Tuning curves and masking functions of auditory-nerve fibers in cat

Single units of the auditory nerve are maximally sensitive to a given frequency, the so-called characteristic frequency (CF). Tuning curves (functions relating the "threshold" for such units to the frequency of a sinusoidal stimulus) have inspired psychophysical measurements of masked thresholds in humans. At low levels, the results bear a striking resemblance to neural tuning curves, suggesting that a small, homogeneous group of units determine the so-called psychophysical tuning curves. This paper describes physiological measurements modeled after the psychophysical forward-masking paradigm. A masking tone was used to reduce the response in auditory-nerve fibers elicited by a probe tone that followed the masker. The level of the masking tone was varied to maintain a criterion level of responding to the probe. Plotting level of the masker as a function of its frequency yields a fiber-masking function. For low probe-tone levels, fiber-masking functions of low CF fibers and the tips of functions from high CF fibers were similar in shape to tuning curves obtained with single tone bursts. The fiber-masking function depends upon the response criterion and upon the level, duration, and frequency of the probe. Varying the level of the probe can also alter the shape of the fiber-masking function. These results have implications for interpreting psychophysical masking functions.

Auditory nerve fiber response to wide-band noise and tone combinations

1. Responses of single auditory nerve fibers to combinations of noise and tone were obtained. The results were found to depend on the relative effectiveness of each stimulus when presented alone. 2. When the response rate to one stimulus presented alone was considerably greater than the response rate to the other stimulus presented alone, the more effective stimulus dominated the responses when the two stimuli were combined. The more effective stimulus captured the response of the neuron. Thus, intense noise was found to mask responses to weaker tones, and intense tones were found to mask responses to weaker noise. This masking of the weaker stimulus is thought to enhance the signal-to-noise ratio of the most prominent response component. 3. When the two stimuli had similar effectiveness, complex interactions occurred. When the tone was near best (characteristic) frequency, partial summation effects occured. The tone partially suppressed the responses to the noise if other frequencies were used. Tones above best frequency caused particularly powerful suppression. 4. The bandwidth of the noise was varied somewhat. While bandwidth affected the effectiveness of the noise, it did not affect the types of interactions observed. 5. For a neuron which was essentially silent in the absence of acoustic stimuli, adding a weak level of noise lowered the threshold of responsiveness to the tone.

Pitch memory: an advantage for the left-handed

In an auditory or musical task, subjects made pitch recognition judgments when the tones to be compared were separated by a sequence of interpolated tones. The left-handed subjects performed significantly better than the right-handed and also had a significantly higher variance. Further analysis showed that the superior performance was attributable largely to the left-handed subjects with mixed hand preference.

Auditory-nerve response from cats raised in a low-noise chamber

A litter of four cats, born and raised in a soundproofed chamber, was studied in an attempt to determine which, if any, features of the auditory-nerve response from routinely available cats might be due to the chronic effects of noise exposure. Two features of routine-normal response were especially suspect in this regard: (1) a "notch" in the distribution of single-unit thresholds centered at characteristic frequencies (CF's) near 3 kHz and (2) a compression of the distribution of rates of spontaneous discharge for units with CF above 10 kHz. A third feature of response in routine animals was the presence of a small number (roughly 10%) of units with virtually no spontaneous discharge and very high thresholds, sometimes 80 dB less sensitive than high-spontaneous units of similar CF. In the data from chamber-raised animals, the high-spontaneous units showed exceptionally low thresholds at all CF regions, however, there were signs of the midfrequency notch in the threshold distribution of at least two of these animals. The compression of the spontaneous rate distribution was not seen in any of the three most sensitive animals. The data suggest that there is a significant amount of "normal pathology" in the high-CF units from routine animals. Low-spontaneous, high-threshold units were present in all four chamber-raised ears with the same characteristics as in routine animals (exceptionally narrow tuning curves and exceptionally low maximum discharge rates) and at roughly the same percentage of the unit sample. A class of units with medium spontaneous rates and intermediate thresholds could also be identified. The possible significance of a classification of auditory-nerve units according to spontaneous rate is discussed.

Hemispheric asymmetry in making absolute judgments of monaurally presented pure tones

Some recent studies have suggested that hemispheric asymmetry may be related to the quantity of information contained in a stimulus. To test this possibility, 20 subjects were given an auditory absolute-judgment task in which they were required to identify 3, 5, or 7 pure tones recorded on a tape. The presentation was monaural, and headphone placement was randomly alternated across subjects. The left ear showed a statistically significant advantage tones. The result is discussed in terms of previous work on auditory channel-capacity and the desirability of using an absolute-judgment paradigm to study hemispheric asymmetry for other kinds of stimuli is indicated.

