The effects of attention and context on the spatial and magnitude components of the early responses of the event-related potential elicited by a rare stimulus.

Sokolov's (1963) model-comparator theory of orienting and attention theorizes that different events underlie passive and active processing of sensory information presented in different contexts. The following study investigates changes in event-related potentials (ERPs) related to this theory by presenting stimuli inter- and intra-modally during passive and active processing tasks. Model-comparator theory proposes that novel events are detected by a mismatch discrimination process made between incoming and previously presented stimuli during preattentive processing. Central processing is engaged when this mismatch is relevant to the organism. To explore how engaging central processing, induced by instructional priming, affects preattentive processing, a 'truly' passive task was compared with a standard active task. This 'truly' passive task is different from the distracted one normally used to control direction of attention in ERP experiments in that it did not instruct subjects attention towards any task. ERP data were modeled as a dipole whose trajectory moved through voltage space. Our results suggest that both the spatial components and the magnitude of the dipole trajectory changed as functions of both passive and active processing and the context in which stimuli were presented. Our results also suggest that the trajectory and magnitudes of certain ERP components reflect processes proposed by model-comparator theory.

Three dimensional vector analysis of the spatial components and voltage magnitudes of the P300 response during different attentional states and stimulus modalities.

The spatial orientation and vector magnitude of the P300 event related potential (ERP) were investigated under both passive and active attentional states using stimuli presented to two sensory modalities. Three orthogonal electrode pairs were used to establish a tri-axial reference frame. By combining the separate single channel data recordings into a Cartisian coordinate system, both the spatial orientation and the vector magnitude of the P300 response could be derived and described as a hypothetical dipole. Results suggested that changes in amplitudes, recorded with individual electrode pairs, can be attributed largely to changes in the spatial orientation of a rotating P300 response dipole, and not simply to the altered magnitude of the response. The combined-axes P300 response occurs in two separate spatial domains, depending on stimulus modality and attentional state.

The Q10 of auditory brain stem responses in rats under hypothermia.

Auditory brain stem responses (ABRs) were recorded from normal Sprague-Dawley rats ranging in age from 15 to 120 days. Increased latencies for all four waves of the ABR were noted for animals of all ages as their body temperatures were reduced from 38 to 28 degrees C. The Q10S of the latency changes were highly variable but were still significantly higher for the first three waves of the ABR in animals less than 20 days of age compared with older animals. The Q10S for the fourth wave in both groups were alike. Wave heights tended to decrease during cooling, but this effect was inconsistent. The cochlear microphonic was recorded in some animals and tended to decrease in size with cooling. As seen in this study, the significant changes in wave latencies due to small temperature differences underscore the need to carefully control this variable when taking evoked responses from animal preparations and humans subject to variations in body temperature. The interanimal variability of the Q10 precludes any simple 'correction' of latency differences due to temperature for research or clinical purposes.

Planar curve analysis of three-channel auditory brain stem response: a preliminary report.

Recording of auditory brain stem responses (ABRs) were made in guinea pigs by means of far-field computer averaging from three orthogonal electrode pairs. The 'Y' locations were mouth and nuchal ridge, 'X' locations left and right mastoids, and 'Z' locations vertex and throat. Three-dimensional models were constructed to plot simultaneous points from the three averages in a voltage-voltage-voltage space. In all five animals tested, at least three subsets of sequential data points lie in separate planes. The data points in each plane roughly correspond to waves I to III in the guinea pig ABR as recorded from the vertex-throat electrode pair. The planarity of data points is a new observation and is not predicted from the single-channel recordings. Three-dimensional planes are also found by means of vectorcardiography, which suggests some underlying principle, such as synchrony and homogeneity of anatomical position of the generators. Planar analysis of ABRs may be useful in analyzing and utilizing the additional information obtainable from 'non-standard' electrode pairs.

Effects of undernutrition on development of far-field auditory brain stem responses in rat pups.

Timed-pregnancy Sprague-Dawley rats were placed on special diets on day 15 of their pregnancy. One group received a normal, 24% protein diet, and the second a deficient, 8% protein diet. After birth the litters within each group were cross-fostered, thinned to 8 pups each, and maintained on their respective diets. Beginning with postnatal day 16 the ABR (auditory brain stem response) was repeatedly recorded from the pups until day 43. Undernourished rat pup body weights were less than one-third of those of the well-nourished litters at the conclusion of testing. ABRs taken from the undernourished group were significantly delayed in early development compared with the controls, but the differences decreased with age despite continuation of a deficient diet. A second experiment was run to rule out body temperature differences between the two groups and showed that the ABR effect was not solely due to this variable. These results indicate that undernourishment is one of the factors that can affect the rate of maturation of the ABR.

The effects of ototoxicity on the auditory brain stem response and the scalp-recorded cochlear microphonic in guinea pigs.

The objective determination of hearing impairment with evoled response techniques may be of value in the early detection of ototoxic reactions, especially in infants and other "difficult" patient populations. In this study the auditory brain stem response to "clicks" at two intensities and two repetition rates were recorded from guinea pigs that had been given one of two dosage levels of kanamycin or no drug. The scalp-recorded cochlear microphonic to continuous tones was also recorded. Hearing loss was seen in one-third of the animals receiving kanamycin. The temporal course of the deficit varied considerably between animals. Auditory system change was most readily detected in response to low-intensity stimuli, while change in repetition rate of the clicks did not affect the results. The cochlear microphonic potentials correlated with the auditory brain stem response.

Brain stem auditory evoked response and brain stem compression.

For the study of brain stem auditory evoked response (BAER) changes due to acutely expanding mass lesions, posterior fossa balloon catheters were inflated slowly in anesthetized cats while supratentorial and infratentorial intracranial pressure and vital signs were monitored. Reliable changes in BAER occurred before the Cushing response and were reversible by balloon deflation. Because BAER changes precede the agonal changes of the Cushing response, serial BAER recording in patients with known or suspected posterior fossa masses may be useful in the management of these lesions.

Spatio-temporal distribution of auditory-evoked far field potentials in rat and cat.

Short latency (less than 5 msec) auditory-evoked 'far field' potentials were mapped over the head of anesthetized rats and cats using a monaural 'click' stimulus. Significant activity was found over most of the head and, in the rat, on other parts of the body. Different components of the observed waveforms have different spatial distributions, supporting earlier evidence that they come from different generators. No area was found on the head which could be considered as an electrically neutral reference point for all the waves. There was considerable asymmetry in the spatial distribution of the early waves, with the area near the contralateral ear showing larger magnitudes than the area near the ipsilateral ear. It was found that most of the head of the rat is not in the far field, as previously defined. Significant activity was found on the tongue in the cat, which tended to increase the apparent magnitude of waves I and II at the vertex when the tongue was used as a reference. The area near the ear canal was also found to show significant activity, raising questions as to the use of the ear lobe as a reference point in human studies. We conclude that the placement of the electrodes can markedly influence the waveforms obtained, in some cases enhancing detection of early components.

Human auditory evoked potentials: possible brain stem components detected on the scalp.

Auditory potentials recorded from the vertex of humans by a modified averaging technique have very short latencies and are probably generated by brain stem structures located at a considerable distance from the recording point. The evoked waves, which shOW considerable detail and consistency within and across subjects, may be clinically useful in evaluating subcortical function.