The perception of dichotic chords by hemispherectomized subjects

Hemispherectomized subjects display a strong ear dominance in their perception of dichotically presented two-tone chords. The frequency of the tone presented to the ear contralateral to the remaining hemisphere dominated the pitch mixture of the chord. The described effect does not vary with the age at which the hemispherectomy was performed. These results are consistent with the hypothesis, developed to account for findings in normal subjects, that the pitch mixture of a dichotic chord is determined by a subcortical pitch processor. The effects observed in hemispherectomized subjects may result from an interruption of an efferent pathway from the cortex to the subcortical pitch processor or from an asymmetrical degeneration of the processor which may be located in the thalamus (medial geniculate).     

Right hemisphere comprehension of verbs in patients with complete forebrain commissurotomy: Use of the dichotic method and manual performance

The paradigm of dichotic listening was used to investigate verbal comprehension in the right, so-called “nonverbal,” hemisphere. Verbal commands were presented to the right and left ears in the simultaneous (dichotic) paradigm. There were striking instances, especially when the left hemisphere was occupied with some extraneous task, in which the right hemisphere understood the verbal command and executed the appropriate motor responses. In those instances the left hemisphere gave no overt response. Although the left hemisphere was usually dominant, it can be nevertheless concluded that not only can the right hemisphere understand verbal commands but can also express itself manually by executing actions more complex than object retrieval or pointing. As has been known for some time, the blockage of the ipsilateral pathway seems so complete during dichotic listening in the commissurotomy patient that there is no report of the words in the left ear—only of those presented to the right. At the same time there is normal report when words are presented to the left ear alone. It was found in the present study, however, that this model is too simple and only applies to the verbal response paradigm of dichotic listening. Under circumstances of dichotic presentation where the stimulus in the left ear (ipsilateral pathway) is necessary or important to the left hemisphere for completing a task, words from both pathways are reported. One may conclude that there exists a gating mechanism in each hemisphere that controls the monitoring of each auditory pathway and the degree of ipsilateral suppression.     

Gain control in the auditory system: absolute identification of intensity within and across the two ears

A number of studies have suggested that the auditory system employs nonlinear gain control to modulate its response to a broad range of stimulus intensities (e.g., Parker, Murphy, & Schneider, 2002), but the precise location and nature of this mechanism remains uncertain. To address these issues, we investigated the extent to which gain control in one ear was influenced by auditory events in the other ear. Listeners were asked to identify which of four sounds, varying in intensity (25, 30, 35, and 40 dB [SPL]), had been presented to the left ear. Subsequent test sets included an 80-dB (SPL) stimulus presented to either the left (ipsilateral) or the right (contralateral) ear. Ipsilateral presentations of the 80-dB (SPL) stimulus produced a greater reduction in identification accuracy among the original four sounds than did contralateral presentations, but both presentations caused a reduction in gain. In a second experiment, the perceived laterality of a sound was manipulated by presenting the same sound to both ears with an interaural time delay. When the 80-dB (SPL) stimulus was added to the set of quiet sounds (which were presented to both ears but were perceived to be in the left ear because of the interaural time delay), identification accuracy was reduced by the same amount, independently of whether the 80-dB (SPL) stimulus was perceived only in the ipsilateral ear or only in the contralateral ear.     

Duplex perception: some initial findings concerning its neural basis

Duplex perception is the simultaneous perception of a speech syllable and of a nonspeech "chirp," and occurs when a single formant transition and the remainder (the "base") of a synthetic syllable are presented to different ears. The current study found a slight but nonsignificant advantage for correct labeling of the fused syllable when the chirp was presented to the left ear. This advantage was amplified in the performance of a "split-brain" subject. A subject with a left pontine lesion performed at chance level when the chirp was presented to her left ear. These findings suggest that some, if not complete, ipsilateral suppression does occur in the dichotic fusion procedure, and that identification of the fused syllable is maximal when the left hemisphere fully processes the linguistic characteristics of the base (through contralateral presentation), and at least minimally processes the frequency transition information of the chirp (through ipsilateral presentation).     

