5.3 Event-related potentials
When a sense organ (eye, ear, etc.) receives a stimulus, the event eventually causes neurons to ‘fire’ (i.e. produce electrical discharges) in the receiving area of the brain. The information is sent on from these first sites to other brain areas. With appropriate apparatus and techniques it is possible to record the electrical signals, using electrodes attached to the scalp. The electrical potentials recorded are called event-related potentials (ERPs), since they dependably follow the triggering sensory event. In fact a whole series of electrical changes are detected, first from the receiving brain areas, then later from subsequent sites. The timing of the ERPs gives a clue as to where in this sequence they are being generated.
Woldorff et al. (1993) examined ERPs evoked by sounds. These included signals occurring as soon as 10 ms after the auditory event. To generate a response so quickly, these ERPs must have originated in the brain stem, in the first ‘relay’ between ear and auditory cortex. The earliest stages of registration at the auditory cortex were detected after about 20–50 ms. It was of particular interest that, whereas the 10 ms signal was not affected by attention, the magnitude of the electrical activity in the cortex was smaller when the sounds were played to an unattended ear. This shows that, at a very early stage of cortical analysis, attending away from a stimulus actually reduces the intensity of the signal in the brain. The result lends a good deal of support to the theory that attention is exercised by controlling a filter early in the processing sequence (see Section 1.3). Note, however, that the unattended signal is only attenuated, not eliminated.