Left Ear Stimulus Research Articles

Complex pitch perception after callosal section: Further evidence for a right hemisphere mechanism

It has recently been suggested that right hemisphere auditory function provides a mechanism for complex pitch perception. This possibility was examined in two patients who underwent surgical section of the corpus callosum. Both patients were right handed, had bilaterally normal hearing, and were assessed to have left hemisphere language dominance. Each patient was tested using the Complex Tone Test, which consists of 112 trials in which a dichotic pair of 550-ms square wave stimuli are followed after 1 s by a binaural probe item. Subjects responded manually, indicating whether or not the probe matched either dichotic item. Both patients revealed above average accuracy on left ear stimuli, but performance on right ear stimuli that was significantly worse than chance. This pattern of results is similar to that found for dichotically presented speech stimuli, but in the opposite direction. These data add further support to the claim that the right hemisphere provides a mechanism for complex pitch perception. [Work supported by USPHS grant number 2 R01 NS15053-02 and the Alfred P. Sloan Foundation.]

Laterality effects in response to offsets of tonal stimuli

In two experiments employing quite different procedures it was found that the offsets of tones presented in the right ear elicit significantly later responses than do the offsets of tones presented in the left ear. However, no ear differences were observed for responses to the onsets of tones in either experiments, attentional explanations of the results seem unlikely. The results may reflect an echoic memory for right-ear stimuli which is of longer duration than that for left-ear stimuli.

The importance of the time constant in duration patterns

Twenty young adult right‐handed subjects, responded half with the right hand and half with the left, to triads of long and short tone bursts. The 175 patterns were subdivided into 9 sets, each with a different pair of durations. Long (L) ranged from 500 to 100 ms; short (S) ranged from 350 to 50 ms. Interburst interval was 200 ms. A significant difference between ear/hand combinations occurred only during the first set of 25 patterns when right hand response to left ear stimuli was superior to left hand response to right ear stimuli. This practice effect disappeared after the first set. The most important finding was the lack of significant differences among sets sharing the same time constant relationship between long and short bursts, but with different durations of long and short bursts. Differences between sets were significant only when this time constant varied; performance was significantly better for longer time constants. There were no significant differences in performance among patterns L‐S‐L, S‐L‐S, L‐S‐S, and S‐S‐L. However, subjects performed significantly better on L‐L‐S and significantly poorer on S‐L‐L.

Single-unit activity in the auditory cortex of monkeys selectively attending left vs. right ear stimuli

Single-unit responses were recorded from the auditory cortex of rhesus monkeys that were performing an auditory selective attention task. Acoustic stimuli were presented randomly to either the left or right ear through headphones. In a given block of trials one ear was selected as the ear to be attended, and was indicated by lighting either a left or right response key. The animal's task was to press the lighted key whenever stimuli were presented to the attended ear, but to make no response to stimuli presented to the other ear. The attended ear was alternated on successive blocks of trials. Fourteen of 77 units showed significantly greater rates of evoked discharges for an attended stimulus than for an identical non-attended stimulus. The increase in stimulus-evoked activity was not accompanied by any increase in spontaneous activity or by any changes in the pattern of a unit's response. Changes in firing rate occurred at latencies as early as 20 msec.

Left ear performance in dichotic listening following commissurotomy

Contribution of the left ear stimulus to dichotic listening performance following commissurotomy was studied in five patients. In two tasks, subjects were asked to identify in writing both members of a pair of competing stimuli, either digits or CV syllables. A third task required subjects to integrate high and low frequency components of a single word presented dichotically. Left ear performance was at chance level for CV syllables but exceeded 80% for four out of five patients on the digit stimuli. All patients showed evidence of being able to utilize left ear information in the dichotic fusion task. Results indicate that apparent supression of left ear material in the dichotic task is a function of spectral-temporal overlap between competing stimuli.

Dichotic signs of the recognition of speech elements in normals, temporal lobectomees, and hemispherectomees

When patients with hemispherectomies or temporal lobectomies listen to dichotic pairs of equal-intensity C-V syllables, they do poorly identifying the stimuli presented to the ear contralateral to the lesion. This effect is similar to that seen for normals, who in the same circumstances, perform poorly on the left-ear stimulus. (The ear contralateral to a lesion for patients and the left ear for normals will be designated the "weak ear", the ear ipsilateral to a patient's lesion and the right ear for normals will be called the "strong ear".) To further explore these phenomena we investigated the ability of stimuli other than C-V's in the strong ear to suppress the perception of C-V's in the weak ear. Suppression was found when the strong-ear stimulus was a vowel. Somewhat more suppression was seen when the strong-ear stimuli were computer-generated signals with acoustic features similar to C-V's ("bleats"). Suppression was seen even if the strong-ear vowels and bleats were 20-40 dB less intense than the syllables in the weak ear. A model was developed that interprets weaker suppression to be, in part, a consequence of the interaction of the auditory features of the dichotic signals prior to phonetic processing.

The Effect of Memory Load on the Right Ear Advantage in Dichotic Listening

The right ear advantage for reporting speech signals observed in dichotic listening tasks was further investigated to examine the effects of memory load on this phenomenon. The time interval between stimulus presentation and response was varied. In all experiments subjects went through a no-delay condition (immediate recall) and different delay conditions that sampled intervals between 5 and 15 sec. Normal adult listeners were stimulated with dichotic triads composed of one-syllable common English words. The first experiment consisted of a free ear-order recall task, while in the second experiment subjects were cued as to which ear to report from. The results from both experiments show an overall right ear superiority, which was most pronounced in the longest delay condition. There was a sharp drop in accuracy for both ears at about 5–6 sec delay. With increasing delay times, the number of correct responses for the left ear stimuli continued to decrease, while the number of correct responses for the right ear stimuli either leveled off or showed an increase. A third experiment investigated the effect of reducing memory load by providing the subject with a chart listing all the stimuli (chart condition). The effect of delay on the right ear advantage with no chart was replicated, but the right ear effect was not observed in any of the chart conditions. The absence of a right ear effect in the chart conditions was due to an increase in the accuracy of the left ear scores while the right ear scores remained at the same level of accuracy under chart and no chart conditions. These findings are interpreted to demonstrate that the right ear advantage is not solely a perceptual phenomenon but that the recall of verbal material is also lateralized and complements the perceptual asymmetry. [Supported by PHS Post Doctoral Fellowship and research grants from NINDS.]