[Dynamics of evoked potentials at different points in the cat somatosensory cortex during elaboration of conditioned reflexes]

In chronic experiments on cats a study was made of the spreading of evoked potentials (EP) in the somatosensory zones S1 and S2 in response to biologically significant tonal stimuli. The formation of conditioned connections enhances the capacity for polysensory reactions in different cortical somatic fields outside the zones of local projection of the unconditioned stimulus. As stable conditioned connections are formed, this capacity diminishes in relation to reinforced and, particularly, easily differentiated sounds: EP are more localized in response to the former, and disappear altogether when the latter are presented. The wide EP spreading in zones S1 and S2 is the longest in response to medium and fine differentiation tones, which points to the involvement of these zones in the integrative analysis of signals by their biological quality.

[Organization of integrated systems of conditioned reflexes in dogs with a damaged temporal cortex]

By means of the classical alimentary secretory method, a study was made of the characteristics of formation and preservation of complex systems of conditioned reflexes in dogs with a preliminarily lesioned cortex of the auditory analyser. It has been shown that a rhythmic-mozaic stereotype of conditioned reflexes to acoustic signals can be elaborated in the operated dogs, although conditioned activity is on a very low functional level. However, integration of two complex stereotypes into one single system is not achieved. The formation of a new stereotype in the operated dogs, in contrast to the intact animals, is not accompanied by an enhanced functional state (tone) of the brain, which points to an insufficiency of the self-regulating mechanisms of the cerebral cortex. It is suggested that special activating mechanisms which participate in controlling the functional state of the auditory system, are present in the cortical structures of the auditory analyser.

Hemispheric differences in simple reaction time to auditory stimuli of different frequencies

To determine whether reaction time varies as a function of tone frequency and/or ear stimulated 27 adult subjects were presented with two two-tone series (1,000 Hz vs 2,500 Hz and 1,500 Hz vs 4,000 Hz) in a simple reaction-time paradigm. The analyses clearly indicated that the higher tone in each series and stimulation to the right ear resulted in significantly shorter reaction times. These results were interpreted as indicating that cerebral asymmetries are apparent in tasks and with stimuli that do not seem to require higher-order inferences regrading the functional organization of the cerebral hemispheres.

Bisensory stimulation: Inferring decision-related processes from the P300 component

Three experiments were conducted to evaluate the P300 component of the human evoked response as an index of bisensory information processing. On different blocks of trials, subjects were presented with auditory stimuli alone, visual stimuli alone, or with audiovisual compounds. In each series there were two possible stimuli, one of which was presented less frequently than the other; the subjects' task was to count the infrequent stimuli. In the first two experiments the information in the two modalities was redundant, whereas in the third the modalities provided nonredundant information. With redundant information, the P300 latency indicated bisensory facilitation when the unimodal P300 latencies were similar; when the unimodal latencies were dissimilar, the bisensory P300 occurred at the latency of the earlier unimodal P300. Reaction times paralleled P300 latency. When the information in the two modalities was nonredundant, both P300 amplitude and reaction-time data indicated interference between the two modalities, regardless of which modality was task relevant. P300 latency and reaction time did not covary in this situation. These data suggest that P300 latency and amplitude do reflect bisensory interactions and that the P300 promises to be a valuable tool for assessing brain processes during complex decision making.

Language patterns of adolescent autistics

The language samples of seven verbal autistic adolescents were analyzed. Linguistic deficits were compared to characteristics reported for preteen autistics and described structurally. Only four subjects demonstrated linguistic impairments. These clustered primarily in the area of prosodic features, semantic cooccurrence constraints and general disfluency. No such clustering had been reported for the preteen population. No correlation between linguistic deficits, IQ, and age was found. However, performance on the Seashore Test for Musical Ability correlated highly with linguistic performance. Results suggest that (a) autism includes liguistically, and possibly etiologically, distinct subtroups; (b) the basic linguistic deficits in autism may be more specific than thought previously; and (c) perception of prosodic features may be crucial for decoding and encoding linguistic signals. Autistic children may be lacking in this ability.

