Effects of spontaneous cortical slow potentials on semantic information processing.

Adult subjects were tested on a choice reaction time (RT) task for decisions of word pairs as synonyms or unrelated. For each trial the word stimuli were presented contingent upon computer-detection of a predetermined negative or positive EEG baseline shift, recorded from either parietal or frontal midline electrodes. With parietal slow potential (SP) shifts, RTs were significantly faster and less variable under negative than positive polarity conditions. No appreciable RT differences were found between negative and positive SP shifts from frontal electrodes or for control subjects. The parietal negative SP shifts are considered to reflect enhanced semantic processing. The present findings in combination with previous results (Born et al., 1982, Electroencephalogr. Clin. Neurophysiol., 54: 668-676) demonstrate a double dissociation between the functional properties of negative SP shifts from parietal and frontal cortical areas, with respective involvements in semantic processing and response selection and execution.

Event-related potential and behavioral correlates of attention in reading retardation.

Auditory selective and sustained attention was examined in 12-year-old reading-disabled and normal boys. The reading-retarded group obtained generally low scores on verbal and achievement measures. For selective attention, two tone pip series of differing frequencies were presented, one to each ear. Infrequent signal tone pips of varying pitch were interspersed in each series. As the subjects counted the signals in one ear, event-related potentials (ERPs) were recorded. To evaluate the waveforms, the average amplitude within latency ranges that encompassed the ERP peaks was computed. For the ERPs to the signals, the retarded readers showed significantly less positivity than did the controls in the P3 latency range. The ERPs to the tone pip series displayed a 400 msec period of greater negativity to the attended than to nonattended stimuli, which was similar for the groups. Also, no major group differences were found for the behavioral measures of button-press responses to the selectively attended signals, vigilance, motor coordination, lateral preference, and dichotic listening. Attentive abilities appear intact in children with reading retardation, contrary to those with hyperactivity, and may distinguish these learning disorders.

The monkey's prefrontal cortex functions in motor programming.

A new experimental approach is presented which resulted in clarification of the specific functions of the monkey’s prefrontal cortex. Monkeys with chronically implanted transcortical nonpolarizable electrodes were trained on delayed response (DR) and visual delayed matching-to-sample (DMS) tasks. The onset of the trial for each group depended upon on-line computer detection of one of the specified events: FN – surface-negative steady potential shifts (SPS) from principalis cortex; MN – a similar SPS from precentral cortex; FB – near baseline SP from principalis cortex; LEM – rightward eye deviations; and YC – controls, with intertrial intervals yoked to those of other monkeys. Monkeys were trained with 1-s cue presentations. on successive delays of 2 to 12 s. The DR acquisition rate by the FN group was substantially faster than that of any other group, as indicated by its mean error that was only 17.24 percent the YC group’s error. The MN and LEM monkeys acquired the task at the same rates as the YCs, while the FB monkeys were the slowest learners. The correct DR performance transferred to testing with constant intertrial intervals (without preconditions). Subsequent on-line tests with brief (0.1 s) cue duration showed high DR performance by the FN, but not by other groups. No similar rapid learning was found with the DMS task. The findings from this, and other experiments, suggest that the major function of principalis cortex is the selection, or programming of delayed spatial choice responses. The view seems consonant with interpretations for the role of the human prefrontal cortex.

Auditory evoked potentials and selective attention in formerly hyperactive adolescent boys.

Although many of the prominent symptoms of minimal brain dysfunction (MBD) subside during pubescence, adolescents who had MBD during childhood may have persistent neuropsychological dysfunctions. The authors studied the auditory average cortical evoked potentials and behavioral responses of 9 formerly hyperactive adolescent boys and 9 matched control subjects who performed a selective attention task. Experimental subjects showed indications of impairment on both electrophysiological and behavioral measures of selective attention. Although the findings are consistent with a neurodevelopmental lag interpretation of MBD, they may also imply a persistent dysfunction related to the frontal association cortex.

Learning and retention by monkeys with epileptogenic implants in anteromedial temporal cortex.

