Intracerebral amplitude distributions of the auditory evoked potential

Intracerebral auditory evoked potentials were recorded from multi-contact electrodes chronically implanted in frontal, temporal, and parietal regions of the brains of 10 epileptic patients. Binaurally presented tones produced a large widely distributed biphasic potential with a close latency correspondence to the scalp vertex response. In each hemisphere, this intracerebral response inverted in polarity from posterior temporal to frontal sites. This response also showed large amplitude changes in the inferior parietal lobe. The distribution of this response is compatible with bilateral posterior temporal sources with a dorso-frontal orientation.

Human intracerebral potentials associated with target, novel, and omitted auditory stimuli

We recorded late auditory potentials from lateral and medial regions in the frontal, temporal and parietal lobes of patients with temporal lobe epilepsy implanted with horizontal depth electrodes. Tone sequences were presented in three tasks: 1) auditory target detection in a tone sequence, 2) target detection with interspersed novel stimuli, and 3) detection of stimulus omissions. At frontal sites, potentials to targets showed a triphasic response with peak latencies around 200, 270 and 350 ms. At temporal sites, potentials consisted of a generally positive 285 ms peak which was sometimes accompanied by a negative peak at 200 ms or at 400 ms. At parietal sites, potentials were generally triphasic with latencies of about 230, 300, and 370 ms. At most sites, potentials evoked by novel stimuli had shorter latencies than those evoked by targets. The frontal and parietal potentials were either absent or strongly attenuated during stimulus omissions. The results lend further support to the multiple generator hypothesis of late potentials and suggest that some of the cerebral sources of the late potentials are stimulus dependent while others are not.

[Decision strategy as a function of uncertainty of the result: I. Latency of the decision]

Until now, the decision strategy used by human beings faced with a dichotomous situation was studied considering that speed of reaction was the main dependent variable. It was found that, in a two choice reaction time (CRT) task, subjects used a very conservative decision strategy. That is, when compared with events of 0.5 probability, only those events with a probability of no less than 0.9 caused a significant decrease in CRT. In real sport situations however, the tennis player can not only increase his speed of response but can also change his position on the court in order to defend more appropriately the area of the court which appears more likely to be attacked. Two experiments were conducted in order to verify this hypothesis. In both experiments, a two CRT task was used; the independent variables being: (a) event probability and (b) time allotted to complete the appropriate response (TA). Results indicate that as soon as the probability of an attack reached 0.7, subjects started initiating defensive responses closer to the most probable area of attack. It thus appears that subjects were not as conservative as that reported in studies where CRT measures only were considered. On the other hand, subjects used a strategy which enabled them to reduce their total response time without increasing choice errors. In fact it seems that, when facing event uncertainty, the main concern of the human being is to keep his error rate as low as possible even if this is not the best strategy to use.