Within mode quality and intensity changes of habituated stimuli

Habituation of skin conductance response (SCR) magnitude was established by a number of presentations of a short burst of moderate intensity white noise. The 6th, 11th, or 21st stimulus event was either a pure tone of unchanged loudness or another noise pulse of greatly reduced loudness. Mean SCR magnitude to the reduced intensity noise pulses was consistently smaller than that to the tones, but the difference of each of these response from the habituation curve varied with the number of prior habituation trials. The qualitative changes in the stimulus produced a consistent increase in SCR magnitude; the greater the number of prior habituation trials, the larger the difference from the habituation curve. The reduced intensity noise stimulus reduced SCR magnitude below that of the habituation curve at the 6th trial, equalled the curve at the 11th trial, and yielded a greater response than the habituation curve at the 21st trial test position. The response to the tone was superior to that to the reduced intensity noise by a constant amount over the three habituation curve test points.

Response bias and physiological reactivity

Sixteen Ss were selected from a group of 30 adult males for low and high levels of physiological reactivity, as defined by several measures of electrodermal activity (EDA). They were subsequently tested on two occasions on a tone discrimination task in which confidence ratings were required. High EDA Ss discriminated a significantly greater number of tones, but this apparent superiority was not due to their superior sensitivity, but to a greater bias towards responding positively to signals. The results were discussed in relation to possible mechanisms mediating individual differences in response bias and physiological reactivity.

Binaural beats at high frequencies

Binaural beats have long been believed to be audible only at low frequencies, but an interaction reminiscent of a binaural beat can sometimes be heard when different two-tone complexes of high frequency are presented to the two ears. The primary requirement is that the frequency separation in the complex at one ear be slightly different from that in the other--that is, that there be a small interaural difference in the envelope periodicities. This finding is in accord with other recent demonstrations that the auditory system is not deaf to interaural time differences at high frequencies.

Free-operant comparisons of wild and domestic Norway rats

Wild and domestic rats (Rattus norvegicus) were compared on nondifferential appetitive variable interval responding, discrimination, and discrimination reversal procedures. The effects of strain, sex, deprivation, preexperimental handling, and sessions on response rate were examined. Performances during the 60 days of variable interval training showed prolonged increases over that period for all strain-sex groupings, with domestic subjects responding at higher rates than wild. Males also tended to respond at higher rates than females. During the discrimination procedure wild subjects showed more resistance to extinction than did the domestic subjects, although these differences generally diminished after the 30 days of training. During the discrimination reversal procedure domestic subjects reversed their responding pattern more readily than did the wild subjects, and domestic females reached criterion significantly sooner than domestic males. The effects of deprivation and handling were not significant during the experimental procedures.

The cardiac responses of schizophrenics to orienting, signal and non-signal tones

The cardiac responses of schizophrenics were examined under three conditions: (1) to repeated 75 dB, 1000 Hz (orienting) tones of no attentional significance; (2) to 75 dB, 1000 and 2000 HZ tones presented randomly and requiring a motor response to the 1000 HZ (signal) tone; (3) to 85 dB, 1000 HZ orienting tones. Responses of non-psychotic psychiatric patients were examined to the 75 dB tones. All patients were subdivided upon the basis of electrodermal responsivity during conditions (1) and (3), into groups of responders - no habituation of responses; non-responders - no responses; habituators - responses habituated to criterion. All groups exhibited a decelerative response with a latency of less than 1 sec. The groups were differentiated by the second component of the response to the orienting tones: responders - acceleration; habituators - deceleration; non-responders - predominantly no response. All exhibited accelerative responses to the signal tones. Parallels between cardiac and electrodermal responsivity are outlined.

On the independence of the CNV and the P300 components of the human averaged evoked potential

We report an experiment designed to assess the interactions between the CNV and the P300 components of human event-related potential. Eight subjects were each presented with series of experimental trials on all of which either a 1200 c/sec or an 800 c/sec tone was presented. There were three independent variables: (a) The presence or absence of a warning flash 1000 msec prior to the tone. (b) The task assigned to the subject--that is subjects were either to make a discriminative response to the tone or, on half the series, to predict prior to the tiral which of the two tones would be presented. (c) The predictability of the tone frequency. On half the series high and low tones alternated from trial to trial. On the other series, tones were chosen randomly on each trial. The data show that the amplitude of the P300 component is not affected by the presence or absence of a warning stimulus. Furthermore, the distributions of P300 and the CNV over the scalp are quite different. These conclusions are supported by a principal component and a discriminant analysis of the data. We conclude that the CNV and the P300 reflect the activity of functionally distinct cortical mechanisms.