Identification of mirror-reversed and nonreversed facial profiles in same and opposite visual fields

The hemispheric lateralization for processing geometric word/shape combinations: the Stroop-shape effect

The authors conducted 4 experiments to test whether hemispheric lateralization occurs for the processing of geometric word-shape combinations. In 3 experiments, participants responded to geometric shapes combined with geometric words (square, circle, triangle). In the 4th experiment, stimuli were combinations of geometric shapes and non-geometric words. The authors predicted that it would take longer to respond in incongruent conditions (e.g., the word "square" combined with the shape of a circle) than in congruent conditions. The authors found the strongest incongruency effects for the dominant hemisphere--that is, the left hemisphere for responding to words and the right hemisphere for responding to shapes. A Shape Interfering Properties hypothesis (SIP) is a possible explanation for these results.

Mirror writing: neurological reflections on an unusual phenomenon

Mirror writing is an unusual script, in which the writing runs in the opposite direction to normal, with individual letters reversed, so that it is most easily read using a mirror. This writing is seen in healthy individuals; it is also associated with various focal lesions that most commonly involve the left hemisphere, as well as with certain diffuse cerebral disorders. Mirror writing is nearly always undertaken with the left hand, and left-handers, and those whose languages are written leftwards, have an unusual facility for this writing. Concerning possible underlying processes, the implications of using the left hand when writing are considered first. Motor pathways that may be important, the surrogate model of bimanual mirror movements and the contribution of the corpus callosum are then discussed. The reasons why left-handed writing is mirrored, and the factors that tend to inhibit mirroring, are outlined. After commenting on mirrored motor and visual engrams, the possibility that the right hemisphere may play an important part is entertained, and Leonardo da Vinci's unique, habitual mirror writing proves to be of unexpected relevance. Further investigations, ranging from epidemiological to functional imaging studies, may provide valuable insights into mirror writing.

The "Poffenberger" and "Dimond" paradigms: interrelated approaches to the study of interhemispheric dynamics?

Brown and Jeeves (1993) found that an evoked potential estimate of interhemispheric transfer time, from the left to the right hemisphere, correlated negatively with a bilateral field advantage (BFA) in a response-choice letter matching task. We implemented a go no-go dot size matching task to determine whether the crossed-uncrossed difference (CUD) in reaction time, commission errors and omission errors (estimates of the "cost" of interhemispheric transfer) would correlate with BFAs in data from the same experiment and whether the type of decision ("same" versus "different") would modulate the CUDs and/or BFAs. Sixteen normal right-handed subjects were tested. The CUDs were negatively correlated with the BFAs. Canonical correlation analysis of this set of relations was highly significant (r = .95). Estimates of left-to-right relay were far more strongly related to BFA (p = .0001) than were estimates of right-to-left relay (p = .03). "Same" decisions yielded a unilateral field advantage and "different" decisions a bilateral field advantage in omission error data, this crossed interaction reaching significance. More efficient interhemispheric relay favored BFAs, i.e., strongly suggesting in such cases an advantage of interhemispheric over intrahemispheric integration. This effect appeared to be markedly asymmetric. Furthermore, resource sharing within and between the hemispheres was a function of the "same" versus "different" dimensions of the decision to be made, especially in omission errors. The opposite dissociation occurred less markedly in the reaction times, this double dissociation reaching significance, revealing presence of a subtle speed-accuracy trade-off in interhemispheric dynamics.

Interhemispheric cooperation of left- and right-handers in mental calculation tasks

Relations between handedness and interhemispheric processing in cognitive tasks were examined. Thirty six right-handers and thirty left-handers (familial and nonfamilial left-handers) were asked to add two numbers which were presented tachistoscopically. Two numbers were displayed either to one visual field, or one to the left and one to the right visual field simultaneously. In Experiment 1, the numbers were displayed in Arabic numerals, and in Experiment 2 one of the numbers was displayed in Kanji and one in Arabic numerals. The results of Experiment 1 showed a bilateral advantage, however no subject group difference was shown in the performance of the three (left unilateral, right unilateral, and bilateral) presentation conditions. Though familial left-handers showed a weaker tendency to different patterns than right and non-familial left-handers, the results of Experiment 2 were largely similar to those of Experiment 1. These findings suggest that interhemispheric processing may not differ between left- and right-handers.

