the stimulus information in the two situations is equated, but if the perceptual arrays cannot be unitized (e.g., consonant sequences), a pattern-level advantage does not occur. In addition, the experiments provide substantial evidence to indicate that if letter arrays cannot be unitized, then they are processed on a componentby-component basis, rather than holistically. Finally, the appropriate definition ofholistic processing is considered.     

Lateral differences in tachistoscopic recognition of bilaterally presented verbal material.

When subjects have to report verbal material presented tachistoscopically simultaneously on both sides of a fixation point, right visual field superiority has been obtained in several experiments using a central task technique, i.e. where a stimulus presented at the fixation point had to be identified before reporting the rest of the material. Without a central task, left visual field superiority has generally been obtained. It has been suggested to attribute the difference to control of eye fixation by the central task. An alternative interpretation, that the central task modifies the order of attentional scanning was put to the test in Experiment I. With two normally printed words, right visual field superiority was obtained with a central task and left visual field superiority without it. It was predicted that with mirrored words, the opposite pattern would be obtained, yet here right visual field superiority was obtained both without and with a central task. Experiment II shows that the latter result is nevertheless dependent on scanning order, for it can be completely inverted through recall order instructions. It is concluded that lateral differences observed with bilateral presentation cannot be explained without taking account of optional processing priorities, but that the factors on which the latter depend are not yet fully understood.     

Heart-Rate responses (HRR) to lateralized visual stimuli

Direction of changes in heart-rate responses (HRR) were investigated in three separate experiments as a measure of differential cognitive and emotional specialization of the cerebral hemispheres. Visual stimuli were presented via the visual half-field technique in all three experiments. Slides with different contents were flashed for 200 msec on each trial either to the left or right of a center LED fixation point. The LED went on 5 seconds prior to slide onset. HR changes were scored as second-by-second deviations during 10 seconds after LED onset from pre-LED base line. In the first experiment it was hypothesized that emotionally relevant stimuli initially projected to only the right hemisphere would result in more anticipatory acceleration than when the same stimulus was initially projected to the left hemisphere. A picture of a snake and of a geometric figure were repeatedly briefly flashed to the right of the LED for half of the subjects, and to the left for the other half. There were 25 trials with an intertriai interval of 25–40 seconds. Results showed significant effects of deceleration as a function of the slide stimulus in all groups on seconds 5, 6, or 7 after onset of the center LED. Furthermore, an anticipatory acceleration was observed during the first trial-block on seconds 3 and 4 in the right hemisphere groups only with no differences between the neutral and emotional stimuli. In Experiments 2 and 3, a letter-string of six letters and a complex symmetric pattern were used as stimuli. These stimuli were chosen because previous research has clearly implicated the hemispheres to be differentially specialized in their ability to process verbal and visuo-spatial stimuli. The set-up was identical to Experiment 1, with the exception that differences in response to the two types of stimuli were evaluated on a within-subjects basis. The results from Experiments 2 and 3 showed stimulus-related deceleration, peaking on seconds 5–7 in all groups and an anticipatory acceleration peaking on seconds 3 and 4 in the right hemisphere groups, with decelerations during the corresponding seconds in the left hemisphere groups. The results are discussed in relation to recent findings by Walker and Sandman (1982) about the possibility of hemispheric specialization in psychologic influences on heart rate changes in response to environmental demands.     

On the Distribution and Growth of Intelligence

The effects of the visual pathways and acuity dominance on visual half-field (VHF) recall of verbal stimuli were investigated with 36 right-handed male undergraduates, with the use of the Hines-Satz short-term memory paradigm under monocular conditions. Scores for stimuli transmitted via the left temporal and right nasal pathways were significantly greater than those for stimuli transmitted via the left nasal and right temporal pathways. No other significant differences were found for either the pathways or acuity dominance variables. The results were hypothesized to reflect the primary influence of hemispheric asymmetry of function on the VHF recall task.     

Mirror-reading in right- and left-handers

In mirror-reading, words are read from right to left and letters are read in a reverse right-to-left orientation. In one experiment we compared the ability of normal right- and left-handed subjects to mirror-read and found that the left-handers made fewer errors and could read mirror print more rapidly. In a second experiment we attempted to learn whether there is a hemifield superiority for reading mirror words and whether there are any differences between left- and right-handers in a hemifield. We found that although both right- and left-handers more rapidly detected mirror words projected to the left visual half-field, there were no differences between groups. However, in the right visual half-field, the performance of left-handers was superior to that of the right-handers. The results of the hemifield study suggest that left-handers may be superior at reading mirror words because they can more easily reverse their scanning pattern.     

