Ray Johnson

On how P300 amplitude varies with the utility of the eliciting stimuli

The bulk of the relevant evidence indicates that stimuli which elicit the P300 component of the human event-related potential (ERP) tend to be unexpected and task relevant. Yet, contradictory evidence has been presented which implied that P300 amplitude is modulated by the intensity difference between two equally relevant, equally probable feedback stimuli (Adams and Benson, 1973). The two experiments reported here were designed to clarify this contradiction. Subjects were instructed to press a button one second after a cue light. A tone at one of two intensities was presented 200 msec later indicating whether the time estimate was correct or incorrect. The magnitude of P300 varied as a function of the difference in intensity between S+ and S-; the smaller the difference, the smaller was P300, regardless of stimulus meaning (S+ or S-). We attribute this effect to the degree to which the subject chooses to utilize the tones in the performance of the time estimation task. In support of this hypothesis, we found that performance of the subjects as time estimators degraded as the difference between S+ and S- decreased. Moreover, a study of choice reaction times taken for the different pairings of S+ and S- showed that the stimulus pairs were not equally discriminable. It appears that as the S+/S- pairs become increasingly difficult to discriminate, their feedback value decreased and this decrease is positively correlated with a decrease in P300 amplitude. This interpretation received support from Exp. 2 in which the stimulus conditions were the same as in Exp. 1, but the subjects task was to count the s+ stimuli. Here, when subject had no option but to process each individual stimulus, the P300 amplitude remained constant despite variations in S- intensity. Thus, it appears that task relevance must be defined in terms of the subjects behavior rather than by the experimenters instructions to the subject.

Differential amygdala activation during emotional decision and recognition memory tasks using unpleasant words: an fMRI study

This study used fMRI to examine the response of the amygdala in the evaluation and short-term recognition memory of unpleasant vs. neutral words in nine right-handed healthy adult women. To establish specificity of the amygdala response, we examined the fMRI BOLD signal in one control region (visual cortex). Alternating blocks of unpleasant and neutral trials were presented. During the emotional decision task, subjects viewed sets of three unpleasant or three neutral words while selecting the most unpleasant or neutral word, respectively. During the memory task, subjects identified words that were presented during the emotional decision task (0.50 probability). Images were detrended, filtered, and coregistered to standard brain coordinates. The Talairach coordinates for the center of the amygdala were chosen before analysis. The BOLD signal at this location in the right hemisphere revealed a greater amplitude signal for the unpleasant relative to the neutral words during the emotional decision but not the memory task, confirmed by Time Course x Word Condition ANOVAs. These results are consistent with the memory modulatory view of amygdala function, which suggests that the amygdala facilitates long-term, but not short-term, memory consolidation of emotionally significant material. The control area showed only an effect for Time Course for both the emotional decision and memory tasks, indicating the specificity of the amygdala response to the evaluation of unpleasant words. Moreover, the right-sided amygdala activation during the unpleasant word condition was strongly correlated with the BOLD response in the occipital cortex. These findings corroborate those by other researchers that the amygdala can modulate early processing of visual information in the occipital cortex. Finally, an increase in subjects state anxiety (evaluated by questionnaire) while in the scanner correlated with amygdala activation under some conditions.

Somatosensory event-related potentials to painful and non-painful stimuli: effects of attention.

&NA; In order to determine the effects of attention and distraction on painful and non‐painful stimuli, the amplitude changes of 3 components (N150, P200, P300) of the Somatosensory event‐related potential (SERF) elicited by painful and non‐painful electrical stimuli were investigated. Painful and non‐painful stimuli were determined using a visual analog scale. SERPs were recorded from 16 healthy volunteers at 5 midline and 4 left and 4 right hemispheric sites. The differences between the amplitudes of attended and ignored stimuli were quantified with a baseline‐to‐peak measure. ANOVA results revealed no significant attention or stimulus intensity effects for N150 but highly significant differences in P200 and P300 amplitudes between attended and ignored stimuli. In addition, P200 and P300 amplitudes were larger for strong stimuli than for weak stimuli, with no significant differences between non‐painful and painful stimuli. These findings are consistent with the existence of a relative, rather than an absolute, relationship between SERP component amplitudes and subjective pain reports. Furthermore, the data give evidence that attentional manipulations represent a powerful method to decrease the perception of pain and that, when used with subjective and behavioral measures, the SERP represents a valuable asset in the multidimensional approach to pain measurement and assessment.

Graded Changes in Evoked Response (P300) Amplitude as a Function of Cognitive Activity

Using a stepwise discriminant analysis, we demonstrate that the amplitude of P300 is a graded function of the complexity of information processing required of a S following a stimulus. This relationship between cognitive complexity and P300 is apparent only when the S is not pressed to generate fast and accurate discriminant responses to the stimuli. Under a reaction-time regime, a large P300 is elicited independently of stimulus predictability or cognitive complexity. It seems that P300 is a measure of the amount of activity of a general-purpose cortical processor.

Magnetic resonance imaging functional activation of left frontal cortex during covert word production.

Six normal volunteers underwent 4‐T functional magnetic resonance imaging while performing a covert letter fluency task. An echo planar imaging sequence was utilized to detect activation based on deoxyhemoglobin contrast. All 6 subjects showed consistent activation in the frontal operculum and premotor and primary motor cortices. Activation was also detected in the supramarginal gyrus and the posterior part of the superior temporal gyrus. These results show that magnetic resonance functional neuroimaging can be used to investigate cerebral activity noninvasively during performance of complex cognitive tasks.