Bilateral visual field processing and evoked potential interhemispheric transmission time

The relationship between the efficiency of interhemispheric interactions via the corpus callosum and the speed and accuracy in making comparisons of information simultaneously presented to the right and left visual fields was studied by comparing bilateral (vs unilateral) advantages in matching letters, with evoked potential measures of interhemispheric transmission time (EP-IHTT). The primary finding was a strong correlation suggesting that larger bilateral field advantages in reaction time are associated with faster EP-IHTT. However, the association between EP-IHTT and bilateral advantage was strong only for transmission speed from left hemisphere to right hemisphere, but not for speed of transmission in the opposite direction. The data are consistent with a hypothesis of asymmetric homologue enhancement, i.e. a directionally asymmetric callosal influence which facilitates processing of letter stimuli in the right hemisphere allowing for increased response speed and accuracy of bilateral visual field comparisons.

The bilateral field advantage on a letter-matching task

A series of studies have reported that responding is faster when letter pairs to be matched are projected to two hemispheres rather than one. Four experiments described here tested this bilateral field advantage and identified factors that influence its extent. Subjects were shown letter pairs drawn from the ensemble "AaBb", and classified the letter pairs as "Match" if the letters had the same name (regardless of case) and "No Match" if they did not. In the first two experiments the letter pairs were presented unilaterally (both letters in one visual field), bilaterally (one letter in each visual field), or centrally (both letters on the vertical midline, above and below fixation), in order to investigate how the bilateral field advantage is influenced by screen location. The third experiment added a bilateral-diagonal position (to check for artefacts related to horizontal scanning strategies), and the fourth experiment added distractor digits (to equate initial processing demands in the bilateral and unilateral conditions). Results indicate that the bilateral field advantage is a robust phenomenon, although several manipulations reduced its magnitude. Implications of these findings for models of hemispheric collaboration and interhemispheric processing are discussed.

Hemispatial factors in mirror writing

The effect of the hemispace in which writing was performed was assessed in two left hemisphere stroke patients who demonstrated left-handed mirror writing. Both patients produced significantly more mirrored words when writing in right, as compared to left (body) hemispace. We suggest that writing in the right hemispace activates the left hemisphere. Further, we propose that mirror writing in the right hemispace is attributable to activation of the damaged left hemisphere spatial system, which fails to assist in the translation of right hand motor programs into those appropriate to the left hand. Writing in left hemispace, in contrast, activates the intact right hemisphere based spatial system which guides the execution or monitoring of motor productions in left hemispace. The result is writing which remains directionally correct when the left hand is used in left hemispace.

Hemispheric interactions: the bilateral advantage and task difficulty

Twenty-five normal subjects made "same-different" responses to dot patterns presented in the LVF, RVF or bilaterally. Task difficulty was manipulated in each condition by varying the number of dots in the two patterns presented from two to four to six. The pairs of patterns always had the same number of dots on a given trial. Response latency and accuracy worsened as the number of dots increased for all three presentation conditions and for both "same" and "different" judgements. Overall, responding was faster and the number of errors lower on Bilateral presentations. For response latencies to identical patterns of dots, the size of the bilateral advantage increased relative to RVF responding as task difficulty increased but did not change significantly relative to LVF responding. When the two patterns were not identical the size of the advantage did not change as task difficulty increased. "Same" judgements were faster but less accurate than "different" judgements. A model of hemispheric interactions is proposed to account for the findings.

