Herbert G. Vaughan

Orienting and habituation to auditory stimuli: A study of short terms changes in average evoked responses ☆

Abstract Short term habituation in human subjects was studied by a method which provided a stimulus by stimulus analysis of averaged evoked responses. Tones delivered every 2 sec resulted in a rapid drop, during the first few stimuli, in the amplitude of the positive component of vertex responses which peaks between 150 and 200 msec; but no similar change was found for tones delivered every 10 sec. The rapid drop for the faster rate of stimulation was considered to have only the appearance of habituation, and was viewed as reflecting refractoriness within the auditory system. On the other hand, when the first stimulus was presented in an unpredictable manner it elicited a large positive component with a peak latency of about 300 msec. Similar responses were obtained when an unpredictable pitch change was presented in an effort to elicit dishabituation. Predictable pitch changes did not produce these results. The 300 msec component was seen as reflecting a shift of attention associated with the orienting response.

Differentiation of negative event-related potentials in an auditory discrimination task ☆

Several different negative potentials elicited in auditory perceptual tasks make spatially and temporally overlapping contributions to the scalp-recorded event-related potential. Frequent non-target tones in a 2-stimulus oddball pitch discrimination task, when compared with the same stimuli in ignore or simple reaction time conditions, elicit a negative deflection with two peaks (NA1 and NA2) differing in their latency and topography from the exogenous N92-P156 deflections. Oddball tones, when compared with the frequent ones, elicit mismatch negativity (MMN) in both ignore and discrimination conditions; MMN displays a frontocentral-posterolateral polarity inversion. In the discrimination condition, MMN is followed by N2 and P3b; the former has a more central amplitude maximum than MMN, and no posterolateral polarity inversion. When the pitch discrimination task was made more difficult, there was no effect on NA1 or NA2, but the latency of MMN, N2, P3b, and reaction time all increased in parallel. It is hypothesized that MMN reflects the outcome of an automatic mismatch detection process, and that the subsequent processing of targets is related to the event of mismatch detection.

An investigation of the auditory streaming effect using event‐related brain potentials

There is uncertainty concerning the extent to which the auditory streaming effect is a function of attentive or preattentive mechanisms. The mismatch negativity (MMN), which indexes preattentive acoustic processing, was used to probe whether the segregation associated with the streaming effect occurs preattentively. In Experiment 1, alternating high and low those were presented at fast and slow paces while subjects ignored the stimuli. At the slow pace, tones were heard as alternating high and low pitches, and no MMN was elicited. At the fast pace a streaming effect was induced and an MMN was observed for the low stream, indicating a preattentive locus for the streaming effect. The high deviant did not elicit an MMN. MMNs were obtained to both the high and low deviants when the interval between the across-stream deviance was lengthened to more than 250 ms in Experiment 2, indicating that the MMN system is susceptible to processing constraints.

The sources and intracerebral distribution of auditory evoked potentials in the alert rhesus monkey

Average auditory evoked potentials (AEP) to click stimuli were recorded from chronically implanted electrodes in alert rhesus monkeys. The cortical sources of each AEP component were identified on the basis of transcortical polarity inversions. The active loci were limited to the supratemporal plane (STP) and to a region within precentral motor cortex. Small and inconstant responses were generated in the lateral superior temporal gyrus and no locally generated activity was detected in frontal granular cortex. The responses from STP were complex, with early components generated mainly within auditory koniocortex and later components in surrounding regions. Each of these components was volume-conducted widely, both above and below the STP, and they projected onto various regions of dorsolateral surface cort(x. The volume-conducted potentials from the STP summed with the potentials generated in motor cortex to produce a complex resultant waveform. Barbiturate anesthesia abolished all but the primary AEP and reduced the volume-conducted projection of this response at the cortical surface. The configuration and intracranial distribution of the monkey AEP were compared to human intracerebral and scalp AEP recordings.

Topographic analysis of human cortical potentials preceding self-initiated and visually triggered saccades

The topography of human brain potentials preceding self-initiated saccadic eye movements was compared with those preceding visually triggered saccades to examine differences in presaccadic activity under different programming requirements. A slow negative shift beginning approximately 650 ms before eye movement and largest in amplitude over the frontal region was recorded only in the self-initiated condition. This potential presumably reflects preparatory activity occurring predominantly within the frontal eye fields. A ramp-like positivity preceding eye movements by 100-250 ms and a sharp positive potential peaking 10-40 ms before saccades were recorded under both conditions. These potentials were of maximum amplitude over the parietal region. In order to examine the extent to which presaccadic enhancement of parietal activity was related to executive or to attentional mechanisms, the presaccadic visual evoked potential components were compared with activity at the same latency when stimuli were counted without saccades or were ignored. Enhancement of parietal potentials was seen both with counting and with saccades, but was greater in the eye movement condition. Occipital enhancement predominated with counting and parietal enhancement with saccades. These findings suggest that attentional and command mechanisms contribute distinct complementary increments in posterior cortical presaccadic activity associated with visually triggered saccades.