The significance of crossed and uncrossed projections for the acquisition and retrieval of visual engrams in hooded and albino rats

Summary Occlusion of one eye and functional hemidecortication by cortical spreading depression (CSD) were used to examine the relative importance of crossed and uncrossed visual projections for pattern discrimination learning in hooded and albino rats. Whereas functionally hemidecorticate albino rats were unable to learn in 200 trials a horizontal-vertical discrimination when using the eye contralateral to the depressed hemisphere, hooded rats needed under similar conditions three times as many trials to criterion (167) than when using opposite eye and hemisphere (59). An engram established in intact animals with both eyes could be retrieved in functionally hemidecorticate rats of both strains even with the eye ipsilateral to the intact hemicortex, but retraining required about twice as many trials as when using the contralateral eye. Retrieval of monocularly acquired engrams was less efficient under similar conditions. Pattern discrimination formed through crossed or uncrossed visual projections during monocular occlusion and contralateral or ipsilateral CSD was completely lateralized, since relearning with naive eye and hemisphere proceeded at the same rate as the original learning. On the contrary, a black-white discrimination trained with the eye contralateral to the depressed hemisphere could be partly retrieved by the untrained eye and hemisphere. It is concluded that the ipsilateral visual projection is more effective in hooded than in albino rats and that it can be more efficiently used for retrieval than for acquisition.This research was partly supported by the Foundations' Fund for Research in Psychiatry, Grant G69-453.     

The effects of contralateral noise on reaction time to monaural stimuli

Experiment I measured reaction time (RT) to monaural tones of six frequencies presented along with white noise to the contralateral ear. RT with the hand ipsilateral to the stimulus was an average of 9.63 msec faster than RT with the contralateral hand. Contralateral RT was significantly affected by the stimulus frequency. Experiment II measured ipsilateral and contralateral RT to monaural tones with and without contralateral noise. Noise-on results agreed with the results of Experiment I, while noise-off results showed no difference between ipsilateral and contralateral RT. No right-ear advantage was found. The ipsilateral-contralateral RT difference found with noise on is interpreted as being due to callosal transmission time as well as other factors. The finding of no right-ear advantage is discussed in relation to other studies which did report a right-ear advantage.     

Effects of hemispheric asymmetry on electrodermal conditioning in a dichotic listening paradigm

It was predicted that when an auditive verbal conditional stimulus (CS+), previously associated with an aversive unconditional stimulus (UCS), is presented to theright ear (initial left hemisphere input), in a dichotic listening task, with the CS-simultaneously presented in the left ear, the result would be greater resistance to extinction than when the same CS+ is presented to the left ear (initial right hemisphere input). In the same vein, it was predicted that, when a tonal CS+ is presented to theleft ear, greater resistance to extinction would occur as compared with when it is presented to the right ear. During a dichotic test phase, each group was split into two subgroups, and the CS+ and CS—repeatedly were presented simultaneously to each ear. Half of the subjects in each subgroup had the CS+ presented to the left ear. The other half had the CS+ presented to the right ear. Skin conductance responses were recorded from both hands. Results showed significantly larger responding in all groups to the CS+/UCS compound during the acquisition phase. However, during the dichotic extinction phase, the verbal CS+ right-ear subgroup showed significantly larger resistance to extinction as compared with the verbal CS+ left-ear subgroup. No differences were found between the two tonal subgroups.     

