Age-related differences in visual perception: a PET study.

To assess age-related differences in cortical activation during form perception, two classes of visual textures were shown to young and older subjects undergoing positron emission tomography (PET). Subjects viewed even textures that were rich in rectangular blocks and extended contours and random textures that lacked these organized form elements. Within-group significant increases in regional cerebral blood flow (rCBF) during even stimulation relative to random stimulation in young subjects were seen in occipital, inferior and medial temporal regions, and cerebellum, and in older subjects, in posterior occipital and frontal regions. Group by texture type interactions revealed significantly smaller rCBF increases in older subjects relative to young in occipital and medial temporal regions. These results indicate that young subjects activate the occipitotemporal pathway during form perception, whereas older subjects activate occipital and frontal regions. The between-group differences suggest that age-related reorganization of cortical activation occur during early visual processes in humans.

Spatial characteristics of visual-auditory summation in human saccades.

Bimodal (auditory + visual) stimuli reduce saccade latencies in human observers to a degree that exceeds levels predictable by probabilistic summation between parallel, independent unimodal pathways. These interactions have been interpreted in terms of converging visual and auditory afferents within the oculomotor pathways, specifically within the superior colliculus (SC). The present work describes the spatial tuning of auditory-visual summation in human saccades, using diagnostics derived from stochastic models of information processing. Consistent with expectations based on the electrophysiology of the SC, the magnitude of facilitation varied with the degree of spatial correspondence, and the spatial tuning was quite coarse.

Spatial scale interactions and visual-tracking performance.

It has previously been observed that low spatial frequencies (< or = 1.0 cycles deg-1) tend to dominate high spatial frequencies (> or = 5.0 cycles deg-1) in several types of visual-information-processing tasks. This earlier work employed reaction times as the primary performance measure and the present experiments address the possibility of low-frequency dominance by evaluating visually guided performance of a completely different response system: the control of slow-pursuit eye movements. Slow-pursuit gains (eye velocity/stimulus velocity) were obtained while observers attempted to track the motion of a sine-wave grating. The drifting gratings were presented on three types of background: a uniform background, a background consisting of a stationary grating, or a flickering background. Low-frequency dominance was evident over a wide range of velocities, in that a stationary high-frequency component produced little disruption in the pursuit of a drifting low spatial frequency, but a stationary low frequency interfered substantially with the tracking of a moving high spatial frequency. Pursuit was unaffected by temporal modulation of the background, suggesting that these effects are due to the spatial characteristics of the stationary grating. Similar asymmetries were observed with respect to the stability of fixation: active fixation was less stable in the presence of a drifting low frequency than in the presence of a drifting high frequency.