Alumina cream was implanted bilaterally in anteromedial temporal cortex of monkeys. After the onset of epileptic discharges eight implanted and eight normal monkeys were trained on tasks of visual and somesthetic discriminations and delayed alternation. All monkeys were tested for retention on the three tasks at 3-week intervals, for a total experimental period of 52 weeks. The normal monkeys received epileptogenic implants after their first retention tests. Serial scalp EEGs were recorded and several monkeys were implanted with cortical recording electrodes. On acquisition training the epileptic group was significantly retarded in comparison with the normals, but also exhibited significantly greater variabilities in individual learning rates. On the retention tests the majority of epileptic monkeys showed little or no deficits. In these monkeys the epileptic discharges remained restricted to the implanted areas. The retention deficits in several monkeys could be related to propagations of epileptiform patterns and secondary foci. These results are consistent with earlier findings with focal epileptogenic implants in different cortical areas of deficient acquisition rates on tasks specific to the focal areas, but unimpaired retention performance. The finding can be explained in relation to labile and consolidated mnemonic processes.

Interhemispheric functional differences in prefrontal cortex of monkeys.

Monkeys had nonpolarizable electrodes implanted bilaterally in prefrontal (principal sulcus), precentral, and occipital cortex. They were trained on a spatial delayed-response (DR) task (8-sec intratrial delay), while cortical potentials were recorded. Three groups of monkeys were trained to 90% criterion: (A) 4 monkeys with only the right hand (the left wrist was attached to the testing chair); (B) 2 monkeys with only the left hand; and (C) 2 monkeys with the left and right hands on alternate sessions. Intermanual transfer tests were then given. Averaged steady potential (SP) shifts of several seconds duration were found in prefrontal cortex during cue presentation and the early portion of the intratrial delay and from the precentral area during the choice response. Evaluations of these SP shift magnitudes indicated: (1) Training with only one hand resulted in substantially larger SP shifts in the prefrontal and precentral areas contralateral to the responding hand; (2) alternate hand training resulted in somewhat larger prefrontal SP shifts in the right hemisphere; (3) intermanual transfer had marked effects on the precentral SP shifts, with larger magnitudes in the hemisphere contralateral to the responding hand, but had little effect on the magnitudes of both prefrontal SP shifts. (4) Subsequent training of Group C monkeys with only one hand resulted in greater SP shifts in the prefrontal area contralateral to the responding hand and in decreased SP shifts in the ipsilateral prefrontal area; and (5) additional intermanual transfer tests had no effects on SP shift magnitudes from both prefrontal areas. These findings indicate a dissociation in interhemispheric functions between the precentral and prefrontal cortical areas, with the formal implicated in motor organization for the contralateral limb, and the latter in mediation of mnemonic processes, primarily in one hemisphere. This hemispheric specialization is affected by the hand-training procedure, but other endogenous or experimential factors may be involved.

Cortical steady potential shifts and anodal polarization during delayed response performance.

Two experiments are described in which monkeys with chronically implanted non-polarizable electrodes in prefrontal, precentral, and occipital cortex were trained on delayed response (DR) tasks for food rewards. In the first experiment, involving recordings of cortical steady potentials (SPs), variations in DR task parameters enabled us to delineate three surface negative SP shifts during the DR trial: (i) before cue presentation, (ii) following the response when reward is eaten, and (iii) at the start of the delay period. The first two shifts are interpreted as indicative of diffuse neuronal processes related to "expectancy" and "reinforcement", respectively, since similar shifts were recorded from other cortical areas. The third shift, recorded only from prefrontal cortex, is interpreted as a correlate of short-term spatial memory formation. Its magnitude was unaffected by variation in duration of either the cue or the delay period, but was significantly correlated with the level of correct perfomance (r's from 0.74 to 0.90 for four monkeys). In the second experiment, surface anodal polarization was applied to prefrontal cortex in three monkeys during acquisition sessions of 4-sec or 8-sec DR. In all monkeys higher acquisition rates were obtained under polarization than under comparable non-polarization conditions. The most dramatic facilitation was obtained with 40 pa current applied throughout several successive sessions. These findings support the concept of excitability changes of prefrontal neurons during DR performance.