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In noninvasive neuroimaging, neural activity is inferred from local fluctuations in deoxyhemoglobin. A fundamental question of functional magnetic resonance imaging (fMRI) is whether the inferred neural activity is driven primarily by synaptic or spiking activity. The answer is critical for the interpretation of the blood oxygen level-dependent (BOLD) signal in fMRI. Here, we have used well-established visual-system circuitry to create a stimulus that elicits synaptic activity without associated spike discharge. In colocalized recordings of neural and metabolic activity in cat primary visual cortex, we observed strong coupling between local field potentials (LFPs) and changes in tissue oxygen concentration in the absence of spikes. These results imply that the BOLD signal is more closely coupled to synaptic activity.

For binocular animals viewing a three-dimensional scene, the left and right eyes receive slightly different information, and the brain uses this 'binocular disparity' to interpret stereoscopic depth. An important theoretical conjecture in this mechanism is that coarse processing precedes and constrains finely detailed processing. We present three types of neurophysiological data from the cat's visual cortex that are consistent with a temporal coarse-to-fine tuning of disparity information. First, the disparity tuning of cortical cells generally sharpened during the time course of response. Second, cells responsive to large and small spatial scale had relatively shorter and longer temporal latencies, respectively. Third, cross-correlation analysis between simultaneously recorded pairs of cortical cells showed that connections between disparity-tuned neurons were generally stronger for coarse-to-fine processing than for fine-to-coarse processing. These results are consistent with theoretical and behavioral studies and suggest that rapid, coarse percepts are refined over time in stereoscopic depth perception.

Functional magnetic resonance imaging is an important tool for measuring brain function noninvasively, but the vascular and metabolic changes on which its measurements are based are not fully understood. Here, we examined the relationship between these changes and neural activity on a fine spatial scale through simultaneous measurements of tissue oxygen and extracellular neural activity in the cat lateral geniculate nucleus. Our findings indicate that activity-dependent increases in cerebral blood flow and oxidative metabolism occur on different spatial scales, and that the ratio between the two depends on the size of the activated neural population.

Motion and stereoscopic depth are fundamental parameters of the structural analysis of visual scenes. Because they are defined by a difference in object position, either over time or across the eyes, a common neural machinery may be used for encoding these attributes. To examine this idea, we analyzed responses of binocular complex cells in the cat striate cortex to stimuli of various intra- and interocular spatial and temporal shifts. We found that most neurons exhibit space-time-oriented response profiles in both monocular and binocular domains. This indicates that these neurons encode motion and depth jointly, and it explains phenomena such as the Pulfrich effect. We also found that the relationship between neuronal tuning of motion and depth conforms to that predicted by the use of motion parallax as a depth cue. These results demonstrate a joint-encoding of motion and depth at an early cortical stage.