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N E Loveless

Publications and source records attributed to N E Loveless.

11 recordsLinked to original sources
PubMed1993

Auditory evoked fields covary with perceptual grouping.

Abrupt acoustic events evoke a transient magnetic response (N100m) at the supratemporal plane. Such responses decrease in amplitude as the interval between successive stimuli decreases to about 1 s. However, when pairs of stimuli are separated by still smaller intervals the second stimulus evokes a larger response than the first. This enhancement depends on the duration of the pause between the offset of the first stimulus and onset of the second. The range over which enhancement of N100m is observed agrees quite well with the range over which subjects experience perceptual grouping of the two stimuli with loudness enhancement of the second. Recordings from multi-channel SQUID gradiometers show that the effect involves not only a change in source strength but also a change in source location within supratemporal cortex. The results suggest that inhibition induced by onset of the first stimulus may be disinhibited by its offset. Physiological and psychological implications are discussed.

Attention↗
PubMed1986

Potentials evoked by temporal deviance.

Infrequent stimuli deviating qualitatively from a train of standard stimuli appear to evoke two 'N2' potentials: A modality-specific 'mismatch negativity' (N2a), and a sharp peak forming part of a biphasic vertex response (N2b/P3a). Previous investigations leave it uncertain whether the same pattern of potentials is elicited by temporal deviance. In the present experiment a chequerboard was flashed at a standard ISI of 2000 msec, with occasional flashes occurring early at ISIs of 500, 1000 or 1500 msec. Detected deviants evoked N2b/P3a at an amplitude which increased with degree of deviance, but it was also evoked, at lower amplitude, by missed deviants and by standards. Its amplitude seems to depend upon the discriminability of the deviant stimuli, rather than upon whether they are actually detected. Moreover deviance from a repetitive background does not appear to be a necessary condition, since the complex can be elicited by isolated stimuli presented at irregular intervals. These considerations suggest that temporal uncertainty is an important factor. Temporal deviance produced no discernible mismatch negativity, modality-specific or otherwise, suggesting that it may involve processes different from those engaged in qualitative deviance.

Arousal↗
PubMed1977

Event-related brain potentials in selective response.

Event-related potentials were recorded during performance of a simple reaction task and two selective response tasks differing in the intensity of the non-key stimulus. Known differences in reaction between these tasks were confirmed, but there were no corresponding differences in amplitude of the slow potential shift which develops in the EEG in anticipation of the imperative signal. Since this anticipatory potential is not affected by event uncertainty, it seems to be related to motor preparation rather than to expectancy. Performance differences between simple and selective response tasks appear to depend not upon differential preparation, but upon selective processing of the imperative signal, which is reflected in the N120 component of the visual evoked response to that signal.

Brain↗
PubMed1975

The impact of warning signal intensity on reaction time and components of the contingent negative variation.

Slow EEG potentials were recorded during performance of a simple reaction task in which warning signal intensity was varied from trial to trial under foreperiod durations of 1, 3 and 8 sec. As shown by speed of reaction, the warning signal had an activating effect which increased with its intensity and decreased with foreperiod duration. This effect was related to the amplitude of a slow potential which appears in the EEG shortly after presentation of the warning signal. This potential is interpreted as a component of the orienting response regulating sensitivity to subsequent stimulation, so that reaction time is affected through change in the effective intensity of the imperative signal.

Acoustic Stimulation↗
PubMedDate not supplied

Stimulus deviance and evoked potentials.

In many studies on the effect of selective attention and stimulus significance on evoked potentials, the target, or otherwise significant, stimuli were also infrequent stimuli. Repetitive homogeneous auditory stimuli were presented at short constant intervals. One of two deviant stimuli, one slightly lower and the other slightly higher in pitch than the repetitive stimulus ('standard'), randomly replaced it at the same low probability. One at a time of these two physically equally deviant and equally infrequent stimuli was designated as a target and the subject's task was to count its number in a run and to ignore the other deviant stimulus. The two deviant stimuli elicited very similar potentials; hence, no target effect was obtained. On the other hand, both potentials were much larger and more complex than those elicited by standard stimuli. Additionally, a 'probe' stimulus, a widely deviant auditory stimulus randomly replaced, with a very small probability, a standard stimulus in these conditions. Even this irrelevant, physically widely deviant, stimulus elicited a wave complex basically similar to that elicited by slightly deviant stimuli, but of much larger amplitude. The comparison of these brain-wave sequences to those elicited by the same stimuli in reading subjects led to the conclusion that in detection conditions, deviant stimuli elicit two overlapping sequences of brain events: exogeneous and endogeneous. The former sequence, mainly including the processes producing the N1 and the N2 (neuronal mismatch) components, is an automatic, inflexible set of brain processes which appears as if providing a central-level stimulus to the endogeneous sequence. The latter seems to include a triphasic frontocentral complex overlapping the mismatch N2 and preceding the parietal late positive component and frontal late negative component. This endogeneous set of brain waves was regarded as a sign of detection of stimulus deviance. Consequently, it did not occur in response to the slightly deviant stimuli in reading, but the widely deviant stimuli which were also (involuntarily) perceived by the subject tended to elicit it in attenuated and delayed form.

Adolescent↗