Psychology, Experimental

Sensorimotor coordination is used in everyday behavior. This includes discrete reactive behaviors, such as maneuvers made to avoid a predator that was heard in the distance, or continuous rhythmic behaviors, such as riding a bicycle. Researchers have studied the behavioral aspects of sensorimotor coordination for over a century and various models have been proposed to account for these findings in terms of the nervous system. The purpose of this thesis was to use behavioral measures and electroencephalographic (EEG) recordings in humans to address several of the remaining issues regarding the spatiotemporal dynamics of cortical activity involved in continuous sensorimotor coordination. First, are the spatiotemporal patterns of cortical activity different for discrete and continuous coordination behaviors? To investigate discrete coordination, a simple reaction time (RT) task was used: upon each random presentation of the visual stimulus (2.5--3.5 sec ISI), subjects responded with a unimanual index finger flexion. Continuous coordination was studied via a synchronization-continuation paradigm, which used the same visual stimulus (1 sec ISI) and the same unimanual index finger flexion as in the reaction time task. By keeping the stimulus and motor properties constant for the two types of coordination it was hypothesized that differences in cortical activity would relate to an internal timekeeping system responsible for pacing the rhythmic movements made during continuous coordination. Several models postulate that oscillatory activity is used by the brain for maintaining task timing information (see Miall, 1989, and Church and Broadbent, 1991, for example). Frequency analysis revealed phase-locking of the alpha rhythm in the occipital lobe. This rhythm appears to play a role as a neural timekeeper mechanism: it was found that the degree of alpha phase-locking was predictable from the expected dependence on neural timekeeping, i.e. continuation was greater than synchronization, which is in turn was greater than reaction. These results also support the concept of modality specificity in neural timekeeping mechanisms (reviewed in Matell and Meck, 2004). Furthermore, the behavioral and EEG results support the theory that continuous sensorimotor coordination is largely influenced by timekeeping mechanisms, with sensory stimulation being employed occasionally to keep timing relatively accurate (Hary and Moore, 1987).

This study investigated the viability of a transient mechanism for the detection of counter-changing luminance, argued by Hock, et al. (2002) to be the informational basis for the perception of apparent motion. A series of experiments verified assumptions of the proposed mechanism and provided additional support for counter-changing luminance as the basis for apparent motion perception. It was found that: (1) The likelihood of perceiving apparent motion is best predicted by the product of local changes in luminance. This provided the basis for the multiplicative combination of subunit responses in the proposed mechanism (i.e. there is no motion signaled without coincident activation of both subunits). (2) When a brief interval of time separates a sequence of luminance onsets and offsets, or a sequence of luminance offsets, at a single element location, subunits exhibit summation of excitatory/inhibitory and excitatory/excitatory responses, respectively. This was consistent with the output of each subunit being determined by its biphasic temporal impulse response. (3) Apparent motion is specified only when there is a luminance offset at one location accompanied by a luminance onset at another location, and the likelihood of perceiving motion decreases with increases in the interval of time (ISI) separating the luminance offset from the luminance onset. Evidence that motion is not perceived beyond a limited range of ISI values indicated that the subunits respond transiently to luminance change. Accordingly, the effects of (ISI) are attributed to a reduction in the temporal coincidence of transient responses. This was supported by evidence that motion can be perceived when a luminance onset (indicating the end of the motion path) occurs before a luminance offset (indicating the start of the motion path). Computational simulations based on the product of transient responses to luminance offsets and onsets provide good qualitative matches to the experimental findings.