Mechanisms of human motion perception: combining evidence from evoked potentials, behavioural performance and computational modelling.

Based on single cell recordings in monkey, it has been suggested that neural activity can be related directly to psychophysically measured threshold behaviour. Here, we investigated in humans whether evoked potentials correlate with behavioural measurements like discrimination thresholds and reaction time. Subjects were asked to report the perceived direction of object motion stimuli which contained variable amounts of coherent motion. Simultaneously, we recorded evoked potentials with a multielectrode array, or measured the reaction time. We show here that motion coherence had a strong influence on both amplitude and latency of the evoked potential. Stronger motion signals evoked stronger and faster cortical responses. The latency reduction of the motion onset response with increasing coherence correlated very well with the concurrent decrease in reaction time. Taken together, these results suggest that temporal integration is an important step in analysing motion signals to generate a reliable behavioural response. We stimulated a two-dimensional array of correlation-type motion detectors with the same motion sequences, and analysed the distribution of local motion signals according to signal detection theory. Performance resembled that of human subjects when the decision strategy was optimized so as to exclude small signals and, in particular, when the ideal observer had some knowledge about a region of interest in which the object was to be expected.

The cortical representation of object motion in man is interindividually variable.

Cortical areas processing visual motion have been well investigated in monkeys, but comparatively little is known about these areas in man. In order to define such cortical areas in the brains of individuals, the magnetic field was recorded while subjects were watching motion-defined static and moving objects. The magnetic response showed a transient component with a clear dipolar magnetic field followed by a sustained component which exhibited some variation in magnetic field structure over time. For the transient component, the single equivalent current dipoles superimposed upon magnetic resonance images for individual subjects were clearly localized outside the primary visual areas. In most cases the neural generator was found in the region of the temporo-parieto-occipital junction of the lateral cortex. The results also suggest that the activated cortical area show interindividual variations in location.

Cortical potentials reflecting motion processing in humans.

Motion processing is a fundamental task of visual systems, and in the monkey cortical areas can be identified which appear to be functionally specialized for motion processing. The human visual system is expected to be organized in a similar way. A noninvasive method to study the functional organization of the visual cortex is the recording of scalp potentials generated by neural activity of the underlying cortical areas. In the present study, we recorded slow cortical potentials from normal subjects in order to investigate how motion stimuli are processed. Three classes of object motion were realized as random dot kinematograms, namely Fourier motion, drift-balanced motion, and theta motion, because they require mechanisms of increasing complexity in order to be extracted. Large-field motion and counterphase flicker were used as control stimuli. Three basic results were obtained: (1) The responses evoked by the three classes of object motion do not differ significantly in their course and distribution of activation. (2) The distributions of cortical activation evoked by object motion, and the control stimuli are different. During object motion the maximum activation occurs over the superior parietal cortex. Large-field motion activates occipital and parietal locations to the same extent, and during counterphase flicker the activity is maximum over the occipital lobe. Thus, the parietal slow potentials are interpreted to specifically reflect the cortical processing of object motion. (3) The time course of the activation reflects changes in the spatial position of the object: the amplitude of a transient negative component (TNC) which occurs 240 ms after motion onset decreases with increasing eccentricity of motion onset. The consecutive sustained negative component (SNC), which persists until the movement stops, decreases during centrifugal and increases during centripetal object motion. These results can be understood on the basis of physiological and anatomical knowledge about the mapping of the visual field on the cortex.

Cortical potentials in humans reflecting the direction of object motion.

In the monkey, cortical areas can be localized which are specific for the processing of form, colour, or motion, and it is expected that the human visual cortex is organized in a similar way. The recording of scalp potentials generated by neural activity of underlying cortical areas is a non-invasive method which can be used to study the functional organization of the visual cortex with a high temporal resolution. In the present study we recorded slow cortical potentials from normal subjects to investigate how motion stimuli of variable complexity are processed in human visual cortex. The results show that the pattern of cortical activation is dependent on the type of stimulus. When random dots were moving within the entire stimulus field, or during counterphase flicker, maximal activation occurred over occipital electrode sites. During object motion a pronounced activation is recorded at parietal locations, with the direction of object motion being reflected by the time course of this activation.