Functional specialization in the lower and upper visual fields in humans is analyzed in relation to the origins of the primate visual system. Processing differences between the vertical hemifields are related to the distinction between near (peripersonal) and far (extrapersonal) space, which are biased toward the lower and upper visual fields, respectively. Nonlinear/global processing is required in the lower visual field in order to pergeive the optically degraded and diplopic images in near vision, whereas objects in far vision are searched for and recognized primarily using linear/local perceptual mechanisms. The functional differences between near and far visual space are correlated with their disproportionate representations in the dorsal and ventral divisions of visual association cortex, respectively, and in the magnocellular and parvocellular pathways that project to them. Advances in far visual capabilities and forelimb manipulatory skills may have led to a significant enhancement of these functional specializations.

Functional Specialization in the Lower and Upper Visual Fields in Humans: Its Ecological Origins and Neurophysiological Implications

Neural dynamics of adaptive timing temporal discrimination during associative learning

Event-related potentials to conjunctions of spatial frequency and orientation as a function of stimulus parameters and response requirements.

Conflicting accounts of the neurobiology of consciousness have emerged from previous imaging studies. Some studies suggest that visual consciousness relates to a distributed network of frontal and partietal regions while others point to localized activity within individual visual areas. While the two positions seem mutually exclusive, timing issues may help reconcile the two. Networks that appear unified in functional magnetic resonance imaging (fMRI) studies may reflect processes that are widely distributed in time. To help resolve this issue, we have investigated timing across a network correlating with consciousness in parallel fMRI and evoked potential (EP) studies of grating stimuli. At threshold, a stimulus is perceived on some occasions but not on others, dissociating sensory input and perception. We have found correlates of consciousness in the occipital lobe at 100 ms and in parietal, frontal, auditory and motor regions from 260 ms onwards. The broad temporal and spatial distribution of activity argues against a unified, distributed fronto-parietal correlate of consciousness. Instead, it suggests that correlates of consciousness are divided into primary and secondary network nodes, with early activity in the occipital lobe correlating with perception and later activity in downstream areas with secondary processes contingent on the outcome of earlier perceptual processing.

The Neural Correlates of Conscious Vision

Air Force Office of Scientific Research (90-0175, 90-0128); Defense Advanced Research Projects Agency (90-0083); National Science Foundation (IRI-87-16960); Office of Naval Research (N00014-91-J-4100)

K. Durden

Papers

Transient visually evoked potentials to the pattern reversal and onset of sinusoidal gratings