More things in heaven and earth than are dreamt of in the initial letter acuity hypothesis

Schwartz, Montagner, and Kirsner (1987 Brain and Language, 31, 301-307) claim that length X visual hemifield interactions for recognition of horizontal words reflect acuity differences for the initial letters of short and long words rather than different methods of lexical access for words presented in the LVF and in the RVF. They argue that Young and Ellis did not satisfactorily eliminate this possibility because they (Young & Ellis, 1985, Brain and Language, 24, 326-358, Experiment 3) allowed the use of sophisticated guessing strategies. We demonstrate that the presentation conditions of the Schwartz et al. (1987) experiment are different from those of our (Young & Ellis, 1985) experiments in potentially important ways, and that the acuity gradient explanation proposed by Schwartz et al. to account for word length X visual hemifield interactions is inadequate in terms of both the existing literature and a reanalysis of data from our own (Young & Ellis, 1985) Experiments 1 and 3.     

Laterality differences and practice effects under central backward masking conditions

Two experiments were conducted to examine laterality differences and practice effects under various central backward masking conditions. Critical stimulus onset asynchrony (SOA) was determined for subjects on 3 consecutive days using single letters as target stimuli (TS) and a pattern masking stimulus (MS). There was a right visual field (RVF) advantage on Day 1 but no difference between the visual fields on following days. The decline in the RVF advantage appeared to be dependent upon prior experience with laterally located letters, to be independent of initial experience with a particular set of letters, and to be more pronounced for females than for males. In addition, large improvements in performance were found, particularly between the first and second testing sessions. These practice effects were discussed in terms of the possible development of strategies for enhancing TS features or attenuating MS features.     

Hemispheric asymmetry and electrodermal activity in orienting and pavlovian conditioning paradigms

Three experiments on lateralization of electrodermal orienting and conditioned behavior are reported. The basic findings show effects of hemispheric asymmetry on initial magnitude and rate of habituation of phasic stimulus-elicited electrodermal responses. In summary, responses to verbal stimuli repeatedly flashed to the right visual half-field are larger than corresponding responses in the left half-field. Similarily, spatially relevant stimuli initially presented to the left half-field result in larger responses than when the same stimuli are repeatedly presented in the right half-field. In the auditory modality, data show asymmetrical control of responding to verbal material in a dichotic conditioning paradigm with greater resistance to extinction when the conditioned stimulus is initially fed only to the left hemisphere as compared to when it is fed only to the right hemisphere. It is argued that the present approach taps on basic mechanisms for the lateralization of attentional and associative functions.     

Letter versus dot stimuli as tools for “splitting the normal brain with reaction time”

Filbey and Gazzaniga (1969) found simple dot-present or -absent reports averaged 35 ms slower for the left than for the right visual field. Other data suggests that verbal processing efficiency differences between the cerebral hemispheres, rather than transcallosal transfer time alone, must be tapped to obtain half-field differences as large as 35 ms. Three experiments were conducted. The first failed to show any half-field differences in vocal RT for dot detection; the second replicated previous reports of significant right field superiority of vocal RT to letter stimuli for right handers, and also showed a substantially smaller half-field difference for left handers; the third experiment, utilizing the fixation control procedure of the second experiment, again failed to show half-field differences for the dot detection paradigm. Differences between the Filbey and Gazzaniga and present results probably reflect important procedural differences. We conclude that transcallosal transfer time for simple dot information is much smaller than assumed by Filbey and Gazzaniga and that the letter report-time task taps hemispheric asymmetries of verbal processing efficiency.     

Cognitive influences on perceptual processing

These experiments investigate the influence of frequency of occurrence of a visual stimulus (stimulus probability) on encoding processes, in an attempt to discover what sorts of mechanisms allow cognitive processes to modify perceptual processes. Experiments 1 and 2 show that frequently occurring visual letters do not facilitate encoding of visually similar letters. This implies that stimulus probability does not directly affect the feature detectors used in encoding the letters. Four more experiments provide evidence that stimulus probability has its effect on the availability of an abstract code that is generated by the encoding process from the visual input. Results from the experiments with letter stimuli could be interpreted using a model similar to the logogen model of Morton. Experiments with nonsense forms suggest that subjects use abstract codes in dealing with the forms only when the stimuli are constructed from a set of orthogonal features. A secondary finding was that visual quality has an effect that extends past the feature analysis stage and into a stage in which the visual input activities an abstract code. This result calls into question the common practice of interpreting the interaction of a factor with visual quality as evidence that the factor affects visual feature analysis.     

Dual hemispheric processing in a letter matching task

It has been reported that performance on recognition, detection, and matching tasks is enhanced if stimuli are projected to both sides of the visual field rather than to one side alone (Dimond, 1972). The present study investigated the claim that this phenomenon is due to the distribution of the burden of perceptual processing between the hemispheres. Three experiments were carried out using a matching paradigm in which RT and response errors were recorded. In all experiments, subjects were required to match two letters that were displayed separately on either side of a central fixation point (bilateral presentation) or were displayed together on the same side of the visual field (unilateral presentation). It was found that although lateral interference between adjacent stimuli was significantly implicated in the phenomenon, a strong residual effect, which could be tentatively ascribed to hemispheric mechanisms, remained in relation to letter name matches. It is argued that a model based on parallel hemispheric decision processes provides a better account of the data than does one based upon the notion of distributed perceptual processing.     