Effects of visual-field and matching instruction on event-related potentials and reaction time

Vertical letter pairs were presented randomly in the left and right visual hemifields in a physical identity match and name identity match condition. The reaction times showed a right visual field superiority for name matches, and a left visual field superiority for physical matches. Event-related potentials to letter pairs showed a sequence of three waves: a negative wave (N2, around 270 msec), a positive wave (P3, around 500 msec), and a broad positive slow wave (SW, around 600-700 msec), respectively. P3 and SW amplitudes were consistently larger at the left hemisphere than at the right hemisphere, regardless of the field of stimulation. At both hemispheres, N2 waves were always larger to stimuli presented in the visual field contralateral to a hemisphere than stimuli presented in the visual field ipsilateral to a hemisphere. The positive waves (P3, SW) showed the opposite pattern: smaller amplitudes to stimuli that were presented contralaterally than stimuli that were presented ipsilaterally to a given hemisphere. These results were attributed to a shift in sustained negativity on the directly stimulated hemisphere, relative to the indirectly stimulated hemisphere, reflecting either sensory at attentional processes in the posterior cerebral hemispheres.     

The influence of stimulus properties on multisensory processing in the awake primate superior colliculus.

Multisensory integration is a process whereby information converges from different sensory modalities to produce a response that is different from that elicited by the individual modalities presented alone. A neural basis for multisensory integration has been identified within a variety of brain regions, but the most thoroughly examined model has been that of the superior colliculus (SC). Multisensory processing in the SC of anaesthetized animals has been shown to be dependent on the physical parameters of the individual stimuli presented (e.g., intensity, direction, velocity) as well as their spatial relationship. However, it is unknown whether these stimulus features are important, or evident, in the awake behaving animal. To address this question, we evaluated the influence of physical properties of sensory stimuli (visual intensity, direction, and velocity; auditory intensity and location) on sensory activity and multisensory integration of SC neurons in awake, behaving primates. Monkeys were trained to fixate a central visual fixation point while visual and/or auditory stimuli were presented in the periphery. Visual stimuli were always presented within the contralateral receptive field of the neuron whereas auditory stimuli were presented at either ipsi- or contralateral locations. Many of the SC neurons responsive to these sensory stimuli (n = 66/84; 76%) had stronger responses when the visual and auditory stimuli were combined at contralateral locations than when the auditory stimulus was located on the ipsilateral side. This trend was significant across the population of auditory-responsive neurons. In addition, some SC neurons (n = 31) were presented a battery of tests in which the quality of one stimulus of a pair was systematically manipulated. A small proportion of these neurons (n = 8/31; 26%) showed preferential responses to stimuli with specific physical properties, and these preferences were not significantly altered when multisensory stimulus combinations were presented. These data demonstrate that multisensory processing in the awake behaving primate is influenced by the spatial congruency of the stimuli as well as their individual physical properties.     

Effects of attention and unilateral neglect on auditory stream segregation

Two pairs of experiments studied the effects of attention and of unilateral neglect on auditory streaming. The first pair showed that the build up of auditory streaming in normal participants is greatly reduced or absent when they attend to a competing task in the contralateral ear. It was concluded that the effective build up of streaming depends on attention. The second pair showed that patients with an attentional deficit toward the left side of space (unilateral neglect) show less stream segregation of tone sequences presented to their left than to their right ears. Streaming in their right ears was similar to that for stimuli presented to either ear of healthy and of brain-damaged controls, who showed no across-ear asymmetry. This result is consistent with an effect of attention on streaming, constrains the neural sites involved, and reveals a qualitative difference between the perception of left- and right-sided sounds by neglect patients.     

The role of the left hemisphere in motor control of touch: a functional magnetic resonance imaging analysis

Previous research using a simple finger-touching task has shown greater blood oxygenation level dependent (BOLD) activation volume in the motor cortex of the right hemisphere for contralateral finger touching compared to ipsilateral finger touching, but no significant contralateral advantage for the left hemisphere. Such equal involvement of the left hemisphere for both contralateral and ipsilateral finger touching suggests a special role of the left hemisphere for finger touching. In contrast, we found a contralateral advantage in the motor cortex of both hemispheres in a majority of participants (14/16) when consistently activated BOLD volumes were examined. However, participants who did not show a clear contralateral advantage for the left hemisphere did show activation in the left inferior frontal gyrus (IFG; Broca's Area) and in the left insular cortex, which suggests that verbally mediated sequencing of finger movements can account for our less frequent result.     

