Encoding and Attentive Modulation of Dynamic Motion Stimuli

Several theoretical and experimental methods are employed to investigate the neural representation of dynamic motion stimuli. Starting out with an investigation of the behaviour of macaque area MT neurons, the key processing stage of visual motion, the current study presents single cell electrophysiological data that shows that the representation of dynamic motion stimuli is reliable over different stimulus statistics. Employing rapidly changing motion stimuli with different statistics of directional changes, performing extracellular recordings in area MT, reverse correlating spiketrains with the motion impulse sequence and computing direction tuning curves out of the resulting spike-triggered averages provides evidence that neural tuning curves obtained with random stimulus sequences can be directly used to predict neural tuning curves in quite different stimulus contexts (chapter 4).Extending this view on neural reliability, a reconstruction and modelling framework is applied to predict neural responses of area MT neurons. Reconstruction is useful in estimating how much information about a physical variable is present in the activity of a neuronal population. It is shown that neurons in area MT do not adapt to the statistics of dynamical stimuli, neither in direction nor in time domain (chapter 5).Having shown the reliability of neural coding for different stimulus statistics the modulatory influence of visual attention on spike-triggered averages and tuning curves is investigated (chapter 6). Using a visual stimulation paradigm similar to the one that has been used for the previous experiments, combining it with a spatial attention task; it is shown that attention alters the neural activity in macaque area MT in a multiplicative fashion, providing evidence in favour of a �gain modulation� effect of attention (McAdams and Maunsell 1999; Treue and Martinez Trujillo 1999).In a final functional imaging study (chapter 7) the functional network of cortical areas involved in representation of dynamic motion stimuli is presented. Two macaque monkeys have been trained to perform an attention task similar to the one described in chapter 6 within a 3T MR scanner. Investigation of the blood-oxygen-level-dependent signal change during performance of the attention task identified the whole set of brain areas modulated by spatial attention. In agreement with early selection accounts of visual attention (Broadbent 1958), modulated activity is found as early as V1 and continued along both parietal and temporal pathways.