Cortical responses to speech sounds and their formants in normal infants: maturational sequence and spatiotemporal analysis

Cortical auditory evoked potentials (AEPs) to the synthesized syllables [da[ and [ta[ and to the isolated 3 formants of [da[ were obtained from 32 normal infants studied at monthly intervals from birth through 3 months and at 6 months of age. A bilateral array of 16 electrodes referenced to the mid-occiput permitted a topographic analysis of the cortical AEPs at selected latencies. A differential maturational sequence was seen: a predominantly negative cortical AEP wave form became positive, first over the frontocentral region (around term), and then over the temporal region (at 1-2 months). The timing of these electrophysiological changes coincides with a differential anatomical maturational sequence in the auditory cortex, as myelination and synaptogenesis are more advanced in primary than secondary auditory areas at term. All infants in this study followed this developmental sequence. However, there was no systematic effect of the center frequency of the formant stimuli on the maturational level of their respective cortical AEPs, suggesting a relative maturational equivalence in those regions of auditory cortex responding to stimuli across the frequency range present in human speech. In term infants, an initial midline positivity and bitemporal negativities were asynchronous in their peak and offset latencies. suggesting independent generators for each of these components. In infants from 3 to 6 months of age, cortical AEP wave forms consisted of 2 initial positive peaks followed by a negative peak. While the wave forms were similar over midline and lateral scalp, spatiotemporal analysis revealed differences in the latency of onset, duration and in the spatial extent of these components, again suggesting that 2 bilateral, temporally overlapping generators contribute to the cortical AEP. No systematic topographic difference was observed in the cortical AEPs elicited by each of the 3 formants, which differed in center frequencies.

An electrophysiologic indication of auditory processing defects in autism

Averaged evoked potentials (EPs) to clicks, random pitch changes (signals), and random deletions of stimuli within a regular train of tones were examined in five autistic and five normal children. Brainstem auditory EPs were abnormal in one of the autistic patients. The early cortical EP components P60 and N100 showed no differences across groups, whereas the P200 component of the cortical responses to clicks, as well as the late positive component (P300) to the pitch changes and deleted stimuli, were significantly smaller in the autistic subjects as a group. Furthermore, when P200 and P300 amplitudes were averaged across conditions for the individual subjects, these components were smaller in every autistic subject than in any of the normal subjects. These results are consistent with the view that there are auditory defects in autism that may sometimes involve lower levels of neural transmission as manifested by abnormalities in the brainstem and auditory EP, but are more consistently manifest in higher aspects of processing that involve the registration and storage of stimulus information. It is suggested that the severe language disorder in childhood autism may be secondary to the basic deficits in higher auditory processing.

Cellular generators of the cortical auditory evoked potential initial component

Cellular generators of the initial cortical auditory evoked potential (AEP) component were determined by analyzing laminar profiles of click-evoked AEPs, current source density, and multiple unit activity (MUA) in primary auditory cortex of awake monkeys. The initial AEP component is a surface-negative wave, N8, that peaks at 8-9 msec and inverts in polarity below lamina 4. N8 is generated by a lamina 4 current sink and a deeper current source. Simultaneous MUA is present from lower lamina 3 to the subjacent white matter. Findings indicate that thalamocortical afferents are a generator of N8 and support a role for lamina 4 stellate cells. Relationships to the human AEP are discussed.

Patterns of Behavioral Deficit Associated with Visual Spatial Neglect

Summary Visual-spatial neglect as measured by an empirically derived response position preference score on the Raven Coloured Progressive Matrices (RCPM) was examined in 70 patients with cerebral lesions and 63 controls. Patients with right cerebral lesions showed poorer RCPM performance and a higher incidence of response position preference for the side ipsilateral to their lesions than did patients with left lesions. Position preference was significantly associated with homonymous visual field defects, constructional dyspraxia, and in patients with left cerebral lesions mixed or receptive aphasia. The pattern and severity of deficits seemed alike in both right and left lesioned groups. Electroencephalographic evidence indicated that the right lesion group contained a greater number of patients with severe or post-rolandic lesions than did the left lesion group.

Effects of the amount of stimulus information processed on negative event-related potentials

Event-related potentials were recorded during simple reaction time and 3 discrimination conditions which varied in the amount of stimulus information that needed to be processed. It was found that NA became longer in duration as the amount of stimulus information that required processing was increased. Using sequential topographic mapping, it was concluded that there are at least 3 overlapping deflections that comprise NA. The experimental effect appeared to be mainly on the third deflection of NA. The problem of overlap between NA and later positive-going components, P380 and P3b, is discussed in terms of their relative latencies and scalp distributions.