Synonym and antonym pairs in the detection of dichotically and monaurally presented targets: Competing monaural stimulation can generate a substantial right ear advantage

Aligned pairs of spoken words were presented, and the subject timed in detecting the presence or absence of a prespecified target. The target, if present, was paired with either its synonym or its antonym or an unrelated word. If the target was absent, mutually synonymous, antonymous or unrelated word pairs occured. In experiment 1, presentation of the word pairs was dichotic, and in experiment 2 the same stimuli were now systematically presented to a single ear (competing monaural stimulation). In both cases a strategy of divided attention was imposed with respect to the words. A powerful REA was obtained in the second experiment, demonstrating that this phenomenon does not depend upon occlusion of the ipsilateral by the contralateral auditory pathways (Kimura 1961), but that competition within ears is sufficient. Secondly, the nature of the effect (facilitatory or interfering) from a co-present synonym or antonym of the target depends markedly upon how the ears are stimulated, in partial contrast to and extending the results of Lewis (1970).     

Ear of input as a determinant of pitch-memory interference

Six experiments to evaluate the effect of presentation ear on pitch-memory interference were conducted using undergraduates as listeners. The task was to compare the pitch of two tones that were separated by an interval that included eight interpolated tones; the interpolated tones were presented either ipsilaterally or contralaterally to the presentation ear of the comparison tones. When the ear of interpolated-tone presentations was blocked, and therefore predictable, ipsilateral interference was greater than contralateral. In contrast, when the interpolated-tone presentation ear was varied randomly from trial to trial, ipsilateral and contralateral interferences were equivalent. These results are analogous to results found in previously reported auditory backward recognition masking (ABRM) experiments and suggest that the ABRM effect may be due, in part, to pitch-memory interference. Implications for theories of auditory processing and memory are discussed.     

A computational model of the Simon effect

Even though stimulus location is task irrelevant, reaction times are faster when the location of the stimulus corresponds with the location of the response than when it does not. This phenomenon is called the Simon effect. Most accounts of the Simon effect are based on the assumption that it arises from a conflict between the spatial code of the stimulus and that of the response. In this paper a computational model of this hypothesis is presented. It provides a computationally explicit mechanism of the Simon effect. Consistent with human performance, the model provides reaction times that indicate both an advantage for the ipsilateral, corresponding response (i.e., facilitation) and a disadvantage of the contralateral, noncorresponding response (i.e., inhibition). In addition, the model accounts for the fact that the size for the effect depends on task difficulty.     

Monaural right-ear advantage in a target-identification task

Subjects were monaurally presented with consonant—vowel syllables to the right or left ear, with or without simultaneous noise to the other ear. The subject's task on each trial was to indicate whether or not the item presented was the target item /ba/. A right-ear advantage in reaction time was obtained: 14 msec for target items, 6 msec for nontarget items. The size of this effect was comparable in the presence vs. absence of competing noise. No consistent individual differences were found in the size of the right-ear advantage for this task, although such differences were obtained in a dichotic-perception pretest.It is argued that data of this type do not permit inferences about the use of particular ear/hemisphere neural pathways.     

Concurrent processing of spatial relations and objects in two cerebral hemispheres

This study examined hemispheric asymmetry for concurrent processing of object and spatial information. Participants viewed two successive stimuli, each of which consisted of two digits and two pictures that were randomly located and judged them as identical or different. A sample stimulus was presented in a central visual field, followed by a matching stimulus presented briefly in a left or right visual field. The matching stimuli were different from the sample stimuli with respect to the object (digit or picture) or spatial (locations or distances of items) aspect. No visual field asymmetry was found in the detection of object change. However, a left visual field advantage was found in the detection of spatial change. This result can be explained by the double filtering by frequency theory of Ivry and Robertson, who asserted that the left hemisphere has a bias for processing information contained in relatively high spatial frequencies whereas the right hemisphere has a bias for processing information contained in relatively low spatial frequencies. Based upon this evidence, the importance of interhemispheric integration for visual scene perception is discussed.     