Interhemispheric interaction affected by computational complexity

The present study investigated whether dividing information between the hemispheres becomes more advantageous to task performance as computational complexity increases. We hypothesized that interhemispheric processing would benefit performance especially for computationally complex tasks, whereas it would hinder performance for relatively simple ones. A letter-matching task was given to 23 subjects at three levels of computational complexity. Complexity was varied either by increasing the number of inputs to be processed or by the nature of the decision to be made. The results indicated that each of these manipulations of complexity influenced performance by making it more advantageous to have both hemispheres involved in processing rather than just one. Furthermore, the effects of each manipulation were separable.

Mirror phenomena in language and nonverbal activities--a case report

A case is described of a 38-year-old, left-handed female who demonstrated mirror phenomena for both language and non-motor, nonverbal material. She mirror-wrote with her left hand with normal right-hand writing. Mirror-reading as well as object and spatial reversals were revealed. These data are consistent with bilateral representation of visual engrams, with one hemisphere containing a mirror-reversed representation of the other.

Effects of processing speed on cerebral asymmetry for left- and right-oriented faces

A free-vision chimeric facial emotion judgment task and a tachistoscopic face-recognition reaction time task were administered to 20 male right-handed subjects. The tachistoscopic task involved judgments of whether a poser in the centrally presented full-face photograph was the same or different poser than in a profile photograph presented in the left or right visual field (LVF, RVF). The free-vision task was that used by J. Levy, W. Heller, M. Banich, and L. Burton (1983, Brain and Cognition, 2, 404-419) and involved judging which of two chimeric faces appeared happier, in which the two chimeras were mirror images of each other and each chimera consisted of a smiling half-face joined at the midline to a neutral half-face of the same poser. For the tachistoscopic task, subjects were divided into groups of Fast and Slow responders by a median split of the mean reaction times. For the Fast subjects, judgments were faster in the LVF than in the RVF, and there was a significant interaction between visual field and profile direction, such that responses were faster for medially oriented profiles; i.e., LVF responses were faster for right-facing than for left-facing profiles, with the reverse relationship in the RVF. The Slow responders did not show these effects. Only the Fast group showed the bias for choosing the chimera with the smile on the left as happier, and mean response speed and the LVF advantage on the tachistoscopic test correlated with the leftward bias on the free-vision task for all subjects combined. It was suggested that overall response speed on the face-matching task reflected the extent to which specialized and more efficient right hemisphere functions were activated.

Interhemispheric interaction: how do the hemispheres divide and conquer a task?

The present studies investigated how dividing processing between the hemispheres affects task performance. In particular, they examined whether dividing processing between the hemispheres leads to a performance advantage only when task demands exceed a certain threshold. In Experiment 1 processing demands were manipulated by varying the difficulty of the decision process. In the more difficult task, subjects decided as quickly as possible whether two of three letters had the same name (e.g. A a), whereas in the less difficult task they simply decided whether two of the three were physically identical (e.g. A A). As expected, dividing processing between the hemispheres aided performance for the more difficult name-identity task whereas it actually hindered performance for easier physical-identity task. In Experiment 2, subjects made a physical-identity decision about a different stimulus, digits. The pattern of results found in Experiment 1 for the physical-identity task was replicated; interhemispheric processing hindered task performance. These results indicate that the physical characteristics of a stimulus have minimal influence on the extent to which interhemispheric processing aids task performance. In Experiment 3, subjects were required to make more difficult decisions about digits. In one task, they decided whether the sum of two of the three digits was greater than or equal to 10, and in the other they decided if the value of a particular digit was less than either of the other two. Dividing processing between the hemispheres led to faster performance for both tasks, similar to the results for the name-identity condition. In sum, these experiments suggest that when task requirements are demanding, performance is enhanced by distributing processing across the hemispheres.

