The challenge of non-invasive cognitive physiology of the human brain: howto negotiate the irrelevant background noise without spoiling the recordeddata through electronic averaging

Brain mechanisms involved in selective attention in humans can be studied b y measures of regional blood flow and metabolism (by positron emission tomo graphy) which help identify the various locations with enhanced activities over a period of time of seconds. The physiological measures provided by sc alp-recorded brain electrical potentials have a better resolution (millisec onds) and can reveal the actual sequences of distinct neural events and the ir precise timing. We studied selective attention to sensory inputs from fi ngers because the brain somatic representations are deployed over the brain convexity under the scalp thereby making it possible to assess distinct st ages of cortical processing and representation through their characteristic scalp topographies. In the electrical response to a finger input attended by the subject, the well-known P-300 manifests a widespread inhibitory mech anism which is released after a target stimulus has been identified. P-300 is preceded by distinct cognitive electrogeneses such as P-40, P-100 and N- 140 which can be differentiated from the control (obligatory) profile by su perimposition or electronic subtraction. The first cortical response N-20, is stable across conditions, suggesting that the first afferent thalamocort ical volley is not affected by selective attention. At the next stage of mo dality-specific cortex in which the sensory features are processed and repr esented, responses were enhanced (cognitive P-40) only a very few milliseco nds after arrival of the afferent volley at the cortex, thus documenting a remarkable precocity of attention gain control in the somatic modality The physiology of selective attention also provides useful cues in relation to non-target inputs which the subject must differentiate in order to perform the task. When having to tell fingers apart, the brain strategy for non-tar get fingers is not to inhibit or filter them out, but rather to submit thei r input to several processing operations that are actually enhanced when th e discrimination from targets becomes more difficult. While resolving a num ber of such issues, averaged data cannot disclose the flexibility of brain mechanisms nor the detailed features of cognitive electrogeneses because re sponse variations along time have been ironed out by the bulk treatment. We attempted to address the remarkable versatility of humans in dealing wit h their sensory environment under ecological conditions by studying single non-averaged responses. We identified distinct cognitive P-40, P-100, N-140 and P-300 electrogeneses in spite of the noise by numerically assessing th eir characteristic scalp topography signatures. Single-trial data suggest r econsiderations of current psychophysiological issues. The study of non-ave raged responses can clarify issues raised by averaging studies as illustrat ed by our recent study of cognitive brain potentials for finger stimuli whi ch remain outside the subject's awareness. This has to do with the physiolo gical basis of the cognitive unconscious', that is, current mental processe s lying on the fringe or outside of phenomenal awareness and voluntary cont rol, but which can influence ongoing behaviour. Averaged data suggest that, in selective auditory attention, the subject may not notice mild concomita nt finger inputs. The study of non-averaged responses documents the optiona l and independent occurrence of the cognitive P-40, P-100 and N-140 (but no t P-300) electrogeneses while the finger inputs remain outside phenomenal a wareness. These results suggest that the subject unconsciously assigns limited cognit ive resources to distinct somatic cortical areas thereby submitting finger inputs to an intermittent curtailed surveillance which can remain on the fr inge or outside consciousness. The study of cognitive electrogeneses in sin gle non-averaged responses is making possible a neurophysiology of cognitio n in real time.