Serial position effects in the identification of letters, digits, symbols, and shapes in peripheral vision

Three experiments measured serial position functions for character-in-string identification in peripheral vision. In Experiment 1, random strings of five letters (e.g., P F H T M) or five symbols (e.g., λ Б Þ Ψ ¥) were briefly presented to the left or to the right of fixation, and identification accuracy was measured at each position in the string using a post-cued two-alternative forced-choice task (e.g., was there a T or a B at the 4th position). In Experiment 2 the performance to letter stimuli was compared with familiar two-dimensional shapes (e.g., square, triangle, circle), and in Experiment 3 we compared digit strings (e.g., 6 3 7 9 2) with a set of keyboard symbols (e.g., % 4 @ < ?). Eye-movements were monitored to ensure central fixation. The results revealed a triple interaction between the nature of the stimulus (letters/digits vs. symbols/shapes), eccentricity, and visual field. In all experiments this interaction reflected a selective left visual field advantage for letter or digit stimuli compared with symbol or shape stimuli for targets presented at the greatest eccentricity. The results are in line with the predictions of the modified receptive field hypothesis proposed by Tydgat and Grainger (2009), and the predictions of the SERIOL2 model of letter string encoding.     

Within-modal and cross-modal reliabilities of two laterality tests

Sixteen right handers were given four tests of a dichotic listening recall task (concrete words) and a visual half-field recall task (letters). There was a significant right ear and right visual half-field superiority for the four test sessions, with improvement in recall scores over sessions. Pearson correlations for each ear on the four dichotic tests ranged from .74 to .90. Correlations on the visual half-field scores, however, were significant only after the first session. Analysis of stability in lateral preferences across modalities revealed stable and reliable performance, particularly in the last two testings, where 81.3% of the subjects had a right-sided asymmetry. Cross-modal correlations, however, were not significant. Implications regarding predictions of speech laterality are discussed in the light of these findings.     

Lateral eyes or lateralized? Hemiretinal and ocular dominance effects on visual field asymmetries for the lexical decision task

The effects of hemiretinal stimulation and ocular dominance on a visual half-field lexical decision task were investigated. Twelve right-eyed and 12 left-eyed subjects made word/nonword decisions about stimuli presented in the left and right visual field under binocular, left-eye alone, and right-eye alone viewing conditions. Both accuracy (d') and response time measures were recorded. The nasal hemiretina advantage for response time and temporal hemiretina advantage for accuracy found for face recognition (Proudfoot, 1983, Brain and Cognition, 2, 25-31) were not present when lexical decisions were made. An overall right visual field advantage was present for both eye-dominance groups. The results support a hemispheric interpretation of visual field differences for the processing of words during lexical decision.     

On the Relation Between Visual Spatial Attention and Visual Field Asymmetries

In the typical visual laterality experiment, words and letters are more rapidly and accurately identified in the right visual field than in the left. However, while such studies usually control fixation, the deployment of visual attention is rarely restricted. The present studies investigated the influence of visual attention on the visual field asymmetries normally observed in single-letter identification and lexical decision tasks. Attention was controlled using a peripheral cue that provided advance knowledge of the location of the forthcoming stimulus. The time period between the onset of the cue and the onset of the stimulus (Stimulus Onset Asynchrony—SOA) was varied, such that the time available for attention to focus upon the location was controlled. At short SO As a right visual field advantage for identifying single letters and for making lexical decisions was apparent. However, at longer SOAs letters and words presented in the two visual fields were identified equally well. It is concluded that visual field advantages arise from an interaction of attentional and structural factors and that the attentional component in visual field asymmetries must be controlled in order to approximate more closely a true assessment of the relative functional capabilities of the right and left cerebral hemispheres.     

Illusory temporal order for stimuli at different depth positions

We used four experiments to examine how the perceived temporal order of two visual stimuli depends on the depth position of the stimuli specified by a binocular disparity cue. When two stimuli were presented simultaneously at different depth positions in front of or around a fixation point, the observer perceived the more distant stimulus before the nearer stimulus (Experiments 1 and 2). This illusory temporal order was found only for sudden stimulus presentation (Experiment 3). These results suggest that a common processing, which is triggered by sudden luminance change, underlies this illusion. The strength of the illusion increased with the disparity gradient and the disparity size (Experiment 4). We propose that this illusion has a basis in the processing of motion in depth, which would alert the observer to a potential collision with an object that suddenly emerges in front of the observer.     

Dividing attention between auditory and visual perception

To what extent is simultaneous visual and auditory perception subject to capacity limitations and attentional control? Two experiments addressed this question by asking observers to recognize test tones and test letters under selective and divided attention. In Experiment 1, both stimuli occurred on each trial, but subjects were cued in advance to process just one or both of the stimuli. In Experiment 2, subjects processed one stimulus and then the other or processed both stimuli simultaneously. Processing time was controlled using a backward recognition masking task. A significant, but small, attention effect was found in both experiments. The present positive results weaken the interpretation that previous attentional effects were due to the particular duration judgment task that was employed. The answer to the question addressed by the experiments appears to be that the degree of capacity limitations and attentional control during visual and auditory perception is small but significant.