Signs of cognitive change in HIV disease An event‐related brain potential study

We recorded event-related brain potentials (ERPs) from 2 groups of human immunodeficiency virus (HIV)-positive men with no physical illness or neurologic involvement: 9 asymptomatic (AS +) and 9 classified as having either acquired immunodeficiency syndrome (AIDS; 7) or AIDS-related complex (ARC; 2). In separate choice reaction time tasks, the subjects pressed buttons to randomly presented auditory or visual stimuli at probabilities of either 20/80 or 50/50. There were no group differences on any of the neuropsychological tests commonly used in screening batteries for HIV patients. In contrast, reduced P300 amplitudes and increased P300 latencies occurred in ARC/AIDS patients in response to both auditory and visual stimuli, while in AS+ patients such alterations occurred only in the visual modality. Significant delays in P2 latency were found only in the auditory modality and then only in ARC/AIDS patients. ARC/AIDS patients alone showed delayed response times, and only in the auditory modality. The P300 results demonstrate alterations in stimulus evaluation and processing speed in the earliest stages of HIV disease, even before cognitive deficits can be detected by more traditional measures.

Event-related brain potentials and subjective probability in a learning task

In a paired associate learning task, subjects responded to each presentation of a nonsense syllable by typing both a three-letter associate and a rating of their confidence that this response was correct or incorrect. Average event-related potentials (ERPs) elicited by the subsequent presentation of the actual paired syllable varied with the interaction of confidence and trial outcome. A larger amplitude P300 was elicited by syllables that informed subjects that they were correct when they thought they were incorrect or that they were incorrect when they thought they were correct than by syllables that confirmed subjects expectations. That this average ERP result was indeed an effect on P300 amplitude, and not an artifact of single-trial variability in P30O latency, was confirmed with a trial-by-trial latency adjustment procedure. Consistent with findings from other tasks, P300 amplitude varied inversely with the subj ective probability of the ERP-eliciting events.

Attention and short-term memory in chronic fatigue syndrome patients: an event-related potential analysis.

We recorded event-related brain potentials (ERPs) from 13 patients with chronic fatigue syndrome (CFS) and 13 matched normal controls. To assess attentional and memory deficits in CFS patients, we used a short-term memory task in which events occurred in different spatial locations and the patients made a rapid response (RT) when a letter in a relevant location matched a letter in the prememorized set (Attention paradigm). Time-on-task effects on the ERP and behavioral measures were assessed over the 2 1/2-hour duration of this task. Both groups also performed a visual Oddball paradigm, with an RT, before and after the Attention paradigm. The patients RTs were much more variable and, in nine of 13 cases, slower than the mean RT of the controls in both paradigms. The patients memory performance was not significantly different from that of the controls and there were no group differences in the overall amplitude, latency, or scalp distribution of the N1, P2, N2, or P300 components of the ERP in either paradigm. The ERP and performance data from both paradigms suggest that perceptual, attentional, and short-term memory processes were unaffected in CFS patients and that the differences were limited to response-related processes.

Progressive supranuclear palsy Altered sensory processing leads to degraded cognition

We studied the latencies, amplitudes, and scalp distributions of the early and late components of the event- related brain potential (ERP) in patients with progressive supranuclear palsy (PSP) and matched normal controls. In separate choice reaction time (RT) tasks, the subjects pressed buttons to visual stimuli presented randomly at probabilities of either 20/80 or 50/50. Compared with normal controls, PSP patients had significantly reduced amplitudes and increased latencies for both the visual P2 and P300 components at all levels of probability. RTs and percent errors were significantly greater in the patients compared with controls. Neither the amplitude nor latency of the visual N1 component was significantly altered in these patients. There were no significant group differences in the distribution of electrical activity over the scalp for any of these ERP components, a finding which suggests that the neural structures responsible for generating these potentials were intact in these patients. The decreased ERP component amplitudes and increased ERP component latencies, combined with intact scalp distributions and increased RTs and error rates, present a pattern of results suggesting that the stimulus identification or categorization processes in these patients are significantly degraded.

Altered brain development in Turner's syndrome An event‐related potential study

We compared event-related brain potentials (ERPs) and reaction time (RT) measures from untreated young (prepubertal and peripubertal; ages 9 to 14) and old (postpubertal; ages 15 to 20) Turners syndrome (TS) subjects with those from normal age-matched controls. Comparisons among groups permitted the assessment of the relative roles of congenital and maturational brain alterations as possible bases of the cognitive deficits in TS. All subjects were presented with series of auditory stimuli, and they either counted one of the two stimuli or made rapid discriminative button presses to both. The results indicated that, whereas the ERPs in young TS females were essentially the same as those in their age-matched controls, the ERPs in old TS females more closely resembled those in both young groups than those in their age-matched controls. Specifically, a late (400 to 900 msec) frontal negative slow wave (Nc) in the old TS subjects failed to show the normal maturational course in which the amplitude and duration of this component steadily decreases with age. Except for slightly greater amounts of N1 amplitude in the young TS group, the latencies, amplitudes, and scalp distributions of the other ERP components (N1, P2, N2, P300) were all the same at all ages in the TS subjects and their controls. Behaviorally, both TS groups had longer RTs than their controls but, consistent with the ERP results, they did not make more errors. The ERP and RT results point to the existence of two kinds of abnormalities in TS females, each from a different mechanism: the ERP results suggest an age-dependent maturational defect while the RT results suggest an age-independent congenital defect.