Neuromagnetic measurements of responses to auditory stimuli consisting of pure tones amplitude-modulated at a low frequency have been used to deduce the location of cortical activity. The evoked field source systematically increased in depth beneath the scalp with increasing frequency of the tone. The tonotopic progression can be described as a logarithmic mapping.

Noninvasive magnetoencephalography makes it possible to identify the cortical area in the human brain whose activity reflects the decay of passive sensory storage of information about auditory stimuli (echoic memory). The lifetime for decay of the neuronal activation trace in primary auditory cortex was found to predict the psychophysically determined duration of memory for the loudness of a tone. Although memory for the loudness of a specific tone is lost, the remembered loudness decays toward the global mean of all of the loudnesses to which a subject is exposed in a series of trials.

The minimum norm least-squares approach based on lead field theory provides a unique inverse solution for a magnetic source image that is the best estimate in the least-squares sense. This has been applied to determine the source current distribution when the primary current is confined to a surface or set of surfaces. In model simulations of cortical activity of the human brain, the magnetic field pattern across the scalp is interpreted with prior knowledge of anatomy to yield a unique magnetic source image across a portion of cerebral cortex, without resort to an explicit source model.

Haidinger's brush was used as a method of locating fixation positions on a display. The various experimental patterns studied showed: (1) It is the already organized cortical representation of shape which governs fixation, rather than the peripheral input per se; (2) Acute angles near 20 deg are the most effective angular stimuli: (3) For figures subtending angles less than 5 deg, the eye is directed toward the center of the figure. and not toward its edge; and (4) Removing one segment from a completely enclosed figure may not alter the mean fixation position.Eye movements and fixation behavior have received considerable attention in recent years. Yet, we have very little information about where people fixate when examining simple forms. Hebb (1949) has suggested that the development of perception is based upon the sequential viewing or scanning of lines and corners. Is the adult, like Hebb's hypothetical infant, also a scanner of outlines of objects? This view seems to be a part of our current prejudice about how people look. Following the modern vogue in computer technology, we have become used to thinking in terms of a piecemeal scanning of parts, which are somehow integrated over time in order to create the perception of form. Supporting this viewpoint is our knowledge of the rapidity with which the fovea scans the world of pattern.Certainly, some kind of sequential processing must occur for complex scenes having a large visual extent. When viewing a complex scene, the eyes make rapid saccades-as opposed to smooth movements between the points of interest which are fixated for relatively long periods. Because of the impairment of vision associated with saccadic movements (Volkmann, 1962;Latour, 1962;Zuber, 1966, the primary processing of pattern information must occur during the pauses. However, given a local region which is being examined, what are the constraints governing the fixation tendencies? Will "corners," "edges," and "lines" or other feature detectors playa role? If they do, then it is plausible that the eye should be preferentially attracted towards these elements. To begin to answer these questions, we decided to study the spontaneous fixation tendencies of the eyes when they are confronted with very simple forms. METHODThe entoptic phenomenon known as Haidinger's brush was used to study spontaneous fixation behavior. This phenomenon is probably caused by thousands of blue-light absorbing, radiallyoriented crystals in the macular pigment. Due to their radial pattern the absorption axis of each body lies approximately along a radius from the center of the fovea. Incident linear polarized light will therefore be absorbed more strongly in some parts of the pattern than in others. The long axis of the brush is approximately perpendicular to the direction of electric vibration in the polarized beam. If the beam has a constant angle of polarization the bru sh is a stabilized image which will therefore tend to disappear rapidly. However, by continually rotating the angle of polarization of the light (...

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