[Temporary mirror writing and mirror reading as disinhibition phenomena? A case study]

After removal of a left parietal meningioma a 74-year-old right-handed woman developed temporary mirror writing and mirror reading. Further analysis of the mirror phenomena came from tachistoscopic hemifield stimulation showing a right hemisphere superiority for the recognition of words presented in mirror fashion. These neuropsychological results are discussed in the context of common concepts. Finally the hypothesis of bihemispheral mirror-image engrams according to Orton will be suggested as a comprehensive explanation and supported by evidence from the literature.

The effect of retention interval upon hemispheric processes in recognition memory

Simple concrete nouns were presented unilaterally in a continuous recognition memory procedure. Each word was presented twice in a session, and subjects were required to signal the second occurrence of a word. Retention interval was manipulated by varying the lag separating word presentations, and lags of 1, 4, 8, and 32 items were employed in the design. Words were projected, on first and second presentations respectively, to the following visual fields: LL, LR, RR, and RL. No visual field effects were observed at lag 1, but an advantage for RVF probes was evident from lag 4 onwards. At lag 8, a relative superiority emerged from uncrossed versus crossed presentation. The results support the idea that lateral asymmetries develop as processing engages deeper and more complex levels of representation. There was also evidence that memory representations are more salient following direct, rather than transcallosal, stimulation of a hemisphere.

Interhemispheric effects in short-term recognition memory for single words

Twenty undergraduate men participated in a short-term recognition memory experiment in which single words of four types, classified by high and low imagery value and high and low Thorndike-Lorge frequency, were each presented twice unilaterally to the right and left visual fields (RVF, LVF). Stimuli were projected either to the same or to the opposite visual field on successive presentations. Results showed that: (1) imagery value affected responses to initial presentations, but not to repetitions; and (2) the speed and accuracy of recognizing repetitions in the LVF were the same whether the stimuli had been presented initially to the LVF or the RVF, whereas speed and accuracy in the RVF were significantly poorer for words initially presented to the LVF than for words initially presented to the RVF. The latter findings are consistent with differential encoding or with asymmetrical storage of verbal information in the two hemispheres, but not with the hypothesis that the memory store for words is confined to the language-dominant hemisphere.

Mirror-reading and writing in association with right-left spatial disorientation

A left-handed patient suddenly developed a right hemiparesis, mirror-reading, and mirror-writing. Although he could discriminate between right and left on himself, he demonstrated right/left spatial disorientation. We propose that his right/left spatial disorientation was induced by a scanning defect. It has been demonstrated that mirror-image engrams are normally available. We believe that a reversal of the learned left-to-right scanning process with the availability of mirror engrams induced mirror-reading. Similarly, reversal of the normal left-to-right writing pattern, with the availability of mirror engrams, induced mirror-writing.

The perception and identification of mirror-reversed patterns

Many species have difficulty in discriminating between mirror-image stimuli, especially those about a vertical axis, and when identificatory rather than purely perceptual processes are involved. Various theories are reviewed. In two experiments involving same-different judgments for pairs of stimuli, triangles or semicircles, these were simultaneously presented either unilaterally or bilaterally, and in mirror or aligned orientations with respect to each other. Mirror oriented stimuli presented to opposite cerebral hemispheres were no more readily matched than those possessing the same orientations (aligned), thus suggesting that at the perceptual level there is no interaction between mirror-corresponding points in the two visual cortices. Foreknowledge of stimulus orientations failed differentially to affect the findings. Two other studies were performed involving manual identification of single letters, correct or mirror oriented, in either visual field. Here, for most subjects, mirror reversal proved either less disruptive or even advantageous when in the visual field which is normally inferior with correctly oriented material. It was concluded that mirror-image confusion at the level of memory is almost certainly a consequence of reversed coding, in some form, in the opposite sides of the brain. A number of incidental findings were made with respect to visual field effects.

Hemispheric factors and scanning dynamics in the processing of briefly exposed stimuli by human subjects: bibliography

268 journal articles on hemispheric factors and scanning dynamics involved in the encoding of briefly exposed stimuli by human Ss are grouped into the categories of experiments with normal Ss, experiments with various types of patients, and review and theoretical articles.