Is there parallel and independent hemispheric processing of intonational and phonetic components of dichotic speech stimuli?

The possibility of hemisphere interaction in the processing of spoken language was studied in two dichotic listening experiments. The stimulus material consisted of six CV syllable triplets each spoken with each one of six intonation contours. In Experiment I, 15 aphasic patients, 8 patients with unilateral right hemisphere lesions, and 10 normal controls were asked to identify the four components of a dichotic item from a multiple-choice (MC) set comprising all possible CV triplets and intonation contours. In Experiment II, 30 normal subjects were required to identify either the right or left ear stimulus alone from an MC set comprising the right and left ear stimulus together with the two wrong combinations of right ear CV triplet with left ear intonation and vice versa. It is concluded from the results that the left hemisphere is capable of processing both phonetic and intonational information and that there is neither the necessity nor the tendency for right hemisphere participation in the perception of spoken language.     

Manual asymmetry in weight discrimination: hand or spatial-field advantage?

Sixty subjects who were either left-handed or right-handed performed one-handed weight discrimination using their left and right hands in the left and right spatial fields. Differential thresholds, for the left and right hands of both left-handed and right-handed subjects, were found to be lower in the spatial field contralateral to the ear which proved superior in a dichotic listening test. It is concluded that manual asymmetry for weight discrimination results primarily from the mapping of sensorimotor events in the spatial fields onto contralateral cerebral cortex, with an advantage in the spatial field contralateral to the nonlanguage cerebral hemisphere.     

Bilaterally recorded multiple-unit activity of the cingulate cortex during head turning conditioning with unilateral medial forebrain bundle stimulation

Cats were conditioned to turn their heads using a tone conditioned stimulus (CS) and medial forebrain bundle stimulation (MFB) unconditioned stimulus (US). The CS+ was delivered to one ear at a time, in random order, followed by the US. A tone of a different frequency was used as a CS-. The cats learned to respond differentially to the CSs showing head movements of greater acceleration to the CS+ than CS- over sessions. Bilateral recordings of cingulate cortex multiple-unit activity showed increased response amplitudes over sessions and larger responses in the hemisphere ipsilateral to the US. Since ipsilateral multiple-unit responses did not differ for the CSs, the asymmetry was probably due to the sensitizing effect of the unilateral US. Although increases in cingulate cortex neural activity coincided with increases in conditioned head movements, larger activation of the cingulate cortex ipsilateral to the US suggests that the neural changes were independent of these movements.     

Hemispheric asymmetry and human associative learning: Interactions with attention

In a previous study (Hugdahl & Brobeck, 1986) it was shown that Pavlovian conditioning to an auditory verbal conditioned stimulus (CS) initially presented only to the left cerebral hemisphere was stronger than when the same CS was presented to the right hemisphere. This was followed up in the present study by controlling for the possibility that the effect was caused by laterally biased attention. The study was performed using the “dichotic extinction paradigm,” which consists of three different phases. During the habituation phase, the CS+ and CS- were presented binaurally and separated in time. During the acquisition phase, the CS+ was followed by a white-noise unconditioned stimulus (UCS). During the dichotic extinction phase, the CS+ and CS- were presented dichotically, i.e., simultaneous presentations on each trial. Half of the subjects had the CS+ in the right ear, and half had the CS+ in the left ear. Each group was further divided into two subgroups, with one subgroup instructed to attend only to the right ear input, and the other subgroup to attend only to the left ear input. During acquisition, larger electrodermal responses were obtained to the CS+ than to the CS-. During dichotic extinction, the CS+ right ear group showed superior resistance to extinction compared to the CS+ left ear group, with no effect of the manipulation of attention. The effect was, however, attenuated when levels of acquisition was used as covariates in an analysis of covariance. There were overall larger responses from the left hand recording.